TEMPORAL TRENDS OF SELECTED AGRICULTURAL CHEMICALS IN IOWA GROUNDWATER, 1982-1995 - ARE THINGS GETTING BETTER

Since 1982, the Iowa Groundwater Monitoring (IGWM) Program has been us ed to sample untreated groundwater from Iowa municipal wells for selec ted agricultural chemicals. This long-term database was used to determ ine if concentrations of select agricultural chemicals in groundwater have changed with time, Nitrate, alachlor loro-2'-6'-diethyl-N-(methox ymethyl)-acetanilide], atrazine -chloro-4-ethylamino-6-isopropylamino- s-triazine), cyanazine 6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-meth yl- propionitrile)} and metolachlor [2-chloro-N- -ethyl-6-methylphenyl )-N-(2-methoxy-1-methylethyl) acetamide] were selected for this tempor al analysis of the data. Conclusive temporal changes in frequency of d etection and median chemical concentrations were found only for atrazi ne (decrease) and metolachlor (increase), The greatest temporal chemic al changes occurred in the shallowest wells and in alluvial aquifers-b oth relating to groups of wells generally having the youngest groundwa ter age, The temporal patterns found for atrazine and metolachlor are consistent with their patterns of chemical use and/or application rate s and are suggestive of a causal relation. Only continued data collect ion, however, will indicate if the trends in chemical concentrations d escribed here represent long-term temporal patterns or only shortterm changes in groundwater, No definitive answers could be made in regards to the question of overall improvements in groundwater quality with r espect to agricultural chemical contamination and time, due to the inh erent problems with the simplistic measurement of overall severity (su mmation of alachlor + atrazine + cyanazine + metolachlor concentration s) examined for this study, To adequately determine if there is an act ual decreasing trend in the overall severity of contamination (improvi ng groundwater quality), the collection of additional water-chemistry data and the investigation of other measures of severity are needed.