Long-term phosphorus loading from onsite wastewater systems to surface waters

Abstract

Accelerated eutrophication caused by oversupply of nutrients from anthropogenic sources has impaired surface waters, especially lakes, in many places in the United States and worldwide. Nitrogen and phosphorus oversupply to surface waters has frequently caused overgrowth of aquatic plants and blooms of phytoplankton (algae) that damage fisheries, recreation, and property values. In many surface waters, phosphorus is the "limiting" nutrient, without which this overgrowth or bloom does not occur. Efforts to prevent, remediate, and mitigate the effects of phosphorus oversupply generally focus on surface sources and transport pathways of phosphorus, but review of the research literature suggests that phosphorus transport from domestic onsite wastewater systems (OWSs) via groundwater has not been considered a significant source because of sorption, and this source is not effectively addressed in lake management. We hypothesize that, with increasing time-in-use of an OWS, phosphorus may be transported via the groundwater surficial aquifer to a down-gradient surface water in ecologically relevant amounts. Here we develop a model of this transport and quantify the total phosphorus load transported in a time-variable manner from all the OWSs in a watershed via the groundwater to surface waters, especially lakes. The results suggest that the phosphorus load from OWSs may be significant and should be considered in efforts to manage the effects of lake eutrophication. PREFACE My research was motivated by some experience as a Conservation Commissioner in the town of Hanson, Massachusetts, USA. One of the principal duties of this municipal government position is to protect the surface water resources of the town from loss or deterioration due to human land-use and development. The town is home to a number of lakes, both natural and impounded, and shares several lakes with other towns. In the course of my tenure as a commissioner, I participated in an ad hoc committee to address the problem of eutrophication of Oldham Pond, a lake shared by Hanson and the neighboring town of Pembroke. The lake had been the subject of studies in the past to determine the sources and to estimate the loading of nutrients, especially phosphorus (P), which had caused annual algae blooms for several years. Although these studies did not explicitly identify onsite wastewater systems (OWSs) as a major source of P, OWSs were acknowledged as a potential source, if close to the lake and especially if in a "failed" state. Management efforts to remediate the condition of the lake by reducing the P load had been undertaken by Pembroke, aimed at stormwater runoff , and "failed" OWSs. These efforts had not been too successful, as annual algae blooms continued. The ad hoc committee was established to obtain participation in this effort by v Hanson. Since I had prior experience as an OWS installer and inspector, I surmised that P loading from OWSs may not only be attributed to "failed" systems, but may come from working systems. I found that this view is not pop...