The Great Flood of 1993 had far-reaching impacts upon surface waters in the State of Missouri. Many stations along the Missouri River were above flood stage for months during the summer of 1993. Researchers conducted extensive sampling during the flood and discovered high levels of agricultural contaminants in many of the samples. This result was surprising, as scientists had previously always assumed that the large volumes of water carried by floods dilute contaminants to lower-than-normal concentrations. The objectives of this study were to locate a series of wells along the Missouri River that could be used to track hydrological and contaminant trends in the alluvial aquifer, monitor those wells approximately monthly during the period of study, determine hydrological and biological/chemical trends in those wells, relate the monitoring results to water level and biological/chemical quality in the Missouri River, and make predictions with respect to impacts of future floods. Groundwater samples obtained during the study were analyzed for chemical and biological constituents used to indicate groundwater contamination. Parameters that are used to "fingerprint" waters--cations, anions, pH, conductivity, and temperature--were also determined. These parameters were obtained to determine if unusual amounts of surface waters had entered the groundwater system. Water level data showed a downward trend that was probably on-going at the beginning of the study. The trend appeared to be in excess of normal seasonal changes. However, there are too few sampling rounds to conclude that the downward trend was solely a remnant of the flood; part of the trend may be normal seasonal variation. The study showed that there were changes in most of the water quality parameters investigated. Statistical analysis indicated the "after flood" data distribution indicated a different population from the "before flood" data. Analysis also indicated that the aquifer chemistry for wells close to the river more closely resembled the chemistry of the river than did water from wells farther from the river; this pattern did not change over time. During the period of the study, coliforms were detected only intermittently and at low levels; pesticides were not detected.Project # G-2029-05 Agreement # 14-08-0001-G-2029-0
