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Numerical modelling and isotopes underline climate impacts on groundwater nitrate in temperate agricultural settings

Congress: 2008
Author(s): Martine M. Savard, Harold Vigneault, Daniel Paradis, René Lefebvre, Anna Smirnoff, George Somers

Article: Poster:
AbstractGroundwater is the only source of drinking water on Prince Edward Island (PEI), Canada, but currently, nearly 5% of domestic wells in the island have nitrate concentrations exceeding the drinking water guideline of 10mg/L N-NO3-. The objectives of this study are to evaluate: (1) how climate conditions and potential changes in agricultural practices could impact groundwater nitrate concentrations in PEI; and (2) the source, timing and proportions of nitrate transfer from agricultural soils to aquifers on a seasonal basis. A three- dimensional numerical model of groundwater flow and nitrate transport (FEFLOW) was build to reproduce hydraulic conditions and nitrate concentrations over the island (5684 km2) for present-day and for climate change (CC) scenarios selected following the Canadian Circulatory General Model (CCGM); CGCM2 A1 reflects the driest conditions and CGCM2 B1, the wettest ones. Groundwater recharge is estimated with a one-dimensional flow model (HELP) coupled with GIS capabilities to represent the daily amount of water reaching the aquifer. Nitrogen available to leaching is estimated on a watershed scale with the agricultural indicator Residual Soil Nitrate. Water samples obtained for d15N and d18O analyses were collected from private wells distributed throughout a representative agricultural watershed (Wilmot) once every season during the 2003-2005 period. Nitrate was concentrated and extracted using an ion-exchange resin protocol; analyses of the d15N and d18O values were performed with on-line combustion and pyrolysis-IRMS systems, respectively. When simulating maintenance of present-day nitrate loading (2001) on PEI, the average groundwater concentrations in nitrate predicted for 2050 increase by about 11%. Climate change would add between 0 and 6% to this average (0% for CGCM2 B1, 4% for HadCMB2a and 6% for CGCM2 A1 and HadCMA2a). The foreseen increases in concentrations imply that, in 2050, proportions of domestic wells higher than the current one would exceed the drinking water criterion. Nitrate isotope results in the Wilmot watershed indicate that nitrification occurs all year long, leaching to aquifers takes place whenever recharge is occurring and winter nitrate production is important partly due to an insulating snow cover. The main sources of nitrate are chemical fertilizers during summer (~64% of total load) and residual crop material during winter (~62% of total load). Our study shows that CC is expected to modify the N cycle and to exacerbate the nitrate problem of PEI. Adaptation of agricultural practices to prolonged growth season has the potential to highly influence the gate of nitrate in groundwater, and increased frequency of winter thaws could enhance the winter transfer rate of crop-derived nitrate to groundwater. Major changes in agricultural practices are required shortly and should be maintained in the future to see improvements in water quality. Truly effective strategies aimed at a reduction of N leaching will need to focus not solely on optimizing fertilizer application rates, but also on carefully considering the management of residual crop material. This is likely to pose special challenges for annual production as it will be necessary to balance the benefits of organic matter in soils against its contribution to winter N-leaching to aquifers.
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