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Daily-Resolution 2001-2017 Time Series of Total Nitrogen Load to Narragansett Bay from Bay-Wide Treatment Facility and Watershed Sources

posted on 07.07.2020 by Narragansett Bay Estuary Program, Dan Codiga

Time series for the load of total nitrogen (TN) to Narragansett Bay, from 18 wastewater treatment facility (WWTF) and land-based runoff sources bay-wide, have been estimated at daily resolution for 2001-2017. The motivation for daily resolution is to enable including nitrogen load as a candidate influence (with others such as river flow, temperature, tidal conditions, etc; e.g., Codiga et al 2009) in statistical analyses investigating drivers of hypoxic events at the short timescales of days to weeks on which events are known to vary. Time series were estimated back to 2001 because such analyses rely on Narragansett Bay Fixed Site Monitoring Network time series oxygen observations, which began that year. The WWTF and riverine sources treated are generally the same as those in the annual-budget analyses in the State of Narragansett Bay and Its Watershed (NBEP 2017). The 18 sources include 11 WWTFs (nine in Rhode Island and two in Massachusetts) discharging directly to the bay; 6 rivers where they enter the bay, which include load from WWTFs located upstream on them; and runoff direct to the bay from ungauged riparian areas. The observations were obtained from the Rhode Island Department of Environmental Management, the Narragansett Bay Commission, the Fall River treatment facility, and the US Geological Survey. Load was computed as the product of concentration and flow. For concentration, linear interpolation was necessary as the nominal frequency of observations ranged from weekly or biweekly, for most of the largest sources, to monthly. However, temporal variations in load are dominated by flow variability, which spans multiple orders of magnitude while concentration variations are less pronounced, and daily flow measurements were available for many of the largest sources. During periods when TN was not directly measured, it was computed as the sum of other measured concentrations (e.g., total Kjeldahl nitrogen plus nitrate plus nitrite), or using a correlation between TN and other constituents during periods when both were measured, or as the longterm mean seasonal cycle of TN computed from sampled years. As a check on the reasonableness of the results, the annual-mean loads from the new time series, computed by averaging the daily values, were compared to results of earlier budgets reported in NBEP (2017) for three periods: 2000-2004 (Nixon et al 2008), 2007-2010 (Krumholz 2012), and 2013-2015 (NBEP 2017). The differences are notable, as expected given the divergent methods, but the agreement is acceptable for the intended use of the new daily time series; for ascertaining longterm changes in loading, the NBEP results are more appropriate. It is recognized that the estimated daily TN loads are approximate, particularly for the several earliest years when fewer concentration measurements were made. A number of suggestions for how to improve the methods are given. The new time series were used to investigate hypoxia in a companion report (Codiga 2020) and are expected to be of use for various other studies.

NOTE: The data for the companion report are available for download in spreadsheet (.xlsx) and Matlab (.mat) file formats, with documented supporting code, at https://figshare.com/s/7d51f2540df6638a4552.

The views expressed in this report are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency or the New England Interstate Water Pollution Control Commission. Any mention of trade names, products, or services does not imply an endorsement by the U.S. Government, the U.S. Environmental Protection Agency, or the New England Interstate Water Pollution Control Commission. The EPA and NEIWPCC do not endorse any commercial products, services, or enterprises.


USEPA CE00A00004