EVALUATION OF THE ORIGIN AND FATE OF NITRATE IN THE ABBOTSFORD AQUIFER USING THE ISOTOPES OF N-15 AND O-18 IN NO3-

Extensive NO3- contamination of groundwater in the Abbotsford aquifer to levels above drinking water limits is a major problem in the Eraser Lowlands of southwestern British Columbia, Canada. Nitrate concentrations in the aquifer ranged from 0 to 151 mg/l NO3-, with a median concentration of 46 mg/l NO3-. Of 117 wells sampled, 54% had NO3- concentrations exceeding the drinking water limit of 45 mg/l. Approximately 80% of the study area had groundwater NO3- concentrations exceeding 40 mg/l NO3-. Potential NO3- source materials were poultry manure N and synthetic NH4 based fertilizers. The delta(15)N of solid poultry manure samples ranged between +7.9 and +8.6 parts per thousand (AIR). Four brands of synthetic fertilizers commonly used had delta(15)N values between -1.5 and -0.6 parts per thousand. Ammonia volatilization caused the delta(15)N of groundwater NO3- produced from poultry manure N to range between +8 and +16 parts per thousand. The delta(18)O values of groundwater NO3-, by contrast, mostly ranged between +2 and +5 parts per thousand (SMOW). This narrow range of delta(18)O values fell within the expected range of NO3- produced by nitrification of reduced N forms such as poultry manure N and NH4 fertilizers, and had a similar range of delta(18)O values as NO3- in the upper part of the unsaturated zone below raspberry fields and beneath former manure piles. The delta(15)N - NO3- and delta(18)O-NO3- data confirmed that NO3- in the aquifer was predominantly derived from poultry manure and to a lesser extent from synthetic fertilizers. The delta(18)O-NO3- data further suggested the nitrification process occurred mainly in the summer months, with the soil NO3- produced subsequently flushed into the aquifer during fall recharge. The delta(15)N-NO3- and delta(18)O-NO3- data conclusively indicated that no significant bacterial denitrification is taking place in the Abbotsford aquifer.