Numerical groundwater flow and contaminant
transport modeling was conducted to evaluate alternative remedial actions and
predict contaminant concentrations in an unconfined glacial aquifer located
in the Village of Milford, Michigan, USA. The distribution and transport of
three contaminants, cis-DCE, benzene, and MTBE, were modeled. Limited subsurface
control contributed uncertainty to the geologic model with respect to the geometry
and continuity of a regional aquitard unit underlying the aquifer and the extent
to which infiltration from two manmade surface water bodies influenced the groundwater
flow field at the site. Three alternative conceptual models were therefore constructed
and independently calibrated to evaluate this uncertainty: A) a regional aquitard,
B) an extended aquifer, and C) a reduced infiltration model. Advective transport
modeling (MODFLOW and MODPATH) demonstrated significant differences in predicted
transport pathways between the regional aquitard and extended aquifer conceptualizations.
However, subsequent simulation of advective-dispersive-reactive transport (MT3DMS)
using these models predicted similar order-of-magnitude breakthrough curve responses
in the remedial pumping wells and municipal water supply wells for each contaminant
species. Thus, while the choice of conceptual model demonstrably affected predicted
contaminant pathways, it had only a limited effect upon transport model predictions
that were integrated over pumping wells screened across multiple model layers.
Nevertheless, the choice of conceptual model has important practical implications
for the
placement of monitoring wells with smaller screened intervals.