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AbstractThe Metro Wastewater Reclamation District (Metro District) in Denver, Colo., applied biosolids resulting from municipal sewage treatment to farmland in eastern Colorado beginning in December 1993. In mid-1993, the U.S. Geological Survey in cooperation with the Metro District began monitoring water quality at the biosolids-application area about 10 miles east of Deer Trail, Colo., to evaluate baseline water quality and the combined effects of natural processes, land uses, and biosolids applications on water quality of the biosolidsapplication area. Water quality was characterized by baseline and post-biosolids-application sampling for selected inorganic and bacteriological constituents during 1993 through 1998, with some additional specialized sampling in 1999. The study included limited sampling of surface water and the unsaturated zone, but primarily focused on groundwater.The baseline data indicate that major-ion, nutrient, trace-element, and bacteria sources other than biosolids were present in the study area and that water in the study area was of variable quality before biosolids ever were applied. Large concentrations of chloride, sulfate, nitrate, iron, and manganese were detected in some baseline samples. Concentrations of other major ions, phosphorus, cadmium, chromium, nickel, and fecal streptococcus bacteria also were detected above the minimum reporting level in baseline samples.Variability in water quality of the study area was evident from the baseline data as well as from the data collected after biosolids applications began. The hydrology of the study area likely accounts for some of the variability in water quality; groundwater recharged preferentially from ponds and through desiccation cracks and coarse-grained deposits and outcrops, not uniformly through the unsaturated zone beneath all fields. Spatial variability was largest for concentrations of sulfate, nitrate, iron, and manganese. Soil pore-water data indicated that chloride, nitrate, and copper reservoirs in the upper 3 feet of the unsaturated zone could eventually migrate to the saturated zone. Temporal...