PRELIMINARY ASSESSMENT OF A FMGP IN EASTERN SOUTH DAKOTA

A former manufactured gas plant (FMGP) provided the City of Watertown, South Dakota, with illuminating gas and gas for fuel from approximately 1906 to 1945. The FMGP is believed to have utilized either the carburetted water-gas process or the oil-enriched water-gas process.

The FMGP site is located in the flood plain of the Big Sioux River, and overlies the Big Sioux aquifer. The sediments comprising the flood plain are unconsolidated sand and gravel outwash overlain by alluvium. The Big Sioux aquifer is unconfined, is a major aquifer in eastern South Dakota, and is the source of numerous municipal water supplies, including the City of Watertown municipal water system.

No previous assessment has been conducted at the FMGP site, although the EPA conducted an assessment in the vicinity of the FMGP site in 1994. Numerous semi-volatile organic compounds commonly associated with coal-tar residuals were identified in the analyses of river sediment samples obtained from locations near the FMGP.

Ground water monitoring wells constructed as part of an environmental assessment unrelated to the FMGP site are present in the site vicinity. The South Dakota Department of Environment and Natural Resources (SDDENR) believes that non-aqueous phase liquid present in one of the monitoring wells, and dissolved hydrocarbons identified in the other monitoring well, could be due to wastes from the FMGP. Therefore, the SDDENR is requiring an assessment to determine the extent of potential wastes related to the FMGP.

The purpose of the preliminary assessment is to obtain additional information regarding the processes employed at the FMGP and the associated waste streams, and to determine if the contamination identified in the two ground water monitoring wells is due to subsurface wastes originating from the FMGP. The results of the preliminary assessment will be utilized for planning a more detailed site investigation of the FMGP.