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Monitoring contamination in surface water and groundwater in a catchment with an unconfined porous aquifer overlying fractured bedrock Chen, Yaming


The factors influencing the design of an integrated surface water and groundwater contaminant monitoring network are examined for a system where a permeable unconfined aquifer overlies fractured bedrock in a headwater stream catchment. The unconfined porous aquifer is modeled as a homogeneous and isotropic porous medium with deterministic properties. The fractured bedrock is modeled as a lower permeability rock mass, but with a stochastic fracture network composed of three orthogonal fracture sets. The surface domain consists of a V-shaped overland flow zone and a linear stream channel with constant width. A continuous contaminant source zone is situated close to the land surface. The analysis is based on a steady state representation of the groundwater and surface water flow system and transport of a non-reactive solute in both the surface and subsurface domains. The probability of plume detection is defined in terms of the likelihood of detecting contamination in a performance monitoring network prior to its detection at a downstream compliance boundary. The ratio of the permeability of the unconfined porous aquifer to the bulk permeability of the underlying fractured bedrock is a key factor influencing detection probabilities. A higher permeability unconfined aquifer reduces detection probabilities in the stream, and increases detection probabilities in the subsurface. Fracture network connectivity and aperture size have a significant influence on detection probabilities in the subsurface, especially in the bedrock. Dilution of contamination in the headwater stream by inflowing non-contaminated water has a strong influence on detection probabilities. Due to the effect of dilution, sampling both stream water and streambed groundwater will enhance detection probabilities in the stream. The contaminant detection threshold is a key factor in influencing the stream water and groundwater monitoring. A lower detection threshold will increase detection probabilities in the surface water and groundwater systems, especially in the stream water. The results in this study demonstrates the importance of integrating stream water and groundwater samples in the design of a performance monitoring network and documents the factors that determine contaminant detection probabilities in the surface stream, the unconfined aquifer, and the underlying fractured bedrock.

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