Nitrogen Transport and Attenuation in the Connecticut River Basin, New Hampshire, Vermont, and Massachusetts

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 the 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.

[Summary provided by the USGS.]