|
Nutrient Sources and Loads in the Connecticut, Housatonic, and Thames River Basins
Water-Resources Investigations Report 99-4236
By Elaine C. Todd Trench
Sources and loads of total nitrogen and total phosphorus in streams of the Connecticut, Housatonic, and Thames River Basins study unit were evaluated as part of the U.S. Geological Survey's (USGS) National Water-Quality Assessment Program. The study area encompasses 15,758 square miles and extends from northern New Hampshire and Vermont to coastal Connecticut.
Annual nutrient loads and yields were estimated using data from 25 USGS water-quality monitoring stations in and near New England. Major sources of nutrients include atmospheric deposition, agricultural fertilizer and manure, urban nonpoint runoff, and municipal wastewater discharges. Nutrient yields for drainage basins with relatively homogenous land use, along with data on point-source loads, were used to estimate nutrient yields for general categories of forested, agricultural, and urban land in selected basins.
Yields of total nitrogen in undeveloped forested drainage basins, for which data are limited, are generally less than about 2,000 pounds per square mile per year (lb/mi 2 /yr), and yields of total phosphorus are generally less than about 100 lb/mi 2 /yr. Nutrient yields are somewhat higher in forested drainage basins that have larger percentages of developed land or that receive point-source discharges, with total nitrogen yields generally less than 4,000 lb/mi 2 /yr and total phosphorus yields generally less than 400 lb/mi 2 /yr. Nonpoint agricultural sources, including fertilizer and manure, constitute substantial nutrient inputs in some drainage basins but do not always result in high stream nutrient loads. Average basin yields of total nitrogen for three agricultural drainage basins were generally less than 4,000 lb/mi 2 /yr, similar to forested basins, and total phosphorus yields were generally less than 300 lb/mi 2 /yr. Total nitrogen yields for the small Broad Brook Basin, however, exceeded 10,000 lb/mi 2 /yr, similar to nitrogen yields in urban basins in Connecticut that receive point discharges. Conditions in the Broad Brook Basin may not be representative of typical agricultural areas.Municipal wastewater discharges are major sources of stream nutrient loads in urban areas of Massachusetts and Connecticut. Point-source discharges are the most important source of nutrient loads in four highly urban basins—the Pequabuck, Hockanum, Quinnipiac, and Naugatuck River Basins in Connecticut—even though 46 to 67 percent of the land in these drainage basins is undeveloped. These findings emphasize the importance of point discharges in urban coastal areas as sources of nutrients to Long Island Sound.
As wastewater flows increase in proportion to streamflow in urban basins, basin yields of total nitrogen and total phosphorus increase. The largest total nitrogen and total phosphorus yields were found in the Pequabuck, Hockanum, Quinnipiac, and Naugatuck River Basins, where wastewater discharges constitute from 9 to 19 percent of the annual mean streamflow in a median flow year. Basin yields of total nitrogen generally ranged from 8,000 to 14,000 lb/mi 2 /yr, and basin yields of total phosphorus generally ranged from 1,000 to 1,400 lb/mi 2 /yr.Nutrient yields from urban drainage basins with no point sources cannot be adequately characterized with the limited amount of monitoring data available, but yields appear to span a wide range, based on different types and intensities of urban land use. Yields for two urban basins that receive no point discharges were much smaller than yields in drainage basins that receive point discharges.
Although yields are moderately low in large, primarily forested drainage basins, rivers draining these large basins transport most of the nutrient load. The Connecticut River, measured at the Thompsonville station, transports about two-thirds of the total nitrogen exported from the major monitored drainage basins and about three-fifths of the total phosphorus.
The major sources of nutrients differ in the northern and southern parts of the study area. Forested land accounts for most of the stream nutrient load in northern areas, whereas municipal wastewater discharges are the dominant source of nutrients in urban areas of Massachusetts and Connecticut.
Export of total nitrogen from the major monitored drainage basins in the study area has ranged from about 32,000,000 to 63,000,000 lb/yr during the 1980's and 1990's, and in recent years has typically been about 37,000,000 lb/yr. Export of total phosphorus from major monitored basins during the 1980's and 1990's ranged from about 2,300,000 to 4,700,000 lb/yr and in recent years has typically been about 2,400,000 lb/yr. Historical maximum loads of total phosphorus do not represent currently expected maximum conditions because of the observed downward trends in phosphorus concentrations on many streams.
Although the 25 drainage basins represent about 86 percent of the study area, the load from these basins probably represents considerably less than 86 percent of the total nutrient load because (1) the monitored area contains only 55 percent of the population in the study area, (2) the monitored area includes essentially all of the least developed land with low nutrient yields, and (3) many point discharges in urban coastal areas are either in unmonitored basins or discharge directly to Long Island Sound.
Back to CONN NAWQA Home Page |
URL: http://ct.water.usgs.gov/NAWQA/abstracts/wrir994236.htm |