Determining Phosphorus Loading Rates Based on Land Use in an

Chapter 3

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Determining Phosphorus Loading Rates Based on Land Use in an Urban Watershed 1

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Zachary Μ. Easton and A. Martin Petrovic 1

Departments of Biological and Environmental Engineering and Horticulture, Cornell University, Ithaca, NY 14853 2

Non-point phosphorus (P) loading to surface water can degrade water quality and impair habitat. As urban areas continue to grow in the United States, critical Ρ source loading areas need to be identified to assess their impact on water quality. A 332 ha urban watershed in Ithaca, N Y was selected and monitored for two years, with above average precipitation. Runoff collected from 98 precipitation events and three land uses was analyzed for dissolved Ρ (DP), particulate Ρ (PP), and total Ρ (TP), with mass losses calculated. Monitored land uses included fertilized lawns (FL), urban barren (UB) areas and wooded (FR) areas. Stream gauges were installed at the stream entrance to the urban area and the watershed outlet to monitor the impact of the urban area on stream water quality. A multivariate analysis of the data revealed that the F L land use had higher DP losses than the other land uses on the shallow, low storage, runoff prone urban soils. If runoff volumes were low, the F L land use had similar or considerably lower DP losses. Particulate Ρ mass losses were highest from the FR and U B land uses due to little or no ground cover to prevent erosive Ρ losses. Total Ρ losses from the land uses were highest on the shallow, low storage soils and surprisingly similar among land uses. As the stream flowed through the urban area, there was a two fold increase in Ρ loads when compared to the undeveloped upper area of the watershed. This indicated that the urban area was contributing Ρ to the stream. Best management practices in this and other similar © 2008 American Chemical Society In The Fate of Nutrients and Pesticides in the Urban Environment; Nett, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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44 watersheds should focus on reducing runoff and P loss from source areas to realize the largest reduction in P loading to surface water. Ultimately, it is imperative to assess land use performance under varying conditions in order to reduce P loads in surface water.


Introduction Due to its role as a limiting nutrient for aquatic plant growth, phosphorus (P) is the primary concern in fresh water systems. Recent work done in the New York City Watershed indicates that P levels as low as 0.024 mg L" can cause the growth and subsequent proliferation of cyanobacteria (/). Unlike predominately agricultural watersheds, urban areas are comprised of a mosaic of land uses including both impervious surfaces and pervious land uses (2), coexisting at a fine spatial scale (5). The impact of each land use within an urban area can be expected to vary in a spatial-temporal manner and independently of each other (4). Therefore, it is unclear how, if, and when urban areas impact water quality. The function of these areas must be studied in greater depth, and much more intensively, to draw conclusions as to the role of urban areas in water quality and ecosystem function. 1

The contribution of urban areas to large scale water quality is generally unclear. However, some studies have shown that urban areas can be a considerable source of P. Phosphorus concentrations in urban stormwater flows have been measured in excess of 9 mg L" (5). The United States Geologic Survey (USGS) (6, 7) concluded that concentrations of TP were generally as high in urban streams as in agricultural streams, with more than 70 percent of sampled urban streams exceeding the U S E P A desired goal (0.1 mg L" of TP) for preventing eutrophication. The density of development has been shown to influence P loading (8). Medium density residential sites (e.g., single family homes) had annual TP loadings of 0.6 kg ha' , while high density, urban sites had considerably higher TP loads of 1.1 kg ha" yr" - presumably due to a higher fraction of impervious surfaces (8). In two urban Wisconsin basins, lawns were estimated to contribute greater than 50% of the DP and TP loads to surface water (P). Wooded areas were also identified as potential P source areas. However, runoff was not flow weighted, nor sampled year round (P), making it difficult to draw conclusions about P loading rates. This study was conducted to examine the contribution of three different land uses (fertilized lawns (FL), urban barren (UB), and wooded (FR) areas in a small urban watershed to stream P losses via runoff. This watershed also presents a unique opportunity to compare P losses from two dichotomous land uses, urban and undeveloped, within the same watershed. 1

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In The Fate of Nutrients and Pesticides in the Urban Environment; Nett, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Materials and Methods


Watershed Description Located in Ithaca, New York (42°48'N, 76°46'W), the experimental watershed is 332 ha in area and a subwatershed in the Cayuga Lake basin. The region is typified by steep hillslopes with shallow permeable soils underlain by a restrictive layer of glacial origin. Soils in the experimental watershed are generally deeper in the upper watershed (>100 cm) and underlain by bedrock, while soil depth decreases near the watershed outlet (

Determining Phosphorus Loading Rates Based on Land Use in an

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