Hydrogeologic Framework


Groundwater occurs in sediments and rock beneath the land surface. Geologic materials that transmit significant amounts of water are called aquifers, and materials that transmit water poorly are called aquitards. Geologic units generally are delineated based on how and when they were deposited, but a geologic unit may contain both aquifers and aquitards. A saturated aquitard that is areally extensive and serves to confine an adjacent artesian aquifer or aquifers is called a confining unit. Leaky confining units may transmit appreciable water to and from adjacent aquifers. A hydrogeologic framework is constructed by representing the distribution of geologic units and separating, or combining, these units into hydrogeologic units that have similar hydraulic properties. 


Although the Mosier area geologic units are well defined at the land surface, their location in the subsurface where groundwater occurs is poorly understood. Using data collected as part of this study and previous studies, the depth, thickness, and extent of important sediment and rock units beneath the study area were mapped. Insufficient subsurface data exists to define the geometry of geologic units to the south of the Chenoweth thrust fault (fig. 2) reliably, so the constructed geologic model does not cover the entire Mosier-Rock-Rowena Creek watershed.


Geologic Setting


The Mosier basin was inundated with flood basalts in Miocene time, followed by deposition of volcaniclastic deposits of mostly Tertiary age (Newcomb, 1969; Swanson and others, 1981; Bela, 1982; and Lite and Grondin, 1988). A small part of the study area is covered with Quaternary fluvial sediments consisting of catastrophic Missoula Floods deposits and modern river and stream deposits. The pre-Miocene basement rock has not been encountered in wells, nor is it exposed in outcrop. Tectonic forces have deformed the system, resulting in faulted and folded basalt. 


The geometry of the system is dominated by the Mosier syncline and Columbia Hills anticline (fig. 2) which deform all hard rock units and form the troughs within which the sedimentary overburden is emplaced. The axes of these two features are approximately parallel, with a southwest to northeast trend. As these folds developed, a series of hydraulically important faults also developed, including the Rocky Prairie and Chenoweth thrust faults and a wrench fault whose trace crosscuts the entire watershed starting in the northwest corner of the model area and trending to the southeast (fig. 2). Each of these faults has significant offset over at least a part of their lengths. The wrench fault is associated with the Maupin Trend (Anderson, 1987), and is hereafter referred to as the Maupin wrench fault in this report.


The oldest CRBG lavas were sheet flows that resulted from a high-volume of lava flowing over flatter terrain, which resulted in a laterally extensive continuous coverage under the entire groundwater model area. As the rock was deformed and the syncline-anticline pair developed, the Mosier syncline became a trough through which later CRBG lavas flowed. The geometry of the valleys during deposition and the low volume of lava resulted in flows, called intracanyon flows, which do not cover the entire watershed area. Between periods of deposition of CRBG lavas, sedimentary deposits accumulated on the surface of the previous lava flow, and where these deposits are preserved and covered by a later lava flow, they are called sedimentary interbeds or interbeds. After CRBG volcanism stopped depositing lava in the area, volcaniclastic deposits associated with Cascadian volcanism flowed from the southwest across much of the watershed. The volcaniclastic deposits are highly heterogeneous and poorly delineated, but generally consist of debris flows and volcanic ash. 
Large floods associated with failure of ice dams near Missoula, Montana, during the last ice age deposited coarse-grained glaciofluvial deposits in a limited area of the lower watershed (fig. 2). The youngest sedimentary deposits in the system are associated with modern erosional processes, and they typically occur near the creeks.


Geologic Model Units


Geologic model units for this study (fig. 3) consist of sedimentary deposits and basalt units of the CRBG, overlying volcaniclastic deposits, and catastrophic flood deposits. The geologic model units used to create a three-dimensional geologic model were selected based on data availability. Mapped or previously identified geologic units were sometimes simplified into simpler geologic model units if data density was insufficient to define the geometry of the units. 


The geologic deposits overlying the CRBG aquifers consist of a variety of alluvial and volcaniclastic deposits, referred to hereafter as overburden. These deposits were grouped into younger Glaciofluvial Deposits and (mostly) older Undifferentiated Overburden (fig. 3) based on preliminary groundwater flow modeling results. The thickness of the overburden is highly variable. No estimates of maximum thickness are available because the thickest sequences are likely in the trough of the Mosier syncline, and no thickness data are available near the syncline axis. 


Below the overburden, a series of CRBG lava flows covers the watershed. Because water wells typically penetrate the minimum depth in the aquifer system that meets water‑usage needs, more information is available for the shallower units at any given location. The three youngest lava units (Pomona, Lolo, and Rosalia) and the two uppermost interbeds (Selah and Quincy-Squaw Creek) are identified with regularity and reasonable confidence in most well logs, allowing identification of each of these units in the geologic model. Each of these lava units consists of a single intracanyon flow that partially covers the study area. Flow thicknesses are variable, pinching out at the margins, but with typical thickness of about100 ft in many areas.


The composition of the sedimentary interbeds is highly variable. The Selah interbed lies between the Pomona and Lolo Basalt units and the Quincy-Squaw Creek interbed lies between the Rosalia and Sentinal Gap Basalt units. Sedimentary interbed thickness is highly variable and may be discontinuous over short distances, with thickness depending on paleotopography of the surface over which the overlying basalt flowed. The Selah interbed thickness is apparently correlated (with high variability) to thick sections of the Pomona Basalt, which is likely because sedimentary deposits tend to be thickest in valley bottoms that the lava filled. Recorded thicknesses of the Selah interbed in well logs range from 0 to about 100 ft. The Quincy-Squaw Creek interbed thickness typically ranges between 10 and 30 ft in well logs, with no apparent correlation to overlying lava unit thickness.


The next youngest basalt unit is the Roza, a single flow deposited during the same period when the Quincy-Squaw Creek interbed was deposited (Tolan and others, 2009). Some of the Quincy-Squaw Creek interbed deposits may be older and some may be younger than the Roza flow. For modeling purposes, the Roza basalt unit was assumed to underlie the interbed (fig. 3). The Roza flow is of limited areal extent, occurring only near Rowena Creek (Lite and Grondin, 1988). 


Three of the Frenchman Springs units are mapped in the Mosier area (Tolan and others, 2009), accounting for at least four lava flows: one Sentinal Gap flow, two Sand Hollow flows, and one or more Gingko flows (Kenneth Lite, Oregon Water Resources Department, written commun., 2010). Because of insufficient data on the geometry of the Roza flow and the Frenchman Springs flows, these flows have been lumped into the Frenchman Springs geologic model unit (fig. 3). Almost all wells that penetrate the Frenchman Springs units are near the crest of the Columbia Hills anticline. In this area, where the Roza likely is absent, the total thickness is estimated at about 400 ft.


The Grande Ronde Basalt likely underlies the entire study area, even though it is only identified in wells near the Columbia Hills anticline. The total number of flows and total thickness are not known, although a thick sequence of Grande Ronde Basalt is exposed in the Columbia River Gorge on the northeast boundary of the study area. The top of this unit forms the lower bound for the geologic model.


Three-Dimensional Geologic Model


The hydrogeologic framework was developed using a three-dimensional geologic model (figs. 4 and 5). The geologic model was constructed for the area where geologic maps and geologic interpretation of 318 well logs from previous studies (Newcomb, 1969; Grady, 1983; Kienle, 1995; Jervey, 1996) provided sufficient information to define the three-dimensional geometry of the geologic units constituting the aquifer system (fig. 2). To the south of the Chenoweth thrust fault, because volcaniclastic deposits cover the area, the geometry of the underlying geologic units is poorly understood. As a result, the geologic model and the derivative groundwater-flow simulation model domains do not extend to the south of the Chenoweth thrust fault.


Because potential errors exist in all of the data, trend interpolation methods were used to develop the three‑dimensional geologic model from the data. Inductive methods were used for construction of the geologic map and interpretation of well stratigraphy, where geologists identified the likely location of geologic contacts based on contacts identified at other locations. For some wells, the geologic interpretation of stratigraphy (termed ‘geologic pick’) differed between studies, so one or both conflicting interpretations of the geology contain errors. For wells with conflicting geologic picks, a single “best” pick was made using available data. Geologic unit tops and bottoms were simulated using two-dimensional surface trend models to ensure that the final geologic model matches most of the data well, preserving the important features of the system that control the storage and transmission of groundwater. The details of constructing the surfaces and the geologic model are described in appendix A.


Preliminary groundwater-flow simulation results indicated that the hydraulic conductivity of the glaciofluvial deposits was possibly an important parameter for understanding aquifer leakage through commingling wells, so zonation was used to separate the glaciofluvial deposits from the remainder of the overburden. Wherever glaciofluvial deposits exist, the geometry of any older buried overburden is poorly understood, so it was assumed that if glaciofluvial deposits are mapped at land surface, they are the only overburden unit present (fig. 5A–A′). This is a poor geologic assumption, but it allows testing of the role of the glaciofluvial deposits in the groundwater-flow simulation model.


Hydrogeologic Units


Following creation of the three-dimensional geologic model, geologic units were divided into hydrogeologic units, where the flow controlling features were identified (fig. 3). This conceptual model allowed the identification of geologic features that are believed to control the response of the groundwater system. 


Hydrogeologic units were defined based on their hydraulic characteristics. If adjacent geologic units have similar abilities to store and transmit water, then they can be grouped into a single hydrogeologic unit. Conversely, if a geologic unit has zones of significantly different hydraulic character, then geologic units can be divided into multiple hydrogeologic units. Geologic material that is very permeable to water is called an aquifer, and significantly less permeable units are called aquitards. Laterally extensive aquitards are called confining units. High permeability corresponds to high hydraulic conductivity, a measure of how easily water is transmitted through geologic material. Similarly, low permeability corresponds to low hydraulic conductivity. For this study, 23 hydrogeologic units (aquifers and confining units) were defined (fig. 3). 


The overburden geologic units were divided into three hydrogeologic units based on the hydrologic properties of these units and their potential influence on important groundwater flow processes (fig. 3): the glaciofluvial aquifer, the upper undifferentiated overburden-confining unit, and the locally productive lower overburden aquifer. The uppermost hydrogeologic unit is the glaciofluvial aquifer, consisting of very permeable gravel and other coarse sediments deposited during the Missoula Floods. These deposits are of limited extent (fig. 2), and where they occur to the south of the Rocky Prairie thrust fault, the permeability may control the rate at which water leaking vertically through commingling wells would return to Mosier Creek. This unit was separated from the undifferentiated overburden in the geologic model to evaluate the role of the glaciofluvial deposits in restricting the flow from commingling wells.


The remainder of the overburden is undifferentiated, but the largest part of this geologic model unit consists of Cascadian volcaniclastic deposits that are older than the glaciofluvial deposits. Previous investigators (Newcomb, 1969; Lite and Grondin, 1988) recognized that, although these deposits typically have low permeability, coarser deposits forming productive aquifers may occur in the lower parts of the unit. For this reason, the undifferentiated overburden geologic model unit is divided conceptually into an upper confining unit and a lower aquifer that may be discontinuous. 


Generally, each CRBG lava flow consists of a dense flow interior and irregular flow tops and flow bottoms with a variety of textures (fig. 6) (Reidel and others, 2002). Flow top textures are formed as the lava develops a crust while the liquid center continues to flow. Flow bottom textures are controlled by the lava properties (for example, temperature and chemical composition) and the properties of the surface over which the lava is flowing. A variety of joint patterns, fractures, and lithologic textures can occur in any single basalt flow. Although flow interiors have joints and fractures, they typically do not transmit water easily. Flow tops and bottoms are commonly vesicular or brecciated, and they may or may not be permeable. Local permeability of flow tops and bottoms may be highly variable over short distances as a result of depositional processes, but the complex connectivity of the open conduits tends to be high over long distances, resulting in highly transmissive aquifers at the regional scale. The variability in lithologic textures implies that even though a flow top or bottom is intersected when drilling, there is no guarantee that this zone will be open and connected to the aquifer system. Within the study area, flow tops generally tend to transmit water easily, forming productive aquifers, but the only documented transmissive flow bottom is at the base of the Pomona Basalt flow (Lite and Grondin, 1988). The Pomona Flow Bottom aquifer does not occur at all locations where the Pomona Basalt occurs, but to the south of the Rocky Prairie thrust fault, it was estimated to cover an area of 4–6 mi², generally coincident with the OWRD groundwater administrative area (fig. 1). In this area, the aquifer may be as much as 40 ft thick (Lite and Grondin, 1988). The transmissive flow bottom is postulated to have formed when the lava flowed over wet sediments in the paleo-valley bottom. 


As a group, the CRBG is a stack of laterally extensive lava flows with relatively thin permeable, productive zones at flow tops and flow bottoms separated by relatively thick flow interiors of low permeability. Individual CRBG lava flows may be tens to hundreds of feet thick, with a typical thickness of about 100 ft in the study area. Thickness of each part of each flow is highly variable locally, but the thin permeable aquifers commonly occupy about 10 percent of the total thickness. The aquifer system can transmit and yield water easily from the thin flow tops and bottoms, but has low storage capacity in the flow interiors, which make up a large part of the aquifer system. Flow interiors have low permeability and low storage characteristics, and they form effective confining units between permeable flow tops.


Sedimentary interbeds between CRBG lava flows are porous and able to store water but are less permeable than the adjacent basalt aquifers, so they form confining units in the Mosier study area. The combined thickness of flow top, interbed, and an overlying flow bottom is called an interflow. If a continuous interbed exists between a permeable flow bottom and permeable flow top, the interbed typically functions as a confining unit, dividing the interflow into two aquifers. In the absence of an interbed, the flow top and overlying flow bottom are hydraulically indistinguishable, so a single aquifer exists. Whether the single aquifer is comprised of only a permeable flow top or the combination of a permeable flow top and permeable flow bottom, the hydrogeologic nature of the aquifer is the same, and these aquifers are designated as flow top aquifers in the terminology of this report (fig. 3).


Three-Dimensional Hydrogeologic Framework


A three-dimensional hydrogeologic framework suitable for input to a groundwater-flow model was constructed by dividing the geologic model into groundwater-flow model units. To the maximum extent practicable, each geologic model unit was divided into separate groundwater-flow model units representing hydrogeologic units (fig. 3). This was accomplished for all geologic model units except the Frenchman Springs unit, which was grouped into groundwater-flow model units representing the bulk properties of the five or more basalt flows within the unit. Each groundwater-flow model unit is represented as a single layer in the groundwater-flow model with the exception of the overburden.


The overburden was divided into two layers in two zones in the groundwater-flow model. The undifferentiated overburden was divided into an upper confining unit and a lower aquifer in one zone and the homogeneous glaciofluvial aquifer occupied both layers in the other zone. The geometry of the aquifer that locally occurs at the base of the Chenoweth Formation is poorly understood, but generally is assumed to be thin relative to the entire thickness. To allow this unit to be represented in the groundwater-flow model, it was arbitrarily assumed that the lower undifferentiated overburden aquifer occupied 10 percent of the total thickness. The glaciofluvial aquifer layers share the same percentage split in thickness as the undifferentiated overburden, but both layers are assigned the same properties, so that this unit is modeled as homogeneous.


Each major sedimentary interbed is represented as a single hydrogeologic unit, and the Pomona, Lolo, and Rosalia Basalt flows were subdivided into aquifers and confining units as described in the Hydrogeologic Units section. The basalt flow top aquifers were assumed to occupy 10 percent of the total thickness. The Pomona Basalt flow bottom aquifer geometry was modeled to match estimates of extent and thickness estimated by Lite and Grondin (1988). The areal extent was defined by identifying a thickness threshold such that the thickness of Pomona basalt exceeding this threshold occupies about 4 mi² to the south of the Rocky Prairie thrust fault near the OWRD groundwater administrative area. Thickness of this aquifer was defined as a fraction of the total thickness exceeding the threshold such that the thickest part of the aquifer in the OWRD administrative area is approximately 20 ft thick. The remainder of the thickness of each basalt unit was defined as a flow interior confining unit. 


Conversely, the Frenchman Springs geologic model unit represents a sequence of five or more basalt flows. For flow modeling purposes, this unit is divided into two flow model units, with each unit represented by a single flow model layer. The upper unit represents the group of flow top aquifers that are associated with the Frenchman Springs geologic model layer. The lower unit represents the group of low permeability flow interiors. The lumped flow top aquifer is modeled as the upper 10 percent of the total thickness, with the lower 90 percent being modeled as a lumped confining unit.


The Grande Ronde Basalt unit top was the lower bound of the geologic model. A single 20 ft thick groundwater model layer was used to simulate a single flow top aquifer associated with the Grande Ronde aquifer system. The flow interior below this aquifer is not simulated in the groundwater‑flow model because it is likely a barrier to flow and no wells penetrate it.


Supporting data and additional details of division of the geologic model into groundwater-flow model units are contained in appendix A.


First posted March 1, 2012