Application of municipal biosolids to a semiarid grassland soil: Effects on soil water quality
Shanks, Christopher Howard
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Land application of biosolids has become an important disposal option. It has been estimated that over six million metric tons are produced annually in the United States. In general biosolids, a by-product of waste water treatment, contain many macro and micro nutrients as well as some toxic heavy metals and trace inorganics. Currently, a commercial land application project is underway in which biosolids are applied to native rangeland on the Sierra Blanca Ranch, near Sierra Blanca, Texas. A study was initiated to evaluate the impact of land application of biosolids on soil water quality and how biosolids influence water movement in the soil. Soil micro lysimeters constructed from PVC pipe were used to obtain soil core samples to depths of 45 and 90 cm for two contrasting soils on the Sierra Blanca Ranch. The soils studied were the coarser textured Armesa fine sandy loam (90 cm columns) and the finer textured Stellar loam (45 cm columns). Biosolids were applied at rates of 0, 7, 18, 34, and 90 dry Mg/ha and incorporated into the top 10 cm of the soil. Water was added to simulate rainfall and leachate was collected from the lysimeters. Quality of water was analyzed using a colorimeter for pH, nitrate-nitrogen, orhtophosphate, sulfate, total hardness, calcium hardness, and chlorides. An additional 20 water quality parameters such as heavy metals, trace inorganics and other elements were tested using Inductively Coupled Plasma (ICP). There were four leaching trials conducted over 13 months. For the Armesa soil, data were analyzed using a repeated measures ANOVA. Of the 27 water quality parameters tested, 13 were significantly affected by an interaction of leaching date and biosolids rate. Of the 14 elements remaining, 3 were significantly affected by leaching date, 2 elements were significantly affected by biosolids rate, and 6 elements were nonsignificant for all factors. Cadmium was above EPA drinking water quality standards at the 0 and 90 Mg/ha rates, while nitrate-nitrogen was above water standards at all rates during all leachings. Generally, as biosolids application rate increased, infiltration, moisture retention, and water yield increased. For the Stellar loam, infiltration was reduced to a point where no water yielded from many of the lysimeters. The data were analyzed using a one-way ANOVA each leaching individually because of the missing data encountered throughout the leaching experiments. During the first leaching, 10 parameters were affected by biosohds rate. The second leaching had 3 parameters affected by biosohds rate, while the third and fourth leachings 5 and 10 parameters were affected by biosolids, respectively. During all leachings, infiltration was low while biosolids did not significantly enhance infiltration. Generally, for those parameters that were significant, as biosolids rate increased, concentration of that parameter increased. Nitrate-nitrogen was above EPA guidelines for drinking water at all rates during all leachings. Some movement of cadmivun and lead occurred in both soils, but concentrations were barely above detection limits. Anions such as nitrate-nitrogen and sulfate were found to be highly mobile and leachate concentrations were increased by the incorporation of biosolids. Incorporation of biosolids in this ecosystem is not a desirable disposal method of biosolids because of the adverse effects on the soil's physical and chemical properties.