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.

When onsite wastewater treatment and disposal systems (OSTDS) are not sited appropriately or installed properly, wastewater constituents can be a source of adverse environmental impacts to soil and groundwater, which can lead to potential public health risks. A paired monitoring design developed to compare water quality in sewered and non‐sewered areas is presented here. It is suggested as a possible monitoring scheme for assessing the impact of sewer installation projects. As such, two sets of single‐family, rural residential Florida neighborhoods were evaluated over a two‐year period to gain insight into the effects of small‐community use of OSTDS on coastal water quality. One set of two neighborhoods were connected to the sanitary sewer network and the other set of two were served exclusively by OSTDS. Water quality sampling was conducted at the paired sites during seasonal high water table (SHWT) and seasonal low water table (SLWT) events. Measured surface water quality during the SHWT showed indications of environmental impacts from OSTDS in terms of nutrients, microbial pathogen indicators, and other water quality measures, such as turbidity and conductivity. However, during the SLWT events, no obvious impacts attributable to OSTDS were detected. The water quality results indicate that OSTDS impacts may be measureable in rural areas. Other factors, such as microbial indicator survival and regrowth potential, may confound the understanding of water quality impacts of sewer projects. For example, the microbial indicators Escherichia coli and enterococci were found to persist over time and therefore did not always represent true comparisons of OSTDS and sewered areas between seasons. The timeframe for evaluating the effects of sewer projects may be longer than anticipated because of this survival and regrowth phenomenon.

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