Comparing the small scale backyard raising of chickens to full scale agricultural operations as I did is not an appropriate or fair comparison because scale and density are important elements of their environmental impact. Nitrogen and phosphorus are essential nutrients of the growth of living organisms in our yards and in the Chesapeake Bay. However, excessive nitrogen and phosphorus degrade the water quality of our groundwater, surface water and the Chesapeake Bay-the entire water shed. As population density has increased in the watershed, the amount of nitrogen, phosphorus and sediment entering the bay has increased tremendously.
Each year, approaching 300 million pounds of nitrogen reaches the Chesapeake Bay. The majority of nitrogen pollution comes from sewage treatment plants, large-scale animal operations, agriculture, and air pollution from vehicle exhaust and power plants and other industrial sources. Other sources of nitrogen pollution include septic systems, runoff from roadways, development, residential and commercial lawn fertilizers, and small scale animal and agricultural sources (the keeping of horses, poultry, and other animals and growing vegetables in predominately suburban or exurban locations. Solutions to nitrogen pollution include upgrading sewage treatment plants, proper operation of septic systems, using nitrogen removal technologies on septic systems, and decreasing fertilizer applications to lawns and controlling suburban and exurban animal waste.
Over the past quarter century the excess nutrient contamination to the Chesapeake Bay has decreased in total, but the Bay’s waters remain seriously degraded and considerably short of attaining the 2010 water quality goals set forth in the Chesapeake 2000 agreement. As a result US EPA is developing a new federally mandated Total Maximum Daily Load (TMDL) plan to establish and apportion an allowable pollution budget among the states.
Over the past 25 years, nitrogen released to the Chesapeake Bay has fallen about 33% from agriculture, fallen about 40% from waste treatment plants, but increased about 15% from septic and mixed open use. During that same period of time phosphorus released into the Chesapeake Bay has fallen about 29% from agriculture, fallen about 65% from waste treatment plants and increased about 14% from septic and mixed open use. The population in the region has increased by more than 20% (including the urban core) during this time period.
According to the Delaware Department of Natural Resources and Environmental Control, the typical household generates 10-15 pounds of nitrogen per year and 1-2 pounds of phosphorus per year. According to a Maryland state study, each chicken generates approximately 0.41 lbs of Nitrogen per year and around 0.35 pounds of phosphorus per year. Thus, each household with 10 chickens would generate 4.1 pounds of nitrogen and 3.5 pounds of phosphorus per year. This is a significant increase in the nutrient load of a typical house hold, a more than three fold increase in phosphorus load and an increase of nitrogen load by more than 30%. This additional waste is delivered in an uncontrolled manner. The poor location of a chicken coop could potentially impact ground water and well heads both on and off site and should be subject to the same off sets as septic systems. In addition the nutrient load has to be addressed.
Dutchess County New York did a study to monitor the effectiveness of septic set backs and studied nitrate concentrations. They chose to use nitrate concentrations at half the drinking water level as a proxy for adequate dilution and natural attenuation of all contaminants. Historically, horizontal and vertical setbacks were developed without consideration of the dilution for wastewater components like nitrate and phosphorus. The NY Department of Health separation distances were assumed (and these are almost identical to the Virginia setbacks), but the overall regional density of septic systems was examined to ensure that groundwater resources would not be overwhelmed by the total load of contaminants. The density recommendations were developed based on the nitrate concentrations. Nitrate was used as a proxy because all humans produce nitrate, it does not easily break down and there is a drinking water standard. The target concentration was half the drinking water level to ensure all outcomes are safely below the standard since household size can vary tremendously. The Dutchess County study found that overall average density of on-site waste disposal should not exceed one unit per 2-3 acres for an average size household to ensure water quality.
Adequate dilution, soil filtration and time are necessary to ensure sustainable water quality. Unfortunately, by adding 10 chickens to a yard you have increased the nutrient load significantly, tripling the phosphorus and increasing the nitrogen by 30%. The geology of this area consists of an interbedded sequence of sedimentary and basaltic rocks. The rocks of the Culpeper basin are highly fractured and overlain by a thin cover of overburden. The lack of overburden limits natural protection to the aquifer. The sedimentary rocks create a productive aquifers, but allow contaminate to easily wash into the groundwater basin. Allowing backyard chickens represent a significant threat to the groundwater basin.
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