A risk assessment approach to identifying constituents in oilfield produced water for treatment prior to beneficial use.

A risk assessment approach incorporating exposure pathways and calculated risk quotients was applied to identifying constituents requiring treatment prior to beneficial use of oilfield produced water (OPW). In this study, risk quotients are ratios of constituent concentrations in soil or water to guideline concentrations for no adverse effects to receptors. The risk assessment approach is illustrated by an example of an oilfield water produced from non-marine geologic strata of a rift basin in sub-Saharan Africa. The OPW studied has the following characteristics: 704-1370 mg L(-1) total dissolved solids (TDS), 45-48 mg L(-1) chloride, and 103.8 mg L(-1) oil and grease. Exposure pathways of constituents in OPW used for irrigation include: ingestion of plant tissue, ingestion and direct contact of irrigated soil by livestock, inhalation of aerosols or volatilized constituents, and ingestion of OPW directly by livestock. Applying risk quotient methods for constituents in soil and water, constituents of concern (COCs) identified for irrigation and livestock watering using the OPW studied include: iron (Fe), manganese (Mn), nickel (Ni), zinc (Zn), and oil and grease. Approximately 165,000 barrels d(-1) (26,233 m(3) d(-1)) of OPW from the study site are available for use. Identification of COCs and consideration of water quantity allows for development of reliable treatment design criteria to ensure effective and consistent treatment is achieved to meet guideline levels required for irrigation, livestock watering, or other uses. This study illustrates the utility of risk assessment for identifying the COCs in OPW for treatment, the level of treatment required, and viable options for use of the treated water.