Use of soil leachates to define the extent of brine-contaminated soils at the USGS petroleum environmental research site "B", Osage County, northeastern Oklahoma

Abstract

Soils at an active oil production site with a history of surface releases of produced water were investigated to determine the chemical signature of salt contamination and the spatial distribution of soil salinity in relation to areas of obvious salt scarring. Grab samples of shallow soil (0-15 cm) and selected soil profiles to depths of 0.6 to 1.7 m were collected onsite and offsite using a 5-cm diameter stainless steel soil auger. Air dried and disaggregated soil from 15 cm intervals (100 g) was mixed with 100 ml of distilled-deionized water to dissolve readily soluble salts. After 24 hours the water was separated by centrifugation followed by filtration through a 0.45 micrometer membrane. Specific conductance was measured immediately. Dissolved chloride (Cl) and sulfate (SO4) were initially estimated with Cl titration strips and turbidimetric measurements respectively. More accurate measurements of dissolved Cl, SO4, and bromide (Br) were later performed by ion chromatography. Brine-contaminated soil was indicated by (1) unusually elevated Cl concentrations or relatively low ratios of conductance/Cl in aqueous leachates compared to leachates of uncontaminated soils, and (2) ratios of Cl/Br in leachates (250± 30) that fall within the range of Cl/Br ratios measured in local produced waters (220-320). Leachates of background soils from the area were characterized by low conductance (sandy soils) or dissolved solids dominated by sulfates (shaley soils). At the study site, the surface (0-15 cm) distribution of elevated soil salinity was largely confined to areas of obvious salt scarring. Within the area of salt scars, elevated salinity persisted to depths of refusal by manual augering. The technique will be applied to deeper samples obtained by mechanical augering or direct push techniques.