COMBINING AIRBORNE EM AND SURFACE-WATER ANALYSES TO IDENTIFY NATURAL AND OIL-FIELD SALINITY SOURCES THAT DEGRADE WATER QUALITY IN TWO TEXAS STREAMS

We delineated salinity sources that degrade water quality in the upper Colorado River (west Texas) and Petronila Creek (Texas coast) by combining multi-frequency airborne EM measurements of apparent ground conductivity with chemical analyses of surface water at key stream locations. To reduce the cost of conventional high-resolution airborne surveying over such a large area, we first flew along the major stream axes and then examined preliminary airborne geophysical survey results in the field to identify local apparent conductivity highs and likely salinized stream segments. We then flew more detailed surveys over these areas rather than over the entire stream basin. Stream-axis survey data also helped identify water-sampling locations upstream and downstream from each salinized segment. We used these data to calculate salinity loads and discriminate among possible natural and oil-field salinity sources.

Increases in salinity loading in the upper Colorado River below Spence Reservoir occur along four segments of elevated apparent conductivity, each several kilometers long, identified from airborne geophysical survey data. Each segment encompasses areas of baseflow salinity contributions to the river from oil-field produced water (1 site) and natural dissolution of evaporite minerals from Paleozoic strata (3 sites). Significant increases in salinity load in Petronila Creek on the coast are also associated with shallow baseflow contributions along three segments of elevated apparent conductivity, each several kilometers long. The dominant mechanism contributing salinity within two upstream segments includes historic discharge of produced water into unlined drainage ditches, infiltration into exposed Pleistocene channel deposits, lateral migration as far as several kilometers, and discharge into the stream. Geophysical and chemical data suggest a mixture of oil-field and seawater salinity contributions at the most downstream Petronila Creek segment.