The historic stream flooding and intense rainfall across the upper Mississippi River Basin during summer 1993 had an immediate effect on near-surface unconsolidated aquifers by raising the water levels closer to the land surface. The objective of this study was to determine if this flooding also had immediate effects on ground-water quality. Water samples were collected during September and October 1993 from 110 wells completed in near-surface unconsolidated aquifers and were analyzed for herbicides, herbicide metabolites, inorganic nutrients, and volatile organic compounds. The results of these samples were compared with those obtained during summer 1991 or 1992. Water samples from the Missouri River alluvial aquifer had a fourfold increase in the frequency of herbicide detection. There also appears to be a relation between increases in total herbicide concentration and the occurrence of stream flooding near a well. Water samples from wells that had at least a 20% increase in dissolved-oxygen concentration had the greatest frequency of substantial changes in total herbicide concentration and substantial increases in nitrate concentration.
From mid-June through early August 1993, flooding was severe in the upper Mississippi River Basin following a wet-weather pattern that persisted over the area for at least 6 months before the flood. The magnitude and timing of several intense rainstorms in late June and July, combined with wet antecedent climatic conditions, were the principal causes of the flooding. Flood-peak discharges that equaled or exceeded the 10-year recurrence interval were recorded at 154 streamflow-gaging stations in the upper Mississippi River Basin. At 41 streamflow-gaging stations, the peak discharge was greater than the previous maximum known discharge. At 15 additional gaging stations, peak discharges exceeded the previous maximum regulated peak discharge. At 45 gaging stations, peak discharges exceeded 100-year recurrence intervals.
Excessive precipitation produced severe flooding in a nine-State area in the upper Mississippi River Basin during spring and summer 1993. Following a spring that was wetter than average, weather patterns that persisted from early June through July caused the upper Midwest to be deluged with an unusually large amount of rainfall. Monthly precipitation data were examined at 10 weather-station locations in the flood-affected region to illustrate precipita tion patterns and amounts in the flood-affected area. In 1993, all 10 of the selected locations received greater than the normal rainfall for January through June 1961-90, 8 of the 10 locations received more than 200 percent of the normal rainfall for July 1961-90, and 3 received more than 400 percent of the normal rainfall for July. (The average rainfall for any given 30-year period is termed the "normal" rainfall for the given period.) May through July 1993 was the wettest or nearly the wettest such period on record at many locations in the flooded area. Of the 10 locations, 6 received more rainfall in the first 7 months of 1993 than generally is received in a year.
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