CGRG Bibliography of Canadian Geomorphology
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Author : Allen, D.M.; Scibek, J.; Chesnaux, R.; and Mc Athur, S.
Date : 2006.
Title : Development of a trans-national groundwater flow model for investigating nitrate transport, Abbotsford-Sumas aquifer, BC and WA.
Publication : Joint Annual Meeting of the Geological Association of Canada and the Mineralogical Association of Canada. University of Quebec in Montreal (UQAM) May 14-17, 2006.
Issue :
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Abstract
A three-dimensional groundwater flow model was developed for the Abbotsford-Sumas unconfined aquifer in the central Fraser Valley along the boundary of British Columbia and Washington State. The model was developed for two purposes, to investigate future impacts of climate change on groundwater resources, and to simulate nitrate transport. In this presentation we focus on the development of the model itself and briefly summarize the approach being used for nitrate transport modelling. Nitrate contamination of the Abbotsford-Sumas aquifer in the central Fraser Valley has become a significant problem over the last 30 years, because the aquifer is a source of water for approximately 120,000 people in Canada and the US. Nitrate concentrations above the maximum allowable concentration (10 mg/L NO3-N) have been recorded in many of the aquifer’s wells since the 1970’s. Agricultural land-use above the aquifer is primarily raspberries, and although application practices for fertilizer have improved in recent years, nitrate concentrations in the aquifer have not dropped. The aquifer system is 160 km2 in area and consists of unconfined and semi-confined units comprised of heterogeneous glaciofluvial/glaciolacustrine sediments, which infill depressions in glaciomarine sediments that overlie Tertiary bedrock. After extensive review of geologic and hydrogeologic information for the Fraser Valley, the hydrostratigraphy was modeled in 3D using 2500 standardized, reclassified, and interpreted well borehole lithologs from both sides of the border. All known surface water features (including all rivers, streams, ditches, canals, lakes) in central Fraser Valley were included as boundary conditions. Spatially-distributed and temporally-varying recharge zonation was mapped for the surficial aquifer. Recharge variability takes into account soil distribution, vadose zone depth and hydraulic conductivity, the extent of impermeable areas, surficial geology, as well as strong precipitation gradients across the aquifer extent. Recharge is driven by physically-based daily weather inputs generated by a stochastic weather generator and calibrated to local observed climate. Nitrate modeling is being conducted using two separate codes. A vadose zone transport code is used to determine loading at the water table for the different soil columns used to estimate aquifer recharge. Fertilizer application rates are currently being considered for raspberries, but managed and unmanaged forage, and point sources from poultry barns may also be considered. For nitrate modeling, residual soil nitrate measurements are used to calibrate a vadose zone transport model. Nitrate concentrations from monitoring wells are used to calibrate the saturated flow and transport model.
Bibliography of Canadian Geomorphology