RAN: Redesigning the American Neighborhood Project
Stormwater Basics: Nutrients
Stormwater also is the chief mechanism by which nutrients such as phosphorus and nitrogen are flushed from the landscape and carried to Lake Champlain. These elements act as fertilizers for the algal populations in the lake; in the summer months high nutrient levels lead to algal blooms, which may have associated health risks and often impair water quality.
In addition to carrying the fertilizer that we put on our lawn and wastes from our pets, stormwater also flushes out the nutrients which naturally occur in the soil:
"In addition to obvious sources of nutrients, like cows, unstable streambanks account for a large proportion of the non-point phosphorus loads. Most phosphorus is bound in the upper soil. When this soil moves into the water through overland erosion or eroding stream banks (often caused by the loss of vegetation), the phosphorus comes with it. Erosion can be amplified by upstream construction projects..."*
Additionally, phosphorus is often transported on or in association with sediment or slightly charged particulate matter found in the suspended solids and sediments. Therefore, by creating higher volumes of stormwater from our impervious surfaces, which leads to more erosion, we increase the potential for phosphorus (typically in the form of phosphate) transport from the land into the lake.
We often assume that the most significant phosphorus loading is from agricultural sources. In fact, 37% of the phosphorus contribution in Lake Champlain is from urban areas and other developed land.**
In the Lake Champlain basin, phosphorus continues to be the key limiting factor. It is important that all sources of phosphorus within the basin be controlled to the extent possible.
Research to date by the RAN team indicates an average total phosphorus concentration of 0.134 mg/L at the lower monitoring station in Butler Farms/Oak Creek Village in Tributary 7, which is below the national average of 0.26 mg/L.
Yet, there is a nearly 30% increase in total phosphorus concentration from the upstream to the downstream monitoring stations, which demonstrates the impact of development within the watershed. While this level of phosphorus is below national levels of concern, any accumulation of phosphorus in Lake Champlain is a significant concern.
Inorganic nitrogen occurs in water as nitrate and nitrite. It can also be measured as Total Kjeldahl Nitrogen (TKN). This is the sum of ammonia and organic nitrogen found in the water. Typically, nitrogen enters surface waters from farming and agricultural practices and faulty sewer systems. It is also washed off lawns and gardens (which may be over fertilized) during storm events.
Nitrogen is a concern in the Potash Brook watershed in that it (along with phosphorus) helps fuel algal blooms in Lake Champlain. The data collected for Tributary 7 at the Butler Farms/Oak Creek Village lower station show an average TKN level of 0.82 mg/L. This is below the 1.47 mg/L national stormwater average; however, at the lower monitoring station, we have observed levels as high as 1.9 mg/L of TKN in isolated samples.
It has been demonstrated that there are ways by which both phosphorus and nitrogen loading can be significantly reduced. These will be explored in the Best Management Practices (BMPs) section of the General Interest page.
*Vermont Agency of Natural Resources; Department of Conservation.Winter 2003, Out of the Blue Newsletter.)
Last modified October 16 2009 09:50 AM