The SACVW is underlain by consolidated sedimentary rocks and filled with poorly consolidated to unconsolidated basin-fill materials (fig. 16). Consolidated sedimentary rocks include the Miocene to Pliocene Monterey Shale, Sisquoc Formation, and Foxen Mudstone which are combined and shown as a single undifferentiated unit (“consolidated bedrock”) in figure 16. Stratigraphically above the consolidated bedrock are poorly consolidated to unconsolidated basin-fill sedimentary materials that include, from oldest to youngest, the upper Pliocene Careaga Sandstone, the early Pleistocene and upper Pliocene Paso Robles Formation, the Pleistocene Orcutt Sand, Pleistocene to recent terrace and alluvial deposits, and young surficial sediments (Woodring and Bramlette, 1950; Dibblee and Ehrenspeck, 1988, 1989, 1993a, b; Dibblee and others, 1994a, b; Sweetkind and others, 2021; fig. 16). The Orcutt Sand and terrace and alluvial deposits are limited in extent and are mapped as part of the Paso Robles Formation and old alluvium on figure 16. Young surficial sediments are labeled as channel alluvium in figure 16.

The Careaga Sandstone is a shallow marine sandstone (Muir, 1964; Dibblee and Ehrenspeck, 1988, 1989, 1993a, b; Dibblee and others, 1994a, b). The Paso Robles Formation is unconsolidated non-marine clayey sand with scattered pebbles and lenses of pebble and cobble gravel with white siliceous shale clasts of the Monterey Shale (Dibblee and Ehrenspeck, 1988, 1989, 1993a, b; Dibblee and others, 1994a, b). The Paso Robles Formation includes discontinuous beds of pebbly sand, clay, and freshwater limestone, including a thin layer of light gray to tan pebbly sand with freshwater marly limestone and shale stratigraphically dissects the formation (mapped as a sub-unit of the Paso Robles Formation with map-unit abbreviation QTps by Dibblee and Ehrenspeck [1988, 1993a] and Dibblee and others [1994a]; fig. 16). The Orcutt Sand, and terrace and alluvial deposits mapped as part of the Paso Robles Formation (fig. 16), are geographically limited remnants of weakly consolidated, non-marine alluvial and wind-blown sands and gravels (Dibblee and others, 1994a). Channel alluvium are unconsolidated deposits of gravel, sand, and clay and are present along San Antonio Creek and its major tributaries.

The geologic structure of the SACVW is generally controlled by the Los Alamos syncline, a west-east trending fold that plunges to the east-southeast. The Los Alamos syncline, and associated faults and subparallel anticlinal and synclinal folds, deform the consolidated bedrock sedimentary materials and downfold and preserve the upper, younger part of the geologic section along the axis of the SACVW. Borehole data have shown that the basin-fill sedimentary materials are nearly 3,000 feet thick in the center of the basin to the west of the town of Los Alamos and rapidly decreases in thickness toward the basin margins (Hutchinson, 1980). Anticlinally folded consolidated bedrock is uplifted at the southern edge of the basin in the Purisima Hills and at the northern edge of the basin in the Casmalia and Solomon Hills (fig. 16). Surface geologic maps show that younger basin-fill sedimentary units are steeply north-dipping to vertical and locally overturned along the southern basin margin (Muir, 1964; Martin, 1985; Dibblee and Ehrenspeck, 1988, 1989, 1993a, b; Dibblee and others, 1994a, b). Strata on the north margin of the basin dip moderately to the south, as does the consolidated bedrock surface, toward the basin axis (Muir, 1964; Martin, 1985). At the western edge of the SACVW, upfolded consolidated bedrock at land surface and at shallow depths locally impedes groundwater flow, resulting in the formation of the Barka Slough through groundwater upwelling (Muir, 1964). The complexity of the top of consolidated bedrock surface reflects the prolonged and complicated tectonic history and structural development of the SACVW that affected the older rocks (Woodring and Bramlette, 1950; Namson and Davis, 1990).

Numerous faults have been mapped in the vicinity of the SACVW (fig. 16); several older faults, such as the Zaca, Purisima, and Lions Head faults, and one unnamed fault, are likely to only offset the consolidated bedrock (fig. 16; Sweetkind and others, 2021). Three faults, the Quaternary Pezzoni-Casmalia, Los Alamos, and Garey faults (U.S. Geological Survey and California Geological Survey, 2019; fig. 15), potentially offset the younger, basin-fill sedimentary units based on evidence of fault recency and proximity to basin-center. Other Quaternary faults in the vicinity of the SACVW include the Foxen Canyon fault, the East Huasna fault zone, and the Orcutt Oil Field fault zone (U.S. Geological Survey and California Geological Survey, 2019); these faults likely do not substantially offset the younger basin-fill sedimentary units.

Surface and subsurface geologic data were used to derive the geometry, thickness, and extent of the hydrogeologic units in the SACVW. Geologic map data were compiled from Woodring and Bramlette (1950), Muir (1964), Tennyson and others (1995), Dibblee and Ehrenspeck (1988, 1989, 1993a, b), Dibblee and others (1994a, b), and Jennings (2010). Surface-fault traces were compiled from these geologic maps and from the Quaternary Fault and Fold Database of the United States (fig. 16; U.S. Geological Survey and California Geological Survey, 2019). Ground-based direct-current resistivity surveys were performed in the north-central portion of the study area (fig. 16); the geophysical data from these surveys were interpolated as two-dimensional profiles of the electrical resistivity structure of subsurface materials down to about 650 feet below land surface (bls; Carlson, 2019). Lithologic and geophysical logs were compiled from 247 boreholes (fig. 16; Ely and others, 2022), including logs from water wells that were obtained from the California Department of Water Resources Well Completion Report database (https://water.ca.gov/Programs/Groundwater-Management/Wells/Well-Completion-Reports), the USGS National Water Information System (NWIS: https://waterdata.usgs.gov/nwis; U.S. Geological Survey, 2021), and oil and gas well from the California Geologic Energy Management Division (CalGEM; https://www.conservation.ca.gov/calgem/Pages/WellFinder.aspx). Additional stratigraphic information from oil wells were compiled from the tabulation of Sweetkind and others (2010; fig. 16).

Two multiple-depth, monitoring-well sites (SACR and SACC) and six single-depth, monitoring-well sites (SAHC, SASA, SAGR, SACR, SALS, and SALA) were drilled by the USGS along the main east-west axis of the SACVW and in Harris Canyon (figs. 16, 17; table 1.2). More information on the installation, drilling, and construction of each well site can be found in appendix 1 and in the USGS GeoLog Locator (U.S Geological Survey, 2018). Hydrogeologic data collected from these well-sites include detailed descriptions of drill cuttings, a suite of geophysical log measurements (fig. 18), aquifer slug tests (see “Definition of the Aquifer System” section), and discrete and continuous groundwater-level measurements (fig. 19; see “Long-Term Trends in Groundwater Levels” section). Compiled well data, including locations, well construction, and standardized lithology depth-interval information are available in a data release (Ely and others, 2022)

Interpretation of the geometry, thickness, and extent of the hydrogeologic units was an iterative process based on the conceptual understanding of SACVW geology and the input data described earlier. The structural conceptualization developed in previous studies of the basin (Woodring and Bramlette, 1950; Hutchinson, 1980; Martin; 1985) were incorporated into this study and used as guides from which interpretations of subsurface data were derived. Geologic maps, drillers’ lithology log descriptions, and geophysical logs were the primary data sources used to interpret subsurface hydrogeologic units. Interpretation of these data was generally a two-step procedure: (1) hydrogeologic units were identified and characterized in the two USGS multiple-depth, monitoring-well sites, where high-quality lithology descriptions, geophysical logs, physical-sample material, and groundwater-level data allowed for high-confidence interpretations and descriptions of each unit and (2) hydrogeologic units identified in the multiple-depth, monitoring-well sites were correlated to other boreholes throughout the subsurface of the SACVW using drillers’ log descriptions, geophysical logs, and geologic-map data Subsurface data points of consolidated bedrock were derived from stratigraphic information tabulated by Sweetkind and others (2010; fig. 16). Subsurface data points for the basin-fill hydrogeologic units were derived from drillers’ log descriptions and geophysical logs from water wells, oil wells, and USGS monitoring-well sites, groundwater-level measurements from USGS multiple-depth, monitoring-well sites, structural contours of the Careaga Sandstone and middle Paso Robles Formation, and previously published geologic sections of the channel alluvium.

The Paso Robles Formation was subdivided into three informal members: lower, middle, and upper (fig. 17). The lower and upper members were interpreted to consist of more coarse-grained sediment, and the middle member was interpreted to consist of fine-grained sediment and act as a confining layer where present. Initial interpretation of the three members was based on analysis of drillers’ log descriptions, geophysical logs, and groundwater-level measurements from USGS multiple-depth, monitoring-well sites SACR and SACC (figs. 18, 19); initial interpretations were corroborated using compiled subsurface information from other water wells and oil wells.

Drilling at USGS multiple-depth, monitoring-well sites, SACR and SACC, was initiated in the Paso Robles Formation (fig. 16). SACR was interpreted to intersect the Careaga Sandstone at a depth of about 550 feet bls based on the appearance of very fine-grained sand with shell fragments, and geophysical log data that indicate the presence of well-sorted, fine-grained material (fig. 18A; U.S. Geological Survey, 2018). In contrast, SACC was interpreted to be drilled entirely within the Paso Robles Formation (fig. 18B). The lithologies at each site within the interpreted Paso Robles Formation were generally heterogeneous, with multiple coarse-grained and fine-grained intervals apparent in the lithologic and geophysical logs (fig. 18). In SACR, the deepest well (SACR1) was completed within the Careaga Sandstone, and the remaining four wells were completed in the Paso Robles Formation (fig. 18A). SACR2, SACR3, and SACR4 had groundwater elevations that fluctuated through time, indicating a response to nearby groundwater pumping and a connected aquifer system across these depth intervals (fig. 19A). Groundwater-level differences between wells SACR2 and SACR4 indicated an upward vertical gradient (deep to shallow) from SACR2 to SACR3 to SACR4 within the Paso Robles Formation (fig. 19A). Groundwater levels between SACR1 and SACR2 indicated an upward vertical gradient (deep to shallow) between the Careaga Sandstone and overlying Paso Robles Formation.

Hydrographs from wells at SACC show a distinct difference in measured groundwater levels between the three deepest wells (SACC1, SACC2, and SACC3) and the two shallow wells (SACC4 and SACC5; fig. 19B). The three deepest wells had similar groundwater elevations and similar fluctuations through time, indicating a response to nearby pumping and a connected aquifer system across this depth interval. In contrast, the two shallow wells had groundwater elevations that were about 40 and 100 ft higher than levels in the deep wells, and had little fluctuations over time, indicating a muted response to nearby pumping (fig. 19B). Groundwater-level differences between the wells at SACC indicate a downward vertical gradient (from shallow to deep), and the different responses to pumping between the upper two and lower three wells indicate that they are hydraulically isolated from each other. This isolation is interpreted to primarily occur across about a 150-foot-thick, fine-grained layer identified in the drill cuttings and on the geophysical logs between wells SACC3 and SACC4 (fig. 18B).

Defining the extent of the intra-Paso Robles Formation fine-grained layer that was identified within borehole SACC was essential to understanding the hydrogeology of the SACVW. If found to be extensive, this layer could act as a barrier to vertical groundwater flow across the SACVW. Geologic maps show thin limestone beds within the Paso Robles Formation that are at least locally continuous (Woodring and Bramlette, 1950; mapped unit QTps of Dibblee and Ehrenspeck [1988, 1993a] and Dibblee and others [1994a]; fig. 16). These limestones are known to be associated with shale horizons at the base of the Paso Robles Formation and locally elsewhere in the unit (Woodring and Bramlette, 1950). The fine-grained layer penetrated in SACC (fig. 18B), and the thin limestone plus shale horizons mapped in outcrop as QTps (fig. 16), are interpreted to be one or more fine-grained terrestrial sediment intervals within the Paso Robles Formation, possibly representing a lacustrine deposit.

Single-depth, monitoring-well site SALS is located along San Antonio Creek where the mapped outcrop of unit QTps crosses the valley (fig. 16). The well penetrated abundant clay and silt, with trace gravels, to a depth of 60 feet bls (U.S. Geological Survey, 2018; Ely and others, 2022). A conceptual east-west section from SACR to SACC shows the interpreted subsurface configuration of basin-fill hydrostratigraphy and lithology (fig. 20). The top of Careaga Sandstone in this section was drawn from the contact in SACR (fig. 18A), an interpreted structural contour of the top of the Careaga Sandstone, and compiled elevations of the top of the Careaga Sandstone from the stratigraphic data tabulation of Sweetkind and others (2010). Lithologic data from nearby water wells were projected onto the section (Ely and others, 2022). The Paso Robles Formation was divided into informal lower, middle, and upper units based on drillers’ lithology logs and hydraulic data from SACC and SACR (fig. 20). The middle unit was interpreted as a near-continuous fine-grained interval connecting the fine-grained interval intercepted in SACC with the mapped outcrop of fine-grained unit QTps near monitoring well SALS. The lower and upper units were relatively coarse grained and consisted of substantial sequences of sands and gravels with interbedded fine-grained layers. In general, site SACR penetrates intervals in the Paso Robles Formation that are stratigraphically below most of the section sampled by SACC (fig. 20).

Digital datasets representing the top of each hydrogeologic unit were compiled as input data for the HFM (see “Data Sources” section above). Each hydrogeologic unit dataset contains outcrop information from geologic maps, subsurface picks from drillers’ lithology logs and geophysical logs, and reported stratigraphic picks from oil wells; datasets for selected units also contain structural contours.

Digital fault data for the HFM were created by generating a set of horizontal (x, y) and vertical (z) data points along each surface fault trace and determining x, y, and z at depth from the fault dip-angle and dip-direction. The dip-angle for all faults was assumed to be vertical; however, the faults were incorporated into the HFM with 85-degree dip angles because of limitations of the geologic modeling software. The older Zaca, Purisima, and Lions Head faults, and the one unnamed fault, are likely to only offset the consolidated bedrock (Sweetkind and others, 2021); therefore, these faults were only used to constrain the structure of the consolidated bedrock in the HFM (labeled as figs. 16 and 21). The Quaternary Pezzoni-Casmalia, Los Alamos, and Garey faults are likely to offset the consolidated bedrock unit and the younger, basin-fill hydrogeologic units based on evidence of fault recency and proximity to basin-center (U.S. Geological Survey and California Geological Survey, 2019; figs. 16, 17). These faults were used in the HFM to constrain the structure of all units in the HFM. The Foxen Canyon fault, the East Huasna fault, and the Orcutt Oil Field fault zone (fig. 16) were not included in the HFM.

The mapped locations of the Pezzoni-Casmalia and Los Alamos faults from the Quaternary Fault and Folds Database of the United States (figs. 16, 17; U.S. Geological Survey and California Geological Survey, 2019) are based on inferred locations reported by Sylvester and Darrow (1979) even though these faults are not identifiable at land surface (Sweetkind and others, 2021). Although there is no evidence of these faults at land surface, these structures could be present at depth and therefore are included in the HFM as mapped by the U.S. Geological Survey and California Geological Survey (2019; figs. 16, 17). Additional work was not completed in this study to verify the existence or location of these faults.

The HFM was built using EarthVision, a 3D geologic-modeling software package (Dynamic Graphics, Inc., 2021). EarthVision uses a biharmonic cubic-spline algorithm that utilizes a minimum-tension (minimum curvature) gridding technique designed to generate horizon grids from 3D point data (Dynamic Graphics, Inc., 2021), such as the hydrogeologic unit datasets described earlier. Grid spacing of all hydrogeologic unit horizons was 492.1 ft in the x and y horizontal directions. Each hydrogeologic unit horizon grid was constructed in stratigraphic order from oldest (deepest) to youngest (most shallow), and subsequently younger horizons were built on top of previously constructed horizon grids. The total thickness of each hydrogeologic unit was defined as the difference between a given unit horizon and the upper surface of the unit horizon below. Finally, all horizon grids were clipped at land surface using a discretized digital elevation model (DEM) grid based on the National Elevation Dataset 10-meter DEM (U.S. Geological Survey, 2013).

In EarthVision, the HFM was formed from a set of independent blocks (called fault blocks) that represented hydrogeologic materials within a particular volume defined by faults. Horizon tops were interpolated independently for each fault block, and offsets across each fault were maintained during the gridding process by utilizing data point elevations on either side of the fault surface for each hydrogeologic unit. Each of the fault blocks were modeled independently and then combined to form a single model. The resulting EarthVision horizon grids were exported to Esri ArcGIS version 10.7.1 geographic information systems software and were manually adjusted to enforce a minimum thickness of about 10 ft for each hydrogeologic unit and to ensure that the grid was consistent with basic geologic principles and the geologic understanding of the SACVW.

Maps showing the extent, elevation, and thickness of the hydrogeologic units in the HFM are shown in figures 21 and 22. Figure 21 shows the elevation of the top of the consolidated bedrock unit. Consolidated bedrock is present across the SACVW and defines the east-plunging synclinal structure of the basin. Surface outcrops of this unit in the Purisima and Solomon Hills are at elevations of more than 1,600 feet above the North American Vertical Datum of 1988 (NAVD 88), but in the center of the basin, the unit surface is as deep as about 3,200 feet below NAVD 88. Figure 22 shows map view images of the extent and thickness of the basin-fill hydrogeologic units. Careaga Sandstone is present across most of the SACVW and ranges from about 10 feet thick in the Purisima and Casmalia Hills to upward of 3,000 feet thick in the center of the basin (fig. 22A). The lower, middle, and upper Paso Robles Formation occupy successively smaller areas in the synclinal axis of the basin (figs. 22B–D). Channel alluvium is present along San Antonio Creek and other active stream channels in the SACVW (fig. 22E). Channel alluvium was difficult to distinguish from underlying units; therefore, average thicknesses were assigned to the channel alluvium based on the work of Muir (1964): 82 ft on the valley floor along San Antonio Creek, 49 ft in Harris Canyon and along San Antonio Creek west of Barka Slough, and 16 ft in all tributary streams.

An egg-crate diagram of the HFM is shown in perspective view in figure 23 and two vertical cross-section panels through the HFM are shown in figure 24. The perspective view of the HFM shows the upper surface of the consolidated bedrock horizon as a solid-block and the basin-fill hydrogeologic units as a series of east-west and north-south vertical profiles. The Careaga Sandstone is present across the SACVW and fills most of the volume above consolidated bedrock. The lower member of the Paso Robles Formation is the second most-voluminous unit in the HFM: the unit varies in thickness, follows the structure of the underlying Careaga Sandstone and consolidated bedrock, and thins against the underlying Careaga Sandstone east of well SALA. The middle fine-grained member of the Paso Robles Formation is thin and barely visible at the scale presented in figure 23. The upper member of the Paso Robles Formation fills the uppermost part of the model volume and is the second most voluminous unit in the southeastern part of the HFM. Channel alluvium is also not easily visible at the scale presented in figure 23.

Vertical sections through the HFM (fig. 24) illustrate the thickness, extent, and configuration of hydrogeologic units in the SACVW. The east-west oriented cross-section (A–A’; fig. 24A) shows the along-axis structure of the SACVW; at the west (left) end of the cross-section, consolidated bedrock is exposed at land surface and overlain by a thin layer of Careaga Sandstone and channel alluvium. Barka Slough is located above these uplifted units, west of the lower member of the Paso Robles Formation. The hydrogeologic units dip to the east (right), following the eastward plunge of the Los Alamos syncline (fig. 16). The total thickness of the Paso Robles Formation increases from west to east, until just east of well-site SACC; here, the synclinal plunge is less prominent, and the hydrogeologic units rise to the east. The south-north oriented cross-section (B–B’; fig. 24B) shows the synclinal structure of the SACVW, with all units dipping inward toward the axis of the basin. The syncline is asymmetric, with the consolidated bedrock and Careaga Sandstone dipping more shallowly on the north (right) side of the basin than the south (left) side. The overlying units thin and disappear at the margins of the basin and differ in their dip patterns from the underlying consolidated bedrock and Careaga Sandstone, implying stratigraphic onlap or angular unconformity between the hydrogeologic units.

The thickness of basin-fill hydrogeologic units in the SACVW ranges from 0 to 650 ft at the western and eastern margins of the basin, to more than 3,200 ft east of the Pezzoni-Casmalia fault in the center of the basin (fig. 24). The Careaga Sandstone is thicker than the combined thickness of the lower, middle, and upper members of the Paso Robles Formation, which are thickest in the middle part of the SACVW, just east of well SACC; the synclinal plunge is less prominent, and the hydrogeologic units rise to the east (fig. 24A). In the northern part of the SACVW, the hydrogeologic units dip gently southward toward the fold axis. The unit horizons in the northern part of the SACVW also indicate the presence of at least one additional anticline-syncline fold structure, north of well SACC, consistent with the surface geologic map. In the southern part of the SACVW, hydrogeologic units dip steeply southward toward the fold axis (fig. 24B).

The HFM is a hydrogeologic model that was developed to synthesize geologic structures and hydrologic information, hydrogeologic conditions, and the relative hydrologic importance of hydrogeologic units in the SACVW for use in a numerical hydrologic model (Woolfenden and others, 2022). The consolidated bedrock unit (a combination of primarily Tertiary sedimentary rocks) is assumed to be hydrologically homogeneous and likely provides limited, if any, contribution to the groundwater flow system. The Careaga Sandstone is treated as a geologic and hydrogeologic unit in the HFM and is assumed to be a hydrologically homogeneous, semi-consolidated, fine-grained sandstone that may contribute to groundwater availability in the SACVW.

Subsurface lithologic data and hydrologic data were combined to demonstrate that the Paso Robles Formation is more lithologically and hydrologically heterogeneous in the SACVW than has been shown on geologic maps of the region (Woodring and Bramlette, 1950; Dibblee and Ehrenspeck, 1988, 1989, 1993a, b; Dibblee and others, 1994a, b). Therefore, the Paso Robles Formation was divided into upper, middle, and lower hydrogeologic units. The coarser-grained lower and upper units of the Paso Robles Formation likely provide more groundwater storage within the formation than the informal, fine-grained middle unit. The predominantly fine-grained middle unit of the Paso Robles Formation functions as a semi-confining to confining unit depending on its subsurface continuity within the basin. A similar fine-grained lithostratigraphic interval within the Paso Robles Formation has been identified in the Los Osos Valley groundwater basin (not shown) about 60 mi north of the SACVW in San Luis Obispo County, California (Cleath-Harris Geologists, 2018); however, without age control in both basins, it is unclear whether these stratigraphic sequences are age-equivalent and represent a single event. Available borehole data indicate that the SACVW may have additional local confining layers in the heterogeneous lower and upper units of the Paso Robles Formation (fig. 20; Ely and others, 2022).

Faults in the HFM are known or inferred to be present from geologic and geophysical evidence (see “Faults” section), but their potential influence on groundwater flow in the SACVW is poorly understood. Faults can inhibit groundwater flow in permeable sediments through the presence of fine-grained gouge material, chemical cementation of sediments, and the juxtaposition of transmissive layers across faults. The Pezzoni-Casmalia and Los Alamos faults transect the SACVW and are the faults most likely to influence groundwater flow in the basin because they are mapped as transecting the basin-fill hydrogeologic units (fig. 17).

The conceptual hydrogeology of the SACVW developed based on previously published reports and analysis of hydrogeologic data was central to the construction of the HFM. The conceptual model (which includes the conceptual section in fig. 20) provided an interpretive framework on which to build the digital HFM and guided model development so that the model did not simply rely on extrapolation of input data. The HFM is partly deterministic because the model output is defined by the input data. The HFM is also partly conceptual because the initial horizons generated by the EarthVision geologic modeling software were based on interpolation of hydrogeologic information. These horizons generally represented the input data and provided reasonable geologic representations of each hydrogeologic unit. The SACVW is a relatively narrow basin; therefore, interpolation distances were short and input data were sufficient to constrain horizon gridding. Adjustments to unit extents were required where units thinned out and disappeared, beyond, or fell short of their expected terminations. Unit thicknesses were manually adjusted to be at least 10 feet thick, or 16 feet in the case of the channel alluvium. The resulting HFM provides a balance between closely matching available data and following the conceptual hydrogeology developed for the basin.

Aquifer properties (transmissivity, horizontal and vertical hydraulic conductivity, specific yield, and specific capacity) and textural analysis of grain size for the hydrogeologic units in the SACVW were reported in previous studies and estimated from aquifer slug tests performed at USGS monitoring-well sites, and from borehole lithology data compiled for the HFM (sources of these data are discussed below). Aquifer property and textural data can form the geologic basis for estimating the hydraulic properties within a numerical hydrologic-flow model (Burow and others, 2004; Faunt and others, 2010). A summary of the aquifer property and textural grain-size information collected and compiled for this study are presented below and discussed in more detail for each of the hydrogeologic units.

Previous studies of the SACVW reported estimates of transmissivity, but reported values are often for the basin-fill materials in general and not specific to a particular aquifer unit. Hutchinson (1980) reported transmissivity values ranging from 2,600 to 34,000 square feet per day (ft²/d), but none of the wells used by Hutchinson (1980) to test the aquifer penetrated more than half the estimated aquifer thickness. The highest transmissivity values reported by Hutchinson (1980) were near the perimeter of the SACVW, while the lowest values were reported along the valley floor. In contrast, Martin (1985) reported calibrated transmissivity values from their groundwater model ranging from less than 100 ft²/d along the perimeter of the SACVW, to more than 20,000 ft²/d along the valley floor. Tetra Tech, Inc. (2013) reported horizontal hydraulic conductivity values from a calibrated groundwater model ranging from 0.08 to 400 feet per day (ft/d) and vertical hydraulic conductivity values ranging from 0.005 to 0.1 ft/d. The average specific capacity of wells in the SACVW was reported to be about 11 gallons per minute per foot (gal/min/ft) of drawdown (Muir, 1964).

Aquifer slug tests were completed at each of the USGS single- and multiple-depth, monitoring-well sites, and at monitoring well 8N/34W-21A1 (fig. 17; table 7; Ely and others, 2022), to estimate the hydraulic conductivity of the hydrogeologic units that comprise the aquifer system. Groundwater levels for each well were measured prior to aquifer slug testing and were monitored using pressure transducers during the tests. A physical slug was used for the slug test if the initial measured groundwater level was within about 7 ft of the perforation interval, otherwise an air slug was used. Air slugs were avoided when the groundwater level was near the top of the perforation interval to avoid displacing the water level to below the top of the interval during testing (Arnold, 2015). Pressure transducers were installed at depths ranging from 7 to 15 ft below the initial pre-test groundwater level, and the slug was lowered to approximately 5 ft above the initial groundwater level. Sufficient time, about 15 to 20 minutes for most wells, was allowed for groundwater levels in the well to recover before completing another slug test. Multiple slug tests were performed at each well to check for consistency between the different tests and provide enough data to statistically evaluate the test results.

Evaluation tools developed by Halford and Kuniansky (2002) and Butler and Garnett (2000) were used to help evaluate the slug-test data and estimate hydraulic conductivity (table 7; Ely and others, 2022). The following assumptions were made for data evaluation using both methods: the volume of water injected into or discharged from the well occurs instantaneously at time (t) = 0, the well diameter is finite, and the well fully penetrates the aquifer. The aquifers were assumed to be confined, homogeneous, isotropic, and uniform in thickness, the flow of groundwater within the aquifer was assumed to be horizontal and radially symmetric, and the response within the aquifer was assumed to be influenced across the entire screened interval. Relatively small changes in hydraulic head were observed in all wells; therefore, early-time data (about 2–10 seconds) were removed from the datasets used in the analysis.

The single-depth, monitoring-well sites and shallowest wells of the multiple-depth, monitoring-well sites (SACC5 and SACR5), and monitoring well 8N/34W-21A1 were assumed to be in unconfined aquifers; therefore, the Bouwer-Rice method of analysis (Bouwer and Rice, 1976) was used to evaluate the slug test data (table 7; fig. 17). For these wells, the results from the Bouwer-Rice method were checked against or compared to the Cooper-Greene method of analysis (Cooper and others, 1967; Shapiro and Greene, 1995; Greene and Shapiro, 1998), unless the water level was within the screened interval, and the KGS_High-K analysis method (Butler and others, 2003) for consistency in hydraulic conductivity value, curve match, and root mean square error. The deep wells of the multiple-depth, monitoring-well sites (SACC1 to SACC4 and SACR1 to SACR4; fig. 17; table 7) were assumed to be in confined aquifers, and either the Cooper-Greene or KGS_High-K method was used for analysis. A summary of the analytical results for each well is presented in table 7, shown in figure 25A, and discussed for each hydrogeologic unit below.

An analysis of the textural variability of lithology and grain size was performed for the basin-fill hydrogeologic units using borehole data (figs. 22, 26; Ely and others, 2022). The primary variable used was sediment grain size, tabulated as percentage of coarse-grained texture (Phillips and Belitz, 1991; Burow and others, 2004; Faunt and others, 2010). For each well in the study area, each downhole lithologic interval was classified in a binary fashion as either “coarse-grained” or “fine-grained” based on the well driller’s description (Ely and others, 2022). In this study, coarse-grained intervals were those dominated by gravel- and sand-size clasts with little to no clay matrix; all other lithology classes were considered fine grained. Units that had a considerable fraction of large clasts were classified as fine grained if the clasts tended to be suspended in a fine-grained matrix. Textural variability in the basin-fill units is ultimately a function of the sedimentary facies, environment of deposition, and depositional history of the basin.

Textural data were compiled by digitally intersecting the downhole lithologic interval data (in 16-ft intervals) in each borehole with the elevation of the top of each basin-fill hydrogeologic unit, thereby assigning specific lithologic intervals to a hydrogeologic unit at each borehole. The percentage of coarse-grained material for a borehole within a given hydrogeologic unit was calculated as the aggregate thickness of coarse-grained intervals penetrated by the borehole, divided by the total thickness of the unit in that borehole (fig. 22). The frequency of percent coarse values for each hydrogeologic unit was then normalized by the total thickness of all boreholes that penetrated that unit and plotted as frequency distributions (fig. 26).

Specific capacity data were calculated for 28 water wells with reported aquifer pump test data (fig. 25B; table 1.3). Calculated specific capacity results were assigned to hydrogeologic units for which at least 60 percent of the reported perforation interval intersected a given unit. A value of 60 percent of the perforation interval intersection was chosen to provide a reasonable distribution of specific capacity estimates for each hydrogeologic unit.

Channel alluvium is composed of unconsolidated gravels, sands, and clays and is located proximal to San Antonio Creek and its tributaries (fig. 17). The sedimentary materials that comprise the channel alluvium were difficult to distinguish from underlying aquifer units; therefore, the channel alluvium was assigned thicknesses based on the work of Muir (1964). In the HFM, the channel alluvium has thicknesses of about 82 ft on the valley floor along San Antonio Creek, 50 ft along Harris Canyon and along San Antonio Creek west of Barka Slough, and 16 ft along all tributary streams. Channel alluvium is not considered a major aquifer because it is probably unsaturated across most of the SACVW, except along the valley floor near Barka Slough where groundwater discharges to the land surface.

Aquifer property data for channel alluvium were compiled from hydraulic conductivity results estimated from aquifer slug tests completed in select USGS single-depth, monitoring-well sites (fig. 25A; table 7), the estimated percentage of coarse-grained material from borehole logs (figs. 22E, 26A), and reported estimates of flow rate and specific capacity (Muir, 1964). Estimated hydraulic-conductivity values from aquifer slug tests at three wells along the main channel (SALS, SASA, and SAHG; figs. 17, 25A; table 7) ranged from 0.3 to 8.7 ft/d. Sediment grain size in the channel alluvium, tabulated as percentage of coarse-grained texture (figs. 22E, 26A) had a distinct peak at low values of percent coarse, which could indicate deposition in a relatively low-gradient surface-water flow system on geologic timescales. Wells in channel alluvium in the eastern (upstream) part of the SACVW had average flow rates of about 350 gallons per minute (gal/min) and average specific capacities of about 13 (gal/min)/ft (Muir, 1964). Wells in channel alluvium in the western (downstream) middle part of the valley had average flow rates of about 200 gal/min and average specific capacities of about 8 (gal/min)/ft (Muir, 1964). Specific capacity data were not available in this unit for water wells compiled as part of this study.

The Paso Robles Formation is non-marine in origin, up to about 2,000 feet thick, and is identified by Muir (1964) as the main water-bearing unit in the SACVW. The Paso Robles Formation was partitioned into three informal members (lower, middle, and upper) based largely on drillers’ lithology logs, geophysical logs, and hydraulic data from USGS multiple-depth, monitoring-well sites SACC and SACR (U.S. Geological Survey, 2018; Ely and others, 2022). The middle member is interpreted as a continuous fine-grained interval, while the lower and upper memebrs are coarser grained, consisting of substantial sequences of sands and gravels, with interbedded fine-grained layers (figs. 18, 20). The lower and upper members of the Paso Robles Formation provide most of the groundwater storage within the unit, while hydraulic properties of the middle member inhibit vertical groundwater flow between the upper and lower members. The middle member of the Paso Robles Formation could be related (or similar) to a regional low-permeability unit that impedes groundwater flow identified in another coastal groundwater basin about 60 mi north of the SACVGB that is interpreted to separate the upper and lower aquifers of that basin (Cleath-Harris Geologists, 2018).

Aquifer property data for the Paso Robles Formation were compiled from hydraulic conductivity results estimated from aquifer slug tests completed in USGS single- and multiple-depth, monitoring-well sites (figs. 17, 25A; table 7; Ely and others, 2022), the estimated percentage of coarse-grained material from borehole logs (figs. 22B–D, 26B–D), specific capacities calculated from water well driller’s records (table 1.3; fig. 25B), and reported estimates of flow rate, specific capacity, and specific yield of the undifferentiated unit (Muir, 1964; Singer and Swarzenski, 1970). Measured hydraulic conductivity values from aquifer slug tests for three wells perforated in the upper Paso Robles Formation had a range of values from 0.32 to 1.0 ft/d (fig. 25A). No wells with aquifer slug tests were perforated in the middle Paso Robles Formation. Measured hydraulic conductivity values from aquifer slug tests for eight wells perforated in the lower Paso Robles Formation had a median value of 2.2 ft/d, with a range of values from 0.04 to 25.0 ft/d (fig. 25A; table 7).

Sediment grain size, tabulated as percentages of coarse-grained texture, was estimated for the three hydrogeologic members of the Paso Robles Formation (figs. 22B–D, 26B–D). The lower member of the Paso Robles Formation showed a broad range of percent coarse values, but the middle and upper members of the Paso Robles Formation showed distinct peaks at low values of percent coarse. Specific capacity was calculated for eleven water wells for which 60 percent of the reported perforation interval intersected with the lower, middle, or upper members of the Paso Robles Formation (fig. 25B; table 1.3). one water well was perforated in each of the middle and upper members of the Paso Robles Formation, with specific capacity values of 2.6 and 12.2 (gal/min)/ft, respectively. Nine water wells perforated in the lower member of the Paso Robles Formation had a median specific capacity value of 3.8 (gal/min)/ft, with a range of values from 1.78 to 17.9 (gal/min)/ft. A well near the town of Los Alamos was reported by Muir (1964) to have a flow rate of 1,200 gal/min and a specific capacity of 12 (gal/min)/ft. Flow rates of 500 gal/min and specific capacities of between 5 to 15 (gal/min)/ft were common for the Paso Robles Formation (Muir, 1964). Specific yield estimates of the Paso Robles Formation from nearby groundwater basins were estimated to be about 0.15 (Singer and Swarzenski, 1970; Miller, 1976).

The Careaga Sandstone is a weakly indurated, semi-consolidated, fine-grained sandstone that may contribute to groundwater availability in the SACVW. Muir (1964) observed that this unit yielded only small quantities of water to wells and that the lack of production was attributed to the difficulty of completing wells in this poorly consolidated sand. The Careaga Sandstone may, however, yield substantial amounts of groundwater at depth (Muir, 1964).

Aquifer property data of the Careaga Sandstone were compiled from hydraulic conductivity results estimates from aquifer slug tests completed in USGS monitoring-wells SACR1 and SAHC (figs. 17, 25A; table 7; Ely and others, 2022), specific capacities calculated from water well driller’s records (fig. 25B; table 1.3), the estimated percentage of coarse-grained material from borehole logs (fig. 26E), and reported estimates of permeability (Upson and Thomasson, 1951). Measured hydraulic conductivity values for two wells perforated in the Careaga Sandstone had values of 0.43 and 7.5 ft/d (fig. 25A; table 7). Sediment grain size, tabulated as percentage of coarse-grained texture, showed a broad range of percent coarse values, likely the result of averaging sediment-texture data from coarse-grained and fine-grained parts of the Careaga Sandstone (figs. 22A, 26E).Specific capacities were calculated for seventeen water wells for which at least 30 percent of the reported perforation interval intersected with the Careaga Sandstone; the median specific capacity was 4.2 (gal/min)/ft, with a range of values from 0.3 to 83.3 (gal/min)/ft (fig. 25B). Laboratory tests by Upson and Thomasson (1951) on samples of Careaga Sandstone from the Santa Ynez River Valley groundwater basin (fig. 1) indicated an average coefficient of permeability of about 12 cubic feet per day per square foot (ft³/d/ft²).

In the SACVW, the consolidated bedrock unit is a combination of consolidated, or highly compacted, Tertiary sedimentary rocks, including Monterey Shale, Sisquoc Formation, and Foxen Mudstone. Aquifer property data for this unit were limited; an aquifer slug test in one well (8N/34W-21A1; fig. 17; table 7) had a measured hydraulic conductivity value of 0.36 ft/d (fig 25A; table 7), and specific capacity data were not available in this unit for water wells compiled as part of this study. The consolidated bedrock unit contains small amounts of water in joints and fractures but does not store or transmit water in substantial quantities (Muir, 1964). For the purposes of this study, consolidated bedrock is assumed to be hydrologically homogeneous and likely provides limited, if any, contribution to the groundwater-flow system.

Sources of natural recharge in the SACVW are infiltration of precipitation and infiltration from streams. All estimates of natural recharge are reported for calendar years, except for those by Tetra Tech, Inc. (2012), which are for water years. Martin (1985) estimated the amount of natural recharge during early development (before calendar year 1944, not shown on fig. 27) to be about 6,200 acre-ft/yr, with average annual infiltration from precipitation and streamflow of 4,700 and 1,500 acre-ft/yr, respectively. Estimates of natural recharge to the SACVW during calendar years 1948–2018 ranged from about 5,000 to 30,000 acre-ft/yr (fig. 27). Estimates of natural recharge during calendar years 1948–80 (fig. 27) ranged from about 5,000 to 12,000 acre-ft/yr (Muir, 1964; Hutchinson, 1980; Martin, 1985; Tetra Tech, Inc., 2012); estimates of natural recharge during calendar years 1980–2010 were substantially larger, ranging from about 15,000 to 30,000 acre-ft/yr (Gibbs, 2006; Tetra Tech, Inc., 2012).

Anthropogenic recharge to the groundwater system in the SACVW comes from infiltration of wastewater effluent, agricultural irrigation return, and leakage from municipal water systems (figs. 28, 29). All estimates of anthropogenic recharge are for water years.

Evapotranspiration is the combined water loss to the atmosphere from evaporation of water and soil water and plant transpiration. ET is a function of potential evapotranspiration (PET), water availability, soil texture, vegetation type, vegetation density, and root depth. PET is the rate of ET that is possible given an unlimited supply of water under specific climatic conditions (Maidment, 1993). If water supply is limited, actual ET will be less than PET. All reported estimates of evapotranspiration are for calendar years, except for those by Tetra Tech, Inc. (2012), which are for water years.

The average reference evapotranspiration is the PET for a well-watered cropped grass surface. This value is 46.3 in/yr in the western part of the SACVW (California Irrigation Management Information System [CIMIS] zone 3) and 49.7 in/yr in the eastern part of the SACVW (CIMIS zone 6; California Irrigation Management Information System, 2005). The average total ET of chaparral, cropland, and pasture lands in the SACVW (excluding Barka Slough and San Antonio Creek) was estimated to be about 94,600 acre-ft/yr during 1958–77, and this estimate of average total ET was computed as a residual in the hydrologic budget of Hutchinson (1980).

Natural discharge from phreatophyte evapotranspiration occurs in Barka Slough and along the channel of San Antonio Creek where native vegetation habitually obtains water from areas where the groundwater table is shallow. Prior to substantial groundwater development in the SACVW (1943 and earlier; Martin, 1985), native vegetation in Barka Slough consisted predominantly of marshland plants, including cattail (Typha spp.), bulrush (Scirpus spp.), saltgrass (Distichlis spp.) and tule (Schoenoplectus acutus var. occidentalis)—these plants require abundant water and live where the depth to water is only a few inches (Mower and Nace, 1957; Martin, 1985). Native vegetation along the San Antonio Creek channel consisted of riparian cottonwood (Populus spp.) and willow (Salix spp.) trees—these plants also require abundant water but can extend their roots to deeper depths to reach the groundwater table.

Muir (1964) estimated an average ET rate of about 4.7 acre-ft/yr per acre (acre-ft/yr/acre) for marshland plants in Barka Slough, and an average ET rate of about 3.0 acre-ft/yr/acre for riparian trees along San Antonio Creek. Martin (1985) assumed that these rates were representative of pre-development conditions (1943 and earlier; Martin, 1985) and estimated an average total phreatophyte ET rate of 3,500 acre-ft/yr (3,100 acre-ft/yr for 660 acres of marshland at Barka Slough and 400 acre-ft/yr for 130 acres of riparian vegetation along San Antonio Creek). Muir (1964) calculated a lower rate of total phreatophyte ET of 3,000 acre-ft/yr for 1958 based on a lower amount of marshland acreage (550 acres). Tetra Tech, Inc. (2012) estimated an average total phreatophyte ET rate of 2,900 acre-ft/yr for 1935–2010 based on ET rates of 3.3 acre-ft/yr/acre for marshland in Barka Slough (670 acres) and 2.2 acre-ft/yr per acre for riparian vegetation along San Antonio Creek (700 acres).

Martin (1985) estimated reductions in the total amount of water lost through phreatophyte ET in Barka Slough and along San Antonio Creek using their groundwater-flow model; the reductions in ET were assumed to be the result of increased pumpage in the SACVW leading to a reduction in groundwater levels and available groundwater for consumptive use. The estimated phreatophyte ET rate in Barka Slough was reduced from about 3,100 acre-ft/yr in pre-development conditions (1943 and earlier) to about 1,000 acre-ft/yr in 1977, and the estimated ET rate along San Antonio Creek was reduced from about 400 to 0 acre-ft/yr during the same period (Martin, 1985). The reduction in the amount of water lost through ET at Barka Slough was consistent with an observed increase in vegetation that can survive in drier conditions, such as willow trees (Dial and Pisapia, 1980). The reduction in the amount of water lost through ET along San Antonio Creek was directly attributed to a lowering of the groundwater table below the assumed ET extinction depth (the depth at which ET ceases) of 10 ft below land surface. Phreatophyte ET along San Antonio Creek is probably not zero in present day, despite the estimate of Martin (1985) that phreatophyte ET was 0 acre-ft/yr in 1977; satellite imagery from Google Earth (imagery date August 11, 2018; Google, Maxar Technologies 2021) show riparian vegetation along most of the entire reach of the creek, indicating that at least some groundwater is consumed by riparian vegetation.

Pumpage, or anthropogenic discharge, is the main mechanism by which groundwater is removed from the SACVW. Groundwater pumpage in the SACVW is used to satisfy the needs of four water-use sectors: domestic, municipal, military, and agricultural. The first domestic wells in the SACVW were developed following initial surveying of the town of Los Alamos in 1876 (Muir, 1964). Pumpage for military use by VSFB and municipal use by the Los Alamos CSD began in the mid-1900s, and pumpage for agricultural irrigation began around 1900 (Muir, 1964). Since at least the 1930s, most groundwater pumped in the SACVW has been used to satisfy agricultural water demand (Muir, 1964; Hutchinson, 1980; Martin, 1985; Tetra Tech, Inc., 2012). Total annual groundwater pumpage from the four water-use sectors during 1948–2018 increased from about 3,000 acre-ft/yr in 1948 to about 32,600 acre-ft/yr in 2018 (fig. 30; table 8).

Reported estimates of groundwater pumpage for the four water-use sectors are for calendar years and water years, depending on the data source. The sources of groundwater pumpage data, and whether the reported values are for calendar or water years, are listed in table 8 and summarized below. Estimates of domestic pumpage were for calendar years (Hutchinson, 1980; this study, see below); estimates of military pumpage were for water years (K. Domako, Vandenberg Space Force Base, written commun., 2019), except for 1963, which was for calendar year (Martin, 1985); and estimates of municipal and agricultural pumpage were for water years (Tetra Tech, Inc., 2012; K. Barnard, Los Alamos Community Services District, written communication, 2019; this study, see below) and calendar years (Hutchinson, 1980; Martin, 1985; Gibbs, 2009).

The estimated amounts of domestic pumpage (assumed to be from rural residents living outside the town of Los Alamos, see table 1.1) during 1948–2018 ranged from 90 to 189 acre-ft/yr (fig. 30; table 8). Domestic pumpage was reported to be 90 acre-ft/yr in calendar year 1976 (Hutchinson, 1980, citing Santa Barbara County Water Agency, 1977). Hutchinson (1980) assumed that the estimate from Santa Barbara County Water Agency (1977) was probably the average value for rural domestic pumpage during calendar years 1958–77. In this study, 90 acre-ft/yr is assumed to be the value for rural domestic pumpage during calendar years 1948–77. In 1976, the estimated rural residential population (people living in the SACVW but outside the town of Los Alamos) was 353 people (see appendix 1; table 1.1)—assuming that domestic pumpage in 1976 was 90 acre-ft/yr, the resulting per capita water demand was 0.25 acre-ft/yr. For this study, the amount of rural domestic pumpage during calendar years 1978–2018 was calculated based on the estimated rural residential population of the SACVW (table 1.1) and a per capita water demand of 0.25 acre-ft/yr per person.

Reported and metered municipal pumpage during 1948–2018 ranged from 100 to 375 acre-ft/yr (fig. 30; table 8). The Los Alamos CSD is the sole purveyor of groundwater for municipal use in the SACVW and provides water to customers within its boundary area (fig. 28). The Los Alamos CSD owns five wells within its boundary area, three of which were actively pumping groundwater during water years 2007–18. Municipal pumpage compiled for this study was reported in Martin (1985) and Gibbs (2009) for calendar years 1948–75 and 1976–92, respectively, and monthly data were provided by the Los Alamos CSD for calendar years 1993–2018 (K. Barnard, Los Alamos Community Services District, written commun., 2019). The monthly municipal pumpage data provided by the Los Alamos CSD was converted to annual municipal pumpage for water years 1993–2018.

Groundwater pumpage for military use began in 1963. Groundwater is pumped from several wells in the western end of the SACVW near Barka Slough (pumping area shown in fig. 28). Military pumpage compiled for this study was reported by Martin (1985) for calendar year 1963, and monthly data were provided by VSFB for calendar years 1964–2018 (K. Domako, Vandenberg Space Force Base, written commun., 2019). The monthly military pumpage data provided by VSFB were converted to annual military pumpage for water years 1964–2018. The annual pumpage from calendar year 1963 to water year 1997 ranged from about 320 acre-ft to 4,112 acre-ft (fig. 30; table 8). In 1997, VSFB began receiving delivery of California State Water from northern California to augment its water supply, thereby reducing the amount local groundwater required to meet the demand for water (K. Domako, Vandenberg Space Force Base, written commun., 2019); during water years 1998–2018 the average annual pumpage for military use in the SACVW was about 670 acre-ft/yr (fig. 10; table 8).

Agricultural pumpage accounts for most groundwater pumpage in the SACVW, ranging from 74.9 to 98.5 percent of total annual pumpage during 1948–2018 (fig. 30A; table 8). The amount of groundwater pumpage for agricultural use increased from 2,750 acre-ft in 1948 to 31,900 acre-ft in 2018. Agricultural pumpage compiled for this study was estimated by Martin (1985) and Hutchinson (1980) for calendar years 1948–57, and 1958–77, respectively, and by Tetra Tech, Inc. (2012) for water years 1978–2010. The estimated values from Tetra Tech, Inc. (2012) included groundwater pumped for frost protection; the estimated values from Martin (1985) and Hutchinson (1980) did not include groundwater pumped for frost protection. Frost protection by overhead spray is used in the SACVW to protect grape vines from damage when the air temperature drops below freezing (Tetra Tech, Inc., 2012). Published estimates of agricultural pumpage were not available for water years 2011–18; therefore, agricultural pumpage for these years was estimated by linearly extrapolating the agricultural pumpage data from Tetra Tech, Inc. (2012) during 1978–2010 (table 8). A straight line was fitted to the pumpage data for 1978–2010 using the least squares method and then extrapolated for 2011–18 to estimate pumpage for those years. Estimated agricultural pumpage during water years 2011–18 ranged from 28,600 to 31,900 acre-ft/yr (table 8).

Groundwater discharges as base flow to San Antonio Creek at Barka Slough where shallow consolidated bedrock causes groundwater to rise above land surface (Martin, 1985); the discharge of groundwater as base flow to the stream channel sustains a reach of naturally occurring perennial streamflow west of Barka Slough (fig. 8). During water years 1956–2018, estimated annual base flow at the Casmalia streamgage ranged from about 170 to 4,230 acre-ft/yr and generally declined over time (fig. 12; table 4). A Mann-Kendall test with a level of significance (α) of 0.05 (Helsel and Hirsch, 2002) was used to identify a statistically significant downward trend (n = 50, p-value = 0.000302) in the time series of base-flow estimates (fig. 12). Hutchinson (1980) and Martin (1985) estimated similar declines in base flow during calendar years 1956–77 (fig. 12) and generally attributed this decline as a response to increasing pumpage in the SACVW.

Hutchinson (1980) presented a statistical comparison of net groundwater pumpage in the basin to base flow in the San Antonio Creek groundwater basin using data from calendar years 1958 to 1977. Hutchinson (1980) fit a line to filtered base-flow data for years with average annual precipitation, with the assumption that base flow and pumpage in these years represented “average” conditions. Years with average annual precipitation were defined by Hutchinson (1980) as having measured annual precipitation within plus or minus 25 percent of the long-term average annual precipitation during 1909–77. Hutchinson (1980) calculated the slope of the regression relating base flow to net pumpage to be –0.2, meaning that for every 10,000 acre-ft/yr of net pumpage in the groundwater basin, base flow in San Antonio Creek would decline by 2,000 acre-ft/yr. Tetra Tech, Inc. (2012) evaluated base flow during 1935–2010 and concluded that the patterns in base flow were better correlated with variations in precipitation rather than pumpage and that the effect of increased pumpage in the SACVW was more likely accommodated by a decrease in groundwater storage rather than a decrease in base flow. Tetra Tech, Inc. (2012) also observed that increases in agricultural pumpage in the hills and valleys north of the valley floor, away from San Antonio Creek, likely limited the short-term influence of increasing pumpage on base flow. Variations in base flow are likely due to the delayed effects of increases in groundwater pumpage and variations in precipitation.

A statistical analysis similar to that of Hutchinson (1980) was performed for this study. A best-fit line of net annual groundwater pumpage (described below) to annual base flow (fig. 12) for water years 1956–90 (fig. 31A) and 1991–2003 (fig. 31B) provided quantitative evaluations of the effects of anthropogenic and natural influences on base flow in San Antonio Creek through time. The years 2016 to 2018 were not included in this analysis because of the substantial gap in the period of record from 2003 to 2015 (fig. 12). Best-fit line equations and coefficients of determination (R²) for the 1956–90 and 1991–2003 periods were compared to evaluate the effects of net pumpage and base flow under different climatic conditions (fig. 31). Water years 1956–90 represent a period of alternating average and dry precipitation with occasional wet years, and water years 1991–2003 represent a period of primarily average precipitation with occasional wet years (fig. 4). Net annual pumpage was calculated as the difference between the sum of agricultural, military, municipal, and domestic pumping (fig. 30; table 8) and the sum of return flow from agricultural irrigation (20 percent of agricultural pumpage), municipal system leakage (13 percent of municipal pumpage), and wastewater effluent from spray fields and septic systems (fig. 29). Following the work of Hutchinson (1980), the statistical analyses for each period were fit to filtered data that represent years with average annual precipitation within 25 percent of the mean annual precipitation during 1948–2018 (fig. 4).

The statistical analysis of net pumpage to base flow for years with average annual precipitation during water years 1956–90 had an R² value of 0.82 and a p-value less than 0.00001, indicating a statistically significant correlation between the two variables during this period (fig. 31A) based on an α of 0.05. The slope of the best-fit line relating net pumpage to base flow for 1956 to 1990 was –0.080 (fig. 31A), meaning that for every 10,000 acre-ft of net pumpage in the SACVW, base flow in San Antonio Creek would decline by 800 acre-ft. This estimated rate of base-flow decline was less than half of that calculated by Hutchinson (1980) for the period 1958–77. The longer period of 1956 to 1990 (fig. 31A) may better demonstrate the long-term effects of net pumpage on base flow, while the shorter period of 1958 to 1977 (Hutchinson, 1980) may have been influenced by a few dry precipitation years at the end of that period (1969, 1970–72; fig. 4) and had a more limited set of available data than for the longer period. A steady decline in base flow during 1956–90 was associated with an increase in total pumpage from about 4,600 acre-ft/yr to about 24,300 acre-ft/yr (figs. 12, 30, 31); this period had mostly average and dry years of precipitation (only 7 of 34 years in this period were classified as wet years; fig. 12). Most of the pumpage in the SACVW was agricultural along the middle of the valley, proximal to San Antonio Creek (figs. 6, 30). The increase in agricultural pumpage was due to an increase in agricultural acreage (from 8,674 acres in 1959 to 9,838 acres in 1986) and a shift in farming practices to irrigated crop land (the amount of irrigated cropland increased from 2,118 acres in 1959 to 7,923 acres in 1986). The correlation of net pumpage to base flow for this period is indicative of the influence of groundwater pumping on base flow in the SACVW during periods of primarily average and dry climatic conditions.

The statistical analysis of net pumpage to base flow for years with average annual precipitation during water years for 1991–2003 had an R² value of 0.063 and a p-value of 0.55, indicating that correlations between the two variables were not statistically significant during this period (fig. 31B). The slope of the best-fit line relating net annual pumpage to base flow was –0.064 (fig. 31B), meaning that for every 10,000 acre-ft/yr of net pumpage in the SACVW, base flow in San Antonio Creek would decline by 640 acre-ft/yr. This estimated rate of base-flow decline was less than values for 1956 to 1990 (fig. 31A) and for 1957 to 1977 (Hutchinson, 1980). The period of 1991 to 2003 was associated with relatively steady base-flow values with punctuated increases in annual base flow during wet years (figs. 12, 31B), a relatively moderate increase in total pumpage from about 22,000 acre-ft/yr to about 26,000 acre-ft/yr (fig. 30; table 8), and mostly average and wet years of precipitation (only 2 of 12 years in this period were classified as dry years). The relatively moderate increase in pumpage over this period, and smaller slope of regression, was in response to relatively high amounts of precipitation and the likely resulting reduction in water demand for crops despite the overall increase in agricultural acreage, all of which was irrigated cropland (from about 10,300 acre-ft/yr in 1996 to about 14,200 acre-ft/yr in 2006; fig. 6; table 2). Agricultural lands generally increased in the northern hills and valleys away from San Antonio Creek (fig. 6). The poor correlation between net pumpage and base flow for this period indicates the influence of climatic variations on base flow in the SACVW during periods of average and wet climate; the groundwater system is sensitive to changes in precipitation because the SACVW is relatively shallow and narrow.

Groundwater underflow to the neighboring Santa Maria River Valley and Santa Ynez River Valley groundwater basins (fig. 28) was assumed to be negligible (Hutchinson, 1980; Martin, 1985). Tetra Tech, Inc. (2012) observed that groundwater-level data across the SACVW boundary with adjacent groundwater basins were consistent with previous conclusions that subsurface outflow from the SACVW was unlikely. If hydrologically connected, a substantial lowering of groundwater levels in the neighboring Santa Maria River Valley and Santa Ynez River Valley groundwater basins could potentially induce subsurface underflow from the SACVW through saturated aquifer materials, but the topography and underlying structure of the SACVW would likely prevent such an occurrence. The amount and location of any subsurface outflow from the SACVW can be estimated with a numerical model based on groundwater-head differences across the boundary of the SACVW.

The quality of surface water in the SACVW is controlled by the water quality of storm runoff, groundwater discharged as base flow, and runoff from agricultural irrigation; thus, surface-water quality varied greatly depending on location within the SACVW and the time of year the samples were collected. Water-quality samples were collected from three surface-water sites in the SACVW during 2017–18: San Antonio Creek at 11135800 (hereafter referred to as the Los Alamos streamgage), Harris Creek at surface-water quality site 11136040 (hereafter referred to as the Harris Canyon streamgage), and San Antonio Creek at surface-water quality site 11136100 (hereafter referred to as the Casmalia streamgage; fig. 36; table 9; U.S. Geological Survey, 2021).

The surface-water quality varied depending on the where the samples were taken. The Los Alamos streamgage is in a sandy channel in San Antonio Creek in the eastern valley, and the Harris Canyon streamgage is in a narrow sandy creek that drains from the Casmalia Hills into San Antonio Creek in the western uplands; streamflow is intermittent at each of these sites. The Casmalia streamgage is situated at the westernmost end of the SACVW, west of Barka Slough. Samples were collected from all three sites during the winter, after a storm, and from the Harris Canyon and Casmalia streamgages during the fall, at the end of the dry season (Santa Barbara County, 2019). A sample was not collected from the Los Alamos streamgage during the fall because the creek was dry. The Harris Canyon streamgage sample collected during the winter was not used for this evaluation because the sample appeared to have been compromised with excess sediment as a result of poor sample collection technique or incomplete filtration during sampling (U.S. Geological Survey, 2021).

Groundwater quality in the SACVW is controlled by the chemistry of the recharge water and various geochemical reactions occurring in the subsurface, including dissolution, precipitation of minerals in the subsurface, mixing with agricultural irrigation-return flow, and solute concentration resulting from evaporation. Groundwater quality and source and age of groundwater were characterized for the SACVW using analyses for selected physical properties, inorganic constituents, and stable and radioactive isotopes. Each chemical and isotopic tracer preserves some aspect of the hydrologic history of the groundwater, including the chemical composition of the groundwater (major ions, minor ions, and TDS), reduction/oxidation (redox) conditions of groundwater, time-since-recharge (tritium and carbon-14), and the source of water (stable isotopes of oxygen and hydrogen). This section describes (1) the areal, depth-dependent, and temporal variations in groundwater quality, including groundwater-quality constituents of concern, and (2) the recharge source and age of groundwater.

Tritium is a naturally occurring radioactive isotope of hydrogen that has a half-life of 12.4 years (Michel, 1976, 2005). Approximately 800 kilograms of tritium was released into the atmosphere because of the atmospheric testing of nuclear weapons between 1952 and 1962 (Michel, 1976). As a result, tritium concentrations in precipitation and groundwater recharge increased during that time. Tritium concentrations are not affected by chemical reactions other than radioactive decay because tritium is part of the water molecule. Therefore, tritium is a tracer of the movement and relative age of water on timescales ranging from recent to about 60 years before present (years BP; post 1952). The concentration of tritium is typically reported in Tritium Units (TU; Michel, 1976). In this report, groundwater that had tritium concentrations less than the detection limit of 0.2 TU was interpreted as water recharged prior to 1952. Groundwater that had measurable tritium concentrations was interpreted as water recharged after 1952.

Carbon-14 is a naturally occurring radioactive isotope of carbon that has a half-life of about 5,730 years (Mook, 1980). Carbon-14 data are expressed as percent modern carbon (pmC) by comparing ¹⁴C activities to the specific activity of National Bureau of Standards oxalic acid, where 13.56 disintegrations per minute per gram of carbon in the year 1950 equals 100 pmC (Kalin, 2000). Similar to tritium, ¹⁴C was produced by the atmospheric testing of nuclear weapons (Mook, 1980). As a result, ¹⁴C activities may exceed 100 pmC in areas where meteoric waters recharged aquifers in the 1950s. Carbon-14activities also are affected by mixing younger water that has high ¹⁴C activity with older water that has low ¹⁴C activity. To estimate relative age, the greatest ¹⁴C activity of water that no longer contains tritium is selected as the initial ¹⁴C activity of groundwater recharge, A₀. This approach is more representative of conditions within the study area and provides a better estimate of the initial ¹⁴C activity of groundwater prior to nuclear weapons testing compared to analytical estimates. Using initial ¹⁴C activity does not account for radioactive decay that occurred since 1952; however, the decrease in ¹⁴C activity resulting from radioactive decay since 1952 is small and is considered negligible based on the half-life of ¹⁴C. For the SACVW, the A₀ was about 75 pmC and is consistent with values estimated in other coastal basins, which ranged from 64 to 69 pmC (Izbicki and others, 1992; O’Leary and others, 2015; Teague and others, 2018).

Carbon-14 activities are used to determine the age of a groundwater sample on timescales generally ranging from about 1,000 to about 15,000 years BP. Carbon-14 is not part of the water molecule; therefore, ¹⁴C activities may be affected by chemical reactions that remove or add carbon to solution. Carbon-13 data can aid in the interpretation of ¹⁴C activities and are expressed as ratios in delta notation (δ) as parts per thousand (per mil) differences relative to the ratio of stable isotopes ¹³C to ¹²C in standard Pee Dee Belemnite (Gonfiantini, 1978). Carbon-14 ages presented in this report did not account for changes in ¹⁴C activity resulting from chemical reactions or mixing; therefore, presented ¹⁴C ages were considered uncorrected ages. In general, uncorrected ¹⁴C ages were older than the actual age of the associated water. In this report, groundwater that had ¹⁴C activities less than 75 pmC was interpreted as being recharged before 1952. Groundwater having ¹⁴C activities greater than 75 pmC was interpreted as being recharged after 1952 (Izbicki and Michel, 2003).

For the purposes of age classification in this report, groundwater samples that had tritium concentrations greater than 0.2 TU and ¹⁴C activities greater than 75 pmC were considered modern. Water samples that had tritium concentrations less than 0.2 TU and ¹⁴C activities less than 75 pmC were considered pre-modern (that is, recharged before 1952). Water samples that had pre-modern and modern water designations from tritium and pmC were designated as mixed (U.S. Geological Survey, 2021). Groundwater with ¹⁴C activity that was less than 75 pmC and had a measurable tritium concentration indicated that other geochemical reactions had occurred within the groundwater system; these reactions may have added carbon that did not contain ¹⁴C to the dissolved phase or may have removed carbon, making the water appear older than it is. Groundwater samples in 10 wells were designated as modern, 6 wells were designated as mixed, and 34 wells were designated as pre-modern (U.S. Geological Survey, 2021).

Concentrations of tritium in TU and pmC in groundwater for selected wells are illustrated with depth on four sections through the SACVW from the HFM (fig. 40; locations shown on fig. 36). Each section (same locations as those in fig. 38) shows selected groundwater wells and the perforation interval from which samples were analyzed for each well, and the corresponding hydrogeologic units. Wells with modern groundwater are in the floor of the eastern valley subarea near Los Alamos, in the western uplands subarea along Harris Canyon, and adjacent to tributaries of San Antonio Creek throughout the SACVW; these wells generally have shallow perforations (with approximate well depths of about 270 ft bls), and most were perforated in the channel alluvium, and upper and lower Paso Robles Formation. Wells with pre-modern groundwater were located in the eastern and western uplands subareas, and the eastern and western valley subareas; these wells generally had deep perforations (with average well depths of about 540 ft bls), and most of these wells were perforated in lower Paso Robles Formation, the Careaga Sandstone, or Consolidated bedrock. Wells with mixed groundwater were located throughout with SACVW and generally had average well depths of about 430 ft bls.

The tritium concentrations in groundwater samples from wells sampled during 2008–18 ranged from less than 0.2 TU in numerous wells to 0.91 TU in SACR5 (fig. 40; U.S. Geological Survey, 2021). Groundwater samples from 33 wells did not have measurable ^tritiumconcentrations and were considered pre-modern. Groundwater samples from 15 wells had measurable tritium concentrations and were considered modern. The ¹⁴C activities in groundwater samples from wells sampled during 2008–18 ranged from 0.1 pmC in well 9N/34W-27L1 to 114.9 pmC in well SAGR (fig. 40A; U.S. Geological Survey, 2021). Uncorrected ages for the water samples ranged from recent (between 1950 and present day) to more than 37,000 yrs BP. Water-quality samples with ¹⁴C activities less than 75 pmC that represented older, pre-modern water were collected from 39 wells. Water-quality samples with ¹⁴C activities greater than 75 pmC that represented younger, modern water were collected in 10 wells

Measured delta carbon-13 (δ¹³C) composition of water from wells ranged from 4.25 to 20.49 per mil (U.S. Geological Survey, 2021). Delta carbon-13 values were generally heavier (more positive) in the eastern part of SACVW than in the western part. The heaviest (most positive) δ¹³C value was from groundwater in well 9N/34W-34C2 in the western uplands subarea proximal to the Orcutt Oil Field, and the lightest (most negative) δ¹³C value was from groundwater in well 8N/33W-0R1 in the central uplands subarea (see fig. 36 for well locations). This variability in the δ¹³C data indicates that additional reactions have occurred in the basin; reactions could include (1) the formation of carbonates in an organic rich system which can have produce heavier, more positive, δ¹³C values or (2) the oxidation of biogenic methane (which is depleted in δ¹³C), which can produce lighter, more negative, δ¹³C values.

Oxygen-18 (¹⁸O) and ²H are natural stable isotopes of oxygen and hydrogen, respectively. Delta oxygen-18 (δ¹⁸O) and delta ²H (δ²H) are isotopic ratios of oxygen isotopes ¹⁸O and ¹⁶O, and hydrogen isotopes ²H and ¹H, respectively. These isotopic ratios are expressed as per mil differences, relative to the standard known as Vienna Standard Mean Ocean Water (Gonfiantini, 1978) and are indicators of the hydrologic history of recharge water.

The linear relation between δ¹⁸O and δ²H in natural precipitation throughout the world (Craig, 1961) is referred to as the global meteoric water line (GMWL; fig. 41). The isotopic composition of ocean water undergoes fractionation during the transfer from the ocean surface to the vapor phase. Further fractionation occurs as precipitation condenses from the atmosphere, leaving the remaining water vapor relatively depleted in the heavier isotopes. Latitude, altitude, and air temperature affect the isotopic composition of atmospheric water. Precipitation from a given storm becomes isotopically lighter (more negative) as the storm moves inland and into higher altitudes with cooler temperatures (Fournier and Thompson, 1980). More negative values of δ¹⁸O and δ²H represent enrichment in the lighter isotopes, ¹⁶O and ¹H (or a depletion in the heavier isotopes, ¹⁸O and ²H); less negative values represent enrichment in the heavier isotope of oxygen (¹⁸O) and hydrogen (²H), respectively. Water that has undergone evaporation is enriched in heavier isotopes relative to its original composition and generally has values below and to the right of the meteoric water line, with a slope between 3 and 6 (International Atomic Energy Agency, 1981). Isotopic composition of water does not change further at low temperatures of most groundwater systems after recharged water has migrated below the depth of evaporation. Therefore, any subsequent differences in the isotopic composition of groundwater along a flow line generally reflect only mixing within the aquifer system or concentration by evaporation in a discharge area. The δ¹⁸O and δ²H compositions of groundwater relative to the meteoric water line and the isotopic composition of water from other sources can indicate the source of groundwater.

Isotopic compositions of groundwater samples from wells sampled during 2008–18 ranged from –6.22 to −4.86 per mil for δ¹⁸O and from –42 to −31.7 to per mil for δ²H (fig. 41; U.S. Geological Survey, 2021). The isotopic data mostly plot to the right of the GMWL, indicating possible evaporation prior to recharge, partial evaporation during precipitation, or a “local” meteoric water line that differs slightly from the GMWL (fig. 41). Water samples from wells were categorized into three main groups: group 1 samples with lighter isotopic composition (more negative) that plot below and down to the right of the GMWL, group 2 samples with intermediate isotopic composition that plot nearest to the GMWL, and group 3 samples with heavier isotopic compositions (less negative) that are characteristic of evaporated groundwater that plot below the GMWL with δ¹⁸O values of –5.6 and greater (fig. 41). Stable isotopic groundwater groups for selected wells are shown in map view in figure 42 and in sections on figure 40. The Pezzoni-Casmalia and Los Alamos faults are inferred to transect the SACVW between the western and eastern areas of the valley floor and do not markedly affect stable isotopic compositions of groundwater in the SACVW.

Group 1 groundwater samples (indicated by red symbols in figs. 40, 41, 42) were primarily from deep wells, in the eastern uplands and western valley subareas, and from wells in the Barka Slough subarea or where deep groundwater discharges to land surface. Group 1 samples represent old, pre-modern groundwater, with the exception of well 8N/34W-16J2; this well was located in Barka Slough and had measurable tritium (concentrations greater than 0.2 TU) and contained some modern groundwater (U.S. Geological Survey, 2021). Group 1 samples plot below and down to the right of the GMWL (fig. 41), indicating that the water was locally recharged during different climate conditions was slightly evaporated when recharged. The geographic and vertical distribution of group 1 samples within the SACVW indicates that recharge occurred from different moisture sources in the eastern uplands subarea and groundwater from the eastern uplands subarea has a long, slow travel time to the western part of the SACVW, where the water eventually discharges as base flow at Barka Slough (as indicated by the presence of group 1 groundwater in wells 8N/34W-16C4 and 8N/34W-16J2; figs. 40D, 41, 42). Along this flow path, groundwater may be extracted as groundwater pumpage from deep wells in the western valley or eastern uplands subareas. The presence of pre-modern groundwater in the western part of the SACVW corresponds with the relatively stable groundwater levels observed in wells in the western valley subarea (wells 8N/33W-20Q1 and 8N/33W-20Q2, and 8N/34W-23B1 and 8N/34W-24E1; hydrographs 10 and 11, respectively, in figs. 34, 35) and the upward vertical gradient of groundwater flow at multiple-depth, monitoring-well site SACR (figs. 19, 34; U.S. Geological Survey, 2021).

Group 2 groundwater samples (indicated by purple dots in figs. 40, 41, 42) were from shallow and deep wells primarily in the central and western uplands subareas and are a mix of modern and pre-modern water (U.S. Geological Survey, 2021). Groundwater samples in group 2 generally plot parallel to the GMWL (fig. 41) and likely represent a “local” meteoric water line. The local meteoric water line consists of modern and pre-modern local recharge from precipitation that had relatively short travel times and was discharged as base flow at Barka Slough (as indicated by the presence of group 2 groundwater at well 8N/34W-16F2; fig. 42) or otherwise extracted as groundwater pumpage.

Group 3 groundwater samples (indicated by green dots in figs. 40, 41, 42) were from shallow and deep wells primarily in the eastern and western valleys and were a mix of modern and pre-modern water (U.S. Geological Survey, 2021). Groundwater samples in group 3 plotted below the GMWL (fig. 41), indicating either mixing with evaporated surface water or mixing with and additional evaporation of deep discharge groundwater from group 1 or group 2. Wells SAHC, SALA, SASA, SAHG, SAGR, and SALS are shallow wells (less than 250 ft depth bls) and are proximal to San Antonio Creek or its tributaries (table. 1.2; figs. 40, 42); stable isotopic compositions of groundwater in these wells may indicate evaporated surface flow draining from sediments along the stream channels. The remaining wells in group 3 (such as 8N/34W-24E2, SACC-1, SACC-3, SACC-4, SACR-3, and 8N/32W-29L3) are deep wells (greater than 250 ft depth bls); groundwater in these wells may be mixed with deep groundwater (recharged under different climate conditions) characterized by wells in group 1.