Aquifer Vulnerability Mapping for the Lake Erie Source Protection Region
Client: Lake Erie Source Protection Region– Grand River Conservation Authority:
Mr. Gregg Zwiers, M.Sc. P.Geo., 400 Clyde Road Cambridge, Ontario N1R 5W6
Key Personnel: Asoka Kodippili, E.J. Wexler, and Dirk Kassenaar.
Earthfx Incorporated conducted a groundwater vulnerability study for the Lake Erie Source Protection Region covering Norfolk and Brant counties, the Catfish Creek and Kettle Creek watersheds, and the entire GRCA. The objective was to develop and apply a consistent and defensible methodology to determine aquifer vulnerability across the diverse study area.
Baseline ISI Assessment
For the baseline analysis, the standard MOE Intrinsic Susceptibility Index (ISI) method was applied to each well in the study area. ISI values were calculated based on the depth to the target aquifer and the K-factor of each geologic unit. Geostatistical methods were applied in an attempt to improve consistency and remove outliers. ISI values were interpolated across the entire study area to delineate areas of low, medium and high vulnerability (Figure 1). Although the ISI showed reasonable correlation with surficial geology, a number of factors resulted in a high level of uncertainty including inconsistencies in the K-factors assigned to clays and bedrock units and issues regarding the use of log-scaled hydraulic conductivities in deriving the K-factors.
New SAAT Assessment Methodology
An alternate method was developed to address the ISI issues. The new method involved estimating the “surface to aquifer advection time” (SAAT) with its two components – unsaturated zone advection time (UZAT) and the Water Table to Aquifer Advection Time (WAAT). This methodology takes into account soil properties, recharge rates, aquifer hydraulic conductivity values, anisotropy, and head differences between the aquifers. In developing the SAAT approach a new material classification system was developed to remove bias in the k-factors and improve the assignment of hydraulic conductivity from lithologic descriptions. Values for SAAT determined at each well were interpolated across the study area to delineate areas of low, medium and high vulnerability to contamination (Figure 2). Compared to ISI, the SAAT results are more consistent and logical.
Despite these improvements it was found that both methods showed a degree of spatial variability, partly due to natural causes and partly due to uncertainty introduced in methodologies. Both method identified where groundwater contamination from surface sources is highly probable, but the ISI method exhibited geologic inconsistencies. There was less agreement in areas of medium to low vulnerability. The high levels of intrinsic error (determined in the uncertainty analysis) relative to the suggested thresholds for both the ISI method and the SAAT method suggests that identifying highly vulnerable areas is relatively straight forward, however differentiating between the relative levels of medium and low vulnerability remains challenging..