Enid Isolated Terrace

The Enid Isolated Terrace aquifer is located in north-central Oklahoma in the western half of Garfield County with a small portion in Alfalfa County. The aquifer consists of Quaternary-age alluvial and terrace deposits that are underlain by Permian-age clays, shales, and sandstones. The purpose of this study is to update an existing hydrologic survey completed in 1982. The Oklahoma Water Resources Board (OWRB) is required to review the maximum annual yield of 19,000 acre-feet and equal proportionate share of 0.5 acre-feet per acre (OWRB, 2014). Updated maps published by the Oklahoma Geological Survey (OGS) in 2002 and 2006 were used to define a study area of 174 square miles.

Climate data were analyzed from the Enid Cooperative Observer (COOP) station and the Oklahoma Mesonet weather station in Breckinridge with mean annual precipitation values of 30.73 inches and 31.23 inches, respectively. There were approximately 1,600 groundwater wells and 64 groundwater permits located within the study area in 2013. Depth to water was measured in 72 groundwater wells in March 2014 to produce a potentiometric surface map, which indicated that groundwater generally flows to the east-southeast where at least two streams receive discharge from the aquifer. The saturated thickness of the study area ranged from 0 to about 65 feet. Data from six monitoring wells were analyzed to show long-term water-level changes in the aquifer. Three wells were equipped with water-level recorders to characterize monthly trends and responses to precipitation. A mean hydraulic conductivity value of 50.16 feet per day was calculated for the study area using single-well pumping tests. Two multi-well pumping tests were conducted in January and February 2013 on wells operated by the City of Enid. With the resultant data, the AQTESOLV modeling program estimated transmissivity values to range from 2,330 to 5,030 feet per day, hydraulic conductivity to range from 101 to 132 feet per day, and storativity values to range from 0.006 to 0.01. Water use data from 1967 to 2013 were analyzed for the study area; the main uses for this time period were irrigation and public water supply with an average annual use of 3,243 acre-feet. Mean annual recharge was estimated to be 4.35 inches using climate data from Oklahoma Mesonet weather stations and the Enid COOP station using a Soil-Water-Balance model. Additionally, average annual recharge over the Skeleton Creek drainage basin was estimated to be 2.79 inches using the surface water analysis software RORA on streamflow data from Skeleton Creek (USGS ID 07160350). Groundwater wells were sampled for selected water quality parameters in 2014 as a part of the OWRB Groundwater Monitoring and Assessment Program (GMAP). Water type in the study area was not uniform and nitrates exceeded the U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public water supply in 11 of the 19 wells sampled.

Updates

Since the completion of the previous study on the Enid Isolated Terrace in 1982, the following changes to the area, population, and data have occurred:

1. The study area now includes terrace deposits to the east and west that were identified in recent geologic maps and were not included in the original study area and boundary of 81 square miles determined by Kent and others (1982). The aquifer study area for this investigation is now 174 square miles.
2. Population of the City of Enid has increased from about 45,000 in 1982 (Kent and others, 1982) to about 50,000 in 2010 (US Census Bureau, 2010).
3. The City of Enid has decreased the amount of municipal groundwater use from the Enid Isolated Terrace aquifer.
4. Recharge reported by Kent and others (1982) was 2.3 inches. Estimates in this investigation ranged from 2.79 inches using the RORA method to 4.35 inches using the Soil-Water-Balance model. The differences may be caused by the newer methods used to estimate recharge because precipitation averages have stayed steady.
5. Data yielded the greatest saturated thicknesses near the center of the study area, north of the city of Enid, thinning towards the east and west.
6. The mean value for hydraulic conductivity was estimated to be 50.16 feet per day utilizing data from single-well pumping tests with a range of 0.32-289.7 feet per day.
7. Multi-well aquifer tests estimated transmissivity to range from 2,333 to 5,031 square feet per day, storativity to range from 0.01-0.006, and hydraulic conductivity to range from 101 to 132 feet per day.