An Assessment of Nutrient Improvement in Surface Water Due to the Conversion of Onsite Sewage Treatment and Disposal Systems to Sewerage

Florida represents approximately 12% of all onsite sewage treatment and disposal systems (OSTDS) in the United States. Properly designed, constructed and maintained OSTDS are effective eco- friendly alternatives for the treatment of domestic wastewater from households not serviced by public sewer systems. However, conventional OSTDS are not designed to effectively remove nitrogen. If installed and improperly used, they may contribute other pollutants. OSTDS have also been linked to degraded water quality in some regions. However, it has been difficult to quantify OSTDS impacts on water quality because of various confounding factors such as point and non-point sources of nutrient input. As a result, numerous counties have initiated conversion projects, to transition from OSTDS to sanitary sewers with the intent to improve water quality conditions in surface and coastal waters.Red Bug Slough in Sarasota Florida was selected based on three criteria: located in a high density OSTDS area, a small catchment area and waterbody, and 15+ year history of water quality monitoring samples. Based on a timeline of abandonment permits, 528 OSTDS conversions have been made in the Red Bug Slough subbasin since August of 2010. Monthly grab samples were categorized into pre-and post- eras to investigate water quality improvements as a result of OSTDS conversions to sewerage. Red Bug Slough was examined for potential confounding factors and this study site showed no evidence of impact from changes in fertilizer practices, land use distribution and land use changes. Correlation tests compared upstream concentrations in Mirror Lake and downstream concentrations in Red Bug Slough to determine whether bird influence as a potential confounding source. Time-series analyses of total nitrogen, total phosphorus, chlorophyll-a and turbidity revealed temporal fluctuations, possibly related to Florida’s wet and dry seasons and subsequent hydraulic variations. The post- conversion trendline slope translated to the rate of change in concentration and estimated the amount of mass loading not exported to the slough. Also, water quality parameter deficits were determined for the post-conversion period and regressed on the cumulative number of OSTDS conversions, as estimated impact dates were reached. Time-series analysis indicated an overall improvement in total nitrogen, total phosphorus and turbidity since OSTDS conversions began. Regression statistics predicted that only 18% of TN deficit and 7% of TP deficit was due to a conversion. There was an overall significant reduction in ammonia and nitrite-plus-nitrate as annual means were about 50% less during the post-conversion years and these reductions were statistically significant. Significant reductions in ammonia and nitrite-plus-nitrate concentrations are consistent with the conversion of OSTDS to sewer in Red Bug Slough. However, more data are needed to confirm the causal relationship. As organic nitrogen is generally not considered the nitrogen species contributed by OSTDS, the dominance of organic nitrogen in Red Bug Slough at least suggests the water quality impact from OSTDS at this site may be limited. Based on the post-conversion period and water yield of Red Bug Slough in Sarasota Florida, the time-series analysis estimated a mass loading reduction of 58.2 kg-TN/year or 0.1 kg-TN/year per conversion. There was also a 23.1 kg-TP/year and 0.04 kg-TP/year per conversion to the slough. Based on the initial loading of 7.34x104 kg-TN/year and 2.8x104 kg-TP/year from all OSTDS discharging to RBS, there was a 0.08% TN and 0.2% TP reduction in TN from 528 conversions. However, these improvements were of very small magnitudes and an even smaller fraction of these nutrients may reach Red Bug Slough based on OSTDS proximity to the slough, OSTDS site lot sizes, soil type, groundwater table depths and environmental attenuation and assimilation processes