Washita River Reach 3 and 4 Alluvial Aquifer Study

Science Center Objects

The objectives of this investigation are to describe the hydrogeology, and construct, calibrate, and use a numerical groundwater-flow model to simulate the effects of transient stresses and various pumping and climate scenarios for Reaches 3 and 4 of the Washita River alluvial aquifer.

SUMMARY

The Washita River alluvial aquifer consists of unconsolidated alluvial and terrace Quaternary-age deposits adjoining that river in southwestern Oklahoma (fig. 1). The Washita River in Oklahoma extends from the western Oklahoma border southeast to the Red River that forms the southern border of Oklahoma with Texas. The Washita River alluvial aquifer is subdivided into four sections, termed “reaches” by the Oklahoma Water Resources Board (OWRB). Reach 3 begins upstream near Anadarko, Oklahoma and terminates downstream between the towns of Alex and Bradley, Oklahoma. Reach 4 begins south of Bradley, Oklahoma, and has a downstream endpoint near Dougherty, Oklahoma. These reaches are contained in the Oklahoma Comprehensive Watershed Planning (OCWP) Lower Washita region, and are the focus of this proposed study (fig. 1).

The 1973 Oklahoma Water Law requires the OWRB to conduct hydrogeological investigations of the State’s aquifers to determine an equal proportionate share (EPS) pumping rate, and the effects of pumping and drought scenarios on groundwater resources. The EPS is defined by the State of Oklahoma as the constant rate of groundwater extraction per acre of land that results in half of the aquifer having 5 feet or less of saturated thickness after a minimum basin life of 20 years (Oklahoma Statutes Title 82 Section 1020.5, 2011).

The objectives of this investigation are to describe the hydrogeology, and construct, calibrate, and use a numerical groundwater-flow model to simulate the effects of transient stresses and various pumping and climate scenarios for Reaches 3 and 4 of the Washita River alluvial aquifer. The results of this investigation will provide information about the hydrogeological characteristics of Reaches 3 and 4 of the Washita River alluvial aquifer, which will provide information to water managers about future water supplies. The transient numerical groundwater-flow model will provide a decision-support system for analysis of the effects of groundwater extraction, drought, and projected future water demands on water resources.

This project will be a cooperative study of the USGS Oklahoma Water Science Center (OKWSC) with the OWRB. The USGS will perform field work, data collection and interpretation, numerical groundwater-flow model development, and report writing with support from the OWRB for access to hydrological and hydrogeological background, tabular and geographical data, field water-level data collection, and direction for the emphasis of this study. The proposed study will commence during March 2016 and conclude in March, 2019.

This proposed study will use all available literature and geographic and tabular data with new field data to describe the climate, water use, groundwater quality, and construct the hydrogeological framework and conceptual flow model of the Washita River alluvial aquifer Reaches 3 and 4. To quantify the groundwater-flow system, a numerical groundwater-flow model will be constructed, calibrated, and used to calculate the EPS and effects of drought and future water use. Additional field data will include as many as 40 direct-push Hydraulic Profiling Tool (HPT) profiles, and one aquifer test to estimate hydraulic conductivity and storage properties. Field data collection will include installation of as many as 3 observation wells, measurement of aquifer synoptic water levels during winter when evapotranspiration and water use are minimal, and the installation of groundwater-level logging devices to collect time-series data.

PROBLEM

 Because more than 20 years have elapsed since the issuance of the EPS for the Washita River alluvial aquifer, the Oklahoma Water Resources Board (OWRB) requires an update to the previous hydrologic investigations (Kent and Naney, 1978; Patterson, 1984), which covered Reach 3 and parts of Reach 4. The EPS is defined by the state of Oklahoma as the constant rate of groundwater extraction per acre of land that results in half of the aquifer having 5 feet or less of saturated thickness after a minimum basin life of 20 years (Oklahoma Statutes Title 82 Section 1020.5, 2011). Determination of the EPS requires a comprehensive hydrogeological study and numerical groundwater-flow model of the groundwater system under transient stresses. Thus, to effectively manage the groundwater resources of the Washita River alluvial aquifer Reaches 3 and 4, a hydrogeological study of the groundwater-flow system, interactions with surface water, and an updated numerical groundwater-flow simulation of the behavior of the groundwater system under stresses such as water pumping and extended drought periods is needed.

OBJECTIVES AND SCOPE

The objectives of this proposed study will be to describe the hydrologic system of Reaches 3 and 4 of the Washita River alluvial aquifer, including a description of the hydrogeological framework and a conceptual-flow model, and construction, calibration, and use of a numerical groundwater-flow model to provide estimates of the response of the aquifer to transient stresses and various pumping and drought scenarios. Separate models and EPS rates will be determined for Reach 3 and Reach 4.

RELEVANCE AND BENEFITS

The proposed study will provide information about the hydrogeological characteristics of Reaches 3 and 4 of the Washita River alluvial aquifer, which will inform OWRB water managers about future water supplies. The transient numerical groundwater-flow model will provide an analysis of the effects of groundwater extraction, drought, and projected future water demands on water resources. This proposal is a continuation of a series of groundwater investigations for the alluvial and bedrock aquifers of Oklahoma to assist the OWRB with the determination of the EPS, and the fulfillment of the objectives of the 2012 OCWP update (Oklahoma Water Resources Board, 2011). This study supports the U.S. Geological Survey (USGS) Strategic Science Direction of building a water census of the United States, particularly determining “changes over time in freshwater quantity and quality in response to changes in climate, land use and land cover, and human and environmental water needs” (U.S. Geological Survey, 2007).

APPROACH

This project will be a cooperative study between the USGS Oklahoma Water Science Center (OKWSC) with the OWRB. OKWSC staff will perform field work, data collection and interpretation, and numerical groundwater-flow model development. OWRB staff will provide surface and water usage information, historical climate data, and other hydrological and hydrogeological information. The OKWSC will publish a Scientific Investigations Report and the OWRB will contribute the water-use and climate sections of the report. The investigation will be divided into the tasks described in the following sections. This approach follows those used for the previous alluvial and bedrock aquifer studies for the OWRB.