Biotoxicity Characterization of a Produced-Water Discharge in Wyoming
Authors: Mancini E.R. , Stilwell C.T. 
Published: December 2007

The objectives of this investigation were to document the physicochemicaland aquatic toxicological quality of a physicochemical and aquatictoxicological quality of a "beneficial-use" produced-water dischargeand its effect on a receiving stream in Wyoming. Fish and water-flea survival, growth, and reproduction tests indicated that the discharge and all othersampling stations passed the state effluent biomonitoring acute toxicitytesting endpoints. While benthic macroinvertebrates were absent at thedischarge point designated by the Natl. Pollutant Discharge Elimination System(NPDES), productive and reproducing populations were present at all otherdownstream and mixing-zone populations were present at all other downstream andmixing-zone stations. This investigation confirmed the validity of thebeneficial-use subcategory for this oilfield discharge.

Introduction

Discharging produced waters into surface water in Wyoming is permittedpursuant to the requirements of the NPDES program under permitted pursuant tothe requirements of the NPDES program under the U. S, Clean Water Act. Suchdischarges are permitted under the U.S. Code of Federal Regulations (40 CFRPart 435) for an oil- and gas-extraction point-source subcategory forbeneficial use in agriculture and wildlife propagation. The NPDES program wasoriginally envisioned to allow, and even to encourage, the beneficial use ofproduced water that is relatively fresh and otherwise of sufficient quality tomeet water demands over broad and semiarid portions of Wyoming. Since theinception of the Wyoming Dept of Environmental Quality's (DEQ's) permit programin the early 1970's, the following effluent limitations have applied to theseproduced- water discharges: total dissolved solids (TDS), 5,000 mg/L; chloride,2,000 mg/L, sulfates, 3,000 mg/L; oil and grease, 10 mg/L, and a pH between 6.5and 8.5. Since its implementation, the NPDES program has been subject toconstant revision and refinement by the U.S. Environmental Protection Agency(EPA). The original focus of the program was to apply technology- basedeffluent limitations, like those listed above to the various dischargecategories and subcategories. This goal has largely been achieved. A newer, water-quality-based objective for the control of toxic pollutants was announcedin 1984 and is being implemented in all U.S. pollutants was announced in 1984and is being implemented in all U.S. states with NPDES administrativeauthority, including Wyoming. The water- quality-based program focuses oncompliance with water-quality standards and designated uses of receivingwaters. One of the major objectives of the program is to achieve dischargesfree of acute toxicity at the end of the pipe and free of chronic toxicity atthe edge of the mixing zone.* In Wyoming, whole-effluent toxicity testingrequirements apply only to produced-water discharges into Class I, II, or IIIstreams (i.e., of high aquatic value, called "live" waters). Acceptableperformance for the acute water flea and fathead minnow aquatic toxicity testsspecified by the State of Wyoming is more than 50 % survival for the prescribedexposure periods for both species. Unsatisfactory test results require thattoxicity be reduced or eliminated or that the discharge be discontinued. Discharges into Class IV low-aquatic-value water bodies like Cottonwood Creekare not required to perform effluent toxicity tests. Currently, the Wyoming DEQclassification of Cottonwood Creek is being reconsidered and may be upgradedfrom its Class IV status. The potential application of whole- effluent toxicityanalyses precipated a reassessment of this produced-water discharge at the Hamilton Dome field. produced-water discharge at the Hamilton Dome field. Oiland gas has been produced from Hamilton Dome field since its discovery in 1918. Primary producing horizons are all part of a classic domed anticlinalstructure. Producing formations include the Madison, Basal Amsden, Tensleep, Phosphoria, and Chugwater. Tensleep, the primary producing formation, comprises Pennsylvanian sandstone and carbonate. Although some waterflooding isundertaken in the field, oil is produced primarily by a strong naturalwaterdrive. While three producers currently operate the field-yielding acombined production of 8,000 BOPD and 185,000 BWPD-this study concerns the oneoperation responsible for more than 50% (105,000 B/D) of the field'sproduced-water discharge. The produced water has a TDS concentration generallyless than 4,500 mg/L (produced waters often range from 10,000 to 150,000 mg/L)and, otherwise, is of similar quality to the famous natural hot springs of Thermopolis, WY, 20 miles to the southeast. In response to the regulatoryinitiatives cited above, an assessment of the NPDES discharge and the Cottonwood Creek drainage in the vicinity of the discharge (Fig. 1) wasinitiated in 1989. This investigation focused on important water-qualityparameters, discharge characteristics, water temperature, analyses from benthicmacroinvertebrate communities, and aquatic toxicology (Table 1).