Climate Controls On Groundwater Recharge and Nitrate Loading: Past and Future Perspectives.

Climate change and climate variability have the potential to alter groundwater recharge and nitrate loading in agricultural areas. Monthly observations of elevated nitrate concentrations in the Abbotsford-Sumas aquifer, Lower Fraser Valley, British Columbia, have been recorded over an 18-year period using a network of dedicated monitoring wells operated by Environment Canada. This high-frequency and long-term data record provides information on the spatial and temporal distribution of non-point source, agriculture-derived nitrate contamination. Over the period 1992 to 2009, average groundwater nitrate concentrations have remained fairly stable (~12-16 mg/L NO₃-N). However, non-uniform cyclical variability in nitrate concentrations is observed in most wells over the period of record. Frequency analysis suggests that El-Niño southern oscillation (ENSO) climate variability likely influences water table height and groundwater-nitrate concentrations (+/-30%) over 4-6 year periods. Shallower sampling depths tend to show a more direct link between precipitation, nitrate concentration and water table elevation. Nitrate loading simulations conducted to estimate the timing and concentration of nitrate arriving at the water table (from mid-April fertilizer application) indicate arrival of the peak concentration ~7 months after application (late October), with late-spring and early summer precipitation initiating leaching. Downscaled climate data from four global climate models (GCMs) suggest greater spring precipitation, and less summer precipitation under future climate periods. However, there is significant variability among the GCM projections and resulting simulated groundwater recharge. Continued research is needed to address changes in nitrate loading under scenarios of climate change given the expected alteration of precipitation patterns and, as a result of increased temperature, lengthened growing seasons and increased rates of evapotranspiration, which will place higher demand on water for irrigation.