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Water cleanup is for the birds

DOE/Argonne National Laboratory : 30 March, 2007  (Technical Article)
In an environmental restoration effort that will benefit birds and humans alike, scientists in Argonne's Environmental Research Division are helping to restore a wetland wildlife sanctuary near Utica, Neb., while cleaning up the town's contaminated groundwater. This is the first time that spray irrigation, commonly used on farms, has been used to restore both groundwater and wetlands.
Utica is located in a 4,200-square-mile area of south-central Nebraska known as the Rainwater Basin Region. Twice a year, tens of millions of migrating waterfowl take to the skies over the central United States, and many of them make rest stops in the Rainwater Basin Region's several thousand natural wetlands. However, the past century has seen these wetlands diminished to one-tenth of their former area as land has been increasingly adapted for agriculture.

The wetlands restoration in the North Lake Basin Wildlife Management Area near Utica is a multi-agency effort involving the Commodity Credit Corp. of the U.S. Department of Agriculture, Argonne's Environmental Research Division, and other national, state and local agencies.

In the late 1980s, carbon tetrachloride, a chemical believed to cause cancer, was found in a shallow aquifer beneath Utica. The carbon tetrachloride, which was present in greater concentrations than federal drinking water standards allow, originated from a grain storage facility formerly on the edge of town.

Before its harmful effects were known, carbon tetrachloride was widely used as a fumigant to protect stored grain from insects. The USDA designated the Utica aquifer for cleanup, even though residents were not being exposed to the contaminated water.

Carbon tetrachloride can persist in many aquifers for decades or longer. Cleanup efforts have often involved pumping the contaminated water to the surface, removing the carbon tetrachloride with a device called an air stripper, and discharging the treated water to the surface to evaporate, run off or soak back into the ground.

Utica residents opposed any plan to release treated groundwater in or near the town because of pre-existing drainage problems. As an alternative, the Nebraska Game and Parks Commission suggested that the water could be piped a half mile north of town to the North Lake Basin Wildlife Management Area. The ephemeral wetlands at North Lake Basin lack a consistent water source, and after several years of drought barely 5 percent of the area provided suitable waterfowl habitat.

Implementing the agency's suggestion had the potential to solve Utica's groundwater contamination problem and also benefit to local wetlands. “The idea evolved from there,” said Bob Sedivy, an Argonne hydrogeologist and the project's technical leader.

As part of its broader initiative to develop efficient, cost-effective cleanup methods for carbon tetrachloride, the USDA selected the Utica project for a demonstration of the effectiveness of spray irrigation in cleaning up groundwater.

Spray irrigation is a simple farming technology, readily available in small midwestern communities. A typical center-pivot irrigation rig with motor-driven wheels can range from 100 to more than 1,000 feet long. The rig includes a long feed pipe mounted on a frame several feet above the ground with nozzles regularly spaced along its length. Water is pumped in at the pivot end of the pipe and sprays out of the nozzles as the unit rotates across the field. Operating under appropriate pressure and temperature conditions, the system disperses volatile contaminants, like carbon tetrachloride, from water as it is sprayed, with no additional treatment.

Spray irrigation has the advantage of simultaneously treating water and distributing it over a large area. In contrast, air strippers release treated water at one point, like a fire hydrant, and are typically more complicated and expensive to operate.

An earlier University of Nebraska study demonstrated that spray irrigation can decontaminate water containing a mixture of volatile chemicals. Argonne scientists theorized that carbon tetrachloride could be removed in the same way. They devised a system that pumps water from the contaminated Utica aquifer, transports it through a network of underground pipes and sprays it onto the wetlands in the North Lake Basin.

To ensure that the water entering the wetlands is clean enough, Argonne researchers conducted an 18-month-long pilot test at Utica to determine the effectiveness of spray irrigation treatment under a wide range of seasonal and operating conditions. Even in less-than-ideal conditions, the pilot system eliminated enough carbon tetrachloride to exceed regulatory cleanup goals, Sedivy said.

The scientists also considered potential air quality issues that might arise from the spray discharge. Ambient air sampling during the pilot study demonstrated, however, that the volatilization of carbon tetrachloride to the atmosphere would pose no health risks.

Utica area residents were mainly concerned that the additional water influx to the wetlands might increase the risk of flooding on neighboring properties. Historically, the farmlands around the wetlands have been prone to flooding during wet periods, because a layer of impermeable clay soil that underlies the central portion of the North Lake Basin extends beneath the surrounding fields. This clay layer locally diminishes downward flow of water into the underlying aquifer and helps create the wetlands.

To address the flooding concern, Sedivy and his collaborators studied the water balance of the basin to learn how it would handle the extra water from the Utica aquifer. They combined surface water and groundwater computer models with climate data to predict how the additional water would influence the depth and extent of surface water in the wetlands.

The Utica treatment system operators will use Sedivy's computer model to determine when it is beneficial to run the pumps and when the extra water would be too much. “We explained to the landowners that the system would not be left to run on its own,” Sedivy said. “The goal is to manage the system so that the water is pumped when it will be most beneficial to the ecosystem.”

Sedivy said that the data developed from Argonne's pilot and full-scale studies at Utica can be adapted readily to address similar contamination problems in other communities. Despite its benefit of irrigating parks, playing fields and farmland, groundwater decontamination with spray irrigation should cost no more than other methods, Sedivy said.

The initial site characterization, treatment planning and implementation for the Utica cleanup took more than a decade and involved area residents, environmental conservation groups, and local, state and federal agencies. In addition to funding the cleanup, the Commodity Credit Corp. also paid for extensive earth moving to restore natural terrain features of the wetlands that had been altered by previous landowners.
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