A hydrogeologic and water-quality evaluation of the Springfield aquifer in the vicinity of north-central Washington County, Arkansas

A hydrologic investigation was undertaken on a 10,640 Ha area in north-central Washington County, Arkansas for the purpose of analyzing the water-quality and flow system of the shallow carbonate Springfield aquifer. Since 1990, the population in the study area has grown by a factor of 1.5 to 2 times, and this project is intended to serve as a base-line assessment of the hydrology, so future analyses can focus on system change as the area becomes more urbanized.; Subsurface geologic mapping indicates the aquifer is thickest (87 m) in the northeastern corner of the study area, thinning to approximately 35 m in the southwestern corner of the study area. The confining unit varies from 8--26 m thick, and vertical offset of key stratigraphic markers indicates the study area is situated on a block fault. The Clear Creek fault exists along the southern border of the study area, and is effectively a groundwater dam, impeding groundwater movement to the south. That fault offsets the Chattanooga shale approximately 34 m on the eastern end of the study area. A potentiometric surface interpretation suggests groundwater flows radially outward from near the central part of the study area; major flow is concentrated along zones of secondary permeability. There are fifteen groundwater basins identified and delineated using a combination of water-level mapping, karst reconnaissance, dye tracing and chemical characterization.; Water samples were collected at 62 sites during base-flow conditions, with a spatial distribution of approximately one per 8 km2. Aqueous chemistry suggests three distinct hydrostratigraphic zones, epikarst, Boone aquifer, and Chattanooga confining unit. Water from the Boone aquifer is a calcium bicarbonate-type, with moderate to high mineralization. It represents water nearest to carbonate equilibrium, indicating carbonate dissolution is the dominant process. Na-HCO3 type water near the Chattanooga/St. Joe Contact is caused by cation exchange between clays of the Chattanooga and Ca-HCO3 of the Boone aquifer. The epikarst aquifer is characterized by low-calcium bicarbonate-type mineralized water (<200 mg/L) with low concentrations of all measured dissolved constituents, and a pH generally less than 7.0. Subsequently, there samples fall on a mixing line between precipitation and Ca-HCO3 dominated groundwater, indicating the regolith is not as chemically reactive with groundwater as the Boone aquifer.; To determine the extent of anthropogenic impacts on the groundwater system indicator parameters were assessed, including Cl-, NO 3-, total organics, fecal coliform, and selected trace metals. Whereas no samples indicated levels above EPA maximum contamination levels (MCL) individual sample sampling sites had concentrations greater than background. Throughout the area, fecal coliform counts were generally high, indicating widespread contamination animal and human manure. A site-specific study analyzed water samples from 17 sites in a 10 km2 area encompassing the Waste Management Tontitown Landfill (WTML), indicating there is no significant difference between WTML and the rest of the study area. Whereas selected trace-metal concentrations occurred above background levels, no concentrations exceeded EPA MCL.