How much nitrogen is coming into the Great Bay Estuary?
Total nitrogen loading from 2012 to 2016 was 903 tons per year, which is 26% percent lower than the 2009 to 2011 levels (1224 tons per year). Low rainfall and corresponding streamflow during this period as well as significant reductions in nitrogen loading at municipal wastewater treatment facilities are the primary reasons for this decrease. Since the human population and impervious cover continue to increase, nitrogen management remains a high priority.
The average annual load of total nitrogen into the Great Bay Estuary from 2012 to 2016 was 903.1 tons per year (Figure 3.1). In 2016, the total nitrogen load was 707.8 tons per year, the lowest since consistent monitoring of loads began in 2003. Before 2003, there were three studies that assessed nitrogen loading to the Great Bay Estuary; they relied on data collected between 1987 and 199621 and estimated nutrient loading at approximately 715 tons per year. These three studies all used different methods from each other and from the current approach, but yielded very similar results.
Figure 3.1 indicates that, since 2003, most of the variability relates to nitrogen from non-point sources. Non-point source nitrogen enters our estuaries in two major ways: 1) from stormwater runoff, which carries nitrogen from atmospheric deposition (including mobile transportation sources–cars, trucks, trains; and stationary stack emissions–smoke stacks), fertilizers, and animal waste to the estuaries; and 2) from groundwater contribution, which carries nitrogen from septic systems, sewer leakage and infiltrated stormwater runoff into streams, rivers and the estuary itself.22, 23 These non-point sources (NPS), accounted for 606.6 tons per year or 67% of the nitrogen load for 2012-2016 (Figure 3.2). It is important to understand that NPS loads are much more difficult to manage than point source loads because they come from a variety of sources, many of which are controlled by private land owners.
In addition, there are 17 municipal wastewater treatment facilities (WWTFs) that discharge treated wastewater into the bay or into rivers that flow into the bay. Point sources of nitrogen from these WWTFs account for 296.4 tons per year or 33% of the total nitrogen load for 2012-2016 (Figure 3.2). Of the 903.1 tons of total nitrogen entering the bay annually from 2012-2016, 506.0 tons were dissolved inorganic nitrogen (DIN), which is the most biologically available form of nitrogen. The DIN load was approximately evenly split between point and non-point sources (Figure 3.3). However, during the summer months when plant and algae growth is highest, point sources from WWTFs dominate DIN loading.24, 25
The highest loads since 2003 were seen in the 2005 to 2007 period (1,662.4 tons per year), a time that coincides with the highest total annual precipitation values (Figure 3.1). In comparison, the 2012 to 2016 period exhibited lower rainfall (Figure 3.3), a contributing factor to the 27% decrease in NPS loading since the 2009-2011 period. This underscores the association between nitrogen loading and run-off. Precipitation records26 and forecasts27 suggest that our region will continue to see periods of extreme highs and lows, which will continue to impact non-point source load.
The nitrogen load from WWTFs for 2012-2016 was 296.4 tons, a decrease of 24% since the 2009-2011 period. In 2015 and 2016, the nitrogen load from WWTFs was 264.3 and 256.2 tons per year, respectively (Figure 3.1). Municipalities have made recent, substantial improvements to their WWTFs to reduce the amount of total nitrogen they discharge. Rochester, Dover, and Newmarket have recently completed major upgrades, Durham has reconfigured its facility and Portsmouth, Newington, and Exeter are in the process of upgrading their treatment plants. Each of these upgrades should result in less nutrients in wastewater effluent.
See the Estuarine Health: Stress & Resilience section for more on how nitrogen loading relates to other indicators, such as phytoplankton, seaweed and eelgrass.