TEMPORAL AND SPATIAL MONITORING OF PATHOGEN INDICATORS AND NITRATE IN A VULNERABLE AGRICULTURAL AQUIFER

Elevated nitrate levels in the Abbotsford-Sumas aquifer (ASA) have been attributed to long-term usage of poultry manure from local agricultural practices. However, the loading of harmful bacteria from these same sources to the shallow, vulnerable ASA has not been well characterized. The objective of this field study was to examine temporal and spatial patterns of bacterial loading to the ASA, targeting the rainy fall and winter months when leaching through the soil profile is most likely. Twenty-six monitoring wells were selected for sampling of fecal indicators (total coliforms [TC] and E. coli) and chemical parameters on a biweekly basis. A vadose zone test site was established with pan lysimeters at four 1-by-1 m plots to monitor soil leachate for breakthrough of bacteria and applied tracers (bromide and microspheres). Groundwater monitoring results indicated sporadic bacterial detections in the aquifer in both time and space. E. coli was absent from most monitoring wells except for one-time detections in wells adjacent to a nearby creek, suggesting rainfall event driven groundwater-surface water connections. TC concentrations showed much less spatial variability than nitrate over the entire aquifer, but increased temporal variability. Over a six-month period at the vadose zone site, a 4-log reduction in E. coli was observed in the top 30 cm of soil, and up to a 3-log reduction in microsphere concentrations in the top 50 cm. Although there was significant bacterial retention with depth in the soil, microsphere tracers revealed that bacteria could be transported to depth if environmental conditions were conducive to bacterial survival. The differences in both spatial distribution and timing of bacterial indicators relative to nitrate demonstrated that microbes were subject to drastically different transport and attenuation processes than nutrients from the same source. These findings imply that nutrient and pathogen specific monitoring approaches must be deployed to properly assess the transport and fate of agricultural contaminants in the environment.