Ground-water flow and effects of agricultural application of sewage sludge and other fertilizers on the chemical quality of sediments in the unsaturated zone and ground water near Platteville, Colorado, 1985-89

Abstract

About one-third of the municipal sewage sludge produced in the United States is used as a source of organic matter, nitrogen, phosphorus, and other nutrients for crop production in agricultural areas. The Metro Wastewater Reclamation District applies sewage sludge on selected agricultural land as part of its beneficial-reuse program. From fall 1985 through 1989, 6,431 dry tons of anaerobically digested sewage sludge were applied on about 1 square mile of sandy farmland near Platteville, Colorado. The sludge, which was injected or plowed about 6 10 inches into the soil, commonly is about 17 percent solids and has about 6 percent total nitrogen, analyzed by total kjeldahl nitrogen method, primarily in complex organic compounds. Data collected during 1985 89 were used to determine the rate and direction of groundwater flow and the effects of sewage sludge and other fertilizers on the quality of sediments in the unsaturated zone and ground water. Ground water in the surficial aquifer in section 16 (T. 3 N., R. 66 W., Sixth Principal Meridian) generally moves northeastward at a velocity of about 0.01 foot per day (3 feet per year). Precipitation and irrigation water reaching the ground surface in section 16 infiltrates into the sandy soil and collects in temporary ponds. There is little or no runoff. Most water that infiltrates into the soil evaporates or is transpired by crops. The remaining water, if any, continues to move down to the saturated zone and recharges the surficial aquifer. Generally, there is little recharge by precipitation to the aquifer in this semiarid area. However, where irrigation water is applied or the water table is close to the surface, the surficial aquifer is recharged. Nitrogen concentrations generally increased and trace-element concentrations changed minimally in the unsaturated zone in the irrigated part of section 16 after 4 years of applying sewage sludge and other fertilizers to the soil. Analyses of water samples from multilevel ground-water sampling devices indicate that nitrite plus nitrate as nitrogen concentrations changed with depth and time in the surficial aquifer. Most nitrogen probably moves vertically downward through the unsaturated zone with the water that infiltrates from irrigated areas and low, temporarily ponded areas. The nitrogen then moves laterally through the saturated zone. Trace elements probably have not moved into the ground water from the sewage sludge. Mean nitrite plus nitrate as nitrogen concentrations in the surficial aquifer increased during the period of sewage-sludge application. However, the addition of commercial inorganic fertilizer during this period could have caused at least some of this increase. Areas having the largest concentrations of nitrite plus nitrate as nitrogen were in the northeastern and southwestern quarters of section 16.