ABSTRACT
From Summer, 1992 through Spring, 1994, a 7 km section of Indiana State Road 37 on the Mitchell Plain in Lawrence County, Indiana was widened to four lanes. The Mitchell Plain is developed on a belt of limestones of middle Mississippian age, and is characterized by sinkholes, sinking streams, swallow holes and caverns. Construction work included the placement of storm water drainage structures in eleven sinkholes, three of which were also fitted with peat filters. The structures were designed to facilitate drainage of storm water runoff, and to improve runoff quality through detention, adsorption, dilution and filtration prior to directing the flow underground. Since February, 1993, semiannual storm water sampling has been conducted by Earth Tech under contract with the Indiana Department of Transportation (INDOT) to monitor the performance of the drainage structures consistent with the provisions of a Memorandum of Understanding (MOU) concerning the construction of highway transportation projects in karst regions of Indiana. Results from four sampling events indicate that a peat filter installed at Sinkhole 9 is removing an average 80 percent of suspended particulates (measured as TSS and selected total recoverable metals) and almost 50 percent of dissolved copper and zinc. Rock filters are removing from 35.8 percent to 55 percent of total recoverable metals, and from 32.7 percent to 76 percent of TSS. Rock filters do not appear to remove dissolved metals. In order to be effective and remain effective, rock filters and peat filters must be protected from the introduction of excess sediment. Drainage systems along State Road 37 are provided with upgradient detention basins whenever possible. Small ditch line detention basins have also been installed in a number of areas to intercept and retain spills or releases of hazardous materials. These basins not only offer an additional buffer against spills, they provide protection to the filter systems as well. Ongoing structure inspections suggest that the installed filters will continue to function as intended, at least for the next few years. The actual working life of peat and rock filters remains to be determined.
