Project Summary:

The objective of this project is to estimate the rate of nitrogen loss in selected reaches of the Connecticut River. In-stream loss of nitrogen may influence the total nitrogen loads being input to Long Island Sound (LIS); therefore, an improved understanding of nitrogen attenuation is needed to plan effective strategies for meeting the goals of the LIS Total Maximum Daily Load (TMDL) allocation plan approved by the U.S. Environmental Protection Agency (USEPA) in 2001. The TMDL plan was instituted to reduce the problem of chronic seasonal hypoxia (low dissolved oxygen) that results from excessive nitrogen loading in Long Island Sound.

Two study methods were used to measure nitrogen loss in selected study reaches of the Connecticut River during 2005: a mass-balance study to observe in-stream changes in total nitrogen, and a dissolved nitrogen gas study to measure denitrification. For the mass-balance study, samples were collected from all major tributaries and at the upstream and downstream ends of two 30- to 40-mile study reaches, and were analyzed for total nitrogen (including ammonia, nitrite, nitrate, and organic nitrogen). Streamflow data (from USGS gaging stations or manual measurements) were also taken at the time of sampling so that the mass flux of nitrogen could be computed at each site. To assess the effects of different hydrologic conditions and water temperatures on nitrogen attenuation in the Connecticut River, the study reaches were sampled two times – in spring and summer. The calculations of nitrogen mass flux entering and exiting each study reach will indicate when and where nitrogen removal processes are significant.

The study of dissolved nitrogen gas was performed on a 6-mile sub-reach of the Connecticut River during a period of late summer when warm temperatures and low-flow conditions are most conducive to observing measurable rates of denitrification. Denitrification is estimated by measuring the downstream change in dissolved nitrogen after compensating for gas exchange with the atmosphere and dilution from inflows. Gas exchange is computed from the downstream concentration changes of SF₆ gas and Bromide, which are injected at the head of the study reach.

The data from this study will be useful for verifying predictions of nitrogen inputs, transport, and loss from water-quality models such as the New England SPARROW model and the RivR-N model. The results will assist state resource managers in the development of nitrogen reduction strategies for the Connecticut River Watershed, including the selection of sources in which to target these strategies. Results of the study will be presented in a journal paper in 2007.