By Mark E. Savoca, Eric M. Sadorf, S. Mike Linhart, and Kymm K.B. Akers
U.S. Geological Survey, Iowa City, IA
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Ground-water
samples were collected from monitoring wells at 31 agricultural and 30 urban
sites in the Eastern Iowa Basins study unit during June–August 1997 to
evaluate the effects of land use and hydrogeology on the water quality of
alluvial aquifers. Ground-water samples were analyzed for common ions,
nutrients, dissolved organic carbon, tritium, radon-222, pesticides and
pesticide metabolites, volatile organic compounds, and environmental isotopes.
Calcium,
magnesium, and bicarbonate were the dominant ions in most samples and were
likely derived from solution of carbonate minerals (calcite and dolomite)
present in alluvial detrital deposits. Chloride and nitrate were dominant anions
in samples from several wells. Sodium and chloride concentrations were
significantly higher in samples from urban areas, where roads are more numerous
and road salts may be more frequently applied, than in agricultural areas.
Nitrate was detected in 94 percent of samples from agricultural areas and 77
percent of samples from urban areas. Nitrate concentrations were significantly
higher in agricultural areas than in urban areas and exceeded the U.S.
Environmental Protection Agency maximum ontaminant level for drinking water (10
milligrams per liter as N) in 39 percent of samples from agricultural areas.
Nitrate concentrations in samples from urban areas did not exceed the maximum
contaminant level. Greater use of fertilizers in agricultural areas most likely
contributes to higher nitrate concentrations in samples from those areas.
Tritium-based ages indicate ground water was most likely recharged after the 1950’s at all but one sampling site. Agricultural and urban land-use areas have remained relatively stable in the study area since the 1950’s; therefore, the effects of current land use should be reflected in ground water sampled during this study. Radon-222 was detected in all samples and exceeded the U.S. Environmental Protection Agency’s previously proposed maximum contaminant level for drinking water (300 picocuries per liter) in 71 percent of samples.
Pesticides
were detected in 84 percent of samples from agricultural areas and 70 percent
from urban areas. Atrazine and metolachlor were the most frequently detected
pesticides in samples from agricultural areas; atrazine and prometon were the
most frequently detected pesticides in samples from urban areas. None of the
pesticide oncentrations exceeded
U.S. Environmental Protection Agency maximum contaminant levels or lifetime
health advisories for drinking water. Pesticide metabolites were detected in 94
percent of samples from agricultural areas and 53 percent from urban areas.
Metolachlor ethane sulfonic acid and deethylatrazine were the most frequently
detected metabolites in samples from agricultural areas; metolachlor ethane
sulfonic acid and alachlor ethane sulfonic acid were the most frequently
detected metabolites in samples from urban areas.
Total
metabolite concentrations were significantly higher in samples from agricultural
areas than in samples from urban areas. Total pesticide concentrations (parent
compounds) tended to be higher in samples
from agricultural areas; however, this difference was not statistically
significant. Metabolites constituted the major portion of the total residue
concentration in the alluvial aquifer.
Volatile
organic compounds were detected in 40 percent of samples from urban areas and 10
percent from agricultural areas. Methyl tertbutyl ether was the most commonly detected
volatile organic compound and was present in 23 percent of samples from urban
areas. Elevated concentrations (greater than 30 micrograms per liter) of methyl
tert-butyl ether and BTEX compounds (benzene, toluene, ethylbenzene, and xylene)
in two samples from urban areas suggest the possible presence of point-source
gasoline leaks or spills.
Factors
other than land use may contribute to observed differences in water quality
between and within agricultural and urban areas. Nitrate, atrazine,
deethylatrazine, and deisopropylatrazine concentrations were significantly
higher in shallow wells with sample intervals nearer the water table and in
wells with thinner cumulative clay thickness above the sample intervals. These
relations suggest that longer flow paths allow
for greater residence time and increase opportunities for sorption, degradation, and dispersion, which may contribute to decreases in nutrient and pesticide concentrations with depth. Nitrogen speciation was influenced by redox conditions. Nitrate concentrations were significantly higher in ground water with dissolved-oxygen concentrations in excess of 0.5 milligram per liter. Ammonia concentrations were higher in ground water with dissolved-oxygen concentrations of 0.5 milligram per liter or less; however, this relation was not statistically significant. The amount of available organic matter may limit denitrification rates. Elevated nitrate concentrations (greater than 2.0 mg/L) were significantly related to lower dissolved organic carbon concentrations in water samples from both agricultural and urban areas. A similar relation between nitrate concentrations (in water) and organic carbon concentrations (in aquifer material) also was observed but was not statistically significant
Abstract.
Introduction.
Purpose and Scope
Description of the Study Area
Acknowledgments
Study Design and Methods
Site Selection
Well Installation
Land-use Classification
Statistical Analysis
Effects of Land Use and Hydrogeology on Water Quality
Common Ions
Nutrients
Radiochemicals
Pesticides and Metabolites
Volatile Organic Compounds
Summary and Conclusions
References.
Appendix.
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