Migration of landfill contaminants in a tilted-block mantled-karst setting in northwestern Arkansas

Parson's Landfill, located near Springdale, Arkansas, was a class I landfill in operation from 1983 until 1990 when it was closed because of contamination of neighboring springs and wells. The landfill encompasses approximately 40 acres, and is emplaced in thick regolith weathered from Lower Mississippian, chert-bearing limestone of the Boone Formation, a well-documented regional karst aquifer. The landfill was constructed prior to the implementation of Arkansas Subtitle D and Regulation 22 liner requirements. Unlined or completely lined landfills in this hydrogeologic setting typically leak, and ground-water quality results show that the landfill is leaking and contaminating local ground water. Detailed hydrogeological and geophysical investigations and dye trace analysis were conducted to characterize the hydrogeologic framework that controls flow characteristics, flow paths connected with landfill cells, and extent of the contamination. Field reconnaissance has identified numerous seeps and springs north of the landfill. Ground-water sampling results indicate contaminant migration along north-trending flow paths. Spatial analysis of the data suggests that localized faulting has created a large-scale hydrogeologic framework of tilted blocks, approximately one square kilometer in size, with well-developed flow conduits positioned at block margins that act as drains for the block interior where the landfill resides. Tilting of these blocks controls ground-water gradient, flow directions, and ultimately contaminant movement. Current investigations have begun to delineate fault zones, fracturing, and karst features influencing preferential flow paths of contaminated ground water in this structurally-controlled environment.