We delineated natural and oil-field salinity sources that degrade water quality in the upper Colorado River (west Texas) and Petronila Creek (Texas coast) by combining multifrequency airborne EM measurements of apparent ground conductivity with chemical analyses of surface water at key stream locations. To reduce the cost of high-resolution airborne surveying over such large areas, we first flew along the stream axes and then examined preliminary results in the field to identify likely salinized stream segments. We then flew more detailed surveys over<br>these areas rather than over the entire basin. Stream-axis EM data also helped identify water-sampling locations upstream and downstream from each salinized segment. We used these data to calculate salinity loads, discriminate among possible natural and oil-field salinity sources, and more effectively implement best-management practices to optimize remediation efforts.<br>We acquired stream-axis airborne EM data along the upper Colorado River using a Geophex GEM-2A instrument operating at five frequencies between 450 Hz and 39 kHz. Increases in chloride, sulfate, and total dissolved solids loading in the upper Colorado River basin occur along eleven segments of elevated apparent conductivity identified from airborne EM data. Each segment encompasses areas of baseflow salinity contributions to the stream from natural dissolution of evaporite minerals in the Permian basin, from oil-field produced water, or both. Analyses of surface water confirm increases salinity loading associated with each segment. Airborne EM data acquired on the coast along Petronila Creek revealed three stream segments with elevated ground conductivity. Increases in chloride, sulfate, and total dissolved solids loading are attributed to shallow baseflow contributions. Using airborne EM and hydrochemistry data, we interpret the dominant salinization mechanism to be historic discharge of produced water into unlined drainage ditches and pits, infiltration into sandy Pleistocene channel deposits, lateral migration as far as several kilometers, and discharge into the stream.


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