Phosphorus runs through local rivers
ADDISON COUNTY — Data released earlier this year showed that in 2011 the Otter Creek poured more than 206 metric tons of phosphorus into Lake Champlain — almost twice the 18-year average of 121 annual metric tons.
An increase in phosphorus, which fuels growth of cyanobacteria, or blue-green algae, is not viewed by staff at the Vermont Agency of Natural Resources (ANR) as a positive harbinger in their effort to clean up Lake Champlain.
After a banner year for phosphorus runoff in Addison County, it beckons the question: Where is the “P” coming from?
The short answer is a combination of agriculture and river alterations, according to Ethan Swift, who monitors Addison County waterways for ANR. But the agricultural phosphorus in county waterways isn’t necessarily attributable to today’s farmers, many of whom have taken drastic measures to contain nutrient runoff.
In the first half of the 20th century, Swift said, farmers were subsidized to use phosphorus-heavy fertilizers.
“It wasn’t really until the last 20 years that there was much of a focus on regulating the amount of phosphorus being applied and the amount of manure being applied,” said Swift. “Then there were programs developed around manure storage” and curbing phosphorus application.
Although many farmers began to better manage nutrient runoff, the clay and silt on the banks of Addison County rivers held onto that phosphorus from earlier decades as well as what’s added today, said Swift.
Local river scientist Kristen Underwood explained the phenomenon in an interview last month.
“Silt and clay-rich soils tend to have higher phosphorus levels because those fine particles have a strong affinity for phosphorus,” she said. “It also happens that those broad-valley flat lands (where rivers with such sediments exist) are great spots to have agriculture. So there’s a source of phosphorus to be bound to those materials.”
Indicative of what Swift and Underwood said, ANR data from the past two years show that the clay and silt-lined rivers in the farmland flats of Addison County contain high levels of phosphorus.
The Lemon Fair River in Shoreham has had the highest phosphorus levels over the past two years: 308 micrograms of phosphorus per liter of sample water in June 2012 and 230 micrograms in 2011. Other rivers and streams that have consistently carried higher levels of phosphorus over the past two years are the Little Otter Creek, Mud Creek (which flows into the Little Otter) and Pond Brook, which comes up from Bristol Pond to meet Lewis Creek.
“These are areas that historically received a tremendous amount of phosphorus from fertilizers,” said Swift.
An anomalous data point recorded in early June also showed the Middlebury River at the Shard Villa Road Bridge had 31,600 micrograms of phosphorus per liter of water. In June 2011 that number was 41.6 micrograms.
“That’s really quite unusual to see something that high,” said Swift. “We usually don’t see something that high from wastewater discharges. It could have been due to a collection error or if something happened on the bank, like a big portion falling into the river.”
All of the data compiled by ANR is from samples drawn by the volunteer-driven Addison County Riverwatch Collaborative. These samples are taken at the beginning of a given month in the spring and summer and only reflect the phosphorus levels at the time when the water is sampled.
Those levels are subject to change, and phosphorus levels are highest during storms and flood events. Such weather events triggered the majority of Otter Creek’s phosphorus loads in 2011.
According to figures compiled by ANR Watershed Management Division scientist Eric Smeltzer, 35 percent of Otter Creek’s 206-ton phosphorus load came from last year’s spring floods. Another 17 percent came from Tropical Storm Irene.
During such events, streams are more likely to erode, said Smeltzer.
Two studies conducted by the USDA and the Lake Champlain Basin Program found that 40-50 percent of the phosphorus loads in the Missisquoi River in Franklin County over the past 30 years came from in-stream sources, like erosion.
“I think we saw from the studies conducted in the Missisquoi that there’s a fair amount of phosphorus entering into watersheds from river adjustments,” said Swift. “We’ve often times tried to armor and shape (rivers) to accommodate our needs … and a lot of rivers are still responding to those historic adjustments that occurred long ago.”
Take a river that’s prone to erosion, combine it with the silt and clay-rich soils of Addison County, introduce a flood event and the result is an elevated level of phosphorus flowing into Lake Champlain.
That’s one of the major cycles of Addison County phosphorus runoff into Lake Champlain, said state scientists.
Editor’s note: This article is the first in a two-part series about phosphorus sources in Addison County waterways. The next article will consider measures modern farmers and state officials are taking to reduce negative environmental effects that have been associated with agriculture.
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