ASSESSMENT OF GROUND-WATER FLOW AND QUALITY: CEDAR RIVER ALLUVIUM, CEDAR RAPIDS, IOWA
PERIOD OF PROJECT:
1992-2003
PROJECT CHIEF:
D.J. Schnoebelen (djschnoe@usgs.gov)
STUDY AREA:
Linn County
COOPERATING AGENCY:
City of Cedar Rapids (Water Department)
Additional research has been provided by USGS Biological Resources Discipline and USGS National Mapping Discipline.
The Iowa Department of Natural Resources has cooperated on a dye-tracing test of the Cedar River.
NEED FOR STUDY:
The City of Cedar Rapids, Iowa obtains its municipal water supply from
shallow (less than 100 feet below land surface) wells completed in the
Cedar River alluvium, an alluvial aquifer adjacent to the Cedar River. A
total of 53 vertical and 2 horizontal-collector wells are installed in 4
well fields (East, Northwest, Seminole, and West). Since 1963, the alluvial
aquifer has provided adequate quantities of generally high-quality water.
Increasing population and industrial development have steadily increased
the demand for water pumped from the alluvial aquifer. The City pumped
about 8,847 million gallons (Mgal) in 1980, about 9,118 Mgal in 1990, and
about 13,140 Mgal in 1997.
Managers of the City of Cedar Rapids Water Department are concerned
about meeting the steady increase in demand for municipal water and
protecting the water quality in the alluvial aquifer to ensure a safe water
supply for their customers. The managers require information to determine
the maximum safe yield of the alluvial aquifer, to plan for additional
withdrawals to meet future demands, to identify possible options to reduce
the infiltration of contaminants from the Cedar River into the alluvial
aquifer, and to satisfy requirements for source-water protection programs
under the Safe Drinking Water Act.
OBJECTIVES:
The objectives of the project are to:
- evaluate the ground-water flow system and to quantify
the interaction between the alluvial aquifer, Cedar River,
and underlying carbonate bedrock aquifer.
- evaluate pumping scenarios in the alluvial aquifer and
to evaluate potential locations for additional water-supply
wells.
- characterize water-quality in the alluvial aquifer,
Cedar River, and underlying carbonate bedrock aquifer.
- identify and evaluate options to reduce infiltration
and associated contaminant transport from the Cedar River to
the alluvial aquifer.
- evaluate the effects of wetlands and ponds on recharge
and water quality in the alluvial aquifer.
- compile and assess data required for source-water
protection programs under the Safe Drinking Water Act.
- assess the fate and transport of nutrients in the Cedar
River Basin using time of travel, synoptic and Lagrangian water sampling.
PROGRESS:
Hydrologic and geologic data from over 300 existing wells
near the municipal well fields have been compiled. A network
of observation wells (consisting of 59 small-diameter wells
[0.14-inch outer-diameter], 15 4-inch outer-diameter wells,
and 13 2-inch outer-diameter wells) has been installed.
Geophysical investigations and lithologic logging have been
used to determine the extent and thickness of the alluvial
aquifer within the study area. Water levels in selected observation
wells are periodically measured and continuously monitored
with pressure transducers. A regional ground-water flow model
covering about 231 square miles has been constructed to
simulate ground-water flow and determine sources of water to
the alluvial aquifer under steady-state conditions. Results
of the regional ground-water flow model indicate about 74
percent of water pumped from the alluvial aquifer is induced
infiltration from the Cedar River, about 21 percent is from
adjacent and underlying hydrogeologic units, and about 5
percent is from precipitation. A detailed ground-water flow
model (with greater resolution than the regional flow model)
is being constructed to simulate ground-water flow under
transient conditions, evaluate pumping scenarios, and
determine capture zones for individual supply wells. Physical
parameters (water temperature, pH, specific conductance, and
dissolved oxygen) have been continuously monitored in the
Cedar River and selected observation wells. Over 300
water-quality samples have been collected from observation
wells, municipal supply wells, and the Cedar River. Nitrite
plus nitrate (nitrate) and herbicides are the primary threats
to water quality in the alluvial aquifer. Most nitrate and
herbicides detected in the alluvial aquifer likely are
transported with induced infiltration from the Cedar River.
Nitrate concentrations in the Cedar River typically are
greatest (<11.0 milligrams per liter) in the spring and fall,
which corresponds to periods of fertilizer and manure
applications to upstream cropland. Nitrate concentrations in
the alluvial aquifer tend to be less than nitrate
concentrations in the Cedar River, but also typically are
greatest in the spring and fall. Triazine herbicides (such as
atrazine and cyanazine) and acetanilide herbicides (such as
acetochlor, alachlor, and metolachlor) are greatest in the
late spring and early summer, following herbicide
applications to upstream cropland. Atrazine is the most
frequently detected herbicide and is detected at the greatest
concentrations (<8.2 micrograms per liter in the Cedar
River). Relatively large concentrations of dissolved iron
(<20 milligrams per liter) and dissolved manganese (<7.5
milligrams per liter) have been detected in some wells. The
relatively large dissolved-iron and -manganese concentrations
are produced in localized areas by microbial-catalyzed
oxidation-reduction reactions. A study of ground-water
geochemistry in the Seminole Well Field indicated that
carbonate-equilibrium reactions, weathering of
aluminosilicate minerals, cation exchange, and
oxidation-reduction reactions affect water-quality in the
alluvial aquifer.
The water-quality of the Cedar River has a large effect on
the water chemistry of the Cedar Rapids alluvial aquifer. In particular,
research since 2000 has focused on a better understanding of flow and
transport of nutrients in the Cedar River Basin. A series of synoptic
studies have been carried out to assess bacteria and nutrient concentrations
in the Cedar River Basin, both at low-flow and high flow. In addition,
dye tracing studies using a nontoxic dye have been completed from Waterloo
to Cedar Rapids to better understand the actual time of travel of compounds
in the Cedar River. A Lagrangian sampling (where the same mass of water is
sampled moving downstream) was recently completed in the fall of 2003.
Results from the Lagrangian sampling will help with a better understanding
of how nutrients are processed. Preliminary work on surface water modeling
of the Cedar River has begun.
SELECTED REFERENCES:
- Boyd, R.A., 1998, Characterizing ground-water flow in the
municipal well fields of Cedar Rapids, Iowa with selected
environmental tracers: Journal of the American Water Resources
Association, vol. 34, no. 3, pp. 507-518.
- Boyd, R.A., 1999, Herbicides and herbicide degradates in shallow
ground water and the Cedar River near a municipal well field,
Cedar Rapids, Iowa: in Proceedings of the 1999 U.S. Geological
Survey Toxic Substances Hydrology Program Meeting.
- Boyd, R.A., 1999, Ground-water geochemistry in the Seminole
Well Field, Cedar Rapids, Iowa: Journal of the American Water
Resources Association, vol. 35, no.5, p.1257-1268.
- Hansen, R.E., 1970, Geology and ground-water resources of Linn
County, Iowa: Iowa Geological Survey Water-Supply Bulletin No. 10,
66 p.
- Schnoebelen, D.J. and Schulmeyer, P.M., 1996, Selected
hydrogeologic data from 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., 1995, Effect of the Cedar River on the quality
of the ground-water supply for Cedar Rapids, Iowa: U.S. Geological
Survey Water-Resources Investigations Report 94-4211, 68 p.
- Schulmeyer, P.M. and Schnoebelen, D.J., 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.
- Squillace, P.J., 1996, Observed and simulated movement of
bank-storage water: Ground Water, Association of Ground Water
Scientists and Engineers, vol. 34, no. 1, pp. 121-134.
- Squillace, P.J., Caldwell J.P., Schulmeyer, P.M., and Harvey,
C.A., 1996, Movement of agricultural chemicals between surface
water and ground water, Lower Cedar River basin, Iowa: prepared as
part of the Toxic Substances Hydrology Program: U.S. Geological
Survey Water-Supply Paper 2448, 59 p.
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