Nitrogen Loads to Estuaries from Waste Water Plumes: Modeling and Isotopic Approaches

Abstract

We developed, and applied in two sites, novel methods to measure ground water-borne nitrogen loads to receiving estuaries from plumes resulting from land disposal of waste water treatment plant (WWTP) effluent. In addition, we quantified nitrogen losses from WWTP effluent during transport through watersheds. WWTP load to receiving water was estimated as the difference between total measured ground water-transported nitrogen load and modeled load from major nitrogen sources other than the WWTP. To test estimated WWTP loads, we applied two additional methods. First, we quantified total annual waste water nitrogen load from watersheds based on nitrogen stable isotopic signatures of primary producers in receiving water. Second, we used published data on ground water nitrogen concentrations in an array of wells to estimate dimensions of the plume and quantify the annual mass of nitrogen transported within the plume. Loss of nitrogen during transport through the watershed was estimated as the difference between the annual mass of nitrogen applied to watersheds as treatment plant effluent and the estimated nitrogen load reaching receiving water. In one plume, we corroborated our estimated nitrogen loss in watersheds using data from multiple-level sampling wells to calculate the loss of nitrogen relative to a conservative tracer. The results suggest that nitrogen from the plumes is discharging to the estuaries but that substantial nitrogen loss occurs during transport through the watersheds. The measured vs. modeled and stable isotopic approaches, in comparison to the plume mapping approach, may more reliably quantify ground water-transported WWTP loads to estuaries.

Tidal and Groundwater Fluxes to a Shallow, Microtidal Estuary: Constraining Inputs Through Field Observations and Hydrodynamic Modeling

“…4). This is significantly lower that the expected value, based on the estimate by Kroeger et al (2006). They estimated that only 7 % of the total groundwater discharge occurs seaward of site IMHJ.…”

“…Measured tidal water fluxes and water level were imposed at the seaward boundary, with a reference oceanic salinity of 32 psu. Freshwater point sources representing groundwater were distributed between six locations following Kroeger et al (2006) (Fig. 7).…”

“…Several methods to measure fresh coastal groundwater discharge have been used with varying success dependent on the system and the assumptions. These include radiochemical tracer methods (e.g., Moore 1996;Cable et al 1996), seepage meters (Lee 1977), eddy correlation (Crusius et al 2008), thermal imaging (Portnoy et al 1998), and watershed mass balance of water (Kroeger et al 2006). In all these cases, estimating a whole-system fresh coastal groundwater discharge over sub-annual timescales may be confounded by temporal and spatial variability.…”

Wastewater Nitrogen Contributions to Coastal Plain Watersheds, NC, USA

“…Additionally, modeling studies have attempted to provide insights into watershed-scale nutrient inputs from septic systems (Geza and McCray 2010;Wang et al 2013). There are several published field-based studies that document the watershed-scale link between use of septic systems and nitrogen in surface water bodies (Bowen and Valiela 2001;Bowen et al 2007;Cole et al 2006;Kroeger et al 2006;Valiela et al 1997;Withers et al 2014). However, there are limited published studies that compare the nitrogen treatment efficiencies of septic systems and sewers at the watershed scale (Garcia et al 2013).…”

Public Review Draft: A Method for Assessing Carbon Stocks, Carbon Sequestration, and Greenhouse-Gas Fluxes in Ecosystems of the United States Under Present Conditions and Future Scenarios

“…Where available, results will be compared to estimates of fresh groundwater discharge at particular locations published in journal articles. Concentrations of nitrogen, carbon, and GHG in discharging groundwater will be estimated based on published and unpublished data, including Kroeger and others (1999), others (2004, 2009), Crusius and others (2005), Cole and others (2006), , Kroeger, Cole, York, and Valiela (2006), Kroeger, Swarzenski, Crusius, and others (2007), Swarzenski and others (2007), and Kroeger and Charette (2008), at selected east coast sites and with USGS monitoring data for several thousand wells available through the USGS's National Water Information System (NWIS). The NWIS data will be selected based on well depth and proximity to the coast, and their suitability as an indication of concentrations in discharging groundwater will be assessed based on comparisons to USGS data on concentrations measured at the coast.…”