Effects of a cattle feedlot on ground-water quality in the South Platte River Valley near Greeley, Colorado

Changes in water quality in an alluvial aquifer resulting from the operation of a feedlot stocked with 90,000 cattle have been minimal. Monitoring water quality in 19 observation wells from April 1974, prior to the operation of the feedlot, to June 1978, after about 4 years of operation, indicates that chloride concentrations have increased slightly in one well downgradient from a runoff-retention pond. Chemical analyses of water from two lysimeters installed in the unsaturated zone indicate that leachate from the feedlot has percolated to a depth of at least 5 feet but has not percolated to a depth of 20 feet. The small changes in ground-water quality caused by the feedlot are likely the result of the limited available recharge, a relatively impermeable manure pack, soil clogging under the cattle pens resulting in slow vertical movement of leachate through the unsaturated zone, soil clogging under the unlined runoff-retention ponds, and denitrification in the unsaturated zone.