KARSTIC GYPSUM PROBLEMS AT WASTE-WATER STABILIZATION SITES IN THE BLACK-HILLS OF SOUTH-DAKOTA

The Triassic Spearfish Formation contains numerous gypsum beds in whic h karstic conditions have developed in the Black Hills of South Dakota . The evaporite karst has caused severe engineering problems for found ations and water retention facilities, including wastewater stabilizat ion sites. Two dramatic examples include the former sewage lagoons for the City of Spearfish and a proposed lagoon/wetlands facility for Whi tewood, South Dakota. The Spearfish sewage lagoons were built in 1972 on alluvium above a thick gypsum layer, despite warnings from local ra nchers. At one point during construction a scraper became stuck in a s inkhole and required four bulldozers to pull it out. The lagoons start ed leaking badly within a year after completion; the southern lagoon w as abandoned after four years because of leaks, and the northern lagoo n could not provide adequate retention time for effective sewage treat ment. Attempts at repairs, including a bentonite liner, were ineffecti ve, and poorly treated sewage discharged beneath the lagoon's berm int o a nearby surface drainage. The lagoons were abandoned completely in 1980 after a large lawsuit, and a mechanical wastewater treatment plan t was constructed nearby. The planned Whitewood sewage lagoon and arti ficial wetlands were proposed in 1988 at an area with troubling simila rities to the failed Spearfish site. Field investigation showed gypsum outcrops within proposed wetland cells and 10-m deep sinkholes nearby . After an engineering report cited these difficulties, subsequent dri lling at a proposed wetland cell showed a 9-m thickness of gypsum that was exposed at the surface. The wetlands project and later the lagoon project were tabled after controversy.