Where does the phosphorus in
Lake Champlain come from?
There are many sources of phosphorus and other nutrients entering Lake Champlain. For every square mile of lake area, there are 18 square miles of land area in the watershed that drains into the Lake. Activities in forested, developed, and agricultural lands all contribute nutrients and other pollutants to the Lake. While tributaries carry the largest load of nutrients from the upper portions of the watershed downstream into the Lake, properties along the lakeshore also can have a direct influence on the Lake’s water quality.
The ratio of land area in Lake Champlain’s watershed—or drainage area—to the surface area of the Lake is 18 to 1. With such a large watershed supplying water to the Lake, the challenge of limiting pollution that washes off the land area and enters the Lake is greater—the watershed has a much greater effect on the water quality of the Lake than would occur with a smaller watershed ratio. For example, the Great Lakes in the Midwestern US and Canada are much larger than Lake Champlain, but their watershed ratio is much smaller (between 1.5:1 and 3.4:1). Consequently, acre for acre, the watersheds of those lakes have a much smaller impact on their water quality and there is, proportionately, a much smaller land area to manage.
Nutrients and pollution enter Lake Champlain, its tributaries, and other water bodies within the watershed through runoff from the land, or nonpoint source pollution (Figure 4), along with discharges from wastewater treatment facilities (WWTFs) and other discrete sources (called point sources).
The tributaries that drain into Lake Champlain from its watershed continually replenish the Lake’s water supply. These tributaries also deliver pollutants, including excess phosphorus, nitrogen, and other nutrients, and toxic substances that are washed off the landscape. Not all tributaries are created equal, however. The total phosphorus load estimated to be delivered to the Lake each year from its watershed is 921 metric tons (2.03 million lbs), but some tributaries deliver substantially more pollutants than others. Management agencies in all parts of the watershed have been targeting problem areas for decades, with some successes.
Recent research by the US Geological Survey has determined that these efforts are beginning to show positive effects (Figure 5). Most notably, pollution from the LaPlatte River in Vermont has decreased as a result of improvements in nonpoint source management and upgrades to wastewater treatment facilities. Other tributaries from which the amount of phosphorus delivered to the Lake has been reduced over the past ten to twenty years include the Little Ausable, Putnam, and Mettawee rivers in New York and the Otter, Missisquoi, and Winooski rivers in Vermont.
Although phosphorus pollution loads from some tributaries, such as the Missisquoi River, have been somewhat reduced, they continue to be far higher than their targets, despite years of concentrated efforts by resource management agencies, local watershed groups and residents to curb this pollution. In some tributaries it may take decades for measurable reductions in the pollution loads to occur, despite best management practices that may be employed today and in the future.
Load from Developed Land Use
Land that has been developed can be a substantial source of nutrients and other pollutants to the Lake. Developed lands tend to have large areas with impervious surfaces, such as building rooftops and parking lots, which shed rain water very quickly and rarely provide opportunities for stormwater to infiltrate or soak into the ground. High storm flows in the rivers increase erosion of streambanks, sending more sediment and nutrients downstream toward the Lake. Flashier storm flows, with surges of large volumes of runoff from developed lands, can also increase the severity of flooding downstream, causing damage to public and private property, as demonstrated by Tropical Storm Irene in 2011. Investment in better-designed and more resilient roads, culverts, and “green infrastructure”—that reduce these storm flows—can increase the amount of water that soaks into the ground, reducing and delaying runoff to a tributary downstream.
Approximately 16% (147 metric tons or 323,610 lbs) of the phosphorus delivered to Lake Champlain each year comes from developed lands in the watershed. This is phosphorus delivered to the Lake from nonpoint sources in developed areas, such as parking lots, roofs, and lawns. Of the 147 metric tons, 112 mt are estimated to come from the Vermont portion of the developed lands in the watershed, 28 mt from New York, and less than 8 mt from Québec.
Phosphorus pollution of the Lake from wastewater treatment facilities is a small fraction of the problem of phosphorus in the watershed, amounting to only 6% of the total phosphorus load to the Lake. Total loads from wastewater treatment facilities in each jurisdiction (New York, Vermont, and Québec) have been at or below their respective targets since 2004 or earlier (Figure 6). Regulations banning phosphorus in detergents have greatly reduced the amount of phosphorus entering treatment facilities, further reducing the amount of phosphorus they need to remove from their effluent stream.
Load from Agricultural Land Use
Agriculture is an important part of the identity of the Lake Champlain region, but it also has a very significant effect on the water quality of the Lake. Many farming practices, both conventional and organic, require application of fertilizer to increase crop productivity. Cattle and other animal-based farms generate large amounts of manure that must be disposed of carefully. Runoff and erosion from barnyards, laneways to pastures, and animal congregation areas can carry excessive pollution into nearby waterways. Frequently farmers spread manure on their fields to recycle those nutrients back into their crops and pastures. Most conventional farms also rely in part on commercial fertilizers and feed additives imported to the Basin. A portion of these nutrients is inevitably washed off the land and into a waterway before soaking into the ground.
The fraction of this phosphorus pollution that comes directly from agriculture is estimated to be about 352 metric tons (775,000 lbs) per year, or 38% of the total estimated amount of phosphorus entering the Lake (Figure 7).
Recent research has shown that in the Lake Champlain watershed, the erosion of streambanks contributes an estimated 18%, or 165 metric tons (365,600 lbs) of phosphorus to the Lake. In Vermont, 20% of the phosphorus load is estimated to come from streambanks, more than 24% of the load in Québec, and 9% from New York. The New York load is likely lower because of the greater amount of forest in this part of the watershed, which helps protect streambanks from erosion. The streambank contribution estimate is separate from the phosphorus that already is in the stream as wash-off from the land. Significant erosion of streambanks occurs most often when stream corridors or the adjacent lowlands are altered to accommodate some land use activity such as construction of a culvert for a town road, the cultivation of farmlands right to the edge of a river, or any streamside disturbance that removes or prevents a vegetated woody buffer.
Streambanks lacking vigorous woody plant growth can be very susceptible to erosion, especially in a time of flooding. When streambanks are eroded and sediments collapse into the steam, changing flow patterns result, potentially causing further erosion of streambanks downstream, and ultimately releasing yet more sediment and nutrients into the waters flowing toward the lake. Collapsed streambanks at the edge of agricultural fields may be especially rich in nutrients, including phosphorus, because of their history as fertilized land. The preconditions that increase the vulnerability of lowland terrain to significant streambank erosion result from both historical and contemporary agricultural practices and developed land use practices in these sensitive areas throughout the Basin.