|
|
|  |
Cedar River Project - Abstracts
Groundwater How and Water Quality – A Flowpath
Study in the Seminole Well Field, Cedar Rapids, Iowa
Douglas J. Schnoebelen, Ph.D.
United States Geological Survey Iowa City, Iowa
Michael J. Turco
United States Geological Survey Lincoln, Nebraska
John D. North
Cedar Rapids Water Department
Cedar Rapids, Iowa
In Iowa, alluvial aquifers near major rivers are a source of water for many communities.
The City of Cedar Rapids withdraws water from wells completed in the Cedar River alluvium,
a shallow alluvial aquifer adjacent to the Cedar River. The City of Cedar Rapids is located
within Linn County in east-central Iowa, and water for the City is supplied by four well fields
(East, Northwest, Seminole, and West well fields) along the Cedar River. The City has a
population of about 121,000, and several large industries are major water users. Currently,
per capita water usage in the City is nearly three times the national average. The City is
committed to providing both a high quality and quantity of water to its customers. The USGS
and Cedar Rapids Water Department have been working together in an ongoing research program
to better understand water quality and flow in the Cedar River and alluvial well fields.
Work has been done on both a basin and well-field approach and has involved dye tracing/time-of-travel
studies on the Cedar River, water-quality sampling, geochemical modeling, and groundwater-flow modeling.
The effect of land use in the Cedar River Basin on both surface-water and groundwater quality is
an important issue. The Cedar River Basin upstream from Cedar Rapids is approximately 6,500
square miles. Upstream land use in the Cedar River Basin is over 90-percent agriculture. Com
and soybeans are the major crops. Livestock raised in the area include beef and dairy cattle,
as well as hogs. Runoff from agriculture is of concern, particularly during the spring and
early summer when many chemicals are applied to cropland. Triazine and acetanilide herbicides
are commonly applied in the Cedar River Basin, and these herbicides are water soluble and
can be transported to streams and infiltrate to groundwater. In addition, several studies in
eastern Iowa have identified nutrients as a major contaminant that has impaired water quality
(Goolsby and Battaglin, 1993; Hallberg et al., 1996; Schnoebelen et al., 1999; Kalkhoff et
al., 2000). In general, the majority of nitrogen inputs in the Cedar River Basin are from
chemical fertilizers and animal manure (Becher et al., 2000). High nitrate levels (greater
than 10.0 mg/L) in the Cedar River are of particular concern to municipal water operators.
The lower Cedar River is listed on the Iowa total maximum daily load list for nitrate upstream
of Cedar Rapids, Iowa. The Cedar River is the source of most nitrate detected in the Cedar
River alluvial aquifer because of induced infiltration from the river due to pumping (Schulmeyer
and Schnoebelen, 1998; Boyd, 2000).
An unconsolidated surficial layer of glacial till, loess, and Cedar River alluvium (alluvial aquifer) overlies
carbonate bedrock of Devonian and Silurian age (bedrock aquifer) in the study area. The alluvial aquifer typically
consists of a sequence of coarse sand and gravel at the base, grading upwards to finer sand, silt, and clay near
the surface. The sand and gravel contain carbonate, shale, and ferro-magnesium rich rock fragments. The thickness of
the alluvial aquifer ranges from about 2 to 30 m. The alluvial aquifer is recharged by the infiltration of water
from the Cedar River, precipitation, and seepage from the underlying bedrock and adjacent hydrogeologic units. In
areas under the influence of municipal pumping, groundwater flow is from the Cedar River toward the well fields;
in areas outside the influence of municipal pumping, groundwater flow is toward the Cedar River. Results from a
regional groundwater flow model indicated mat approximately 74 percent of the water pumped from the alluvial well
fields is recharged from the Cedar River, approximately 21 percent is recharged from adjacent underlying hydrogeologic
units, and approximately 5 percent of the water is from infiltrating precipitation (Schulmeyer and Schnoebelen, 1998).
Currently, a more detailed groundwater model in the study area indicates that, in some places, up to 90 percent of me
water pumped from the alluvial well fields is recharged from the Cedar River.
The water quality in the alluvial aquifer within me well field has been characterized with samples collected from both
monitoring and municipal wells at various times since 1992 (Boyd, 2000; Schulmeyer and Schnoebelen, 1998; Schnoebelen
and Schulmeyer, 1996). Calcium, magnesium, and bicarbonate are me dominant ions. In addition, nitrate, sulfate, silica,
iron, and manganese are present in significant concentrations in certain wells or at certain times of me year. Previous
work in me Seminole well field indicated some detections of herbicides and their degradates (breakdown products) in shallow
monitoring wells (3.8- to 6-m deep) completed in me alluvium as water moved from me river into me alluvial aquifer (Boyd,
2000). Atrazine was the most commonly detected herbicide in mis study. Acetochlor, cyanazine, and metolachlor were also
detected, but at smaller concentrations than atrazine. Acetanilide degradates were detected at greater frequencies and at
greater concentrations than their corresponding parent compounds. Fewer numbers of detections of herbicide compounds were
found in wells completed deeper in the alluvium.
The infiltration of water with large nitrate concentrations into the alluvial aquifer from the Cedar River affects groundwater
quality. Recent research was conducted along a flowpath to study RBF through a natural wetland area. Groundwater modeling helped
locate the flowpath study. The study examined the role of a natural wetland in reducing nitrate concentrations as water moves
from the Cedar River. A real challenge for me Cedar Rapids Water Department is the increasing trend of nitrate concentrations
in the Cedar River. Nitrate concentrations in me Cedar River during the spring are often more than 10 mg/L and can reach
20 mg/L. A 2- to 3-mg/L reduction in nitrate often occurs as water moves from me river to the well, but in some wells, this
may not reduce nitrate concentrations below the 10.0-mg/L maximum contaminant level. Sampling in
wells along a flowpath occurred quarterly over a period of about 4 years. A comparison of water
chemistry was made from water analyses from:
-The river.
-A monitoring well upgradient of the wetland area and river.
-Wells in me wetland area.
-Wells between the wetland area and river.
In addition, a comparison of water-chemistry data from a municipal well located near the wetland area and one located nearest the
river were compared in terms of water chemistry from previous sampling work (Schulmeyer and Schnoebelen, 1998). Results show that
nitrate concentrations were 4 to 6 times lower in samples from monitoring wells completed in the wetland area than in the Cedar
River or groundwater in the upland area; however, iron and manganese concentrations in samples from the monitoring wells in the wetland
areas were an order of magnitude higher when compared to the river or upland well. Water samples from the wells and the Cedar River
generally displayed similar trends (high in the spring and low in the fall), while iron and manganese concentrations were more variable.
As water moves from the river towards the monitoring wells, microorganisms obtain energy for metabolic processes by catalyzing the
oxidation of organic matter with a progressive series of reducing reactions (Stumm and Morgan, 1981). Nitrate can be reduced to elemental
nitrogen (Nz) by denitrification (Equation 1) or to ammonium (NH4+) by reduction (Equation 2). Since ammonium was only detected in small
quantities (less than 0.80 mg/L), denitrification most likely is the predominant process.
4N03- + 5CHzO + 4H+ = 2Nz(g) + 5COz + 7HzO (Equation 1)
N03- + 2CHzO + 2H+ = N~ + + HzO + 2COz (Equation 2)
Reduction then proceeds from nitrate (N03-) to Mn+4, Fe+3, S04-z, COz, and Nz. The reduced
forms of iron (Fe II) and manganese (Mn II) are more soluble in water and are more mobile than oxidized forms (Hem, 1985) and, under anoxic
conditions, are stable. As nitrate in groundwater is depleted, iron and manganese reduction begins. The reduction of Fe+3 to Fe+z and Mn+4
to Mn+z from aquifer grain coatings can cause large concentrations of these ions in groundwater. Ferrihydrite and manganite (MnOOH) occurring
as oxyhydroxide coatings on clay and silt particles are the most likely oxidized forms of iron (Fe+3) and manganese (Mn+3 and Mn+4) in the
alluvial aquifer. Oxidized forms of iron and manganese might occur in the aquifer as crystalline minerals, such as hematite (Fez03) and
hausmannite (Mn304). Iron and manganese may co-precipitate with carbonate minerals to cause well fouling.
Research in the Seminole well field indicates that the location of a well in or near natural wetland areas may benefit from the natural
reduction of nitrate concentrations, with the disadvantage of increased iron and manganese concentrations. Future expansions of the well
fields may take advantage of natural wetland areas to help reduce nitrate concentrations. In Iowa, most wetlands have been drained, but
alluvial wetlands associated with bottomland forested and oxbow lake areas may persist as they are subject to periodic flooding and are
often not suitable for sustained agriculture.
REFERENCES
Becher, K.D., D.]. Schnoebelen, and K.K.B. Akers (2000). "Nutrients discharged to the Mississippi River from Eastern Iowa Watersheds, 1996-97."
Journal AWWA, 36(1): 161-173.
Boyd, R.A. (2000). "Herbicides and herbicide degradates in shallow groundwater and the Cedar River near a municipal well field, Cedar Rapids, Iowa.
" The Science of the Total Environment, 241-253.
Goolsby, D.A., and W.A. Battaglin (1993). "Occurrence, distribution, and transport of agricultural chemicals in surface water of the Midwestern
United States." Selected papers on agricultural chemicals in water resources of the midecontinental United States, D.A. Goolsby, L.L. Boyer, and
G.E. Mallard, compilers, U.S. Geological Survey Open-File Report 93-418, p. 1-25.
Hallberg, G.R., D.G. Riley, J.R. Kantamneni, P.J. Weyer, and R.D. Kelley (1996). Assessment of Iowa safe
drinking water act monitoring data, 1988-1995, Iowa City, University of Iowa Hygienic Laboratory Research
Report 97-1,132 p.
Hem, J.D. (1985). Study and interpretation of the chemical characteristics of natural water, third edition, U.S.
Geological Survey Water-Supply Paper 2254, p. 264.
Kalkhoff, S.J., K.K. Barnes, K.D. Becher, M.E. Savoca, D.J. Schnoebelen, E.M. Sadorf, S.D. Porter, and D.J.
Sullivan (2000). Water Quality in the Eastern Iowa Basins, Iowa and Minnesota, 1996-98, U.S. Geological
Survey Circular 1210, 37 p.
Schnoebelen, D.J., K.D. Becher, M. w: Bobier, and T. Wilton (1999). Selected nutrients and pesticides in streams
of the Eastern Iowa Basins, 1970-95, U.S. Geological Survey Water-Resources Investigations Report 99-4028,
105 p.
Schnoebelen, D.J., and P.M. Schulmeyer (1996). Selected hydrogeologic data in the Cedar Rapids area, Benton and Linn Counties, Iowa, October 1992 through March 1996, U.S. Geological Survey Open-File Report
96-471, 172 p.
Schulmeyer, P.M., and D.J. Schnoebelen (1998). Hydrogeology and water-quality in the Cedar Rapids Area, Iowa, 1992-96, U.S. Geological Survey
Water Resources Investigations Report 97-4261,77 p.
Stumm W., and J.J. Morgan (1981). Aquatic Chemistry - An introduction emphasizing chemical equilibrium in natural waters, second edition,
Wiley-Interscience Publishers, New York, 780 p.
DOUG SCHNOEBELEN is a Research Hydrologist with the United States Geological Survey and has worked on a variety of groundwater and surface-water
projects over the last 13 years. He has served as the groundwater specialist on the White River National Water Quality Assessment project in Indiana
and as the surface-water specialist for the Eastern Iowa Basins National Water Quality Assessment project in Iowa. In addition, he has been the Iowa
District Water-Quality Specialist since 1994, and is an Adjunct Professor in the Geoscience Department at the University of Iowa. His research has
focused on the use of isotopes and bore hole geophysics in groundwater and, more recently, on the fate and transport of agricultural chemicals in
surface water and groundwater across eastern Iowa. In particular, he has focused on the fate and transport of several pesticide degradate compounds.
He has been involved with ongoing research in riverbank filtration and geochemical modeling in the Cedar Rapids well field since 1999. Schnoebelen
received a B.S. in Geology from the University of Iowa, an M.S. in Geology from the University of Tennessee, and a Ph.D. in Geology, with a minor in
Environmental Science, from Indiana University.
|
|
|
