QUANTIFYING THE IMPACT OF AN INDIVIDUAL AGRICULTURAL FIELD ON GROUNDWATER NITRATE

Although agriculture has long been recognized as a key contributor to groundwater nitrate, successful mitigation has been limited to only a few regions. It is difficult to relate groundwater nitrate loading to land use or soil and crop management of specific agricultural fields due to the multiple sources of agronomic N inputs, and seasonal variation and non-point nature of agricultural nitrate leaching. It is consequently challenging to identify the management practices that will most effectively mitigate nitrate leaching. We monitored the nitrate fluxes leaving the root zone and arriving at the water table under a commercial raspberry field, in Abbotsford, BC, to evaluate their timing and magnitude over a two-year period. The perennial crop was replanted in spring of 2010, which included an unknown amount of poultry manure in addition to an annual fertilizer application (80-100 kg N ha^-1). Root zone leaching was quantified by bi-weekly passive wick sampling. Groundwater nitrate loading was quantified using monthly high-resolution shallow groundwater sampling directly down-gradient of the agricultural field. Large temporal variations in nitrate flux were observed in both the root zone and shallow groundwater. There were 3-6 month time lags, observed as substantial, short term root zone leaching events (10-60 kg N ha^‑1), occurring in spring and fall, which were subsequently observed in the shallow groundwater as smoothed, seasonal trends with maximum monthly fluxes (year 1: 37; year 2: 9 kg ha^‑1) occurring in March. While estimated annual root zone losses compared favourably with estimated groundwater loading (i.e. 170 vs. 180 kg N ha^‑1 for year 1, and 69 vs. 63 kg N ha^-1 for year 2), there was an improvement in N losses as the effect of the manure application declined. A mass weighted average of monthly δ¹⁵N_NO3 values (8.5 ‰) suggested organic N was the primary source of nitrate loading, but seasonal variation indicated that seasonal variations in nitrate sources and/or transformation processes also occurred. This integrated approach is suited to understanding the timing, quantity and isotopic values of nitrate contributed to groundwater by an individual field, and monitoring the progress of soil and crop management of that field. Such insights are critical to identifying effective mitigation strategies.