Riparian Buffers: Protecting the Mill River by Filtering Fertilizer from the Florence Fields

9 May

Students in Ecohydrology at Smith College have spent the last weeks of the semester taking on challenging hydrologic questions relevant to our community. In this post, Ellena Baum, Alison Grady, Michelle Hannon, and Maya Kutz address the question: What is the effectiveness of the riparian forest buffers along the Florence Fields on reducing the nitrate load to the Mill River?

Excess nitrogen is a form of nonpoint source pollution that can contribute to the growth of algae and the decrease of available oxygen in waterways.(1) Nonpoint source nitrogen pollution stems from a variety of locations, including agricultural fields treated with fertilizer, and it arrives in rivers and lakes through stormwater runoff and subsurface water flow.(2) One way to mitigate this problem is by introducing riparian buffers, which are strips of vegetation along the banks of rivers that filter sediments and pollutants, stabilize eroding banks, and preserve wildlife habitat.(3)

Riparian buffers are commonly used as transition areas between the Mill River and surrounding fields. The Florence Fields consist of four farms, the Florence Organic Community Garden, and the Florence Recreation Area, spanning about 120 acres of land, as shown in Figure 1, below.(4) While synthetic fertilizers were applied to the fields in the first two years of establishment, the City of Northampton plans to use organic methods of fertilizer such as cover crops, compost, and manure to manage the fields in the future.(5, 6)

Figure 1. Map of Florence Fields(4)

After fertilizer is applied to agricultural lands, excess nitrogen can leach into the groundwater as water-soluble nitrate or be carried by surface runoff as either nitrate or as ammonium attached to sediment.(7) Nitrate is more likely to leach into soils that have a high hydraulic conductivity, which is defined as the ease with which water can flow through a material.(8) For example, gravel, with a high hydraulic conductivity, allows water to pass through more easily, while clay has a low hydraulic conductivity, and retains more water. Once water leaches into the groundwater, it travels as subsurface flow to the nearest waterway, which is the Mill River in the case of the Florence Fields. If water does not leach into the groundwater, it travels as overland flow, which is faster and carries more sediment when the slope of the land is steeper.

Riparian buffers can slow down overland flow because the presence of vegetation can interrupt flow and cause the water to release sediment and infiltrate into the soil.(9) Once the nitrogen has infiltrated into the soil of a riparian buffer, whether it arrived there through overland flow or subsurface flow, it can be taken up by plants or denitrified before the groundwater reaches the river.(10) Denitrifying bacteria in the root zone of the buffer changes water-soluble nitrate into nitrogen gas which escapes into the atmosphere.(11) Nitrogen removal is more effective if the water spends more time in contact with the vegetation root zone.(12) The residence time of the water in the buffer zone is affected by slope, soil type, vegetation type, and buffer width, and the effectiveness of nitrogen removal can be quantified by measuring the changes in nitrate concentrations in the water.(12) A twelve-year study in Bear Creek, Iowa found that riparian buffers could reduce up to 80% of surface water nitrogen and up to 90% of groundwater nitrate.(13)

It is unknown which buffer characteristic has the largest impact on pollutant removal, but there are a variety of recommendations for best practices. State and federal guidelines for buffer width suggest covering between seven and one hundred meters from the stream bank, depending on site conditions.(14) The effect of width is inconsistent and suggests that soil type and groundwater flow are also important factors in nitrogen removal.(15) A wider buffer should be used for steep land or where pollutant loads are high, in order to provide more opportunity for pollutant removal.(16) Figure 2 below shows the relationship between buffer width and nitrogen removal effectiveness based on compiled research by the EPA. The dotted lines indicate the probable 50%, 75%, and 90% removal efficiencies based on the fitted non-linear model.(14) According to these data, a buffer width of only five meters would be needed to remove 50% of the nitrogen in the water moving through the buffer. It takes significantly wider buffers to remove 75% or 90% of the nitrogen based on the logarithmic relationship between width and effectiveness.(14)

Fig2Figure 2. Nitrogen Removal Efficiency based on Buffer Width(14)

Vegetation fosters the presence of denitrifying bacteria and absorbs nitrogen through root networks.(17) Various types of vegetation in a riparian buffer zone are needed to absorb nonpoint source pollution in infiltrated water or subsurface flow. Buffer zones are most effective as combinations of different vegetation types because of their complementary canopy heights and root structures.(18) The design of an ideal buffer is shown in Figure 3, below. Tall trees with large canopies are especially beneficial close to the water because their root systems absorb more nitrogen from subsurface runoff, stabilize stream banks, and provide shade to keep stream temperatures cool.(18) Rapidly growing woody species, such as willow, poplar, silver maple, and green ash, ensure rapid uptake of nutrients in a riparian buffer zone.(18) On the outskirts of a buffer zone, native shrubs and grasses can serve as a transition from developed landscape to riparian ecosystem.(18) Forest management could include trimming tree stems to stimulate increased growth and nitrogen uptake.(18) Grass-only buffers have been shown to be more variable in nitrogen uptake, perhaps as a function of buffer width.(14) 

Figure 3. Ideal Design of a Riparian Buffer(18)

Our research provided a theoretical basis for future field testing of the riparian buffer at the Florence Fields. Satellite images show that the riparian buffer between the Florence Fields and the Mill River consists of a mix of vegetative types with a variable width of 30 to 135 meters, which corresponds to 75% nitrogen removal at its narrowest point based on Figure 2. The efficiency of the buffer is difficult to determine because vegetation type and other variables are site-specific and not easily quantified, but it is clear that the benefits of the buffer are not limited to nitrogen uptake. In addition to filtering nitrogen pollution, riparian buffers also contribute to stream bank stabilization, flood control, groundwater recharge, carbon sequestration, and wildlife habitat.(9)

Sources Cited

[1] Chrislock, Michael F. “Eutrophication: Causes, Consequences, and Controls in Aquatic Ecosystems.” The Nature Education. 2013. Web. 3 May 2014. <>

[2] U.S. EPA. “What is Nonpoint Source Pollution?” Water: Polluted Runoff. Web. 2 May 2014.

[3] Connecticut River Joint Commissions. N.p.: Connecticut River Joint Commissions, n.d. CRJC. 1 May 2001. Web. 23 Apr. 2014. <>.

[4] Grow Food Northampton. Northampton Community Farm. 2013. Web. 24 Apr. 2014.

[5] Cain, Chad. Florence Fields to be Managed Organically. 8 Apr. 2014. Web. 24 Apr. 2014.

[6] Lombard, Lilly. Personal Electronic Communication. 16 Apr. 2014.

[7] Follett, Ronald. “Fate and Transport of Nutrients: Nitrogen.” USDA, Agricultural Research Service. Web. 27 Apr. 2014.

[8] Guswa, Andrew J. “Savanna Ecohydrology”. Lecture to EGR 315, Smith College, Northampton, MA. 6 Mar. 2014.

[9] Schultz, R.C., T.M. Isenhart, W.W. Simpkins and J.P. Colletti. “Riparian forest buffers in agroecosystems – lessons learned from the Bear Creek Watershed, central Iowa, USA.” Agroforestry Systems 61: 35-50 2004. Kluwer Academic Publishers. Netherlands.

[10] Aber, John D. et al. “Nitrogen Saturation in Northern Forest Ecosystems.” BioScience, Vol. 39, No. 6. 1989. 378-386. Web. 21 Apr 2014. <>

[11] Rassam, DW; Pagendam, D; Hunter, H . “CRC Catchment Hydrology and CRC Coastal Zones Technical Report 05/9.” CRCCH. 2005. Web. 27 Apr 2014. <>

[12] De la Crétaz, A. & Barten, P. Land Use Effects on Streamflow and Water Quality in the Northeastern United States. Boca Raton: CRC Press, 2007.

[13] Kleiman, Laura, and Corry Bregendahl. Funding Impact Brief #5: Bear Creek Riparian Buffer Project. Spring 2013. Web. 24 Apr. 2014.

[14] United States. Environmental Protection Agency. Office of Research and Development. Riparian Buffer Width, Vegetative Cover, and Nitrogen Removal Effectiveness: A Review of Current Science and Regulations. By Paul M. Mayer, Steven K. Reynolds, Jr., and Timothy J. Canfield. EPA, 1 Nov. 2005. Web. 24 Apr. 2014.

[15] Mayer, Paul M., Steven K. Reynolds, Marshall D. Mccutchen, and Timothy J. Canfield. “Meta-Analysis of Nitrogen Removal in Riparian Buffers.” Journal of Environment Quality 36.4 (2007): 1172. 1 July 2007. Web. 21 Apr. 2014.

[16] Connecticut River Joint Commissions. N.p.: Connecticut River Joint Commissions, n.d. CRJC. 1 May 2001. Web. 23 Apr. 2014. <>.

[17] Aber, John D. et al. “Nitrogen Saturation in Northern Forest Ecosystems.” BioScience, Vol. 39, No. 6. 1989. 378-386. Web. 21 Apr 2014. <>

[18] Environmental Protection Agency. EPA. Bear Creek Iowa, The Case Summary. Web. 24 Apr. 2014.

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