Projected effects of proposed salinity-control projects on shallow ground water; preliminary results for the upper Brazos River basin, Texas

Abstract

As part of the plan to control the natural salt pollution in the upper Brazos River basin of Texas, the U.S. Army Corps of Engineers recommended construction of three impoundment and retention reservoirs. In connection with the proposed reservoirs, the U.S. Geological Survey was requested to define the existing ground-water conditions in the shallow ground-water system of the area and to project the post-construction effects of the reservoirs on the shallow aquifer, especially in relation to aquifer-head changes but also with respect to possible changes in the chemical quality of the ground water. The Corps of Engineers' plan includes a total impoundment reservoir (Kiowa Peak Lake on North Croton Creek) with more than 170,000 acre-feet of storage at the 100-year average pool altitude of 1,550 feet. Croton Lake (23,000 acrefeet of storage at the 100-year average pool altitude of 1,760 feet) on Croton Creek and Dove Lake (no permanent storage) on Salt Croton Creek are projected to hold salt water for transfer to the large Kiowa Peak Lake. The aquifer in the project area is a shallow water-table system with relatively fresh water (calcium sulfate type), in comparison to the saline streamflow, and contains 2,000 to 5,000 milligrams per liter of dissolved solids. The aquifer consists of Permian rocks with very small permeability and is separated from an even less perneable and deeper brine system (sodium chloride type of up to 200,000 milligrams per liter of dissolved solids) by a thin transition zone. Small quantities of infiltration from precipitation in the drainage area constitute the recharge to the aquifer. Discharge from the aquifer consists of the base flow along creeks; well discharge is negligible. Two-dimensional digital-computer models were developed for aquifer simulation of steady and transient conditions in which the density effects of salt water are considered. The models were used to project the effects of the 100- year impoundment of salt water in Kiowa Peak Lake and Croton Lake on the freshwater system. Rises in aquifer head of 10 to 50 feet are projected only for areas near each dan and along each lake shoreline. The maximum migration of salt water downstream from each dam is projected to be about 1 mile. The modeling efforts in this study did not include the effects of hydrodynamic dispersion nor consideration of possible changes in the hydraulic conductivity of the aquifer due to physical and chemical interactions in the salt-water and fresh-water environments.