Acute Toxicity of Saline Produced Waters to Marine Organisms
Authors: Pillard D.A. , Evans J.M. , DuFresne D.L. 
Published: December 2007

Produced waters from oil and gas drilling operations are typically very saline, and may cause acute toxicity to marine organisms due to osmotic imbalances as well as to an excess or deficiency of specific common ions. In order to better understand the relationship between toxicity and ion concentration. laboratory toxicity tests were conducted using mysid shrimp (Mysidopsis bahia), sheepshead minnow, (Cyprinodon variegatus), and inland silverside (Menidia beryllina). For each species the ionic concentration of standard laboratory water was proportionally increased or decreased to produce test solutions with a range of salinities. Individual ions (sodium, potassium. calcium, magnesium. strontium, chloride, bromide, sulfate, bicarbonate, and borate) were also manipulated to examine individual ion toxicity. Organisms were exposed for 48 hours. The three test species differ in their tolerance of salinity. Mysid shrimp show a marked decrease in survival at salinities less than approximately 5 ppt. Both fish species tolerated low salinity water, however, silversides were less tolerant of saline waters (salinity greater than 40 ppt). There were also significant differences in the responses of the organisms to different ions. The results show that salinity of the test solution may play an important role in the responses of the organisms to produced water effluent. Predictable toxicity/ion relationships developed in this study can be used to estimate whether toxicity in produced water is a result of common ions, salinity, or some other unknown toxicant.

Introduction and Background

Common ions in effluents can be toxic to aquatic organisms over a wide range of salinities. Although the toxicity of high salinity (hypersaline) solutions and low salinity (hyposaline) solutions may be due to osmotic conditions, toxicity may also be the result of specific ion concentrations. Therefore, even in effluents of normal freshwater or marine salinity, common ion toxicity to aquatic organisms may occur in the absence of other toxicants. Because little historical data exist on the toxicity of specific ions to marine test organisms, the Gas Research Institute (GRI) sponsored a research program to examine the toxicity of major ions to aquatic test organisms. The goal of this research is to quantify the relationships between common ion concentrations and organism response (mortality), and then use these empirical relationships to develop models that would allow the prediction of organism response based upon known ion concentrations.

The first phase of this research program quantified the toxicity of seven common ions to three freshwater organisms (Ceriodaphnia dubia, Daphnia magna, and Pimephales promelas). It was demonstrated that common ions (e.g., potassium. bicarbonate) in produced waters are toxic to freshwater organisms3. This research resulted in a computer program (the GRI-FWSTRTM Program) that predicts the acute toxicity of an effluent using ion concentrations as input variables.

Because produced waters are also generated and released in marine environments. additional research was initiated to deter- mine the relationship between common ions and toxicity to marine species. In general. produced waters have total dissolved solids (TDS) concentrations higher than most fresh and marine surface waters, although individual TDS values may vary widely (2,000 to 200,000 mg/L TDS; natural seawater is about 32,000 mg/L). Typically. the predominant cation and anion in produced waters are sodium and chloride. Produced waters can vary greatly in composition and salinity, depending on the type of production operation, geologic source of the water, and the treatment of the water once brought to the surface. For example, although chloride is usually the predominant anion and can sometimes reach concentrations of 200,000 mg/L in produced waters, bicarbonate &gt;9,000 mg/L) was reported as the dominant anion in coalbed methane-produced water from Colorado in the United States.

Produced waters are complex mixtures, and it is often difficult to separate and quantify toxicity due to common ions and toxicity due to other chemicals. Toxicity Identification Evaluation (TIE) methods have been developed to systematically identify the causative toxicants present in effluents and other complex mixtures.

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