AGRICULTURAL CHEMICALS IN ALLUVIAL AQUIFERS IN MISSOURI AFTER THE 1993 FLOOD

Intense rains produced flooding during the spring and summer of 1993 o ver much of the midwestern USA including many agricultural areas of Mi ssouri. Because of potential contamination from floodwater, an investi gation was conducted to determine the changes in concentrations of agr icultural chemicals in water samples from alluvial wells in Missouri a fter the flood. Water samples from 80 alluvial wells with historical d ata were collected in March, July, and November 1994, and analyzed for dissolved herbicides, herbicide metabolites, and nitrate (NO3). There were no statistically significant differences in the distribution of alachlor loro-2'-6'-diethyl-N-[methoxymethyl]-acetanilide), atrazine ( 2-chloro-4-ethylamino-6-isopropylamino-1, 3, 5 triazine), and nitrate concentrations between pre- and postflood samples (alpha = 0.05). The detection frequency of alachlor and atrazine in postflood samples was generally lower than the frequency in preflood samples. Analyses of ag ricultural chemicals In water samples from an intensely sampled well f ield indicate significant differences between the distribution of diss olved P concentrations in pre- and postflood samples (alpha = 0.05). H owever, no significant differences were detected between the pre- and postflood distributions of NO3 or ammonia concentrations. Because of t he numerous sources of temporal variability and the relatively short r ecord of water-quality data for the study wells, a cause-and-effect re lation between changes in agricultural chemical concentrations and a s ingle factor of the 1993 flood is difficult to determine. Based on the results of this study, the 1993 flood did not cause widespread or lon g-term significant changes in concentrations of agricultural chemicals in water from alluvial aquifers in Missouri.