IDENTIFICATION AND FATE OF OIL-BRINE CONTAMINATION IN DIFFERENT HYDROGEOLOGIC SETTINGS

Past disposal of oil-field brine at the surface has caused substantial contamination of water resources in south-central Kansas. Natural saline water occurs in and discharges from Permian bedrock in parts of the region, and other anthropogenic sources of saline water exist, requiring clear identification of the different sources. Variations in bromide/chloride and sulfate/chloride ratios relative to time, streamflow, and chloride concentration and end-member mixing are practical methods for source differentiation. Although regulations preventing escape of saltwater from oil wells were first passed in Kansas in 1935, much oil and gas brine was disposed on the surface through the 1940s. Hydrogeologic characteristics of the areas with past surface disposal of oil brine differ appreciably and result in large differences in the ratio of saltwater transported in streams or ground water. Much of the brine disposed during the 1910s to 1940s in an area of silty clay soils overlying shale and limestone bedrock soon ran off or was flushed from the surface by rain into streams. Chloride concentration in the river draining this area often exceeded 1,000 mg/L from after the start of oil production up to the 1950s. Chloride content in the river then generally declined to near 100 mg/L in recent low flows. Oil brine was also disposed in surface ponds overlying the High Plains aquifer in south-central Kansas from the latter 1920s into the 1940s. Most of the surface-disposed brine infiltrated to the underlying aquifer. Where the High Plains aquifer is thin, saltwater migrated along clay layers or the underlying shaly bedrock and either discharged into small streams or flowed into thicker parts of the aquifer. Where the aquifer is thick, saltwater moved downward until reaching clay lenses, migrated latterly to the edge of the clay, and again moved downward if still dense enough. Water-level declines from pumping have increased the lateral migration rate of the saltwater contamination in the aquifer towards water-supply wells. The period of flushing most of the surface-disposed saltwater from the area of shale and limestone bedrock is on the order of many decades, but is at least many centuries for the deeper parts of the High Plains aquifer.