DEQ releases study results for low pH wells near Carlile

The Department of Environmental Quality has released the results of its investigation into a number of water wells in the vicinity of Carlile that suddenly ran dry or acidic in August, 2017. The study did not find any confirmable links between the well issues and the Gillette Madison Water Project, though it was unable to rule out the possibility that discharges from the Gillette wells contributed to the low pH values of the wells in Crook County.

The study investigated whether the construction and testing of Gillette’s city wells was the cause of reduced water levels in the wells near Carlile. The answer was no.

The investigation determined that the domestic wells draw from the Fall River, Lakota and Sundance formations, while the Gillette wells draw from the Madison Limestone, which is “hydraulically separated” by around 1000 feet of low-permeability strata. The groundwater elevation for the Madison Limestone is 200 feet lower than in the aquifers serving the Carlile wells.

“Even under aggressive production from the Madison aquifer by the City wells (only short-term test production has occurred to date), it is highly unlikely significant drawdown impacts could be realized through the intervening strata,” says the conclusion of the study.

The study further points out that it is unlikely there are poor well seals in the Madison wells that would facilitate drainage of the higher groundwater in the aquifers used by the Carlile wells. It also notes that the wells that suffered declining water levels have partially recovered, which is not compatible with continued drainage via a well seal failure.

The study also investigated whether the stimulation of the Gillette city wells were the cause of the low pH conditions in the Carlile domestic wells. Again, the answer was no.

The Gillette wells were stimulated using hydrochloric acid, the study notes, while the acidic conditions in the Carlile wells have been “consistently identified with sulfuric rather than hydrochloric acid”.

“Low pH conditions in the domestic wells are likely the result of oxidation of native pyrite deposits in the host aquifers that supply water to these wells,” the study concludes.

The third question investigated by the study was whether construction and test discharges from the Gillette city wells enhanced recharge to the aquifers supplying the Carlile wells in a way that could have created or contributed to the low pH conditions. The answer to this question was, “unknown”.

According to the study, discharges from the Gillette wells have had the opportunity to directly recharge those aquifers where they outcrop into drainage channels. The discharges have been of “a magnitude and duration comparable to natural runoff events with recurrence intervals,” the study says.

Recharge from surface sources is likely to have higher dissolved oxygen concentrations than native groundwater, which the study points out does provide the potential for acid production via reaction with local pyritic material.

“Whether those natural processes have been locally and temporarily enhanced through discharge from the City wells cannot be determined without substantial additional research,” researchers concluded.

Finally, the study investigated the cause of the low pH conditions in the wells near Carlile and concluded that “local oxidation of native sulfide minerals, chiefly pyrite, is the most reasonable source of the low pH conditions observed in some domestic wells and two natural springs of the study area.”

“Although multiple opportunities for enhancement of that natural process have been identified in the completion and operation of domestic wells, no specific occurrence of low pH groundwater has been linked with specific local mechanisms,” concludes the study.

“Groundwater quality is naturally poor throughout the study area. One element of that poor groundwater quality in select wells is low pH.”