Probing the ground near Lake Geneva, geologists have found that the geologic and chemical conditions behind arsenic in well water there are distinctly different from those in the Fox River Valley to the north.The finding illustrates the difficulty of predicting arsenic concentrations in new or existing wells, experts say, and it highlights the importance of having well water tested annually.

In the Fox River Valley, arsenic in drinking water has concerned people for nearly two decades, according to Madeline Gotkowitz, a hydrogeologist with the Wisconsin Geological and Natural History Survey and an outreach specialist with UW-Extension. Studies in the area have shown that groundwater is most readily accessible in a permeable layer of rock called the St. Peter Sandstone, Gotkowitz said. This aquifer, or water-bearing formation, rises relatively close to the surface in this part of the state, providing a convenient source of water.

However, significant municipal, agricultural, and industrial use of that groundwater has drawn down its level, exposing a thin, fairly well-defined layer of arsenic-bearing sulfide minerals to oxygen. These particular minerals dissolve in the presence of oxygen, releasing the arsenic into the groundwater.

Thus, one solution for well owners in the region is to drill deeper or shallower wells that access water from other aquifers, Gotkowitz said.It may be costly, but it is often practical and effective.

The picture is much different in southeast Wisconsin, and solutions there are more challenging, according to Tara Root, a graduate student working with hydrogeologist Jean Bahr at UW-Madison. For her Ph.D. thesis, Root examined the geochemical conditions behind high levels of arsenic reported near Lake Geneva, in Walworth County.

With support from the UW Water Resources Institute , Root used special drilling methods to collect the first intact core of the area’s primary aquifer. It’s 300 feet of clay, silt, sand,gravel, and boulders deposited by glaciers tens of thousands of years ago and known as the Quaternary formation. Root also obtained sediment samples from previous drilling efforts in the area, and she had all the samples analyzed to determine their chemical composition.

Root found that arsenic in the Quaternary aquifer is associated not with sulfide minerals, but with iron-hydroxide minerals, and these are dispersed widely throughout the aquifer. Further contrasting with Fox River Valley conditions, arsenic bound to these hydroxide minerals enters groundwater when oxygen levels are low, which is the case with the relatively old water throughout the deep Quaternary aquifer.

Consequently, drilling deeper in the Quaternary is unlikely to reach water with lower arsenic concentrations, Gotkowitz says. And drilling below the thick aquifer is prohibitively expensive for residential well owners. Well owners here must often resort to buying bottled water or installing in-home treatment systems.

The finding that the geological sources of arsenic are so different near Lake Geneva than in the Fox River Valley – and the fact that few such detailed studies have been conducted in many parts of the state – makes it difficult for geologists to accurately predict when a new or existing well is likely to yield low-arsenic water, Gotkowitz says.

In light of these findings, Gotkowitz urges all owners of private wells in Wisconsin to have their water tested annually for arsenic and nitrates, a contaminant that comes from fertilizers, animal wastes, and septic systems.

“Getting your water tested should be considered part of the cost of buying a house or drilling a well,” Gotkowitz said. “If you have your own well and you live in Wisconsin, you should have your water tested. It’s that simple,” Gotkowitz said.

For more information on arsenic in Wisconsin, see the Wisconsin DNR arsenic page and the ASC arsenic fact sheet .