# Quantifying Total Phosphorus and Heavy Metals in Residential Septage
## 3 Results and Discussion
### Land Application of Septage and Potential Environmental Impacts  
Land application of domestic septage and sewage sludge is a common practice in the United States. One of the primary benefits of land application is to recycle nutrients and organic matter in the receiving soils [3]. Land application sites are determined by topography, soil characteristics, depth to groundwater, proximity to surface waters, floodplains, wetlands, and other environmentally sensitive areas. While land application of septage can replenish the nutrients in soils, there remain concerns about potential environmental impacts. Septage can contain elevated concentrations of phosphorus, heavy metals (Table 3), nitrogen, and pathogens [15, 16, 17, 18, 19, 20]. Repeated application of septage may result in the accumulation of TP and heavy metals, which increases potential for leaching into the groundwater [18, 21, 22, 23]. Groundwater can serve as a conduit for pollutants to enter surface waters, especially in locations with minimal riparian buffers [88]. Furthermore, land application should not occur during periods of inclement weather or when the ground is frozen or snow-covered because of the increased runoff potential and difficulty in using sludge application equipment [89]. Despite these environmental concerns, land application can provide watershed scale benefits. Septage is commonly treated and applied on land [3]. If land application sites are outside of the watershed of origin, then pumping out tanks acts as a watershed benefit by exporting TP and heavy metals, especially if the original watershed is vulnerable to nutrients, heavy metals, or other pollutants. Nutrients and some heavy metals are essential to biological functioning, but excessive concentrations of these compounds can destabilize aquatic habitats. Lick Creek is classified as a nutrient-sensitive water [90], thus additions of nitrogen and/or phosphorus can degrade the aquatic habitat by facilitating eutrophication, harmful algal blooms, and/or fish kills [91, 92]. Septage may also contain elevated concentrations of heavy metals, many of which (e.g., cadmium, chromium, cobalt, copper, mercury, lead, nickel, and zinc) are highly toxic both in their elemental and soluble salt forms [93]. An abundance of heavy metals in the biosphere endangers ecological functioning of terrestrial and aquatic habitats [94]. In the current study, removal of TP and heavy metals provided environmental benefits for Lick Creek. However, the end-of-lifemanagement of residential septage requires careful planning, test these benefits result in translocation of environmental problems to other watersheds. Land application is a common practice to supplement the use of commercial fertilizers and add value to a byproduct of wastewater treatment processes. Application rates are primarily designed around the crop's nutrient needs compared to the fertility of the arable soil [89]. In the US, most states use soil phosphorus indices to identify fields at risk for phosphorus losses [95]. Soils that exceed the critical level are rated as \"high\" or \"very high\" indicating that these soils will not likely respond to additions of fertilizer [96]. Despite the adoption of soil phosphorus indices, many southern states were unable to limit phosphorus application despite soils being classified as \"very high\", thereby increasing the potential for phosphorus runoff [97]. In North Carolina, a significant relationship was found between higher soil phosphorus index ratings and increased masses of TP and dissolved phosphorus in agricultural runoff [98]. These studies suggest that the application of septage (which may contain up to 760 mg L\({}^{-1}\) of TP (Table 3)) to soils with critical soil phosphorus indices increases the potential for transport of TP to downstream water resources via agriculture runoff. This is a particular concern for land application sites located within nutrient-sensitive waters, since eutrophication can result in environmental [91,92] and economic [99] damages.