Purpose and Scope

The purpose of this report is to describe a hydrologic investigation of the Washita River aquifer in southern Oklahoma. This description includes (1) an updated summary of the hydrogeology with a definition of the hydrogeologic framework (including an updated geographic extent) of the aquifer, (2) development of conceptual and calibrated numerical groundwater-flow models for the aquifer representing the 1980–2017 study period, and (3) results of simulations of groundwater-availability scenarios. The groundwater-availability scenarios use the calibrated numerical groundwater-flow model to (1) estimate the EPS pumping rate that ensures a minimum 20-, 40-, and 50-year life of the aquifer, (2) quantify the potential effects of projected well withdrawals on groundwater storage over a 50-year period, and (3) simulate the potential effects of a hypothetical (10-year) drought on groundwater storage. The calibrated numerical groundwater-flow model and groundwater-availability scenarios were archived and released in a USGS data release (Rogers and others, 2023).

The entire Washita River aquifer in Oklahoma consists of four administrative sections, or reaches, which are managed independently by the OWRB. Reach 1 extends from the Texas border to downstream from Foss Reservoir in western Oklahoma (fig. 1; Ellis and others, 2020), and reach 2 is hydraulically connected to and managed as part of the Rush Springs aquifer in western Oklahoma (fig. 1; Ellis, 2018a); both reach 1 and reach 2 are outside the scope of this report. The modeled area of this report is a 279.6-square-mile (mi²) (178,938-acre) subarea of the 411.5-mi² extent of alluvium and terrace deposits (Heran and others, 2003; Miller and Stanley, 2004; Chang and Stanley, 2010; Stanley and Chang, 2012) of reaches 3 and 4 of the Washita River aquifer in southern Oklahoma (fig. 1). Reach 3 extends from near Anadarko, Okla., to Alex, Okla., and includes 113.6 mi² (72,697 acres, or 40.6 percent) of the modeled area. Reach 4 extends from near Alex to south of Davis, Okla., and includes 166.0 mi² (106,241 acres, or 59.4 percent) of the modeled area. The aquifer becomes narrow and thin in smaller tributary valleys, so this investigation focused on the main body of the aquifer along the Washita River main stem and selected large tributaries (modeled aquifer area, fig. 1). Though sometimes referred to as the “Washita River alluvial aquifer” (Schipper, 1983; OWRB, 2019b), the aquifer is referred to as the “Washita River aquifer” in this report because it consists not only of alluvium deposits but also of terrace and dune deposits. Selected sections in this report are modified from Ellis (2018a, b), Ellis and others (2020), and Smith and others (2021). Though the study areas are different, the organization and wording of this report are largely based on Smith and others (2021).

Description of Study Area

The Washita River aquifer is a long, narrow, unconfined aquifer that includes alluvium and terrace deposits of the Washita River and tributaries in Caddo, Grady, Garvin, McClain, and Murray Counties, Oklahoma (fig. 1). The Washita River is perennial in the study area except during the most extreme droughts as documented by historical USGS streamgaging records for the stream obtained from the USGS National Water Information System (NWIS) database (USGS, 2020; table 1). The Washita River generally flows from northwest to southeast for about 207 miles (mi) (Horizon Systems Corporation, 2015) in the study area. The largest tributaries in the study area are Little Washita River and Sugar, Rush, and Wildhorse Creeks (fig. 1). Many of the smaller tributaries to the Washita River in the study area are dammed by hundreds of Natural Resources Conservation Service (NRCS) floodwater-retarding structures (fig. 2), which were mostly built in the 1950s through 1970s (U.S. Army Corps of Engineers, 2021). Floodwater-retarding structures were designed to impound runoff from headwater catchments and slowly release it downstream with the purpose of reducing the frequency and magnitude of damaging flood flows on the Washita River main stem (Smith and Esralew, 2010). The Washita River main stem, however, is not dammed in the study area.

Land-cover data for the Washita River aquifer study area were obtained from the CropScape database (National Agricultural Statistics Service, 2019) (fig. 3), which includes land-cover characteristics at 30-meter (m) resolution for land overlying the Washita River aquifer for the 2010–17 period. During this period, land-cover type was primarily cropland (44.1 percent), grass or pasture (36.3 percent), forest and shrubland (9.3 percent), or developed (7.7 percent). Winter wheat was the major crop-cover type in the study area, accounting for 57.6 percent of cropland by area. Alfalfa or hay (22.0 percent) was the next largest crop-cover type by area. Corn and soybeans accounted for 7.5 and 6.6 percent of cropland by area, respectively. Cotton (1.3 percent) and fallow or idle cropland (1.2 percent) were the only other crop-cover types that accounted for more than 1 percent of cropland by area (fig. 3). Crop-cover types typically change in response to economic conditions and hydrologic factors, but the percentages of total cropland cover and individual crop types did not change substantially during the 2010–17 period.

Climate Characteristics

The climate of the study area is classified as subhumid with generally southerly prevailing winds (Patterson, 1984). Historical data from selected climate stations in southern Oklahoma (Climate Division 8) have been quality assured and summarized monthly as part of the Global Historical Climatology Network (National Centers for Environmental Information, 2021). These data were used to calculate and graph annual and monthly temperature and precipitation statistics for the study area (fig. 4, table 2). A locally weighted scatterplot smoothing line (Cleveland, 1979) was used to delineate periods of below- and above-mean annual temperature and precipitation (fig. 4). The mean annual temperature during 1895–2020 was 62.2 degrees Fahrenheit (°F) (fig. 4, table 2), and the mean annual temperature differed by about 2 °F from northwest to southeast across the study area (Oklahoma Climatological Survey, 2021a, b). The mean annual precipitation during 1895–2020 was 38.0 inches per year (in/yr) (fig. 4, table 2), and the mean annual precipitation increased by about 7.5 in/yr from northwest to southeast across the study area (Oklahoma Climatological Survey, 2021a, b). The mean annual precipitation for the 1980–2017 study period was 40.1 in/yr (fig. 4, table 2). The 1981–2002 period was an unprecedented wet period in the available record; above-mean precipitation was recorded in 18 of these 22 years (82 percent of the years), contributing to a higher mean precipitation during the study period compared to the mean for the period of record (fig. 4A). May is typically the wettest month, and January is typically the driest month (fig. 5A).

Multiyear to decadal droughts are common for the study area (fig. 4). The 1929–41 (Dust Bowl; Egan, 2006) and 1952–56 drought periods were among the most severe in Oklahoma in the 20th century; two shorter, less severe drought periods also occurred in the late 20th century during 1961–72 and 1976–81 (Tortorelli, 2008; Shivers and Andrews, 2013). The 21st century began with the 2002–6 and 2010–14 drought periods (Tortorelli, 2008; Shivers and Andrews, 2013). The most severe droughts on record developed from extended periods of below-mean precipitation paired with above-mean temperature.

Streamflow Characteristics and Trends

Daily streamflow data, with various and sometimes interrupted periods of record, were recorded at selected USGS streamgages in the study area (figs. 1 and 2) and summarized for the 1980–2017 study period (table 3). Streamflow measured at streamgages is the sum of runoff and base flow originating upstream; base flow is the component of streamflow that is supplied by the discharge of groundwater to streams (Barlow and Leake, 2012). For this report, streamflow-hydrograph data obtained from the NWIS database (USGS, 2020) were separated into runoff and base-flow components by using the standard Base-Flow Index code (Wahl and Wahl, 1995) in the USGS Groundwater Toolbox (Barlow and others, 2015). The Base-Flow Index code uses the minimum streamflow in a moving n-day window as a basis for hydrograph separation; for consistency, a 6-day window was used for all streamgages in this report. The 6-day window was selected by testing multiple n-day windows for the study period at USGS streamgage 07328100 Washita River at Alex, Okla. (hereinafter referred to as the “Alex gage”) and plotting the resulting mean base-flow percentage against n; a slope change was evident at 6 days. Base flow, as computed using this method, accounts for about 35–48 percent of annual streamflow for the period of record at streamgages on the Washita River main stem (table 3). This percentage, known as the base-flow index (BFI) (Wahl and Wahl, 1995; Barlow and others, 2015), generally increased over the period of record at streamgages on the Washita River (fig. 6A–D). When summarized annually using the Kendall (1938) tau test with the Theil-Sen (Sen, 1968) slope estimator and an alpha value of 0.0500 set as the significance level, the upward trends in the BFI were found to be statistically significant (probability value [p-value] = 0.0054) at USGS streamgage 07326500 Washita River at Anadarko, Okla. (hereinafter referred to as the “Anadarko gage”), statistically significant (p-value = 0.0248) at the Alex gage, statistically significant (p-value = 0.0000) at USGS streamgage 07328500 Washita River near Pauls Valley, Okla. (hereinafter referred to as the “Pauls Valley gage”), and statistically significant (p-value = 0.0083) at USGS streamgage 07331000 Washita River near Dickson, Okla. (hereinafter referred to as the “Dickson gage”). Upward trends in base flow (fig. 6A–D) were found to be not statistically significant (p-value = 0.0772) at the Anadarko gage, statistically significant (p-value = 0.0111) at the Alex gage, statistically significant (p-value = 0.0163) at the Pauls Valley gage, and statistically significant (p-value = 0.0177) at the Dickson gage. The overall pattern of upward trends in the BFI and base flow may have developed following the construction of more than 570 NRCS floodwater-retarding structures (dams; fig. 2) on tributaries of the Washita River within the study area. NRCS dam construction in this area began in 1948, peaked at 65 dams per year in 1963, greatly diminished to about 1 dam per year in the 1980s, and concluded in the early 1990s (U.S. Army Corps of Engineers, 2021). NRCS dams temporarily impound runoff and release it over several days with the purpose of reducing peak flows on the river main stem. The flows that are slowly and steadily released by these dams are indistinguishable from groundwater-derived base flows when they reach the main stem and are mostly classified as base flows by the Base-Flow Index code. Though most upward trends in the BFI and base flow were statistically significant for the streamgage periods of record, no statistically significant trends in base flow or BFI were detected for the 1980–2017 study period at streamgages on the Washita River main stem. One interpretation of the upward trends in the BFI and base flow for the period of record is that the construction of the floodwater-retarding structures caused a step change in apparent base flows in the Washita River.

Surface-water use in the Washita River aquifer study area consists of permitted diversions along the main stem of the Washita River and its associated tributaries (OWRB, 2019d). At the end of the study period (2017), 60 active, nontemporary permitted diversions on the Washita River main stem allowed for a maximum total withdrawal of 13,957 acre-ft/yr. All of those permitted diversions were for irrigation with the exception of three permitted diversions that were used for public supply, and all but 15 of those permitted diversions were in reach 3 of the Washita River aquifer. At the end of the study period (2017), 29 active, nontemporary permitted diversions on Washita River tributaries allowed for a maximum total withdrawal of 9,749 acre-ft/yr.

Groundwater Use

The OWRB permits and regulates groundwater use of more than 5 acre-ft/yr for domestic and agricultural purposes and groundwater use for irrigating more than 3 acres of land for growing gardens, orchards, or lawns in Oklahoma (Oklahoma Statutes §82–1020.1[2] [Oklahoma State Legislature, 2021a; OWRB, 2014]; Oklahoma Statutes §82–1020.3 [Oklahoma State Legislature, 2021c]). Groundwater-use data since 1980 are self-reported annually to the OWRB by permitted users (Christopher Neel, OWRB, written commun., 2019). In 2017, about 61 long-term temporary (regular) groundwater-use permits and about 66 prior-right groundwater-use permits were active for the Washita River aquifer (OWRB, 2019d). Each permit may be served by multiple wells that share the allocated groundwater use. Most groundwater-use permits for the Washita River aquifer were allocated for irrigation (38.0 percent) and public supply (61.4 percent), with some allocated for other uses (0.6 percent), including agricultural (nonirrigation); commercial; industrial; mining; power; recreation, fish, and wildlife; or uncategorized uses (figs. 7 and 8, table 4) (OWRB, 2019d; Rogers and others, 2023). Groundwater use for domestic supply (self-supplied directly to a home by a private well) was assumed to be a negligible part of the total groundwater use because the population of the land overlying the aquifer was predominantly rural or concentrated in cities supplied water by municipal or rural water districts. For the purposes of this report, all groundwater use was assumed to be consumptive use (that is, none of the groundwater that is withdrawn returns to the aquifer as recharge).

Mean annual groundwater use for the 1980–2017 study period was 1,214.1 acre-ft/yr, which includes 745.8 acre-ft/yr (61.4 percent) for public supply and 461.8 acre-ft/yr (38.0 percent) for irrigation (table 4) (Rogers and others, 2023). Mean annual groundwater use for the period of record (1967–2017) was 1,590.7 acre-ft/yr, which includes 861.6 acre-ft/yr (54.2 percent) for irrigation and 709.7 acre-ft/yr (44.6 percent) for public supply (table 4) (Rogers and others, 2023). The decrease in groundwater use, particularly in irrigation groundwater use after 1980, is likely a result of the OWRB implementing changes in methods of groundwater-use reporting. Since 1980, the OWRB has required irrigation permit holders to estimate and annually report their groundwater use in terms of number of applications and number of inches of water applied (Christopher Neel, OWRB, written commun., 2019). Prior to 1980, however, inches of water applied during irrigation were not required to be reported. As a result, the OWRB adopted rules to estimate the number of inches of water applied for pre-1980 data on the basis of the number of water applications. This estimate results in what appears to be a step-change decrease in irrigation groundwater use after 1980 (fig. 8). In 2017, regular permits accounted for about 58 percent and prior-right permits accounted for about 42 percent of reported groundwater use from the Washita River aquifer (OWRB, 2019d).

Mean annual groundwater use for the 1980–2017 study period was 309.9 acre-ft/yr in reach 3 and 904.2 acre-ft/yr in reach 4 of the Washita River aquifer (table 4) (Rogers and others, 2023). Mean annual groundwater use for the period of record 1967–2017 was 406.7 acre-ft/yr in reach 3 and 1,184.0 acre-ft/yr in reach 4 of the Washita River aquifer (table 4) (Rogers and others, 2023). Groundwater-use permits for public supply were predominantly for wells near Lindsay, Okla., or Pauls Valley, Okla., in reach 4 of the Washita River aquifer, whereas groundwater-use permits for irrigation were for wells scattered throughout the Washita River aquifer (fig. 7). Reach 4 accounted for 74.5 percent of the groundwater use for the 1980–2017 study period and 74.4 percent of the groundwater use for the period of record 1967–2017.

Historical groundwater yields from wells completed in the Washita River aquifer generally decreased from northwest to southeast as the aquifer materials thin; groundwater yields of about 350 gallons per minute (gal/min) were reported near Lindsay and groundwater yields of about 200 gal/min were reported near Davis in the 1950s (Leonard and others, 1958). Driller-estimated groundwater-yield values, which have been mostly recorded since the 1980s, averaged about 70 gal/min; about 50 percent of the values recorded were greater than 25 gal/min (the median value), and about 9 percent of the values recorded were greater than 200 gal/min (OWRB, 2019a). Historical groundwater-yield values in the Washita River aquifer obtained from the NWIS database (USGS, 2020) averaged about 240 gal/min. The median groundwater-yield value was 175 gal/min, and a groundwater-yield value of 1,200 gal/min was reported at one well northwest of Pauls Valley (USGS station 344625097181001). These groundwater-yield statistics likely were biased high because high-yield irrigation and public-supply wells were the most common type of well to have measured discharges (USGS, 2020).

Alluvium and Terrace Deposits

Alluvium and terrace deposits of Quaternary age are the principal water-bearing units of the Washita River aquifer (fig. 9). These deposits are made up of alluvial gravel, sand, silt, and clay (figs. 9 and 10; Leonard and others, 1958). Most terrace deposits are hydraulically connected to the valley-bottom alluvium, but some are hydraulically disconnected from the other terrace deposits and the valley-bottom alluvium and thus are unable to store water (Leonard and others, 1958). The disconnected terrace deposits are most evident north of Anadarko and northwest of Pauls Valley (fig. 9). The alluvium and terrace deposits are typically about 2–3 mi wide, but where the Washita River Valley constricts, those deposits are less than 1 mi wide (fig. 11). The Washita River Valley is narrowest where it approaches the Arbuckle Uplift (figs. 9 and 11E). In that area, the alluvium and terrace deposits are only about 0.25 mi wide, which the OWRB regards as the southern boundary of the Washita River aquifer (fig. 9; OWRB, 2019a).

Bedrock Units

Bedrock units of Permian age underlie all of reach 3 (fig. 9A) and most of reach 4 (fig. 9B) of the Washita River aquifer. In the northwesternmost part of the aquifer, west of Anadarko, the Whitehorse Group directly underlies the geologic units that contain the Washita River aquifer (fig. 9). The Whitehorse Group consists of the Rush Springs Formation and the Marlow Formation (fig. 10) but is shown in figure 9 as an undivided unit. The Rush Springs Formation is described as orange-brown, cross-bedded fine-grained sandstone, with some dolomite and gypsum beds (Ellis, 2018a; Neel and others, 2018). The thickness of this geologic unit ranges between 186 and 560 ft in the study area (Ellis, 2018a). Stratigraphically below the Rush Springs Formation is the Marlow Formation, which consists of orange-brown, fine-grained sandstone and siltstone interbedded with mudstone, gypsum, and dolomite (Kent and others, 1984; Ellis, 2018a). The Marlow Formation directly underlies the Washita River aquifer near Anadarko and along Sugar Creek (Kent and others, 1984).

Beneath the White Horse Group is the Permian-age El Reno Group, which consists of the Dog Creek Shale, the Chickasha Formation, and the Duncan Sandstone (fig. 10) but is shown in figure 9 as an undivided unit (Heran and others, 2003). The Dog Creek Shale underlies the Washita River aquifer in the western part of Grady County and consists of red, brown, and green gypsiferous shales interbedded with siltstone, sandstone, and dolomite (Kent and others, 1984; Heran and others, 2003; Ellis, 2018a). The Dog Creek Shale has an estimated thickness of 200 ft and is described as having very low permeability (Mogg and others, 1960; Ellis, 2018a), which serves to limit downward flow in the Washita River aquifer. The Chickasha Formation and Duncan Sandstone make up the stratigraphically lower part of the El Reno Group. The Chickasha Formation is characterized as a red-brown mudstone conglomerate with some shale and siltstone and is estimated to be about 200 ft thick (Kent and others, 1984; Ellis and others, 2017). The Duncan Sandstone has a similar thickness to the Chickasha Formation and the Dog Creek Shale and is characterized as a light gray to reddish cross-bedded fine-grained sandstone interbedded with shale (Heran and others, 2003; Ellis and others, 2017). The Duncan Sandstone directly underlies the Washita River aquifer as reach 3 transitions to reach 4 (Heran and others, 2003). The units of the El Reno Group contain the El Reno minor aquifer in the study area.

The Permian-age Hennessey Group directly underlies the Washita River aquifer in the northern part of Garvin County (Kent and others, 1984; fig. 9). The Hennessey Group is made up of the Bison Formation, Purcell Sandstone, and Fairmont Shale (fig. 10) but is shown in figure 9 as an undivided unit. The Bison Formation, estimated to be 50–90 ft thick in the study area, makes up the upper part of the Hennessey Group and is characterized as gray to red-brown shale containing calcium carbonate (Heran and others, 2003). The Purcell Sandstone is a red-brown, fine- to coarse-grained sandstone with some shale and mudstone conglomerate (Heran and others, 2003). The Purcell Sandstone is about 90–150 ft thick in the study area and lies directly above the Fairmont Shale. The Fairmont Shale is similar in color to the Purcell Sandstone, characterized as red-brown and blocky. The Fairmont Shale thickness is 40–80 ft in the study area.

The Permian-age Sumner Group, which includes the Garber Sandstone and Wellington Formation, underlies the Washita River aquifer northwest of Pauls Valley (figs. 9 and 10). The Garber Sandstone is characterized as red-brown fine-grained sandstone with a thickness of about 100–150 ft in the study area (Heran and others, 2003). The Wellington Formation is composed of red-brown shale with several 20–30-ft bituminous sandstones at the base and a thickness of about 100–200 ft decreasing southeastward in the study area (Heran and others, 2003). The Garber Sandstone and Wellington Formation contain the Garber-Wellington aquifer (also known as the Central Oklahoma aquifer, Mashburn and others, 2013) in the northeastern part of the study area, where the Garber Sandstone and Wellington Formation are thicker and more permeable. However, the Garber Sandstone and Wellington Formation are not considered to be a major aquifer near the Washita River aquifer (Hart, 1974; Patterson, 1984).

Underlying the Sumner Group is the Pontotoc Group, a mostly Permian-age red-brown to grayish shale with arkosic sandstone and limestone conglomerates (fig. 10). The Pontotoc Group directly underlies the Washita River aquifer in reach 4 where it approaches the Arbuckle Mountains (fig. 9) and has a relative thickness of 300–500 ft in the study area (Heran and others, 2003). The Pontotoc Group is not sufficiently permeable to yield large quantities of water in most locations and is not considered to be a major water-bearing unit in the study area; however, well yields of 60 gal/min have been reported in a small area in southwestern Garvin County (Hart, 1974).

Pre-Permian Paleozoic-age units make up the Arbuckle Mountains and underlie the southeasternmost part of the Washita River aquifer (fig. 9). These units are composed of gray-brown limestone with shale and some bituminous sandstone and limestone conglomerate (fig. 10) and were greatly faulted and folded by the Arbuckle Uplift (Reeds, 1927; Ye and others, 1996). Some of the pre-Permian Paleozoic-age units form the Arbuckle-Simpson aquifer in southern Oklahoma. It is uncertain, however, whether that aquifer is in direct hydraulic connection with the Washita River aquifer as defined in this report. Igneous units (Wichita Granite and Raggedy Mountain Gabbro Groups [fig. 9], consisting of diallage and olivine gabbro, anorthosite, perthite leucogranites, and diorite composing a layered intrusion [fig. 10]) are present within the study area southwest of Davis; however, these igneous units are not in direct hydraulic connection with the Washita River aquifer.

Groundwater Quality

Groundwater-quality data were collected in the study area during July 28–August 25, 2014, by the OWRB (OWRB, 2017). Groundwater was sampled from 14 wells in the alluvium and terrace deposits of the Washita River aquifer study area (fig. 9; site information is available in Rogers and others [2023]) and analyzed for selected constituents including physical properties (pH, temperature, and specific conductance), major ions, nutrients, and trace metals. Total dissolved solids (TDS) concentrations in the groundwater samples ranged from 215 to 1,070 milligrams per liter (mg/L) with a median concentration of 523 mg/L and a mean concentration of 594 mg/L. The U.S. Environmental Protection Agency (2017) has established a secondary drinking water standard of 500 mg/L for TDS. The State of Oklahoma, however, acknowledges a beneficial domestic use for general use (class II) groundwater with TDS concentrations of less than 3,000 mg/L and limited use (class III) groundwater with TDS concentrations of 3,000–5,000 mg/L (OWRB, 2015). Specific conductance ranged from 316 to 1,879 microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25 °C) with a median value of 955 µS/cm at 25 °C and a mean of 1,051 µS/cm at 25 °C. The hardness ranged from 142 to 1,470 mg/L; the median hardness value was 493 mg/L, and the mean hardness value was 553 mg/L. All groundwater samples were classified as hard water (hardness greater than 180 mg/L) except for two samples that were collected from terrace deposits of northern Garvin County, which had hardness values of 142 and 170 mg/L.

In addition to the groundwater-quality data collected in 2014 from 14 wells by the OWRB (OWRB, 2017), groundwater-quality data from 3 wells sampled during 1947–53 were obtained from the NWIS database (USGS, 2020) (fig. 9). Groundwater from those wells was analyzed for selected constituents including physical properties (pH, temperature, and specific conductance), major ions, and nutrients. TDS concentrations in the groundwater samples ranged from 432 to 7,960 mg/L with a median concentration of 716 mg/L and a mean concentration of 1,471 mg/L. Specific conductance ranged from 195 to 6,990 µS/cm at 25 °C with a median value of 1,120 µS/cm at 25 °C and a mean value of 2,115 µS/cm at 25 °C. The hardness ranged from 160 to 900 mg/L; the median value was 622 mg/L, and the mean value was 219 mg/L. All groundwater samples were classified as hard water except for two samples, which had hardness values of 160 and 180 mg/L, from western Grady County.

Major-ion groundwater-quality data were converted to units of milliequivalents per liter and excluded from further analysis if the difference in the cation-anion balance for the sample was greater than 10 percent; one sample was excluded from the OWRB (2017) data, and two samples were excluded from the USGS (2020) data. The data from the remaining 13 OWRB (2017) and 12 USGS (2020) samples were plotted on a Piper (1944) diagram for visualization of groundwater types and trends (fig. 12). The northern parts of the Washita River aquifer are adjacent to Permian-age bedrock transitioning in the southern parts to the Pontotoc Group, Permian and pre-Permian Paleozoic-age red-brown to grayish shale with arkosic sandstone and limestone conglomerates near the Arbuckle Mountains (figs. 9 and 10). The groundwater samples show a downgradient transition in anions from a sulfate-rich water to a bicarbonate-rich water (fig. 12). Bicarbonate dominates the anions, particularly in the downgradient samples, resulting in the calcium-magnesium bicarbonate water type for a majority of the samples. A calcium-magnesium sulfate-chloride water type was identified in seven of the samples. These samples occurred mostly in the northwestern part of the aquifer (fig. 9A), where interactions with the Permian-age bedrock contribute the sulfate anion through dissolution of gypsum beds (fig. 10).

Aquifer Extent and Thickness

The geographic extent of the Washita River aquifer (fig. 1) was updated from the OWRB (2019b) by using information obtained from relatively new 1:100,000-scale geologic maps (Miller and Stanley, 2004; Chang and Stanley, 2010; Stanley and Chang, 2012). An older 1:250,000-scale geologic map (Heran and others, 2003) was used for a part of the aquifer near Chickasha, Okla., for which finer scale geologic maps were unavailable. Compared to the relatively coarse scale of the older 1:250,000-scale geologic map of the study area, the finer 1:100,000-scale geologic maps showed a narrower extent of the alluvium and terrace deposits that contain the aquifer, so the updated aquifer area presented in this report is smaller than previously documented.

Where present, the top of the Washita River aquifer was defined as the land-surface altitude obtained from a 10-m (horizontal resolution) digital elevation model (DEM) (USGS, 2015) with filled depressions. The altitude of the base of the Washita River aquifer, which was equivalent to the top of the bedrock, was contoured at a 50-ft interval from bedrock depths obtained from drillers’ lithologic logs, well-completion reports, and test-hole data (OWRB, 2019a; USGS, 2020) and direct-push test holes (fig. 13; USGS, 2020). For each of these data sources, the altitude of the base of the aquifer was calculated by subtracting the measured bedrock depth from the land-surface altitude. For consistency, the land-surface altitude was obtained from the 10-m DEM, even when the data source provided a land-surface altitude.

Selected wells and direct-push test holes in the Washita River aquifer were assumed to fully penetrate the aquifer (USGS, 2020; fig. 13), and their depths were assumed to be equal to the bedrock depth. Bedrock depths from these wells and test holes were given the highest priority because they had each been visited and checked by the authors of this report. Other wells and test holes included reports with drillers’ lithologic logs (OWRB, 2019a; fig. 13) that were searched for terms representing consolidated Permian-age bedrock units (such as “redbed” and “bedrock”). The altitude associated with the first occurrence of these terms in the logs was used as the altitude of the aquifer base. For logs that indicated that the well completely penetrated the Washita River aquifer (for example, the well log did not include a bedrock-related term), the altitude associated with the hole depth listed on the lithologic log was assumed to be the maximum (highest) possible altitude of the aquifer base at that location.

The mean thickness of alluvium and terrace deposits in the Washita River aquifer was about 54 ft, estimated by using the land-surface altitude and the aquifer-base altitude defined in this report. The mean thickness of alluvium and terrace deposits in reach 3 was about 56 ft and in reach 4 was about 53 ft.

Potentiometric Surface and Saturated Thickness

Potentiometric-surface maps show the altitude at which the water level would have stood in tightly cased wells at a specified time; the potentiometric surface is usually contoured or spatially interpolated from synoptic water-table-altitude measurements in many wells across an aquifer extent. Potentiometric-surface maps are used to indicate the general directions of groundwater flow in an aquifer. Groundwater generally flows perpendicular to potentiometric contours in the direction of decreasing contour altitudes (Freeze and Cherry, 1979).

In this report, the Washita River aquifer water table was assumed to be equivalent to the potentiometric surface because the aquifer is unconfined (Kent and others, 1984). The potentiometric surface for the Washita River aquifer was mapped by using groundwater-level measurements from selected wells and subtracting the measured depth to water from the land-surface altitude derived from a light detection and ranging (lidar) DEM (USGS, 2019). The selected wells were domestic and irrigation wells that were unused at the time of measurement. Methods described in Cunningham and Schalk (2011) were used to obtain groundwater-level measurements. A January 2020 potentiometric surface (fig. 14) for the Washita River aquifer was contoured primarily from synoptic groundwater-level altitudes measured during January 21–23, 2020, in 66 wells and supplemented with selected water-level altitudes measured mostly during February 2018 and December 1986–January 1987 in 24 wells (USGS, 2020; sites not listed in table 1; instead, site information is available in Rogers and others [2023]). Water-level altitudes measured in 2018 and 1986–87 were used to add control in areas where there was not a measurement in 2020. The mean saturated thickness of the Washita River aquifer was about 47 ft. The mean saturated thickness of reach 3 was about 50 ft and of reach 4 was about 45 ft (Rogers and others, 2023). Local flow in the Washita River aquifer was generally from topographically high areas toward the Washita River and major tributaries with regional flow in the aquifer generally from northwest to southeast (fig. 14).

Hydraulic and Storage Properties of Aquifer Materials

The distribution and variability of textural and hydraulic properties of aquifer materials, especially the horizontal hydraulic conductivity, were assumed to be the primary controls on groundwater flow in the Washita River aquifer. Multiple methods were used to estimate the range and central tendency of horizontal hydraulic conductivity values in the aquifer. These methods included in-place estimation in test holes, a multiwell aquifer test, and summary of data in drillers’ lithologic logs.

Horizontal Hydraulic Conductivity Estimated in Test Holes

A direct-push Geoprobe hydraulic profiling tool (HPT; Geoprobe Systems, 2015) was used at six Geoprobe test-hole sites (table 5, fig. 1) during June 24–July 15, 2021, to obtain in-place estimates of horizontal hydraulic conductivity by using the McCall (2010) method. This method can be used in the saturated zone to estimate horizontal hydraulic conductivity values between 0.1 and 150 feet per day (ft/d). During direct-push drilling, the HPT injects water into the aquifer material while logging electrical conductivity, injection pressure (corrected for hydrostatic gradient in the saturated zone), and injection flow rate at 0.05-ft depth intervals (Geoprobe Systems, 2015). The Direct Image Viewer (version 3.0) software package (Geoprobe Systems, 2020) was used to calculate discrete horizontal hydraulic conductivity values with depth by using the ratio of the injection flow rate and the hydrostatic-corrected injection pressure. The HPT test holes were drilled to a depth of refusal, which was assumed to be the bedrock contact. The 5,595 discrete (every 0.05 ft) HPT horizontal hydraulic conductivity measurements from all six test holes ranged from 0.1 to 89.0 ft/d (fig. 15) with a mean of 12.2 ft/d (table 5; USGS, 2021). These horizontal hydraulic conductivity data may be underestimated, though; small algal mats from the injection-water tank became entrained in the injection line during logging, which may have partially obstructed flow through the injection line, possibly increasing the measured injection pressure and making the aquifer appear less permeable. Progressive obstruction of the injection line may explain why the first two drilled test holes (TH11 and TH12, table 5, fig. 15A) had greater mean horizontal hydraulic conductivity measurements (22.1 and 26.7 ft/d, respectively) than did the others. Most of the HPT test holes showed generally decreasing electrical conductivity and generally increasing horizontal hydraulic conductivity with depth, indicating that the aquifer materials tend to fine upward from fine to medium sand to silt and clay (fig. 9.2 in Boggs [1995]).

Table 5.    

Hydraulic properties calculated from Geoprobe hydraulic profiling tool logs at direct-push Geoprobe test holes in the Washita River aquifer, southern Oklahoma, 2020–21.

[Data from U.S. Geological Survey (USGS; 2020, 2021). M/D/Y, month/day/year; NAVD 88, North American Vertical Datum of 1988; HPT, hydraulic profiling tool; Kh, horizontal hydraulic conductivity; --, not available or not applicable]

Table 5.    Hydraulic properties calculated from Geoprobe hydraulic profiling tool logs at direct-push Geoprobe test holes in the Washita River aquifer, southern Oklahoma, 2020–21.

Map name (fig. 1)	Field identifier	USGS station identifier	USGS site name	Latitude, in decimal degrees	Longitude, in decimal degrees	Reach	Drilling date (M/D/Y)	Land-surface altitude, in feet above NAVD 88	Well or hole depth, in feet	HPT logging date (M/D/Y)	HPT depth, in feet below land surface	HPT depth to water, in feet below land surface	HPT mean Kh, in feet per day
TH01	WYNNEWOOD3	343537097085801	02N-01E-36 DCC 1	34.5936	−97.1494	4	10/5/2020	826	40.6	7/13/2021	33.2	3.4	5.7
TH02	PAULSVALLEY1	344011097114301	02N-01E-03 CCB 1	34.6697	−97.1953	4	10/5/2020	864	48.6	--	--	--	--
TH03	PAULSVALLEY2	344053097101001	03N-01E-35 CDD 1	34.6814	−97.1696	4	10/2/2020	851	46.6	--	--	--	--
TH04	LINDSAY3	344842097283401	04N-03W-23 AAA 1	34.8118	−97.4762	4	9/11/2020	958	46.9	--	--	--	--
TH05	LINDSAY1	344916097311401	04N-03W-16 BDA 1	34.8212	−97.5205	4	10/2/2020	961	63.2	7/14/2021	63.0	15.6	6.8
TH06	LINDSAY4	345122097294001	05N-03W-34 DDD 1	34.8562	−97.4946	4	10/2/2020	956	74.1	--	--	--	--
TH07	BRADLEY1	345402097434901	05N-05W-16 DCB 1	34.9005	−97.7302	4	10/6/2020	1,019	49.2	7/14/2021	48.5	4.3	7.1
TH08	BRADLEY3	345422097421801	05N-05W-14 CBB 1	34.9061	−97.7051	4	10/9/2020	1,013	90.2	--	--	--	--
TH09	ALEX3	345749097484101	06N-06W-26 CBB 1	34.9637	−97.8114	3	10/1/2020	1,038	59.2	--	--	--	--
TH10	ALEX2	345817097470602	06N-06W-24 DCC 2	34.9715	−97.7849	3	10/1/2020	1,038	43.4	--	--	--	--
TH11	ALEX1	345829097484101	06N-06W-23 CCB 1	34.9748	−97.8115	3	10/9/2020	1,043	57.5	6/28/2021	58.9	19.2	22.1
TH12	CHICKASHA2	350513098021301	07N-08W-10 CCD 1	35.0870	−98.0369	3	10/1/2020	1,115	63.6	6/24/2021	63.6	2.8	26.7
TH13	ANADARKO2	350513098115501	07N-10W-12 DDD 1	35.0869	−98.1987	3	10/8/2020	1,164	69.2	7/15/2021	68.5	7.4	4.8
TH14	CHICKASHA1	350606098022501	07N-08W-03 CCC 1	35.1017	−98.0403	3	10/9/2020	1,123	49.3	--	--	--	--
TH15	ANADARKO1	350610098114701	07N-09W-06 CCC 1	35.1028	−98.1965	3	10/8/2020	1,165	98.0	--	--	--	--
TH16	CHICKASHA3	350700098022501	07N-08W-04 AAA 1	35.1167	−98.0403	3	10/1/2020	1,114	35.2	--	--	--	--
TH17	ANADARKO3	350701098121901	07N-10W-01 ABB 1	35.1170	−98.2054	3	10/8/2020	1,172	92.7	--	--	--	--
TH18	CHICKASHA4	350753098022001	08N-08W-28 DDD 1	35.1314	−98.0389	3	10/9/2020	1,117	47.3	--	--	--	--
Mean											55.9	8.8	12.2

Hydrologic Boundaries

Hydrologic boundaries in the conceptual model represent actual sources (inflows) and sinks (outflows) of water to and from the aquifer. Boundaries that act as both inflows and outflows may be referred to as “net inflows” or “net outflows” depending on which flow component dominates.

Recharge

Recharge is the predominant inflow to the Washita River aquifer. Recharge is defined in this report as the amount of precipitation that infiltrates from the land surface through the unsaturated zone and reaches the water table over a given time. This definition of recharge includes irrigation return flows to groundwater. Other processes such as stream seepage or lateral groundwater flow from adjacent hydrogeologic units are not considered recharge and are accounted for separately in the conceptual-model water budget. Recharge rates are controlled by many factors including precipitation rate, land-surface gradient, soil and sediment permeability, evapotranspiration rates, and vegetation cover type. Although recharge rates are difficult to measure because of high spatial and temporal variability, methods that include environmental tracers, physical measurements, streamflow-hydrograph techniques, and computer codes can be used to estimate recharge rates. For this report, a groundwater-hydrograph-based water-table fluctuation (WTF) method (Healy and Cook, 2002) was used to estimate localized recharge rates for 2017–19, and a code-based water-balance-estimation technique (Westenbroek and others, 2010) was used to estimate spatially distributed recharge rates for the 1980–2017 study period.

Water-Table Fluctuation Method

The WTF method was the primary method used to estimate recharge to the Washita River aquifer. The WTF method assumes that rises in groundwater levels in unconfined aquifers that occurred during a relatively short period (hours to a few days) can be attributed to recharge arriving at the saturation zone following a period of precipitation. This method is most appropriately applied to groundwater wells with shallow water tables and hydrographs that display sharp rises in groundwater levels after precipitation (Healy and Cook, 2002). The WTF method cannot account for a steady rate of recharge or recharge from sources other than precipitation. Annual recharge (R), in inches per year, was estimated by using the following equation:

R

=

Sy

× Σ(

Δh

/

Δt

)

(2)

where

Sy

is the specific yield (dimensionless);

Δh

is the rise in groundwater-level altitude, in inches; and

Δt

is the change in time, in years.

Water-level hydrographs from three USGS continuous water-level recorder wells (W01, W02, and W05, table 1; USGS, 2020) were used to estimate annual recharge during 2017–19. W01 was located near Lindsay, W02 was located near Chickasha, and W05 was located near Pauls Valley (fig. 1). The water-level hydrographs from other continuous water-level recorder wells in the study area were not analyzed because they were affected by nearby pumping (in the case of W03, fig. 17A) or because they did not display sharp water-level rises (in the case of W04 and W06, fig. 17B). Annual precipitation data for the 2016–19 period of analysis were obtained from the Chickasha (CHIC) and Pauls Valley (PAUL) climate stations (fig. 1; Oklahoma Mesonet, 2019). Using the daily precipitation record from the nearest climate station and a specific yield of 0.10 from Hemann (1985), the annual recharge estimates for wells W02 and W05 in 2017 were 5.3 inches (in.; about 13.5 percent of the station annual precipitation) and 7.4 in. (about 18.5 percent of the station annual precipitation), respectively (table 7). The annual recharge estimates for wells W02 and W05 in 2018 were 9.0 in. (about 23.7 percent of the station annual precipitation) and 11.5 in. (about 22.8 percent of the station annual precipitation), respectively. In a year-long period from April 1, 2018, through March 31, 2019, the annual recharge estimate at well W01 was 10.3 in. (about 22.6 percent of the station annual precipitation). When those recharge estimates are normalized by the annual mean precipitation of 40.1 in/yr for the 1980–2017 study period and averaged, the resulting station-averaged mean annual recharge for 2017–19 in reach 3 and 4 of the Washita River aquifer was 8.1 in/yr (about 20.2 percent of mean annual precipitation for the 1980–2017 period). Multiplied by the 178,938-acre modeled area and unit converted, the WTF-calculated mean annual recharge for 2017–19 was 120,783 acre-ft/yr; this value was used as the conceptual-model recharge during 1980–2017 (table 6).

Table 7.    

Summary of recharge amounts estimated using the water-table fluctuation method for the Washita River aquifer, southern Oklahoma, 2017–19.

[Continuous water-level recorder wells W03 and W04 were not suitable for analysis with the water-table fluctuation method (Healy and Cook, 2002). Dates for the periods are in month-day-year (MM-DD-YYYY) format; --, data not available or not applicable]

Table 7.    Summary of recharge amounts estimated using the water-table fluctuation method for the Washita River aquifer, southern Oklahoma, 2017–19.

	U.S. Geological Survey continuous water-level recorder well (fig. 1, table 1)
	W02	W01	W05
Annual mean precipitation 1980–2017, in inches per year, southern Oklahoma, Climate Division 8 (National Centers for Environmental Information, 2021)	40.1	40.1	40.1
Climate station (fig. 2, table 1)	CHIC	PAUL	PAUL
Estimated specific yield	0.10	0.10	0.10
Period 1: 01-01-2017 to 12-31-2017
Station annual precipitation, in inches	39.4	--	40.1
Sum of water-level rises, in feet	4.4	--	6.2
Recharge, in inches per year	5.3	--	7.4
Recharge, percent of annual precipitation	13.5	--	18.5
Recharge, in inches per year, normalized to mean annual precipitation during 1980–2017	5.4	--	7.4
Period 2: 01-01-2018 to 12-31-2018
Station annual precipitation, in inches	37.9	--	50.5
Sum of water-level rises, in feet	7.5	--	9.6
Recharge, in inches per year	9.0	--	11.5
Recharge, percent of annual precipitation	23.7	--	22.8
Recharge, in inches per year, normalized to mean annual precipitation during 1980–2017	9.5	--	9.2
Period 3: 04-01-2018 to 03-31-2019
Station annual precipitation, in inches	--	45.5	--
Sum of water-level rises, in feet	--	8.6	--
Recharge, in inches per year	--	10.3	--
Recharge, percent of annual precipitation	--	22.6	--
Recharge, in inches per year, normalized to mean annual precipitation during 1980–2017	--	9.0	--
Mean annual recharge during 2017–19, in inches per year, normalized to mean annual precipitation 1980–2017			8.1

Soil-Water-Balance Code

The Soil-Water-Balance code (SWB; Westenbroek and others, 2010) was used to estimate the amount and spatial distribution of daily groundwater recharge to the Washita River aquifer for each month of the 1980–2017 study period. SWB uses a modified Thornthwaite and Mather (1957) soil-water-balance method on a gridded data structure to compute the daily amount of infiltration (minus losses) that exceeds the storage capacity of the plant root zone. The soil-water-balance equation (modified from Westenbroek and others [2010]) has the form of the following equation:

R

= (

P

+

S

+

R_i

) – (

Int

+

R_o

+

P_et

) –

ΔSm

(3)

where

R

is recharge, in inches per day;

P

is precipitation, in inches per day;

S

is snowmelt, in inches per day;

R_i

is surface runoff inflow, in inches per day;

Int

is plant interception, in inches per day;

R_o

is surface runoff outflow, in inches per day;

P_et

is potential evapotranspiration, in inches per day; and

ΔSm

is the change in soil moisture, in inches per day.

SWB requires climate and landscape characteristic data inputs including precipitation, temperature, soil-water storage capacity, hydrologic soil group, surface-water flow direction, and land-cover type (Westenbroek and others, 2010). Each of these inputs was assigned to a user-specified grid that was 600 by 500 cells of 800 by 800 ft each. The landscape inputs were assumed to remain constant during the study period, but climate data inputs varied daily. Daily climate data including precipitation and minimum and maximum air temperature were obtained for 14 selected climate stations (fig. 2, table 1; National Centers for Environmental Information, 2022) in and near the study area and gridded using inverse-distance-weighted (Esri, 2021b) interpolation. Accumulated snowmelt was added on the basis of the daily minimum and maximum air temperatures. Potential evapotranspiration was calculated by using the Hargreaves and Samani (1985) method for a reference latitude range of 34.5–35.4 degrees. Surface runoff was routed downslope by using a flow-direction grid derived from a 10-m DEM (USGS, 2015). Depressions in the DEM were filled by using the ArcGIS Fill tool (Esri, 2021a) to ensure correct routing of surface runoff and to eliminate areas of internal drainage that can result in unrealistically high amounts of recharge. Soil properties (soil-water storage capacity and hydrologic soil group) were derived from the Soil Survey Geographic database (NRCS, 2022), which is an inventory of generalized soil characteristics. Land-cover types (Fry and others, 2011; Multi-Resolution Land Characteristics Consortium, 2011; fig. 3) were used in conjunction with hydrologic soil groups to partition daily precipitation into plant interception (Int) and surface runoff (R_i and R_o) components and assign plant root-zone depths. The root-zone depths for grass/pasture and cropland (the dominant land-cover types for land overlying the aquifer; fig. 3) varied with soil texture but ranged from about 0.8 to 1.5 ft (40 percent of the values used by Westenbroek and others [2010] for permeable glacial deposits in Wisconsin). The soil-water storage capacity, analogous to specific yield in the saturated zone, multiplied by the root-zone depth determined the maximum volume of water available in the root zone. Changes in soil moisture (ΔSm) exceeding the soil-water storage capacity were assumed to be recharge (R) to the saturated zone. Larger root-zone depths resulted in increased evapotranspiration of water from the root zone and decreased recharge; smaller root-zone depths resulted in decreased evapotranspiration of water from the root zone and increased recharge. Recharge from irrigation was not simulated by SWB but was assumed to be negligible given the relatively small amount of irrigation groundwater use in the study area. The SWB model for the Washita River aquifer study area is included in Rogers and others (2023).

The SWB-estimated mean annual recharge for the 1980–2017 study period was about 4.9 in/yr, or about 12.2 percent of the mean annual precipitation of 40.1 in/yr during 1980–2017 (fig. 18A). The minimum and maximum SWB-estimated annual recharge values were about 1.1 in/yr in 2003 and 11.6 in/yr in 2015, respectively. Recharge efficiency (mean monthly recharge as a percentage of mean monthly precipitation) was greatest from November to February, when evapotranspiration was at a minimum; recharge in these months was greater than 20 percent of precipitation (fig. 18B). Recharge efficiency was lowest in July and August, when evapotranspiration was at a maximum; recharge in these months was less than 6 percent of precipitation. Spatially, mean annual recharge for the study period was greatest in areas of alluvium near the Washita River and tributaries (fig. 19). The SWB-estimated mean annual recharge generally increased from northwest to southeast in the Washita River aquifer; the mean annual recharge for reach 4 was about 14 percent greater than mean annual recharge for reach 3 of the Washita River aquifer. Increased recharge from reach 3 to reach 4 is correlated with increased precipitation from northwest to southeast in the study area (Oklahoma Climatological Survey, 2021a, b).

Mean annual recharge rates estimated from SWB were compared to published estimates of mean annual recharge for the Washita River aquifer. Kent and others (1984) estimated a mean annual recharge of 3.72 in/yr, or about 12 percent of the mean annual precipitation of 32.2 in/yr during a 20-year study period (July 1, 1973–July 1, 1993). That estimate was applied across the entire Washita River aquifer in Oklahoma and included reaches that are outside the scope of this report. Schipper (1983) estimated a mean annual recharge of 3.17 in/yr, or about 13 percent of the mean annual precipitation (24.90 in/yr), during the same 20-year study period for a groundwater model of reach 1 of the Washita River aquifer (in western Oklahoma). Ellis and others (2020) estimated a mean annual recharge for reach 1 of the Washita River aquifer (in western Oklahoma) of 3.15 in/yr, or about 14 percent of the mean annual precipitation of 23 in/yr, during a 36-year study period (1980–2015). The mean annual recharge rate of 4.9 in/yr that was estimated by using SWB for the study area in this report is thus within the range of published estimates of recharge for the Washita River aquifer, particularly when calculated as a percentage of the applicable mean annual precipitation.

The SWB-estimated mean annual recharge was, however, about 40 percent less than the WTF-estimated mean annual recharge used for the conceptual model. SWB is a numerical model, and SWB-estimated recharge must, therefore, be checked against and calibrated to field-based measurements or estimates of recharge like those estimated using WTF. In other reports (Smith and others, 2017, 2021; Ellis, 2018a; Ellis and others, 2020), SWB-estimated recharge was calibrated by decreasing root-zone depths until the SWB-estimated mean annual recharge approximated the conceptual-model recharge. That method was also applied in this report to a point but was halted when the mean root-zone depths began to decrease to less than 1.2 ft. Additional adjustment of SWB-estimated recharge was accomplished during calibration of the numerical model as detailed in the “Numerical Groundwater-Flow Model” section of this report.