MIGRATION OF A NEARLY 80-YEAR OLD SALTWATER PLUME IN THE HIGH PLAINS AQUIFER, KANSAS

The disposal of oil-field brine (mean chloride concentration 96,000 mg/L) in over 200 surface ponds in a 40 km² area near Burrton, south-central Kansas, during the 1930s and 1940s created a large saltwater plume in the High Plains aquifer. Monitoring of the saltwater contamination from the Burrton oil field began in 1939 and has continued to the present. Over 130 monitoring and supply wells at depths of 5-87 m are now sampled for chloride concentration in a 120 km² area surrounding and including the plume. Three-dimensional images of the plume for the last 30 years show the migration of the saline water downward and in the direction of ground-water flow as would be expected for fluid denser than the fresh background water. Silt and clay lenses within the sand and gravel aquifer affect relative rates of vertical and lateral migration of the saline water. The highest chloride concentration now encountered in the monitoring wells is about 2,000 mg/L. Although chloride concentration at different locations and depths has increased or decreased substantially, no statistically significant trend exists over 1980-2010 for the mean of annual measurements from 48 observation wells within and immediately adjacent to the plume. This is expected for the chloride mass, which is conserved within the historical plume extent. The current extent of the 500 mg/L isosurface for chloride concentration is approximately 10.5 km and 6.4 km in the longitudinal and transverse directions, respectively, and 70 m deep. This is larger than the current 6-km longitudinal and 1.4-km transverse extent of the well-studied, Cape Cod sewage wastewater plume. The Kansas plume front has advanced about 2-4 km from 1980 to 2010, depending on the location, and is now approximately 2-3 km from the westernmost portion of a large well field supplying much of the municipal supply of Wichita. Three-dimensional images reveal the location and depth where monitoring of the plume nose is needed for better determination of when the plume will enter the well field.