Denitrification Along Pangborn Creek in the Abbotsford-Sumas Aquifer, Washington

The Abbotsford-Sumas aquifer is a shallow, unconfined aquifer located in the agricultural regions of southwestern British Columbia and northwestern Washington and has a history of nitrate contamination. I monitored nitrate distributions in a study area bisected by a wide- scale peat deposit within a portion of the Whatcom County component of the aquifer to assess the current nitrate distribution, evaluate ground and surface water interactions in the peat, and determine the affect of peat on denitrification. The water quality dataset and statistical analyses showed that nitrate contamination was heavily concentrated upgradient of the peatlands. In general, shallow wells (table) north of the peatlands had higher nitrate concentrations than deeper wells (\u3e10 m below the median water table). Some upgradient wells showed low nitrate concentrations and data suggest they received denitrified ground water from unmapped peat deposits. Nitrogen isotope data (δ15N on nitrate) indicated that nitrate sources included manure and inorganic commercial fertilizers. The contamination south of the peatlands was significantly lower than the contamination to the north and the median nitrate levels within the peatlands were near the detection limit. Nitrogen gas measurements and a combination of nitrogen (δ15N on nitrate) and oxygen (δ180 on water) isotopes confirmed that denitrification occurred in the peatlands and in much of the ground water south of the peatlands. In addition, the gas measurements suggested that iron, manganese, sulfate, and methane occurred throughout the peatlands and may have contributed to redox reductions. The implication of these findings is that a natural means for nitrate reduction exists in this region. Hydrostratigraphic data suggest that peat deposits occur throughout Whatcom County at various unmapped depths. Identifying these peat deposits and quantifying the upgradient ground water nitrate contributions may help facilitate nutrient management in the region