Evaluation of soil water quality following the application of biosolids on a semiarid rangeland in Texas
Brenton, Cynthia Marie
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Biosolids are the by-product of wastewater treatment plants and it is estimated that six million metric tons of biosolids are produced annually in the United States. Biosolids contain large amounts of phosphorus and nitrogen as well as heavy metals and inorganic compounds. The amount of heavy metals and other constituents in the biosolids depends on the source and treatment process of the sewage sludge. At Sierra Blanca Ranch, Texas, biosolids are being applied at a rate of 6.6 dry Mg/ha to rangeland as a reclamation project. Aside from studies on the effects of sewage sludge on vegetation growth, it is also important to study the effects sludge has on soil water quality. In this study, two contrasting soil types on the ranch were chosen: a finer textured soil (Stellar loam) and a coarser textured soil (Armesa taxajunct fine sandy loam). Twenty-five 25.4 cm diameter, tube lysimeters were inserted into each of the soil types and a soil column was extracted. Five treatment rates of 0 (the control), 6.6, 17.6, 34 and 90 dry Mg/ha were randomly applied to the lysimeters with 5 repetitions. Water (simulated rainfall) was applied to each lysimeter to produce 1 liter of soil water leachate for analysis. Leachate was analyzed for 24 metals and other constituents such as sulfate, nitrate nitrogen, pH, ortho-phosphate, calcium hardness, total hardness and chlorides. All processes of simulated rainfall, leachate collection and analysis were conducted initially on freshly applied biosolids and 60-110 days later after the biosolids had aged. Results show that of 31 ANOVA tests preformed on the first leaching of the Stellar loam soil, 17 tests showed statistically significant differences in leachate among treatment rates. In the second leaching of the loam soil, 5 tests showed statistically significant differences among treatment rates. Nitrate nitrogen, pH, sulfate, and ortho-phosphate were significant in both sampling periods. Results from the Armesa taxajunct fine sandy loam were very different from the loam. The first leaching date had only 2 elements (Zn and Ba) that showed statistically significant differences among treatment rates. On the second leaching date, only Mg showed a statistical difference among the 5 treatments. For the commercial application rate (6.6 dry Mg/ha), heavy metals and nitrate nitrogen levels in the soil water leachate from both soils were found to be within guidelines set by the United States Environmental Protection Agency (USEPA) for drinking water on both sampling dates. In general, the leachate quality was statistically unaffected by 0, 6.6 and 17.6 dry Mg/ha rates and only began to show an increase with the 34 dry Mg/ha rate and the 90 dry Mg/ha rate. In some cases, even the 90 dry Mg/ha rate proved to be within drinking water quality standards on many of the metals and inorganic compounds. Finally, during a normal rainfall season, the leachate would not be expected to reach the groundwater table on the Ranch, which is in excess of 152.4 m. Total infiltration of the equivalent of 8.2 cm of precipitation over 24 hours was required to move water through 74.8 cm of the coarser Armesa taxajunct fine sandy loam soil.