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

The contribution of septic systems to watershed nutrient loads is poorly quantified although they are often cited as potentially important nutrient sources. The study used a geospatial model to estimate P 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 developed to automatically locate individual septic systems and to use these locations to estimate P load contributions to tributaries. The model was first tested on three subwatersheds in the Canadian Lake Erie Basin before being applied across the Basin. Present-day basin-wide P load estimates reveal that: (i) only a fraction of septic effluent is currently reaching the tributaries due to slow transport and other delays, as well as (ii) P attenuation in the subsurface, range from 23 ± 11 to 68 ± 32 MT/yr. Based on these estimates, septic systems may currently contribute 1.7 ± 0.8–5 ± 2.3% of the P loads to Lake Erie from Canada. However, maximum P load estimates and transient model results show that the contribution of septic systems to P loads will increase over time as slow moving septic-derived groundwater P plumes reach tributaries 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.