Wastewater Flow and Pathogen Transport from At-Grade Line Sources to Shallow Groundwater

On-site wastewater treatment systems are commonly used in sparsely populated areas where capital-intensive centralized wastewater treatment facilities are not feasible. The primary objective of this work was to investigate vadose zone and groundwater transport of a bromide (Br) tracer and naturally occurring applied to the soil surface in secondarily treated wastewater at a public rest stop in central Alberta, Canada, with seasonally fluctuating water table (between 0.2 and 1.5 m) over a 1-yr period. A transect within the wastewater application field was instrumented with 10 nests of three monitoring wells ( = 30). We found that travel times for Br and were most likely related to vadose zone thickness under wastewater application lines, with Br and initially detected in monitoring wells within 4 d at locations where the vadose zone was 0.2 to 0.4 m thick. When the vadose zone thickness increased to ≥0.9 m, however, levels in the monitoring wells decreased dramatically despite continued high surface application of . The observed travel times were consistent with those calculated assuming piston flow. Therefore, the risk of groundwater contamination from wastewater at this site is greatest during times when high wastewater applications (high facility use) and shallow water table conditions coincide. We recommend that detailed knowledge of vadose zone and groundwater hydrology be used to guide the design of on-site wastewater treatment systems and also to assess the probability of human exposure to and other pathogens that are transported to groundwater.