Tyler Clindsey, a Portage County staff member, told the Planning and Zoning Committee on June 24 that groundwater monitoring wells installed around the Village of Nelsonville showed higher, sometimes highly variable nitrate readings in upgradient wells and consistently low nitrate readings near the river. "That red line is the the 10 milligrams per liter threshold where it's deemed unsafe, by the health department," Clindsey said, pointing to the public-health cutoff on his chart.

County staff said the monitoring program includes multiple channels (depths) within each well and that patterns differ by depth as well as by location. The committee heard that wells labeled 1, 2, 6 and 7 (upgradient of the river) generally showed higher nitrate concentrations and, at some depths, large fluctuations between sampling events; wells 3, 4 and 5 (downgradient, nearer the river) typically showed low nitrate levels and little fluctuation. Clindsey said well 5 showed almost zero nitrate throughout the sampled depths and that wells 2 and 6 sometimes ranged from near zero to about 20 milligrams per liter in the same channel across different samples.

Why the pattern exists is not yet known. Clindsey told the committee the team is considering processes such as denitrification, vertical and lateral mixing between aquifer layers, and dilution from river water near the river. He described denitrification as a biological process in low-oxygen parts of an aquifer in which bacteria can reduce nitrate to nitrogen gas. "It takes in nitrates and ... emits nitrogen gas," Clindsey said.

A committee member asked whether flow rates could be measured to help interpret the patterns. Clindsey replied that vertical flow estimates are possible because the monitoring channels are stacked and the hydraulic head can be measured; horizontal flow is harder to quantify because the wells are relatively far apart.

Clindsey said the May results are broadly consistent with earlier samples, and that staff will continue sampling through next April. He noted that an organic-source-tracer sampling run is scheduled in July and that July will be the last year for organic tracer sampling under the current plan. No formal action was taken; the presentation was for information and follow-up analysis.

The committee did not adopt any new regulatory measures at the meeting; staff said they will continue to analyze the dataset and consult additional experts while preparing a written report.