Simulating the Effects of Groundwater Dynamics on Aquifer Vulnerability

English Abstract:

Groundwater vulnerability mapping is extensively used as an analytical tool for groundwater resource protection and land use management. Vulnerability mapping can however bring some limitation, as the most widely used methods (i.e. DRASTIC, GOD, SINTACS, etc.) are based on semi-objective weighted parameters. The majority of these methods do not take into account groundwater flow and solute transport. The objective of this study was to use numerical modeling to quantitatively identify the hydrogeological parameters which significantly influence contaminant transport and therefore aquifer vulnerability. A synthetic 3D model using MODFLOW and MT3D was built to represent the main characteristics (e.g. watershed scale, hydraulic gradient, recharge rate) of the Becancour watershed (2 900 km²) located in southern Québec (Canada). Diffuses sources of contamination were simulated as contaminant influx with the recharge and computed concentrations in groundwater were interpreted as a proxy of vulnerability. The numerical experiments show that maximum contaminant concentrations in groundwater are more largely influenced by hydrogeological parameters than by the size and density of the contaminant sources. At the regional scale, the simulated concentrations in groundwater are mostly sensitive to recharge rates and to the dilution potential with regional flow. At the local scale, river drainage coefficients and hydraulic gradients are the most influential parameters. As expected, these results suggest that the most vulnerable areas can be found where recharge rates are high and where the contaminant dilution potential is low. In the simulated model, the majority of contaminant input is exported out of the aquifer through the rivers. The modelling also suggests that deep bedrock aquifers could become vulnerable to anthropogenic pollution (e.g. nitrates) in the long-term, as nitrates originating from the beginning of intensive agriculture in the 70-80’s may not yet have reached aquifers deeper than 75 m.