Estimation of phosphorus loads from septic systems to tributaries in the Canadian Lake Erie Basin

摘要

The contribution of septic systems to watershed nutrient loads is poorly quantified although septic systems are often cited as potentially important nutrient sources. The study uses a geospatial model to estimate phosphorus loads from septic systems to the tributaries of the Canadian Lake Erie Basin to inform Lake Erie nutrient management initiatives. There is currently no inventory of septic systems in the Lake Erie Basin (e.g., numbers and locations of septic systems). Therefore a geospatial model was first developed to automatically locate individual septic systems with these locations then used to estimate phosphorus load contributions to tributaries. The model was tested on three subwatersheds in the Canadian Lake Erie Basin before being applied across the Basin. Present-day basin-wide phosphorus load estimates that consider: (i) only a fraction of septic effluent is currently reaching the tributaries due to slow transport and other delays, and (ii) phosphorus attenuation in the subsurface, range from 23 +/- 11 to 68 +/- 32 MT/yr. Based on these estimates, septic systems may currently contribute between 1.7 +/- 0.8 to 5 +/- 2.3% of the phosphorus loads to Lake Erie from Canada. However, maximum phosphorus load estimates and transient model results show that the contribution of septic systems to phosphorus loads will increase over time as slow moving septic-derived groundwater phosphorus plumes reach tributaries and if aging septic systems are not maintained. This study provides widely applicable new knowledge and methodology; as well as specific findings needed to inform nutrient and septic system management in the Lake Erie Basin. (C) 2020 The Authors. Published by Elsevier B.V. on behalf of International Association for Great Lakes Research.