The data used in the dashboard originates from 17 monitoring locations across multiple watersheds from 1995 to 2021. There are two graphical representations of the data – one separated by year and the other separated by seasons. An earlier version of this dashboard was created several years ago with data up until 2016 – this new version adds on five more years of information. 

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“The value of USGS long-term monitoring efforts is the ability to analyze data in an infinite number of ways – and this dashboard provides a visual representation of how nitrogen loading has changed over a 27-year period,” said USGS Hydrologist John Mullaney, who built the dashboard application. “Without USGS’s role documenting water quality and streamflow over long stretches of time, we wouldn’t be able to tell a story of what’s happening – you must have enough long-term data to see the changes.”

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The dashboard shows the nitrogen load estimates calculated from USGS-collected streamflow and concentration data taken at the 17 monitoring locations. These monitoring locations were chosen because they are the furthest downstream streamgages within their subsequent watersheds, and they are not impacted by tidal influence. These characteristics guarantee accurate streamflow estimates and total nitrogen loads for the tributary.

The Connecticut Department of Energy and Environmental Protection (CTDEEP), New York Department of Environmental Conservation, and the U.S. Environmental Protection Agency (EPA) set Total Maximum Daily Loads of nitrogen discharged into Long Island Sound in 2001 to manage nutrient pollution. CTDEEP aimed to reduce nitrogen pollution by setting up a nitrogen-trading program among 79 wastewater treatment plants called the Nitrogen Credit Exchange Program. This program provided incentives to wastewater treatment facilities that removed nitrogen from their wastewater to reduce nitrogen loads into Long Island Sound. Users of the dashboard can see the drastic reductions in nitrogen in some of the stations that are downstream from wastewater treatment facilities.

The statistical methods used to calculate the nitrogen loads for this dashboard were initially developed for the Chesapeake Bay and work best with streamgages that have at least 20 years of monthly or more frequent data. USGS inputs the nitrogen concentration and streamflow data from each gage into a statistical model and runs regression equations to generate the annual mean concentrations and annual nitrogen loads. The line on both the annual and seasonal graphs in the dashboard is not a trend line, but a line connecting the annual nitrogen load estimates adjusted for the year-to-year variability in streamflow.

“The flow adjustment line helps the managers at EPA and the state agencies actually see that there is a change over time,” said Mullaney. “They’re interested in graphs like this because it allows the annual loads to be fairly compared. It’s a much more accessible way for managers and the public to see this information and how the nitrogen management has reduced nitrogen loading into Long Island Sound.”