RIVERBANK FILTRATION AND THE IMPACT OF AN ALLUVIAL WETLAND ON WATER QUALITY

Alluvial aquifers near major rivers in east-central Iowa are a source of water for many communities. Municipal water managers and planners are concerned with increasing concentrations of nitrate and other agricultural chemicals in streams that can infiltrate into alluvial aquifers. The City of Cedar Rapids, Iowa operates public supply wells that are completed in the Cedar River alluvium, a shallow alluvial aquifer adjacent to the Cedar River. A ground-water-flow model, recently completed for the Cedar River and Cedar Rapids wellfield showed that almost 80% of the flow to the wellfield area is from induced infiltration from the Cedar River. Understanding riverbank filtration—as water moves from the river through the aquifer—is important in assessing water-quality concerns.

Induced infiltration from pumping is of concern to municipal water operators due to high nitrate concentrations (often greater than 10.0 mg/L nitrogen) in the Cedar River. The Cedar River upstream of Cedar Rapids is listed on the State of Iowa impared waters list for nitrate. A ground-water flowpath study through natural wetlands in the Cedar Rapids Seminole wellfield was conducted from the fall of 1998 through spring of 2003 to investigate the potential of reducing nitrate concentrations using existing wetland areas and to better understand riverbank filtration processes. Water samples from wells completed in the wetland area contained low nitrate concentrations (0.1 to 4.7 mg/L) compared to the Cedar River (8.0 to 18.4 mg/L) and to an up-gradient well (3.1 to 7.6 mg/L). Conversely, water samples from these wells contained higher iron concentrations (13 to 312 µg/L) and manganese concentrations (4.5 to 1080 µg/L) when compared to the Cedar River (iron—2.0 to 25 µg/L , manganese—1.6 to 37 µg/L). A recent assessment of denitrification potential of soil types in the Cedar River Basin suggests that soils of alluvial wetlands/bottomland forest may be the areas of highest denitrification potential in the basin. Natural wetland areas might provide improvements in ground water quality as pertains to nitrate, a constituent of concern for public water supply.