Groundwater flow delineation in the Toyah Basin of Trans-Pecos, Texas
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The Toyah alluvial basin is a north, northeast-trending trough over 1000 feet (300 m) deep, created by dissolution of underlying Permian evaporites. The Cenozoic alluvial fill and adjoining Cretaceous limestones constitute a single hydrostratigraphic unit hereafter referred to as the Toyah aquifer. Pumpage from this aquifer has created a cone of depression near Pecos which is a principal influence on the groundwater flow system. Potentiometric surfaces and high SO₄/CI ratios (> 2) in the western half of the basin indicate recharge through the Permian Rustler and Castile Formations which crop out in the Rustler Hills. In the central and extreme southwestern parts of the basin lower SO₄/CI ratios (< 2) suggest interbasin flow through the Apache Mountains to the west. Low TDS water (< 1000 mg/l) along the southern margin of the basin indicates recharge from the Davis Mountains. The large springs near the head of the basin discharge a constant, warm (25° C) mineralized flow (> 2000 mg/l) except after heavy rainfall which increases discharge, decreases temperature, and decreases the dissolved mineral content. This change in flow characteristics indicates two separate source areas. The Capitan reef limestone in the Apache Mountains is the probable source for the constant flow. The Capitan reef and the lower Cretaceous limestones, from which the springs issue, are juxtaposed by the Stocks Fault. The chemical signatures of the water in the Capitan Limestone and the water discharging from the springs are nearly identical. Correlation of discharge with rainfall and chemical reaction modeling (PHREEQE) indicate that the Davis Mountains provide the flashy, low salinity component of flow.