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EIWA Abstracts
Quality
of Water in Alluvial Aquifers in Eastern Iowa
Mark E. Savoca, Eric M. Sadorf, S. Michael Linhart, and Kimberlee
K. Barnes
Hydrologists
U.S. Geological Survey
Iowa City, Iowa
Proceedings from Agriculture and the Environment: State and Federal
Initiatives conference at Iowa State University, March 5-7, 2001
The goal of the U.S. Geological Survey (USGS) National Water-Quality
Assessment (NAWQA) Program is to assess the status and trends
in the quality of the Nation's surface and ground water, and to
better understand the natural and human factors affecting water
quality. The Eastern Iowa Basins study unit encompasses an area
of about 50,500 square kilometers (19,500 square miles) in eastern
Iowa and southern Minnesota and is one of 59 study units in the
NAWQA program. Land-use studies are an important component of
the NAWQA program, and are designed to assess the concentration
and distribution of water-quality constituents in recently recharged
ground water associated with the most significant land use and
hydrogeologic settings within a study unit. The focus of the land-use
study in the Eastern Iowa Basins study unit is agricultural and
urban land uses and alluvial aquifers. Agriculture is the dominant
land use in the study unit. Urban areas, although not extensive,
represent important potential source areas of contaminants associated
with residential, commercial, and industrial activities. Alluvial
aquifers are present throughout much of the study unit, and constitute
a major ground-water supply that is susceptible to contamination
from land-use activities.
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. 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. 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. 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
contaminant 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 usage of fertilizers in agricultural
areas most likely contributes to higher nitrate concentrations
in samples from those areas.
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
concentrations exceeded U.S. Environmental Protection Agency maximum
contaminant levels or lifetime health advisories for drinking
water. Pesticide degradates 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 degradates in samples from urban areas.
Total degradate 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. Degradates 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
tert-butyl 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, suggesting that longer flow
paths allow for greater residence time and increase opportunities
for sorbtion, 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 milligrams per
liter. Ammonia concentrations were higher in ground water with
dissolved-oxygen concentrations of 0.5 milligrams 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.
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