|Metrics for Nitrate Contamination of Ground Water at CAFO Land Application Sites -
Arkansas Dairy Study (EPA/600/R-09/044) June 2009
Nitrate is the most common chemical contaminant found in ground water. Recent research by U.S. EPA has shown that land application of manure can cause nitrate contamination of ground water above the maximum contaminant levels (MCLs) of 10 mg NO3-N/L at significant depths. This finding and similar ones across the nation are raising concerns about the potential for manure to degrade ground water quality near concentrated animal feeding operations (CAFOs). The objectives of this research were to determine if nutrient management plans (NMPs) for CAFOs are inherently protective of ground water and which metrics can be used as red flags to identify when the land application practices pose a risk to ground water.
A study was conducted for one year (August 2004 to July 2005) on a typical dairy farm in northwest Arkansas. The dairy had 250 cows and utilized 27 ha (67 acres) for manure application. Effluent from the holding pond was sprayed onto four 4.05 ha (10-acre) fields each year. According to the farmer’s NMP, effluent applications were to occur during the growing season after the soil had dried considerably, which did not occur until August in 2004. Four small (10 x 10 m) plots were established in each of two of these spray fields. Stainless steel lysimeters were installed to a depth of 1 m and sampled weekly. Three soil cores were taken periodically from seven depths (0-5, 5-10, 10-20, 20-40, 40-60, 60-80 and 80-100 cm). Soil samples were collected 20 times throughout the year. These samples were analyzed for soluble components as well as exchangeable ammonium and Mehlich III extractable P. Plant samples were also analyzed for nitrate.
Nitrate levels in lysimeter samples were high, with peaks in excess of 100 mg NO3-N/L. The amount of N applied via effluent averaged 280 kg N/ha (250 lbs N/acre), which was not believed to be sufficient to cause such high levels. Lysimeter P concentrations were also very high. Beginning in November, it was observed that the farmer utilized the 4.05 ha fields as a loafing area or high use area (HUA) for his cows. We estimate that at the observed stocking rate (31 cows/ha) as much as 840 kg N/ha (750 lbs N/acre) was being added to these fields via direct waste deposits from the cows. When coupled with the effluent application, the total N loading to these fields was approximately 1100 kg N/ha (1000 lbs N/acre) in one year.
When state officials were contacted to determine if direct deposits were taken into account when determining maximum N application rates for farms spreading liquid manures in Arkansas, we discovered that these deposits by the cows were not accounted for. Currently, the Arkansas Phosphorus Index is being revised. During this revision we will attempt to change regulations so that growers cannot apply effluent to a HUA unless the direct deposits are properly accounted for. The best predictor of high nitrate in the lysimeter samples was leaf tissue nitrate concentration with R-square of 0.82. Since nitrate toxicity in cows is a problem that will negatively affect production, dairy farmers could easily be convinced to monitor this parameter. High nitrate levels in forage would allow them to know which fields were receiving too much N, and allow them to alter applications accordingly.
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