Concentrations of Selected Constituents in Surface-Water and Streambed-Sediment Samples Collected From Streams In and Near an Area of Oil and Natural-Gas Development, South-Central Texas, 2011–13

Abstract

During 2011–13, the U.S. Geological Survey, in cooperation with the San Antonio River Authority and the Guadalupe-Blanco River Authority, analyzed surface-water and streambed-sediment samples collected from 10 sites in the San Antonio River Basin to provide data for a broad range of constituents that might be associated with hydraulic fracturing and the produced waters that are a consequence of hydraulic fracturing. Among surface-water samples, all sulfide concentrations were less than the method detection limit of 0.79 milligrams per liter. Four glycols—diethylene glycol, ethylene glycol, propylene glycol, and triethylene glycol—were analyzed for in surface-water samples collected for this study, and none were detected. Of the 91 semivolatile organic compounds analyzed for this study, there were six detections, all but one of which were in storm-runoff samples. The base-flow sample collected at the San Antonio River at Goliad, Tex. (SAR Goliad), site contained bis(2-ethylhexyl) phthalate, a plasticizer in polyvinyl chloride and a constituent in hydraulic fracturing fluids. The storm-runoff samples collected at the San Antonio River near Elmendorf, Tex. (SAR Elmendorf), and Ecleto Creek at County Road 326 near Runge, Tex. (Ecleto 2), sites also contained bis(2-ethylhexyl) phthalate. The storm-runoff sample collected at the SAR Elmendorf site contained the plasticizer diethyl phthalate. Both storm-runoff samples collected at the Ecleto Creek near Runge, Tex. (Ecleto 1), and Ecleto 2 sites contained benzyl alcohol, a solvent commonly used in paints. Of the 67 volatile organic compounds analyzed in this study, there were a total of six detections, all of which were in base-flow samples. The surface-water sample collected at the SAR Elmendorf site contained bromodichloromethane, dibromochloromethane, and trichloromethane, all of which are disinfection byproducts associated with the chlorination of municipal water supplies and of treated municipal wastewater. The sample collected at the Cibolo Creek near Saint Hedwig, Tex. (Cibolo St. Hedwig), site contained toluene, a fuel additive, solvent, and industrial feedstock used to produce benzene and a constituent associated with produced waters. The Cibolo St. Hedwig site is upstream from current (2014) oil and natural-gas production areas. Dichloromethane, an industrial solvent with multiple uses, was detected in surface-water samples at both the San Antonio River at State Highway 72 near Runge, Tex. (SAR 72), and SAR Goliad sites.

In streambed-sediment samples, concentrations of total saturated hydrocarbons (TSH) ranged from an estimated 260 micrograms per kilogram (μg/kg) in the less than (<) 2-millimeter (mm) size-fraction sample collected at the SAR Goliad site to 11,000 μg/kg in the <2-mm size-fraction sample collected at the Ecleto 1 site. TSH concentrations were greater in the <63-micrometer (μm) size-fraction samples than in the <2-mm size-fraction samples in streambed-sediment samples collected from 5 of the 9 sites. Total polycyclic aromatic hydrocarbons (PAHs) were calculated as the sum of the individual PAHs and alkylated PAHs. Total PAH concentrations ranged from less than the method detection limit in the <2-mm size-fraction samples collected from multiple sites to 1,600 μg/kg in the <2-mm size-fraction sample collected from the San Antonio River near McFaddin, Tex. (SAR McFaddin), site. Total PAH concentrations were greater in the <63-μm size-fraction samples than in the <2-mm size-fraction samples at 7 of the 9 sites.

During collection of streambed-sediment samples, additional samples from a subset of three sites (the SAR Elmendorf, SAR 72, and SAR McFaddin sites) were processed by using a 63-µm sieve on one aliquot and a 2-mm sieve on a second aliquot for PAH and n-alkane analyses. The purpose of analyzing PAHs and n-alkanes on a sample containing sand, silt, and clay versus a sample containing only silt and clay was to provide data that could be used to determine if these organic constituents had a greater affinity for silt- and clay-sized particles relative to sand-sized particles. The greater concentrations of PAHs in the <63-μm size-fraction samples at all three of these sites are consistent with a greater percentage of binding sites associated with fine-grained (<63 μm) sediment versus coarse-grained (<2 mm) sediment. The larger difference in total PAHs between the <2-mm and <63-μm size-fraction samples at the SAR Elmendorf site might be related to the large percentage of sand in the <2-mm size-fraction sample which was absent in the <63-μm size-fraction sample. In contrast, the <2-mm size-fraction sample collected from the SAR McFaddin site contained very little sand and was similar in particle-size composition to the <63-μm size-fraction sample.

First posted April 29, 2014

Revised January 28, 2015