Simulated effects of projected pumping on the availability of freshwater in the Evangeline Aquifer in an area southwest of Corpus Christi, Texas

This study is an investigation of the continued availability of freshwater in the Evangeline aquifer along the Texas Gulf Coast and the potential for degradation of the water quality by salinewater intrusion. Recharge to the aquifer occurs by the infiltration of precipitation in the outcrop area and by cross-formational flow from deeper aquifers. The predevelopment recharge rate is about 6 to 8 cubic feet per second. The predevelopment flow is toward the coast. The flow is semiconfined in the outcrop area and confined underneath the Chicot aquifer in the eastern two-thirds of the study area. Discharge, under natural conditions, is upward into the Chicot aquifer and to the Nueces River or Gulf of Mexico. Intensive pumping by irrigators, industries, and municipalities over the last 80 years has created a cone of depression as deep as 219 feet below sea level under the city of Kingsville in Kleberg County. The total rate of pumpage in 1982 was 29.6 cubic feet per second.

A mathematical model of the flow and water quality in the Evangel ine aquifer was developed using available data to simulate the historical effect of pumping on the potentiometric surface and water quality, and to simulate the effect of projected pumping on the potentiometric surface and water quality to the year 2020. The water quality in the aquifer is only marginally suitable for drinking water. The chloride concentration before development in the 1930's and 1940's, ranged from 9 to 1,971 milligrams per liter. The mean chloride concentration was 353 (standard deviation 262) milligrams per liter. The potential sources of water-quality degradation on a regional scale are: Salinewater intrusion from under the Gulf of Mexico; movement of poor quality water within outlying sections of the aquifer; and downward leakage from the overlying Chicot aquifer. Leakage from the Chicot is the most likely to cause serious regional water-quality degradation. Other local potential sources of contamination are: Leaky well casings, oil-field brine disposal, water movement along faults, and in-situ uranium mining. These sources might create some local water-quality degradation. The results of the historical period simulation indicate, as do current field data, that little or no significant deterioration has occurred in the water quality of the Evangeline aquifer.

The simulations and the sensitivity tests of the aquifer properties, conditions, and assumptions indicate that vertical conductivity of the Chicot aquifer is the most sensitive and least well known part of the system. The storage coefficient of the Evangeline aquifer and the aggregate thickness of high-conductivity sand layers within the aquifer as well as the vertical distribution of these layers are also important properties that are not well known.

Two simulations of the projected pumping a low estimate, as much as 46.2 cubic feet per second during 2011-20; and a high estimate, as much as 60.0 cubic feet per second during the same period indicate that no further regional water-quality deterioration is likely to occur. Many important properties and conditions are estimated from poor or insufficient field data, and possible ranges of these properties and conditions are tested. In spite of the errors and data deficiencies, the results are based on the best estimates currently available. The reliability of the conclusions rests on the adequacy of the data and the demonstrated sensitivity of the model results to errors in estimates of these properties.