Graduate student Jeffrey Wilcox takes water samples from a well in a new, unsewered subdivision.
UW Water Resources Research
From Farmland to Suburbia 12/05/05
Groundwater and Unsewered Subdivisions
By John Karl
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.
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.
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.
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.
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.
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
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).