Purpose and Scope

The purpose of this report is to describe groundwater-storage changes, trends, and aquifer storage properties in the Rio Rancho area for the period between 2019 and 2023, as determined from groundwater-level measurements and repeat microgravity measurements. Repeat microgravity data are presented for 20 monitoring sites from 11 surveys carried out during the study period. Groundwater-level data are presented for 27 wells. Specific yield is estimated at two locations.

Study Area

The study area is located in the Middle Rio Grande Basin (MRGB) in central New Mexico and includes the City of Rio Rancho (fig. 1). The area is semiarid and has a temperate climate. Average annual precipitation from 2006 through 2022 was 7.9 inches (in.) (National Oceanic and Atmospheric Administration [NOAA], 2022), with an average of 4.5 in. received during the summer monsoon season (June 15–September 30) (NOAA, 2022). Land surface elevation in the study area ranges from approximately 5,000 ft above sea level near the Rio Grande east of the study area to over 6,300 ft above sea level in the far western parts of the study area (U.S. Geological Survey, 2024a). Temperatures in July can range from an average daily low of 64 degrees Fahrenheit (°F) to an average daily high of 96 °F and, in January, from an average daily low of 23 °F to an average daily high of 48 °F (NOAA, 2022). As of December 2022, Rio Rancho and the surrounding area were in a state of moderate drought and have experienced exceptional drought five times since 2000 (U.S. Drought Monitor, 2022).

Geology and Hydrology

The Middle Rio Grande Basin (MRGB) (fig. 1B) is one of a series of alluvial basins created by extensional faulting in the Rio Grande Rift. The primary river in the MRGB is the Rio Grande, which flows north to south through the MRGB from its headwaters in Colorado and has a drainage area of approximately 14,900 square miles (Bartolino and Cole, 2002). The MRGB is bounded on the west by sedimentary rocks of the Colorado Plateau, and on the north, south, and east by uplifted basement rocks. Sediments in the MRGB were generally deposited in fluvial environments and primarily consist of alluvium and colluvium, which, in some areas west of the Rio Grande, are interbedded with volcanic rocks, specifically basalt flows and ejecta derived from local volcanoes (Bartolino and Cole, 2002).

The major structural feature in the study area is the Ziana horst, a zone of relative uplift oriented roughly north-northwest to south-southeast through Rio Rancho (Personius and others, 2000; Koning, 2022). Originally interpreted from geophysical (gravity) data as a plunging anticline, later reports have reinterpreted the feature as an uplifted fault block positioning lowermost middle Santa Fe Group rocks against middle and upper Santa Fe Group rocks (Personius and others, 2000). Borehole evidence confirms the existence and extent of the Ziana horst (Glorieta Geoscience, unpub. data, 2004). The feature potentially affects groundwater flow and availability in the study area, owing to the thinner aquifer and absence of the productive upper part of the Santa Fe Group where uplift is greatest (Koning, 2022).

The basin fill Santa Fe Group, in hydraulic connection with overlying Quaternary deposits, forms the primary aquifer system in the region and ranges from less than 2,000 ft thick at the margin boundaries of the MRGB to over 13,000 ft thick in the middle (Bartolino and Cole, 2002). In the study area, aquifer thickness west of the Rio Grande is greatest in the southern and western parts of Rio Rancho (Koning, 2022). The Santa Fe Group aquifer system is composed primarily of sand and silt, with smaller amounts of clay and gravel (Bartolino and Cole, 2002). Quaternary post Santa Fe Group deposits in the valley include river alluvium, alluvial fill terraces, eolian sand deposits, and colluvium (Williams and Cole, 2007).

The most recent groundwater-level maps in the MRGB indicate groundwater flow away from the Rio Grande throughout the Rio Rancho and Albuquerque area. In southern Rio Rancho, groundwater-level contours indicate a depression in the groundwater-level surface, although data are sparse and the direction of groundwater flow is uncertain. North of the study area, data indicate more north-to-south groundwater flow (Rawling, 2023), but recent groundwater-level maps (Falk and others, 2011; Galanter and Curry, 2019; Rawling, 2023) do not extend to the northern half of the study area. In addition to the regional Santa Fe Group aquifer system, shallow aquifers exist within the Quaternary fluvial sand and gravel deposits underlying the modern Rio Grande floodplain. These deposits range in thickness from 70 to 120 ft (Hawley and Haase, 1992) and serve as a source of groundwater to shallow wells in the area near the Rio Grande, as well as a conduit for recharge to the Santa Fe Group aquifer system (Logan, 1994). West of the Rio Grande, groundwater flow in the shallow alluvium is toward the southwest, and to the east of the Rio Grande, flow is toward the southeast (Rawling, 2023). This pattern is a result of seepage from the Rio Grande (Veenhuis, 2002; Rankin and others, 2016) and the location of major groundwater pumping centers to the west and east of the river (Bexfield and Anderholm, 2002).

Groundwater recharge in the study area occurs through seepage from the Rio Grande, infiltration from precipitation (primarily as runoff in ephemeral channels), leakage from water delivery systems and septic fields, over-watering of parks and golf courses, and direct injection wells (Bartolino and Cole, 2002). As part of the City of Rio Rancho’s water conservation and management program, construction of a managed aquifer recharge facility named the Advanced Water Treatment Facility (AWTF) began in 2010, with the final phase completed in May 2017 (Rio Rancho Utilities Division, 2017). Mountain-front recharge occurs along the west side of the Sandia Mountains east of the study area and east of the Rio Grande (Plummer and others, 2004). Although this source likely has a role in maintaining hydraulic head in the Rio Rancho study area, by reducing the amount of infiltration from the Rio Grande that would otherwise occur, its influence likely diminishes from east to west, and the river acts primarily as a hydraulic boundary between groundwater to the east and to the west (Bartolino and Cole, 2002). Recharge in the form of groundwater subflow may also occur from the north and west, but high-elevation (and therefore high-precipitation) regions in those directions are more distant, and the magnitude of this recharge, if any, is unknown.

Groundwater withdrawals in the study area occur primarily from wells associated with municipal, industrial, and agricultural uses. Well locations were retrieved from the New Mexico Office of the State Engineer (fig. 2; New Mexico Office of the State Engineer [NMOSE], 2023). Abandoned wells and those known to be monitoring wells only were removed. The City of Rio Rancho is permitted for total pumping as much as 24,020 acre-feet per year (acre-ft/yr), but actual annual pumping during the study period averaged approximately 11,000 acre-feet (acre-ft). The largest industrial groundwater user in Rio Rancho, Intel Corporation, is separately permitted to pump 3,250 acre-ft/yr (actual pumping during the study period was about 2,300 acre-ft/yr) from its own wells (NMOSE, 2023), and much of this water is treated and discharged to the Rio Grande. Other major water producers in the study area, which provide water for small farms, housing developments not serviced by large municipal providers, and light industry, are generally permitted to pump from 3 to 2,450 acre-ft/yr. Residential-use wells are permitted to pump as much as 3 acre-ft/yr. Most of the residential wells in the study area are in the Village of Corrales, located between Rio Rancho and the Rio Grande. A smaller number of residential wells exist in lower-density areas in the east-central and south-central parts of Rio Rancho (fig. 2). Treated wastewater in the City of Rio Rancho is used for irrigation, aquifer injection, and is also discharged to the Rio Grande.

Previous Investigations

The importance of the regional aquifer for economic and population growth in the region drove many prior regional investigations that are summarized briefly here. Hawley and Haase (1992) compiled a conceptual hydrogeologic model of the northern Albuquerque Basin on behalf of the New Mexico Bureau of Mines & Mineral Resources to aid understanding of the regional groundwater system, as did Thorn and others (1993) for the USGS. In 1994, expansion of a large semiconductor manufacturing operation in Rio Rancho and an associated request to expand groundwater withdrawals prompted a geohydrologic study by the New Mexico State Engineer’s Office (Logan, 1994). Numerical modeling using groundwater-level data was completed in 1999 to understand the recharge rate from the Rio Grande to the Santa Fe Group aquifer system (Bartolino and Niswonger, 1999).

In 2002, the USGS issued a comprehensive report on the MRGB hydrologic system after completing a 6-year interagency basin study in the area, which synthesized much of the previous work into a broad overview (Bartolino and Cole, 2002). In 2004, Glorieta Geoscience conducted exploratory well drilling, water-quality testing, and geophysical logging to support Rio Rancho municipal water use (Glorieta Geoscience, unpub. data, 2004).

Connell (2008) proposed a revised three-unit stratigraphy for the Santa Fe Group, the major water-bearing formation of the Albuquerque Basin, based on newly available geologic mapping data. Rankin and others (2016) analyzed groundwater flux from the Rio Grande to the surrounding shallow alluvial aquifer and found that flow moved in a vertical gradient that rapidly shifted to horizontal flow with increasing distance from the river, and the aquifer slope was likely influenced by riverside drains and nearby groundwater wells. A 2019 study summarized groundwater depletion of the Santa Fe Group aquifer system in Albuquerque and Rio Rancho from the mid-20th century to 2016 and found that the largest declines occurred in the eastern and northwestern Albuquerque area of the MRGB (Galanter and Curry, 2019). However, the report also noted a recent rebound caused by reduced groundwater withdrawals since 2008. In a report showing groundwater-level change since predevelopment (before 1961), Rawling (2023) indicated decreases of as much as 69 ft in Rio Rancho. The median groundwater-level decline in Rio Rancho from predevelopment conditions was 36.5 ft (n = 10).

In a study similar to this one, Kennedy and Bell (2023) used groundwater-level and repeat microgravity data from 2016 to 2022 to identify spatiotemporal trends in groundwater storage in the Albuquerque area southeast of the present study area. From 2016 to 2019, widespread storage increases occurred in response to reduced groundwater pumping by the Albuquerque Bernalillo County Water Utility Authority, but storage decreased from 2020 to 2022 in response to increased pumping. A strong correlation between gravity and groundwater levels indicated water was being stored in or removed from unconfined storage at the uppermost level of the regional aquifer. Specific yield calculated using water level and microgravity data ranged from 0.04 to 0.39 (mean = 0.22) and was greatest in the central part of Albuquerque, east of the Rio Grande, where the most pumping has historically occurred.

Groundwater-Level Monitoring

For this project, groundwater-level monitoring in the Rio Rancho area began in 2019, although earlier records exist for many locations. At each location, measurements were made relative to a stable measuring point positioned using a survey-grade Global Positioning System (GPS) and the Online Positioning User Service (National Geodetic Survey, 2022). From 2019 to 2023, discrete groundwater levels were collected quarterly using an electric tape at 17 wells, and semiannually at 1 well (table 1). At two locations (LMS and Hunters Ridge), groundwater levels were recorded continuously in nine nested piezometers (multiple wells in a single borehole, open to the aquifer at different depths) using vented pressure transducers and data loggers recording at 15-minute or 1-hour intervals. Groundwater levels were collected, reviewed, and approved following standard USGS technical procedures (Cunningham and Schalk, 2011, and updates) and are available from the National Water Information System database (USGS, 2023).

Aquifer-Storage Monitoring

Changes in aquifer storage were monitored using the repeat microgravity method (Crossley and others, 2013; van Camp and others, 2017; Kennedy and others, 2021a). According to Newton’s Law of Universal Gravitation, the acceleration due to gravity at any point depends on the surrounding mass. In an aquifer, mass may change because of changes in groundwater storage caused either by recharge of the aquifer or by discharge, such as by groundwater pumping. When the mass of an aquifer changes, the gravitational acceleration measured at the land surface also changes; these small changes can be detected with gravity meters. Gravity measurements at a given location are also affected by Earth tides, barometric pressure changes, elevation changes, mass changes from volcanism, and other processes. These processes can be accounted for independently and removed from the signal either prior to or during data processing, or are considered negligible in the context of groundwater studies. Other factors that can affect gravity, often considered in studies interpreting the gravity field itself (instead of changes in the gravity field, as in this study), include latitude, topography surrounding the gravity station, and underlying geology. These factors are typically considered to be unchanging for repeat microgravity studies.

The gravitational effect of groundwater storage changes at the water table may be approximated using the Bouguer slab (that is, the horizontal infinite slab) approximation if the water table is approximately horizontal underneath the gravity station to a distance about 10 times the depth to water (Pool and Eychaner, 1995). Departures from the Bouguer slab approximation are usually small, even in the presence of significant groundwater mounding (Kennedy and others, 2021b). The Bouguer approximation allows for a direct conversion from the change in gravitational acceleration to a one-dimensional thickness of free-standing water:

The significance of equation 1, and the value of the repeat microgravity method for groundwater hydrology, is that storage change can be estimated directly from the measured change in gravity independent of the depth to groundwater or the aquifer’s porosity (Pool, 2008).

Applications of the repeat microgravity method for groundwater investigations include mapping aquifer storage change (Pool and Anderson, 2008; Kennedy and Bell, 2023), estimating specific yield (Pool and Eychaner, 1995, Robertson and others, 2022), and monitoring recharge at artificial recharge facilities (Kennedy and others, 2014, 2016). For the present study, repeat microgravity was used to investigate changes in aquifer storage in the study area and to estimate site-specific specific yield where microgravity stations were collocated with groundwater-level monitoring locations.

Precipitation

Precipitation in the study area can lead to groundwater recharge, primarily in arroyos where concentrated flow can rapidly infiltrate sandy soils (Moore, 2007). The amount of direct land-surface recharge away from channels is unknown, but owing to the high evapotranspiration demand in the semiarid environment in the study area, it is likely minimal. Recharge along arroyos is greatest following relatively large flows and during periods of sustained precipitation lasting days to weeks. Conversely, small, infrequent precipitation is generally lost to evapotranspiration and does not lead to recharge (Moore, 2007).

Annual precipitation in the study area from 2019 to 2022 ranged from 4.8 in. for 2021 to 9.6 in. for 2022. The mean annual precipitation in the study area between 2006 and 2022 was 7.9 in. (NOAA, 2022). Precipitation during the study period followed the typical bimodal temporal pattern prevalent in the southwestern United States (fig. 3), with generally greater precipitation during the summer (July–September) than during other seasons. Summer rainfall is usually generated by convective thunderstorms, which can occur over small areas; the data from a single station shown in figure 3 may not represent average rainfall over the entire study area.

Summer precipitation was generally below the 2006–22 average during 2019–21 but was average to well above average in 2022. Winter precipitation is generally low, and except for November 2019, observed rainfall was near the 2006–22 average for the study period. The year 2021 stands out as a particularly dry period; lack of rainfall during this period may contribute to the aquifer-storage declines observed in the summer of 2022.

Groundwater Additions and Withdrawals

Groundwater Withdrawals

The City of Rio Rancho, like many municipalities in New Mexico, exclusively uses groundwater for water supply (Daniel B. Stephens & Associates, Inc., 2014). Information on Rio Rancho pumping wells in the study area was obtained from NMOSE (2023). Pumping from the City’s municipal wells between 2019 and early 2023 was spread across 12 active locations (NMOSE, 2023). Pumping generally followed a seasonal pattern, with less pumping during the winter and more pumping during the summer, primarily reflecting the demand for irrigation water (fig. 4). Pumping from nonmunicipal production wells has not been estimated here, although the broad spatial distribution of all permitted production wells (fig. 2) suggests this pumping may stress groundwater resources, particularly during years of below average precipitation.

Groundwater Levels

Groundwater levels were generally stable during the 2019–23 study period, although seasonal and longer-term fluctuations were observed (figs. 5 and 6). At the two sites where continuous (hourly) groundwater-level data are available (the LIMS and HRUN well nests), groundwater levels were consistently lower in summer (fig. 5), when groundwater pumping is greatest (fig. 4), than in other seasons when there is generally less groundwater pumping. At both sites, groundwater elevation was highest in the uppermost nested piezometer and lower in the piezometers open at a deeper level in the aquifer, indicating a downward gradient, potential downward flow, and focused recharge from the land surface. At each site, changes in groundwater elevation associated with pumping are visible—both the long-term, seasonal changes and higher-frequency, daily fluctuations—primarily in data from the lower two piezometers, which are open at deeper depths in the aquifer (more than 835 ft below land surface). The absence of these fluctuations in the uppermost screened interval at both sites (fig. 5) suggests that confining units in the Santa Fe Group basin-fill aquifer system between the aquifer being pumped and the uppermost unconfined aquifer may be more laterally or vertically extensive (that is, there is greater separation between units) than has been observed in the same aquifer in the basin-fill sediments underlying the City of Albuquerque about 10 miles to the southeast (Kennedy and Bell, 2023).

Discrete quarterly groundwater-level measurements show generally small changes and no clear trends over the study period (fig. 6). The data are insufficient for showing seasonal or subannual trends. Only two stations, 351542106401001 (monitoring well at well 21) and 351627106435301 (well 8), had large changes (greater than 10 ft) in groundwater elevation (fig. 6). Monitoring well at well 21 is near an active production well that is likely to influence nearby water levels. The decrease in groundwater elevation at station 351909106383001 (well 18) in the winter of 2020–21 is likely related to an issue with the well screen or well construction and does not accurately reflect a change in groundwater elevation. During the study period, groundwater elevations have increased at the westernmost stations in the study area, 351630106481201 (well 13) and 352019106474801 (Phoenix), continuing longer-term trends prior to the present study period. Additional groundwater-level data at these sites are available in the National Water Information System (USGS, 2023).

Groundwater Storage Change and Storage Properties

Gravity surveys were carried out at 20 microgravity stations seasonally from April 2019 through January 2023, usually three times per year (in April, August, and December) to capture differences related to seasonal groundwater pumping; no data were collected in April 2020. Gravity change, and therefore storage change, can be calculated between any two surveys. The average measurement uncertainty (standard deviation) of the network-adjusted gravity values is 3.0 µGal, and the average uncertainty of gravity change between surveys is 5.2 µGal, equivalent to 0.41 ft of free-standing water (eq. 1; Bell and Kennedy, 2025). The microgravity stations are split into two groups for discussion: the stations in the vicinity of the AWTF (AWTF1, awtf2, AWTFW, AWTFE, AWTFS, awtfn), which are grouped together as their general purpose is to monitor groundwater storage near an injection well, and the remaining stations, which are grouped together to monitor groundwater storage over the entirety of the study area. Capitalized identifiers denote absolute-gravity stations (relative-gravity measurements were also made at most of these stations). Uncapitalized identifiers denote stations where only relative-gravity measurements were made. Gravity change at each station, measured in units of microGals, has been converted to units of “thickness of free-standing water,” in feet, using the Bouguer slab approximation (eq. 1).

Advanced Water Treatment Facility

Six stations were located at the AWTF to monitor storage change near a well injecting treated water into the subsurface (fig. 1C). One station was discontinued after the initial survey (AWTF1) owing to its proximity to a nearby station (AWTFW). Gravity measurements at the AWTF stations followed a general seasonal pattern, with increases in gravity, and therefore groundwater storage, during the winter and spring surveys, followed by decreases in gravity and groundwater storage during the summer surveys (fig. 7). At the end of the study in January 2023, following the recovery from summer 2022, storage at all stations near the AWTF was slightly greater than at the beginning of the study in August 2019.

Comparison of the change in aquifer storage at the AWTF with the volume of water injected into the subsurface at the injection well (fig. 7) may indicate a delayed positive response in groundwater storage to injection, but that injected water dissipates quickly, as indicated by the absence of persistent storage increases at these stations. Ultimately, however, the total volume of water injected during the 4-year study period, 712 acre-ft, is much less than the volume pumped (from 500 to 1,000+ acre-ft per month; fig. 4).

Regional Groundwater Storage Change

Over the study period, groundwater storage decreased slightly across the network; average gravity change from August 2019 to January 2023, exclusive of the AWTF stations, was −0.66 ft of free-standing water (fig. 8). Overall, gravity changes between surveys were small, mostly less than ±1 ft of water, but larger declines were measured in the summer of 2022. From April to August 2022, widespread groundwater-storage declines were observed across the network, with the greatest changes occurring in the southeastern part of the study area, at stations nearest to the Rio Grande (stations riov, vihp, and palo). From August 2022 to January 2023, storage rebounded, with increases recorded across the network. Several factors may have contributed to this decrease and rebound. First, groundwater pumping by Rio Rancho was higher in the summer of 2022 than in other periods during the study (fig. 4). Second, at the nearest streamgaging station on the Rio Grande, Rio Grande at Alameda Bridge at Alameda, New Mexico (USGS site number 08329918; USGS, 2023), streamflow was at its lowest during the study period in the summer of 2022, then increased throughout the fall of 2022 (fig. 9). Systematic measurement error in the absolute-gravity measurements, which provide the datum, or reference level, for the relative-gravity measurements, may also be a possibility. However, systematic error would have to affect all the absolute-gravity stations equally for this to be an important factor; that is, the measurements would need to have all been biased toward low values, rather than simply exhibiting increased random error for this survey. Errors at individual stations are usually apparent on the basis of their relation to the relative-gravity measurements during the least-squares network adjustment (that is, they present as statistical outliers). The absolute gravity meter was serviced by the manufacturer in June 2022, including calibration of the laser and clock frequencies that are the basis for the gravity acceleration measurement. Similar low gravity values were not observed on other projects carried out around the same time. Finally, the magnitude of the gravity change, greater than 20 µGal in most cases, is much larger than the expected measurement error, 5–8 µGal (Schmerge and Francis, 2006), and larger than the estimated average station standard deviation for this survey from least-squares statistics, 2.8 µGal (Bell and Kennedy, 2025).

Map view plots show the seasonality of groundwater-storage change. During the April–August period when groundwater pumping is greatest, storage declines occur at almost all stations (fig. 10). Minimal storage declines were observed in the summer of 2019, mostly less than the estimated uncertainty of 0.5 ft of free-standing water. This period had higher flows in the Rio Grande (fig. 9) than subsequent summer periods. The August–December and December–April intervals (fig. 10, left and right columns), were largely periods of increasing storage. Overall, storage changes are relatively small, with the largest changes occurring in the eastern part of the study area near the Rio Grande. Larger storage changes, and higher volumes of groundwater pumping, occur in Albuquerque directly southeast of the study area (Kennedy and Bell, 2023).

Cumulative groundwater-storage change during the study period and groundwater-level change are shown in figure 11 and table 3. At wells with only quarterly discrete data, groundwater levels at the time of the gravity survey were estimated using linear interpolation between the nearest values in time. At wells with rapid groundwater-level changes, such as 351545106462001 (well 14), this need to interpolate in time demonstrates one of the limitations of spatially interpolating groundwater-levels to create head maps, and in particular, head-change maps: groundwater-level measurements collected at different times (for example, over a several-month period) and combined in a single map may not represent a “snapshot” of the aquifer. (The same limitation would apply to gravity data if a survey was carried out over a long time period.) At this well, the large increase in groundwater level (2.7 ft, one of the westernmost wells in fig. 11), viewed in context of the entire record (U.S. Geological Survey, 2024b), appears to represent short-term fluctuations in groundwater head and may not indicate an actual large storage change because of delayed drainage/filling caused by capillary effects, and (or) confined or semiconfined aquifer conditions. In contrast, the large increase in groundwater level at the two wells (sites 350950106434001 and 350918106425401) in the southwestern part of the map area on figure 11 near the southern boundary appears to represent a years-long, steady increase in groundwater level, likely caused by a reduction in pumping in nearby wells, and interpolation in time introduces less error. A further limitation of combining groundwater-level data from multiple wells is introduced when different wells are open to different parts of the aquifer, with different screen lengths, creating a heterogeneous dataset. In contrast, repeat microgravity data, despite having less precision, are more uniform in their region of sensitivity across the network because the instrument is measuring approximately the same volume of the subsurface at each location.

Table 3.    

Linear trends in groundwater-storage change from repeat microgravity data in the Rio Rancho, New Mexico area (2019–23).

[ID, identifier; n, number of observations used in trend analysis; Apr., April; Jan., January; NA, not applicable]

Table 3.    Linear trends in groundwater-storage change from repeat microgravity data in the Rio Rancho, New Mexico area (2019–23).

Station number	Station ID1	Coefficient of determination (R2)	Linear trend in groundwater storage change (feet of water per year)	n	Cumulative storage change, Apr. 2019–Jan. 2023 (feet of water)
1	AWTF1	0.46	0.64	5	NA
2	awtf2	0.08	–0.14	10	–0.11
3	AWTFE	0.6	–0.37	11	–0.79
4	awtfn	0.02	–0.07	10	0.40
5	AWTFS	0.02	0.06	10	0.41
6	AWTFW	0.12	–0.13	11	–0.23
7	cacp	0.62	–0.35	11	–0.99
8	chip	0.24	–0.28	11	0.02
9	CLHS	0.16	–0.21	11	–0.52
10	gatp	0.20	–0.24	11	–0.26
11	hirp	0.01	0.01	11	0.42
12	HRUN	0.46	–0.53	11	–0.42
13	LIMS	0.46	–0.38	11	–0.67
14	MW18	0.27	–0.32	11	–0.64
15	MW23	0.83	–0.61	10	–2.04
16	palo	0.40	–0.56	11	–1.42
17	riov	0.10	–0.30	11	–0.40
18	RNBW	0.55	–0.26	11	–1.28
19	sn2p	0.56	–0.45	11	–0.87
20	vihp	0.13	–0.24	11	–0.26

¹

Uppercase letters indicate sites where absolute-gravity data were collected.

Spatial changes in gravity and groundwater levels generally agree (fig. 11). Declines are greatest in the northern and central parts of the network, and less in the southern and western parts. Groundwater-level changes are near zero in the south-central part of the study area, whereas large increases have occurred in the far southwestern and southeastern parts. These increases were caused by decreased groundwater pumping by the Albuquerque Bernalillo County Water Utility Authority beginning in 2010 (followed by an increase in pumping in 2020) and are supported by repeat microgravity monitoring data (Albuquerque Bernalillo County Water Utility Authority, 2022; Kennedy and Bell, 2023). Groundwater levels in the southern part of the study area reached their maximum in 2020 and have stabilized or declined slightly between 2020 and 2023 (USGS, 2023).

Trends in Groundwater Storage and Groundwater Levels

To evaluate change in groundwater storage during the entire study period, a linear trend was fit to the time-series data at each location (table 3). The coefficient of determination (R²) values are low (mean = 0.36) because of unaccounted-for seasonality in the data, low signal-to-noise ratios, and the large decreases and increases in storage during the last two surveys. Nevertheless, the trends are nearly all negative, ranging from 0.06 (station AWTFS) to −0.61 (station MW23) ft of water per year. The largest negative trends occur along a north-south line in the central part of the study area (fig. 11). The trend for AWTF1 is the highest computed of the study at 0.64 ft of water per year, but is not included in the general comparison discussion because gravity data were collected during only 5 events early in the study, whereas all other stations had gravity data collected during at least 10 events.

The small but widespread decline in groundwater storage over the study period (fig. 12) may be reconciled with the generally stable to slightly increasing groundwater levels (figs. 5 and 6), in part, by a change in water content in the unsaturated zone. Depth to groundwater in the study area is nearly 200 ft below land surface near the Rio Grande and over 1,000 ft below land surface in the western part of the study area. The gravity-change measurement is interpreted as reflecting water-storage change at the water table, but it also includes water stored in the unsaturated zone. Although the change in unsaturated-zone water content is generally small, owing to the high rate of evapotranspiration relative to precipitation, a small change summed over a sufficiently thick unsaturated zone may produce a measurable gravity signal. Drying of the unsaturated zone could manifest as decreasing gravity during the study period without affecting groundwater levels, but over time, this drying will lead to less recharge reaching the aquifer and an eventual lowering of groundwater levels. The timescale of unsaturated-zone drying is difficult to determine but likely occurs over several years to decades (Dickinson and Ferré, 2018).

Specific Yield

Specific yield estimates are possible at locations having collocated repeat microgravity data and water-level data. For any particular increment of groundwater-storage change, the specific yield can be calculated as the ratio of groundwater storage change (from gravity data) to groundwater-level change. Where time-series data exist, groundwater-storage change data for the interval between each gravity survey can be plotted against groundwater-level change for the same interval, and specific yield can be estimated as the slope of the best-fit line.

In the Rio Rancho study area, four locations had collocated gravity and groundwater-level data. Specific yield estimates had high associated uncertainty because groundwater-level (and groundwater-storage) changes were small. At two of the four locations (MW18 and HRUN), specific yield estimates were unrealistic (negative or greater than 1) and are not presented here. A reasonable estimate, 0.20 ±0.07, was obtained at site MW23, which had over 3 ft of groundwater-level change during the study period (fig. 13). An unreasonably high estimate with high uncertainty, 0.71 ±0.23, was obtained at site LIMS, which had about 0.5 ft of groundwater-level change during the study period (fig. 13). Published values of specific yield for basin-fill aquifers similar to the Santa Fe Group aquifer system range from approximately 0.1 to 0.25 (Johnson, 1967). Specific yield values used in groundwater flow models of the Santa Fe Group range from 0.15 to 0.20 (Kernodle and others, 1995; Tiedeman and others, 1998; Barroll, 2001). Nevertheless, the relatively large gravity changes at MW23 and LIMS that are correlated with groundwater-level changes provide evidence that groundwater-storage changes are occurring at the water table in unconfined storage, indicating a degree of connection between the water table and deeper aquifer layers where municipal pumping occurs.