Rush Springs
The study area includes 4,692 square miles in west-central Oklahoma, underlying portions of Blaine, Caddo, Canadian, Comanche, Custer, Grady, Stephens, and Washita counties. The study area for this investigation was expanded from a 1998 study by the US Geological Survey to include two additional areas where well yields are indicative of a “major groundwater basin” as defined by the OWRB.
The study area received an annual average of 28.20 inches of precipitation from 1905–2015. Recharge occurred through diffuse precipitation and discharges through groundwater withdrawals and streams, including Barnitz, Cobb, Deer, and Lake Creeks. Groundwater also supplies baseflow to the Canadian and Washita rivers. Recharge was estimated using the SWB code and the RORA method. Estimates using SWB for the period 1950–2015 ranged from 0.03 inches in 1963 to 4.63 inches in 2007 and an average annual recharge of 1.4 inches.
RORA, which utilizes a base-flow separation technique from streamflow gauging stations, ranged from 0.46 inches in 2006 on the Little Washita River streamflow gauge near Ninnekah to 5.76 inches in 2007 on Cobb Creek streamflow gauge near Eakly. From 1946–2015, at least one station from the study area had streamflow data to estimate recharge using RORA.
Reported groundwater use from the Rush Springs aquifer for 1967–2015 averaged 69,900 acre-feet per year with a median of 62,154 acre-feet per year.
During this period, 91.0 percent of reported groundwater use in the study area was for irrigation, 7.8 percent was for public water supply, and 1.2 percent was for other purposes. The highest total reported annual groundwater use was about 115,016 acre-feet in 2014 and 133,113 acre-feet in 2015, which corresponded to drought conditions during these years. In 1992, only 37,210 acre-feet was reported, which was the lowest reported use for a single year; however, the data for that year may be incomplete. The second lowest reported total use for a single year occurred in 2007 at 40,418 acre-feet. Water use trends for the period of record correspond with changing precipitation patterns, with the highest groundwater use occurring during the 2010-2015 drought period and the lowest groundwater use during the wet period in the late 1980s and early 1990s.
Annual water-level measurements collected by the OWRB since the 1950s were analyzed for long-term trends. Water-level data from 95 wells with a period of record of greater than 12 years provided enough data to assess long-term trends. Water-level trends from 54 wells were determined to primarily fluctuate with climate trends, showing declining water levels during drought periods and increasing water levels during wet periods. Data from 15 sites showed overall increasing water levels and 17 sites showed decreasing water levels; nine sites had indiscernible water levels during the period of record. Measurements at the USGS well 351308098341601 had the longest period of record in the study area and showed a decline of 37.52 feet from September 1948 to April 2015.
Lithologic descriptions from groundwater wells were used to determine the base of the aquifer. Generally most of the descriptions indicated a “red bed,” “dark red bed,” or “red shale” at the bottom of the borehole, which was interpreted to be the base of the aquifer. The contact between the Rush Springs and Marlow formations on geologic maps was used to refine the edges of the aquifer where lithologic logs were sparse; however, this caused the edges of the base of the aquifer to be at higher elevations than what was observed on the lithologic logs independently. Rock cores collected in the study area also show the Marlow Formation consisting of some coarser-grained layers capable of transmitting water that can be considered part of the aquifer system. Therefore, for this study, the Marlow Formation was included as part of the aquifer. Average saturated thickness using the 2013 potentiometric map and base of aquifer is 181 feet with a maximum thickness of 432 feet. The area of greatest saturated thickness occurs along the axis of the Anadarko Basin where the Cloud Chief Formation confines the Rush Springs aquifer.
Hydraulic conductivity was estimated from drawdown analysis, slug tests, aquifer tests, and a percent-coarse analysis from lithologic logs. The minimum hydraulic conductivity for the Rush Springs aquifer estimated from drawdown data was less than 0.01 feet per day, and the maximum was 90.90 feet per day with a median of 1.63 feet per day and a mean of 3.27 feet per day. Hydraulic conductivity estimated from slug tests ranged from 0.13 feet per day to 7.60 feet per day, with a mean of 1.70 feet per day and median of 1.40 feet per day. Hydraulic conductivity estimates from three multi-well aquifer tests were 1.60, 6.40, and 44.9 feet per day. Using lithologic logs and assigning hydraulic conductivity to lithologic descriptions, mean and median hydraulic conductivity were estimated to be 6.3 and 4.0 feet per day, respectively. Transmissivity estimates for the three multi-well aquifer tests were 219, 956, and 4,129 feet squared per day.
Specific yield was estimated from regional methods and aquifer tests. Using base flow discharge and monthly groundwater-level measurements, specific yield was estimated in the Cobb Creek, Deer Creek, and Lake Creek subsurface watersheds. For this method, the ratio of the volume of groundwater discharged to the volume of the aquifer drained is the specific yield for the aquifer drained. The specific yield estimated for Cobb Creek, Deer Creek, and Lake Creek subsurface watersheds was 0.05, 0.07, and 0.07, respectively. Specific yield estimated from three multi-well aquifer tests was 0.04, 0.07, and 0.09, which correlates with the regional method.
The mean total dissolved solids concentration from 79 samples collected from the study area was 1,106 milligrams per liter. Concentrations ranged from 178 to 4,680 milligrams per liter with a median of 485 milligrams per liter. The dominant cation of the samples is calcium and the dominant anion is carbonate/bicarbonate with a secondary bimodal population of sulfates, which were predominantly collected in areas where the Cloud Chief Formation overlies the Rush Springs Formation. Four samples exceeded the maximum contaminant level for arsenic of 10 micrograms per liter; the highest concentration of arsenic sampled was 16.5 micrograms per liter. Thirteen samples reported concentrations exceeding the maximum contaminant level for nitrates of 10 milligrams per liter; the highest concentration of nitrate sampled was 59.2 milligrams per liter.
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