Identification of Sources and Mechanisms of Salt-Water Pollution Affecting Ground-Water Quality; A Case Study, West Texas

Occurrences of ground-water and soil salinization are numerous in the Concho River watershed and its confluence with the Colorado River in West Texas and in other semiarid regions of Texas and the United States. Salinization results from both natural and anthropogenic processes, which can occur together or separately. To recognize regional salt-water sources and understand salinization mechanisms in the Concho River valley, we investigated chemical and physical characteristics associated with saline ground water. Using more than 1,200 chemical analyses of water samples from previous studies and 76 analyses of samples from this investigation, we differentiated salinization mechanisms by mapping hydrogeologically controlled salinity patterns and hydrochemical facies and by graphically analyzing isotopic compositions and ionic ratios.In the eastern part of the study area, located in Runnels County, evaporation of irrigation water and ground water from a shallow aquifer accounts for most salinization. In the western part of the study area, located in lrion and Tom Green Counties, much of the saline to brackish ground water is interpreted as being a naturally occurring mixture of subsurface brine flowing eastward from the Midland Basin and shallowly circulating meteoric water recharged in the Concho River watershed. Test drilling to depths below the base of fresh water confirmed the presence of subsurface brine in shallow Permian formations. Aquifers that contain relatively fresh water in outcropping Permian rocks also contain brine and hydrocarbons at depths as shallow as 60 to 275 m (200 to 900 ft), tens of kilometers to the west.Test drilling also helped document two anthropogenic mechanisms for local mixing of subsurface brine and shallow ground water: (1) upward flowing of brine allowed by abandoned oil and gas exploration holes, where surface casing and borehole plugs are above the base of fresh water, and (2) leaching of accumulated salt from soil beneath former brine-disposal pits, an ongoing process even 20 years after this brine-disposal method was discontinued. A third mechanism, mixing of brine and shallow ground water via abandoned water wells, could not be tested. No records exist on deep water wells that reportedly were drilled into saline portions of aquifers and were abandoned without being plugged.Chemical compositions of subsurface brines make up a continuous array between two endmember groups in Permian and Pennsylvanian formations. The end-member groups are distinguishable by graphical analysis of the (1) calcium, magnesium, sodium, and sulfate concentrations, (2) Br/CI ratio plotted against chloride concentration, and (3) Cl/SO, ratio plotted against sulfate concentration and Na/Ca ratio. Most saline samples of shallow ground water in the Concho River watershed are chemically similar to the Permian formation brine end member. Coleman Junction Formation brines cannot be chemically distinguished from other Permian formation brines. Information about organic-acid anions, minor and trace constituents other than bromide, and isotopes of hydrogen, oxygen, carbon, and sulfur also can be used to differentiate between Permian and Pennsylvanian brines but may not always be useful measurements to determine sources of salinization in shallow ground waters.