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Data Set #058

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    Chicken production and consumption have increased dramatically in the US over the last 40 years; today, about 8 billion broiler chickens are raised in the US each year.  Broiler chickens are typically grown in large metal barns with automatic feed and water systems.  The chicken feed is very rich, to speed weight gain and reduce time to market; the average chicken at slaughter weighs about 3.7 pounds.  One concern among environmentalists is that high quality foodstuffs such as corn, wheat and rice, which can be consumed directly by people, are being fed to chickens instead. Another big environmental concern is that chickens produce a lot of waste, approximately 1 pound of litter (manure, feathers, uneaten food and bedding) per pound of chicken, which equals about 8 billion tons of chicken waste per year in the US.  Disposal areas include chicken house floors, litter stockpiles, agricultural fields where chicken litter is been spread as fertilizer, and dead chicken pits.  All of these areas are potential sources of nitrogen, phosphorous and other chemicals that can pollute groundwater via percolation and/or surface water via runoff.

    Broiler (poultry) farm waste disposal practices are thought to be affecting the quality of groundwater in north-central Florida.  In this study, 18 monitoring wells were installed at five Florida broiler farms and monitored quarterly from March 1992 through January 1993.  Collected data included specific conductance, pH, temperature, and concentrations of potassium, chloride, nitrate nitrogen, nitrite nitrogen, ammonium nitrogen, phosphorous, organic nitrogen and organic carbon.  The wells were placed up-gradient and down-gradient (up-slope and down-slope in the groundwater) of the broiler farms to assess the farms' influence to the above-mentioned variables.  At all sites the chemical constituent with the highest concentration was nitrate, ranging from below the reporting limit of 0.05 mg/L  to 105 mg/L.  The EPA has the set the maximum contaminant level from nitrate in drinking water at 10 mg/ L. Concentrations greater than this this can become a health risk (especially to infants) by reducing the ability of blood to transport oxygen.  The study concluded that increases in nitrates and the other chemicals did occur in the vicinity of the broiler farms, although complications existed in identifying the direction of groundwater flow and accounting for influences of nearby farms.

        The data presented here are from 4 wells at site #1, located in southwest Suwannee County and close to 2 chicken houses containing 14,000 birds each.  As one can see, there is a strong positive correlation between the nitrate and potassium concentrations. (Interestingly, at a second site there was no correlation between nitrates and potassium.)  The researchers state that, "if the chicken house are the source of the nitrates, then the high correlation between the nitrate and potassium concentrations implies that the chicken houses are also the source of the potassium at site 1."  Students may want to consider the following questions:  Do you agree with the researchers' statement listed above? Can you identify the outlier in these data?  What are the correlation coefficients before and after the removal of the outlier?  What would the regression equation tell us if the potassium concentration equaled 2.0 mg/L?

Source:  Hatzell, H.H., 1995.  Effects of Waste Disposal Practices on Ground-Water Quality at Five Poultry (Broiler) Farms in North-Central Florida, 1992-93.  USGS Water Resources Investigation Report 95-4064.

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Well water at a Florida broiler farm  
Source: USGS Water Resources Investigations Report 95-4064
Potassium (mg/L) Nitrate (mg/L)
1.5 18
1.8 19
1.6 20
1.3 21
0.7 9.4
0.7 10
0.6 11
0.6 26
0.4 5.2
0.4 5.4
0.4 5.0
0.1 4.9
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