Multiple working transport hypotheses in a heterogeneous glacial aquifer system

Multiple working hypotheses can be used to evaluate permissible alternative hydrogeological interpretations at sites with limited subsurface control. This approach was applied to test the viability of three conceptual aquifer system architecture models coupled with three hypothesized source locations for a 1,4-dioxane plume in a heterogeneous glacial aquifer system in Washtenaw County, Michigan. The three alternative conceptual models characterized the site hydrogeology with increasingly complex distributions of hydrostratigraphic units: (A) an effective aquifer, (B) a layered confined aquifer, and (C) a discretely heterogeneous aquifer model. Each was incorporated into an independently calibrated numerical ground water flow (MODFLOW) model. Steady-state and transient flow simulations of the alternative models were evaluated using both hydraulic flow field characteristics observed under natural conditions and the perturbed response after local remedial pumping activity began. Three plausible locations where 1,4-dioxane could have entered the aquifer system were identified using historical information at the site: (1) manufacturing waste water disposal lagoons, (2) a 60 foot (18 m) deep kettle lake, and (3) a shallow impoundment on a local stream. Advective transport modeling (MODPATH) was used to assess the consistency of the hypothesized source locations with observed contaminant migration pathways inferred from the mapped location of the plume. Evaluation of the nine combinations of hydrogeologic conceptualizations and 1,4-dioxane source locations led to elimination of four working hypotheses and discounting of two others, leading to reduced overall uncertainty and the development of new insights into the system behavior.