Quantifying contributions to stream salinity using electromagnetic induction and hydrochemistry in a small Texas coastal-plain basin

Abstract

Airborne, ground, and borehole electromagnetic (EM) induction measurements were combined with surface-water chemical analyses to determine the extent of salinization as well as identify salinity sources and migration mechanisms that elevate total dissolved solids (TDS), Cl −, and SO 4 2 - concentrations in Petronila Creek, a shallow stream that drains a small (1600 km 2) basin on the Texas coastal plain. A multifrequency airborne EM induction survey measured apparent electrical conductivity of the ground to depths of a few tens of meters along the axis of the stream and along parallel flight lines within a 150 km 2 area extending from where the stream is fresh to the estuarine mixing zone. Apparent conductivity sections and maps were analyzed to identify stream segments where high conductivity indicates near-surface salinization and where saline baseflow increases the salinity load. Elevated conductivities at shallow depths beneath the stream lie adjacent to extensive conductive areas in oil fields and along ditches that once carried brine produced from the oil fields. These highly conductive areas delineate salinization that dominantly is caused by past discharge of produced brine into ditches and pits, infiltration into sandy, permeable horizons, lateral subsurface migration, and eventual discharge into the stream. Streamflow measurements and chemical analyses show that the TDS load increased along two highly conductive streambed segments upstream from the estuarine mixing zone. Extensive salinization between the ditches and the stream could provide a continuing source of salinity. A third highly conductive segment coincides with the zone of estuarine mixing, where there is geophysical evidence of possible subsurface seawater intrusion. Multifrequency EM induction spectral images (the sectional depiction of apparent conductivity measured at multiple frequencies) along the stream axis, produced with minimal geophysical processing, helped identify stream segments that receive saline baseflow and guided surface-water sampling that allowed quantification of salinity loads. Apparent conductivity maps helped determine the salinization extent and identify salinity sources and migration mechanisms. Both the stream-axis and gridded airborne EM surveys were effective approaches to investigate salinization and interaction between ground water and surface water in this small basin, but multifrequency stream-axis images are a more practical alternative in larger basins where discrete salinity sources are poorly known.