Winter 2005


Graduate student Jeffrey Wilcox takes water samples from a well in a new, unsewered subdivision.
UW Water Resources Research

From Farmland to Suburbia
Groundwater and Unsewered Subdivisions

By John Karl

Across the country, new housing developments are sprawling past city limits – and past city sewerage systems. That’s fueled concern about how dense clusters of septic systems might affect groundwater. Could they contaminate wells used for drinking water? Might those contaminants include pain relievers, antibiotics, and hormones? Will bacteria in drinking water become resistant to antibiotics?

Hard data for answering those questions have been scarce, according to Ken Bradbury, a hydrogeologist at the Wisconsin Geological and Natural History Survey and UW-Extension. But that’s changing with two studies about four miles northeast of Sun Prairie, Wis. They’re both monitoring groundwater before, during, and after 78 acres of farmland are transformed from corn and soybeans to houses, septic systems, and private wells.

In one study, Bradbury, UW geologist Jean Bahr, and graduate student Jeff Wilcox are looking at water quality as the land turns residential. Before housing construction began in 2003, the researchers installed wells for collecting water samples and equipment for monitoring flow rates of surface water, groundwater, and septic system effluent. Wilcox constructed a map of the area’s geology and developed a computer model of groundwater flow that can be adapted to other sites as well.

After five houses were built and residents moved in, the team found that overall nitrate concentrations in groundwater had decreased slightly, probably due to decreased agricultural inputs at the site. Septic system effluent contained elevated nitrate and chloride, as well as acetaminophen, a caffeine metabolite, and two hormones. Nine of 10 samples contained estrogenically active compounds. However, no pharmaceuticals or hormones were detected in the groundwater.

These results are preliminary, and it may take five to 10 years of monitoring before the full story can be told, Bradbury said. Wilcox’s computer model indicates that the septic plumes could eventually reach some wells. However, Bradbury said the chemistry of those plumes may change substantially by that time. As water travels slowly through soil and rock, the contaminants may be removed by attaching to soil particles or by being broken down by microbes – processes by which septic systems purify effluent. The rates and end-products of these processes are well understood for nitrates and chloride, but not for hormones and other pharmaceuticals, Bradbury said.

More details about the project are available at www.geology.wisc.edu/~hydro/SV.

Another project at the site is looking at whether residential development is causing bacteria in groundwater to become more resistant to antibiotics. That could potentially create a "reservoir of antibiotic resistance" that could reduce the effectiveness of antibiotics for humans, according to Trina McMahon, assistant professor of civil and environmental engineering and lead scientist on the project.

McMahon emphasized that this is only a theoretical possibility at this point, and no one yet knows whether septic systems contribute significantly to antibiotic-resistant bacteria in groundwater.

To find out, McMahon and graduate student Trevor Ghylin are using cutting-edge genetic technology to test groundwater for DNA associated with antibacterial resistance.

Any DNA associated with antimicrobial resistance detected in pre-development groundwater samples must come from other sources, such as naturally occurring resistant bacteria or those in runoff from agricultural lands, McMahon said. However, finding more kinds or amounts of DNA from antibiotic-resistant bacteria after the septic systems are in use would indicate a contribution from the septic systems, McMahon said.

Both studies are funded through the Wisconsin Groundwater Research and Monitoring Program (http://www.wri.wisc.edu/wgrmp/wgrmp.htm).










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