Temporal Trends of Selected Agricultural Chemicals in Iowa's Groundwater,
1982-95: Are Things Getting Better?
Dana W. Kolpin dwkolpin@usgs.gov,
Debra Sneck-Fahrer, George R. Hallberg, and Robert D. Libra
ABSTRACT
Since 1982, the Iowa Groundwater Monitoring (IGWM) Program has been
used to sample untreated groundwater from Iowa municipal wells for selected
agricultural chemicals. This long-term database was used to determine if
concentrations of select agricultural chemicals in groundwater have changed
with time. Nitrate, alachlor
[2-chloro-2´-6´-diethyl-
N-(methoxymethyl)-acetamide], atrazine
(2-chloro-4-ethylamino-6-isopropylamino-
s-triazine),
cyanazine
{2-[[chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropionitrile}, and
metolachlor
[2-chloro-
N-(2-ethyl-6-methylphenyl)-
N-(2-methoxy-1-methylethyl)acetamide]
were selected for this temporal analysis of the data. Conclusive temporal
changes in frequency of detection and median chemical concentrations were
found only for atrazine (decrease) and metolachlor (increase). The greatest
temporal chemical changes occurred in the shallowest wells and in alluvial
aquifers -- both relating to groups of wells generally having the youngest
groundwater age. The temporal patterns found for atrazine and metolachlor are
consistent with their patterns of chemical use and/or application rates and
are suggestive of a causal relation. Only continued data collection, however,
will indicate if the trends in chemical concentrations described here
represent long-term temporal patterns or only short-term changes in
groundwater. No definitive answers could be made in regards to the question
of overall improvements in groundwater quality with respect to agricultural
chemical contamination and time, due to the inherent problems with the
simplistic measurement of overall severity (summation of alachlor + atrazine
+ cyanazine + metolachlor concentrations) examined for this study. To
adequately determine if there is an actual decreasing trend in the overall
severity of contamination (improving groundwater quality), the collection of
additional water-chemistry data and the investigation of other measures of
severity are needed.
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