Salinity/Toxicity Relationships To Predict the Acute Toxicity of Produced Waters to Freshwater Organisms

Abstract

Abstract As part of previous research, the Gas Research Institute, ENSR, and the University of Wyoming developed a series of multivariate logistic regression equations (called Salinity/Toxicity Relationships or STRS) that predict acute toxicity to three freshwater organisms based on major ion composition. While these equations did an excellent job of representing responses in the laboratory data set on which they were based, their validity and usefulness lay in their ability to make accurate predictions for independent data sets and real-world samples. Accordingly, the STR equations were applied to a series of data sets from different sources. This exercise shows that the equations generally provide accurate predictions of toxicity, even across waters with very different ionic compositions. STR equations were also able to detect the presence of toxicants other than major ions in the samples. These results indicate that the STR approach will provide a valuable tool for assessing and managing produced waters and other waters with elevated concentrations of dissolved ions. INTRODUCTION AND BACKGROUND During the last decade, regulatory authorities have turned increasingly toward the use of freshwater and marine toxicity tests to evaluate the potential for environmental effects from waterborne chemicals. In particular, effluent toxicity testing ("biomonitoring") has become a centerpiece of effluent regulation under the National Pollutant Discharge Elimination System (NPDES) mandated by the Clean Water Act. Whole effluent toxicity testing and accompanying limits for aquatic toxicity have been incorporated into NPDES permits for many discharge categories. In addition to wastewaters from traditional municipal and industrial facilities, many produced waters are also subject to effluent toxicity limits in areas where these waters are discharged to surface waters. Probably the most pronounced characteristic of produced waters is their tendency to contain elevated concentrations of total dissolved solids (TDS), relate to fresh water. While not typically thought of as "toxics," the ions comprising TDS (e.g., chloride) can cause toxicity to freshwater organisms if present in sufficiently high concentrations. As a result, many produced waters can show toxicity caused by major ions, like chloride, regardless of the presence of hydrocarbons, metals, or other potentially toxic materials. Accordingly, appropriate management and treatment of produced waters can depend, in part, on understanding the role of TDS in causing aquatic toxicity. Sodium and chloride are typically considered to be the predominant major ions in produced water and other waters arising as by-products of the fossil fuel industry. However, sodium and chloride are not universally predominant. For example, many coalbed methane waters from the San Juan Basin have elevated concentrations of bicarbonate (personal communication, D.H. Pope, Bioindustrial Technologies, Inc., Georgetown, Texas); leachates from oil shale may contain high concentrations of magnesium and sulfate. P. 605^