The effects of flooding and large amounts of rainfall during the summer of 1993 on the Sheyenne Delta aquifer were examined. Water levels were measured and water-quality samples were collected at 29 randomly distributed observation wells in the aquifer during late summer 1993. Water levels in an additional 30 wells also were measured every three weeks from November 1993 through May 1994. Additional water-quality samples were collected from 16 of the original 29 wells in November 1993 and in March, April, and May 1994.

Ground-water levels in the Sheyenne Delta aquifer responded quickly to rainfall and flooding. Ground-water levels rose more in areas away from the river where gradients are small and flow toward the river is slower than in areas near the river where gradients are large and flow toward the river is faster. In areas very near the river, however, ground-water levels rose in response to increased river stage caused by flooding, which decreased or reversed the normal ground-water gradients.

Ground-water quality of the Sheyenne Delta aquifer possibly was affected during the flood. Ground-water analyses showed low levels of ammonia and nitrates in the aquifer. Following the flood, water samples from three wells had detectable concentrations of picloram, indicating transport of pesticides to the water table. In addition, almost all samples had detectable concentrations of arsenic and selenium, and, in some cases, the concentrations exceeded State and Federal drinking-water standards. The source of arsenic and selenium to the aquifer is unclear, but potential sources include natural and anthropogenic sources.