VERTICAL HYDRAULIC CONNECTION BETWEEN A PERCHED CARBONATE AQUIFER AND AN UNDERLYING REGIONAL AQUIFER

Even though a perched aquifer may be physically separated from an underlying regional aquifer by a thick unsaturated zone, a lack of integrity in the perching unit may facilitate vertical hydraulic connection between the aquifers. We have investigated a perched flow system in southwestern Wisconsin that is of interest because of the potential for contaminants to migrate from the perched surficial aquifer down through discontinuities in the perching unit to the regional aquifer. We have instrumented a site where an unconfined aquifer in dolomite of the Galena Formation is perched above a regionally-extensive unconfined aquifer in sandstone of the St. Peter Formation. The aquifers are separated by an aquitard composed of shale and dolomite of the Decorah, Platteville and Glenwood Formations that serves as a perching unit.

Discrete openings across the aquitard (e.g., fractures, wells) may be preferential pathways for groundwater flow and chemical migration from the perched to the regional aquifer. In a quarry near the study site, vertical fractures cross the shale and dolomite units that compose the aquitard. However, core of these bedrock units and the hydraulic head profile across the aquitard suggest that fractures do not provide significant vertical flow pathways at the study site. Concentrations of sodium and chloride are much higher in groundwater from the shallow aquifer than in the deep system at the site, supporting the hypothesis that the aquitard is a significant geologic control on contaminant transport from the upper to lower systems. Well casings of water supply wells in this region commonly do not extend into or through the aquitard; this allows shallow groundwater to flow down the well from the perched to the regional aquifer. Measurements of flow through such a multi-aquifer well and results from a numerical model of groundwater flow and contaminant transport suggest that these wells provide preferential pathways for shallow groundwater to reach the deep system and affect the quality of groundwater pumped for domestic use.

The presence of an aquitard reduces the risk of contamination of the regional aquifer, but the construction of wells that perforate the aquitard has reduced that protection and increases risk of contamination. A simple change in well construction could mitigate future contamination.