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Environmental Studies senior maps Carroll County sinkhole density

Environmental Studies professor Mona Becker and senior James Porteous analyze limestone.
February 26, 2013

Senior James Porteous doesn’t graduate until December but already he is putting his Environmental Studies major to work identifying possible sinkholes and mapping their density in Carroll County for his senior capstone project.

Mentoring Porteous’ work sleuthing for karst features – landscape typical of sinkhole formation – brings back memories of her own undergraduate days to Environmental Studies professor Mona Becker, who spent two summers and a winter mapping sinkhole density in south-central Pennsylvania’s York, Adams and Lancaster counties.

“I’ve thought for a while that it would be a good idea to map sinkhole density in Carroll County – and I thought James would be interested in doing that,” Becker says.

She knew that sinkhole and karst land formation maps existed in Pennsylvania because she helped put them together in the early 1990s with geologist Bill Kochanov for the Pennsylvania Topographic and Geologic Survey. But at the time Becker used aerial photos, not the Google Earth Imaging that Porteous has access to.

Sinkholes form when water-soluble bedrock such as limestone and marble is dissolved by groundwater, explains Porteous, whose home state of Florida is littered with sinkholes. 

“If the soil is sandy, it erodes at a steady pace like an hourglass,” he says. “If it is clay, it holds the burden for a time and then collapses all at once.”

Becker and Porteous make frequent field trips to examine sinkholes and even the depressions that indicate one is either developing or filling in. But, with a previous estimate by a hydrogeologist of more than 600 sinkholes in Carroll County, neither professor nor student expects to examine every one.

Still, at the top of their list of sinkholes to visit was the huge one – 10 meters across and more than 100 feet deep – on a farm outside of Hanover, Pa. Becker and Porteous went to see that and other dramatic sinkholes in the Hanover-McSherrystown area near a rock quarry, which no doubt contributed to the formation of the sinkholes as workers lowered area groundwater levels when they pump more and more water out of the quarry.

Mostly, Porteous spends his project time going over the aerial images and circling depressions in the landscape. The goal is to map density.

“Sinkholes are not isolated – they are integrated,” Porteous says. “It’s important to understand the density and where they are so that you can mitigate risk when building.”

“You don’t want to build where there is a high density of sinkholes,” Becker says, explaining that 10 sinkholes per square mile should certainly alert a builder to consider re-locating his site.