Evaluation of water quality impacts of on-site treatment and disposal systems on urban coastal waters

Questions have been raised about the impact of onsite sewage treatment and disposal systems (OSTDS) on adjacent water bodies, particularly in coastal areas. If OSTDS are not properly sited and maintained, they pose a potential risk to public health and may contribute toward degradation of the receiving water body. To quantify the pollutant loading contributions from OSTDS located adjacent to coastal canals, two similar single-family residential neighborhoods were evaluated. One was connected to the sanitary sewer network (Hollywood, FL) and the other was served exclusively by septic tanks (Dania Beach, FL). Water quality sampling, focusing on nutrient and pathogen indicators, was conducted at the paired sites during the seasonal high water table (SHWT) and seasonal low water table (SLWT) events. During the SHWT, measured canal water quality, in terms of nutrient (i.e. nitrate) and microbial pathogen indicators in the adjacent areas appeared to be impacted by OSTDS. However, during the SLWT, no obvious impacts attributable to the septic tanks were detected. This suggests that the OSTDS may operate properly when the water table is low, but that the contributions by OSTDS to coastal pollutant loading may be quantifiable during certain portions of the calendar year when the ground water table elevation is relatively high, although the impact on the downstream marine environment is uncertain as a result of tidal dilution and lack of offshore study.