Nitrogen Loading from Coastal Watersheds to Receiving Estuaries: New Method and Application

Abstract

Abstract. In this paper we develop a model to estimate nitrogen loading to watersheds and receiving waters, and then apply the model to gain insight about sources, losses, and transport of nitrogen in groundwater moving through a coastal watershed. The model is developed from data of the Waquoit Bay Land Margin Ecosystems Research project (WBLMER), and from syntheses of published information.The WBLMER nitrogen loading model first estimates inputs by atmospheric deposition, fertilizer use, and wastewater to surfaces of the major types of land use (natural vegetation, turf, agricultural land, residential areas, and impervious surfaces) within the landscape. Then, the model estimates losses of nitrogen in the various compartments of the watershed ecosystem. For atmospheric and fertilizer nitrogen, the model allows losses in vegetation and soils, in the vadose zone, and in the aquifer. For wastewater nitrogen, the model allows losses in septic systems and effluent plumes, and it adds further losses that occur during diffuse transport within aquifers. The calculation of losses is done separately for each major type of land cover, because the processes and loss rates involved differ for different tesserae of the land cover mosaic. If groundwater flows into a freshwater body, the model adds a loss of nitrogen for traversing the freshwater body and then subjects the surviving nitrogen to losses in the aquifer. The WBLMER model is developed for Waquoit Bay, but with inputs for local conditions it is applicable to other rural to suburban watersheds underlain by unconsolidated sandy sediments.Model calculations suggest that the atmosphere contributes 56%, fertilizer 14%, and wastewater 27% of the nitrogen delivered to the surface of the watershed of Waquoit Bay. Losses within the watershed amount to 89% of atmospheric nitrogen, 79% of fertilizer nitrogen, and 65% of wastewater nitrogen. The net result of inputs to the watershed surface and losses within the watershed is that wastewater becomes the largest source (48%) of nitrogen loads to receiving estuaries, followed by atmospheric deposition (30%) and fertilizer use (15%).The nitrogen load to estuaries of Waquoit Bay is transported primarily through land parcels covered by residential areas (39%, mainly via wastewater), natural vegetation (21%, by atmospheric deposition), and turf (16%, by atmospheric deposition and fertilizers). Other land covers were involved in lesser throughputs of nitrogen.The model results have implications for management of coastal landscapes and water quality. Most attention should be given to wastewater disposal within the watershed, particularly within 200 m of the shore. Rules regarding setbacks of septic system location relative to shore and nitrogen retention ability of septic systems, will be useful in control of wastewater nitrogen loading. Installation of multiple conventional leaching fields or septic systems in high-flow parcels could be one way to increase nitrogen retention. Control of fertilizer use can help to a modest degree, part...

Meteorological Influences on Nitrogen Dynamics of a Coastal Onsite Wastewater Treatment System

“…The general trends were consistent with those published by Valiela et al 1997 (Figure 5). Generally, TDN concentrations in the drainfield and downgradient groundwater were higher during the second year of study when drought conditions were prevalent.…”

“…Measured TDN concentrations versus distance from drainfield for Year 1 (red circles) and Year 2 (blue circles) compared to a range of values documented in an earlier study by Valiela et al (1997).…”

“…Major N inputs are from point and non-point sources, typically agricultural fertilizer, animal manure, atmospheric deposition, and effluents from wastewater treatment systems (Howarth et al 2002). Although nutrient management efforts have focused primarily on agricultural inputs (Heathwaite et al 2000), in many coastal areas research findings have shown that wastewater may be the dominant N source that can affect groundwater and surface water quality (Valiela et al 1997; Kroeger et al 2006a). Estimates of N inputs to coastal watersheds in the eastern U.S. suggest that residential on-site wastewater treatment systems (OWTS, also known as septic systems) may contribute from 4–64 % of the N loading (McClelland and Valiela, 1998; Pradhan et al 2007).…”

“…In a review of eight published studies, Valiela et al (1997) reported that OWTS can retain from 10–90% of N. In situations when N-retention is low, exports from OWTS can affect human and ecological health, locally by affecting drinking water supplies (Harman et al 1996) and regionally by contributing to nutrient enrichment of surface waters (Valiela et al 1997). In coastal settings where sandy soils predominate and water tables are shallow (Fan et al 2013), conventional OWTS are likely to contribute to elevated nitrate (NO 3 − ) concentrations to the shallow groundwater and surface water (Gold and Sims 2001).…”

“…Studies conducted on systems located in sandy soils show that OWTS NO 3 − may cause elevated NO 3 − concentrations in groundwater (Robertson et al, 1991; Humphrey et al 2010a). The extent of NO 3 − contamination is variable and is a function of a number of controls including: water table depth, soil texture, OWTS density, distance from the drainfield, groundwater flow velocity, dilution/dispersion, biomat formation, climate and seasonal factors, presence of riparian buffers, age and design of the system, water use, household practices, and system maintenance (Beal et al 2005, Gold and Sims 2001, Valiela et al 1997, Oakley et al 2010). …”