A vulnearbility-based framework for assessing impact and risk in source water protection

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Source water protection has become an issue of high priority in Canada.  Conventionally, groundwater sources are protected by establishing wellhead protection areas that express the average time a contaminant will travel from a contamination source to a well.  This approach may neglect some processes that can affect the actual impact of contamination on a well, such as dispersion, the effect of heterogeneities, chemical reactions, and dilution.  Also, most of the models that are  used at present make certain assumptions in how they represent the interaction between surface water and groundwater, and how they deal with the unsaturated zone.  Furthermore, they generally assume constant recharge and pumping rates.  The effect of these assumptions is still poorly understood.  Risk is generally expressed in a qualitative or semi-quantitative way, without a ready means for quantitative assessment.      We will take wellhead protection to the next level by applying the new concept of well vulnerability to assess the impact of a contamination source on a drinking water well.  Well vulnerability considers the entire subsurface pathway of a contaminant from source to receptor, taking into account all relevant subsurface processes.  It expresses the impact in terms of the time when drinking water objectives at the well would be breached, the dilution factor relative to the source strength, and the time of exposure to the contamination.  The effect of commonly made simplifying assumptions will be evaluated, and the performance of standard models used in wellhead protection will be assessed.  Models will be compared with respect to reliability and validity in the context of well vulnerability, leading to a better understanding of the controlling processes and the merits of different modelling approaches.  The effect of uncertainty will be evaluated and quantitative approaches for assessing the risk of a well failing drinking water objectives will be developed.  This will lead to more robust and defensible ground rules for source water protection, and an improved basis for decision making in the protection and long-term management of our groundwater resources.