Assessment of Groundwater and Quality - Cedar River Project

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 underlying and adjacent to the Cedar River. A total of 53 vertical and 5 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. Population and industrial development have steadily increased the demand for water pumped from the alluvial aquifer. The City.s water use has increased about 52 percent since 1980 when it pumped about 8,847 million gallons (Mgal; 24 Mgal per day) compared to 13,460 Mgal (37 Mgal per day) during 2014.

Managers of the City of Cedar Rapids Water Division 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 in response to climate change, 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 have varied over the years and have included:

1. Evaluation of the ground-water flow system and quantifying the interaction between the alluvial aquifer, Cedar River, and underlying carbonate bedrock aquifer.
2. Evaluation of pumping scenarios in the alluvial aquifer and potential locations for additional water-supply wells.
3. Characterization of water-quality in the alluvial aquifer, Cedar River, and underlying carbonate bedrock aquifer.
4. Identification and evaluation of options to reduce infiltration and associated contaminant transport from the Cedar River to the alluvial aquifer.
5. Evaluation of the effects of wetlands and ponds on recharge and water quality in the alluvial aquifer.
6. Compilation and assessment of data required for source-water protection programs under the Safe Drinking Water Act.
7. Assessment of the fate and transport of nutrients in the Cedar River Basin using time of travel, synoptic and Lagrangian water sampling.

Since 2012, the primary objectives have been to:

1. Characterize the water quality in the alluvial aquifer, Cedar River, and underlying carbonate bedrock aquifer.
2. Compile and assess data required for source-water protection programs under the Safe Drinking Water Act.
3. Assess decadal changes in land use and nutrient concentrations in the Cedar River Basin through comparison of synoptic water samplings.
4. To assess the effects of climate change on the availability of source water in the alluvial aquifer.

PROGRESS:

A regional ground-water flow model, covering about 231 square miles, was constructed in 1996 to simulate ground-water flow and determine sources of water to the alluvial aquifer under steady-state conditions. Results of the model indicate that 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 more detailed ground-water flow model (with greater resolution than the regional model) was constructed in 2004 to simulate ground-water flow under transient conditions, evaluate pumping scenarios, and determine capture zones for individual supply wells. Over 600 water-quality samples have been collected from observation wells, municipal supply wells, and the Cedar River. Physical parameters, common ions, nutrients, pesticides, and viruses are currently being monitored on a quarterly basis. 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. Relatively large dissolved-iron concentrations (<20 milligrams per liter) and -manganese concentrations (<7.5 milligrams per liter) 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 at different discharge rates. Preliminary work on surface water modeling of the Cedar River Basin has begun in cooperation with the IDNR. Water-quality data from the Cedar River and from monitor wells in the municipal well field were used to study the effectiveness of an alluvial wetland on improving groundwater quality.

CURRENT AND FUTURE ACTIVITIES

A new modeling effort is underway in 2015 to assess the impact of climate change on the future availability of source water in the alluvial aquifer. The model will build on knowledge gained during previous investigations and is scheduled to be completed in 2017. Water-quality monitoring will include nutrients, pesticides, and viruses (which began in 2013 at selected sites).