Graduate Thesis Or Dissertation
 

Nitrogen and heavy metal distribution in soils utilized as sludge disposal sites

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  • Metropolitan areas today must dispose large volumes of sewage sludge produced during the wastewater treatment process. This research was conducted to study the effect of large applications of municipal sewage sludge on (a) the distribution of N and trace elements (Cd, Cr, Cu, Mn, Ni, Zn) in the soil profile, (b) the uptake of these elements by plants growing on the sludge-treated soil, and (c) the potential for groundwater contamination. Five sludge disposal sites in the Willamette Valley were selected for the project, Milwaukie, Eugene, Hillsboro, Forest Grove, and Woodburn, Oregon. The soils were sampled quarterly, July, 1974, December, March, and June, 1975, as a function of depth. Surface soil samples (8 to 10 cores) were collected in September, 1974, from each disposal location to determine the uniformity and amount of sludge applied. Plant samples from the disposal area and water samples from wells adjacent to each area were also collected. The soil, plant, water, and sludge samples were analyzed for total N, NH₄-N, NO3-N, Cd, Cr, Cu, Mn, Ni, and Zn; the water, plant, and sludges were also analyzed for P. The soil samples collected during the winter, spring, and summer, 1975, were analyzed for NH₄-N and NO₃-N. An estimated 500, 290, 96, 72, and 20 dry m tons /ha of sludge were applied to the Eugene, Hillsboro, Forest Grove, Woodburn, and Milwaukie disposal areas, respectively. The total N and P content of the five sludges ranged from 3.9 to 6. 3% and 0.5 to 2. 9%, respectively. The inorganic N was primarily in the form of NH₄-N. The trace element (Cd, Cr, Cu, Mn, Ni, Zn) content of the sewage sludge fell within general ranges reported for municipal sludges, except for the Cr content (17, 700 ppm) of the Milwaukie sludge. The Cd, Cr, Cu, Ni, Zn, NH₄-N, NO₃-N, and total N content increased in the surface soil of each sludge disposal area. The Cd, Cr, Cu, Mn, Ni, Zn, NH₄-N and total N content of the treated soils compared closely to the control soils below 50 cm in the soil profile suggesting restricted movement of heavy metals. The NO₃-N content in the soil profile increased with the sludge application rate. The NO₃-N level of the Eugene and Hillsboro disposal areas was as high as 120 and 20 ppm, respectively. During the winter and spring, 1975, increased rainfall and cooler temperatures combined to decrease the NO₃-N content in the surface soil and increase NO₃-N levels in the lower soil horizons. The pH of the surface soil at the Eugene and Hillsboro disposal areas decreased from pH values of 6.4 to 4.6 and pH 4.8 to 4.4, respectively, a result of the nitrification reaction. The pH values of the other disposal areas compared closely to the control soil. The N and Zn content of the grass growing on the sludge-treated areas increased at the high sludge application rate compared to the grasses growing in the control area. The Cd, Cr, Cu, Mn, Ni, and P concentration increased in the grass sampled from the Eugene disposal area, while the Cd, Cr, Cu, Mn, Ni, and P content of grasses from the other sludge - treated areas compared more closely to the grasses from the control areas. The lower pH at the Eugene disposal area and the high sludge application rate combined to enhance the trace element uptake by plants. The NO₃-N, NH₄-N, P, Cd, Cr, Cu, Mn, Ni, and Zn content in the water samples from the Eugene, Milwaukie, Forest Grove, and Woodburn disposal areas were below the Public Health Service drinking water limits. The long-term disposal of municipal sewage sludges on agricultural land appears to be a viable waste disposal method, providing the sludge application rate and metal content are not excessively high. In any land disposal program for sewage sludge, the heavy metal accumulation in the soil surface and plants growing in the sludge-treated soil should be monitored.
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