The U.S. Geological Survey, in cooperation with the City of Fort Madison, conducted a study of the potential sources and possible processes that might produce ammonia in ground water from the Mississippi River alluvium near Fort Madison, Iowa. High ammonia concentrations of 3.0 to 6.4 milligrams per liter were detected in water produced from municipal well Fort Madison #4 from 1992 through 1996. Total organic carbon concentrations of about 5 milligrams per liter were also reported during the same time period.

Water-level measurements from two municipal wells and one well at Iowa State Penitentiary were used to determine that the general direction of ground-water flow during the two days of sampling was from northeast to southwest toward wells Fort Madison #4 and Fort Madison #5. Continuous pumping of other nearby wells probably influenced the direction of ground-water flow on these days. Because of a till confining unit separating the Mississippi River and the alluvial aquifer, it is uncertain to what extent induced recharge occurs to the alluvial aquifer from the river.

Water samples were collected from three municipal wells, two industrial wells, one watersupply well at the Iowa State Penitentiary, one domestic well, two creeks, and the Mississippi River. Tritium analytical results indicate that the time of recharge was after 1953 for water from the municipal wells (where tritium samples were collected). The ammonia concentration, 5.1 milligrams per liter, was considerably higher in ground water collected from well Fort Madison #4 compared to samples collected from other wells. The dissolved organic carbon concentration was also highest in ground water collected from well Fort Madison #4 (5.5 milligrams per liter) although samples from other wells were also relatively high, 2.6 to 5.3 milligrams per liter. The most likely process producing the high ammonia concentration in ground water in the vicinity of well Fort Madison #4 is decomposition of organic matter (ammonification). Nitrogen isotope values, when compared to nitrogen isotope values found in ground water in the vicinity of some of the other wells, indicate the possibility that a higher percentage of the ammonia found in ground water in the vicinity of well Fort Madison #4 may be from human and(or) other animal wastes. Higher concentrations of dissolved phosphorus, orthophosphate, and a higher chloride-to-sodium mole ratio found in ground water sampled from well Fort Madison #4 could be indicative of a sewage or septic source. Potential sources include existing or abandoned septic systems, leaking sanitary sewer lines, commercial or residential waste-disposal sites, and agricultural runoff upgradient from well Fort Madison #4, or possibly, bed sediments of the Mississippi River.