Hydrogeochemical Interaction Between a Municipal Waste Stabilization Lagoon and a Shallow Aquifer

Unlined municipal waste stabilization lagoons are potential sources of ground-water contamination. Fourteen monitoring wells were installed around the Mc Ville, North Dakota lagoon, a site at which the impoundment is excavated into permeable sediments of an unconfined glacio-fluvial aquifer with a shallow water table. One cell at the site, Cell I, retains waste water continuously, while another, Cell II, is used for periodic overflow discharges from Cell I. Seepage through the bottom of Cell I passes through a strongly reducing organic sludge layer. Sulfate in the waste water is reduced to sulfide and possibly precipitated as sulfide minerals in or below this sludge layer. In the unsaturated or shallow saturated zone beneath the pond, the infiltrating waste water reduces ferric iron in iron oxide minerals to more soluble ferrous iron. Proximal down-gradient well analyses indicate high iron concentrations and very low sulfate levels. Downgradient wells near the lagoon have very high ammonium concentrations. The source of the ammonium is either rapid infiltration from Cell II or denitrification of the nitrate present in ground water upgradient from the lagoon. About 300 feet downgradient from Cell I, ammonium concentrations decline to near zero. The most likely mechanism for this decrease is cation