Impact of Septic Systems on Nutrients in Groundwater and Drainage Canals in South Florida

Abstract

An approach is presented and demonstrated for assessing the impact of septic systems on nutrient concentrations in groundwater and nearby drainage canals. The study covers an area of 1,680 km2 in South Florida where there are more than 81,000 septic systems discharging to a relatively shallow water table. Local spikes occur in the concentrations of total Kjeldahl nitrogen, nitrite plus nitrate nitrogen (NOx−N), and orthophosphate at locations within 100 m of septic systems, with relatively uniform concentrations outside the 100-m zone. It is further shown that septic-system density cannot be used as the sole indicator of locally elevated nutrient concentrations, with the distance to the nearest septic system usually being a coindicator and a better solo indicator. Comparison of nutrient concentrations in the groundwater and drainage channels shows that groundwater has a substantial influence on the nutrient concentrations at most monitored canal locations, with some local anomalies influenced by local nutrient sources and sinks. Policy outcomes that can be derived from this study include required separations between septic systems and drainage channels and the extent to which nutrient concentrations in drainage canals are controlled by nutrient concentrations in groundwater relative to other sources.