The area’s shallow aquifer is the subject of scrutiny this month as the Department of Environmental Quality digs wells in search of the source of PFAS contamination.
Standing on the southeast end of the Robinson Elementary School grounds Thursday, DEQ geologist Paul Knoerr held up 5-foot cylindrical tube — called a sleeve — to show bands of coarse sediment laced amid fine wet sand. The soil sample had just emerged from around 25 feet beneath the school grounds.
Geologists examined the sediment and recorded observations, picking through the pebbles, comparing the sand to a color matching chart.
At 25 and 50 feet, water samples were collected. Knoerr said he expects both shallow and deep samples from the hydrogeologic investigation to reveal PFAS above 70 parts per trillion, as high concentrations were detected at the school in October.
If this happens, the 120th Avenue area investigation will become an official PFAS contamination site — one of 43 around Michigan. Robinson Elementary is the only school in the state with these high readings, causing students and staff to rely on bottled water for drinking and cooking since fall.
In November and December, the DEQ sampled more than 60 residential wells, mostly north of the school along 120th Avenue. Traces of PFAS have shown up throughout the area, but the school’s samples are still the highest concentrations.
This means the source should be at or very near the school, according to Knoerr.
If you dig about 50 feet, you reach a layer of clay at the bottom of the sandy aquifer. Knoerr said the geology is uniform and horizontal, flowing north toward the Grand River. Without a steep gradient, the groundwater moves slowly. But as sediment becomes more coarse and large-grain, it moves faster.
PFAS tends to move toward water, and moves quickly through large-grain sediment.
“The PFAS is going to run through the stuff that moves fast,” Knoerr said. “Wherever it goes, that stuff wants to hook into that water and it’s going to run. That’s why we’re seeing something here that might be impacting something a distance away from here.”
PFAS was also discovered south of the school. Knoerr said agricultural irrigation in the area could temporarily alter the flow of groundwater, drawing it southward. A source could also be located farther south.
The DEQ commonly cleans up well-understood chemicals. Gasoline, for instance, is lighter than water, and is usually found in the shallow soil. Dry cleaning solvent is denser than water, Knoerr said, and will sink below the water table.
“We don’t know what happens with PFAS,” Knoerr said. He said the substances are likely denser than water, due to their large molecular structure, but the large group of chemical compounds linked to numerous health risks remains mysterious.
Drilling deep, even with the powerful Geoprobe, involves a lot of waiting. Metal tubes are lined up to deliver the sleeve to the next layer of earth.
“Every single time, everything’s got to come out of the hole and back down again,” Knoerr said.
Monitoring wells are being placed around the school, and east across 120th Avenue on Robinson Township property. Sampling should last through next week, Knoerr said, and results are expected in about four weeks.
Once the ice melts, the DEQ will also sample water from a few nearby ponds. If those results reveal PFAS, new exposure routes could be considered — such as through contaminated fish and deer.
It’s possible that a source will never be discovered, Knoerr said. He described the investigative method as using Occam’s razor, a problem-solving principle that states that the simplest solutions are more likely to be correct than complex ones.
A lot of the area’s wells lack documentation. Their depths are unknown.
Environmental regulations were not in vogue before the 1970s, and potential sources, such as dump sites, may also lack documentation. The investigation is a journey into the past — to forgotten human activity, and to Michigan’s glacial origin.
“We’re trying to recreate history,” Knoerr said.