How have we monitored groundwater in Seeley Lake?

In 1996, the Missoula County commissioners funded an aquifer carrying capacity study, which identified the sensitive nature of the Seeley Lake groundwater, even when compared with other similarly developed areas of the county without sewer systems. A groundwater study conducted shortly after concluded that the low conductivity, or the degree to which the Seeley Lake aquifer conducts electricity, would lead to elevated nitrate levels in groundwater unless the community managed sewage in a different way.

Since 2004, the Seeley Lake Sewer District has monitored nitrate levels in the groundwater quarterly at three locations. Given the direction of groundwater flow determined in the groundwater study, nitrate concentrations at these sampling points reflect accumulation from the most densely developed areas of town.

Ongoing monitoring suggests the problem is holding steady at best, but is likely worsening, and staff have found violations of contaminant levels for nitrate in groundwater (10 mg/L).

Recently, the Seeley Lake Sewer District has invested resources into extending the network of monitoring wells in the area of concern. They drilled 14 new monitoring wells in 2022, and the new wells have provided useful information. The results show widespread elevated nitrate contamination and more places and tests where the levels exceed the nitrate maximum contaminant levels of 10 mg/L.

In 2024, MPH offered to conduct a groundwater flow direction study, using the new monitoring wells. In addition, MPH will take over the responsibility for testing for a period of about one year. This will not only save the Sewer District money, but it will also provide valuable data that can inform future decisions. This study is ongoing, and will conclude sometime in 2025.