Episode 4 - Groundwater

March 2021

It is National Groundwater Awareness Week, March 7ththrough the 13th, so this month we’re diving into the world of groundwater.

Groundwater is defined as water held underground in the soil or in pores and crevices in rock. An aquifer is the underground layer of water-bearing sediment or permeable rock from which groundwater can be extracted using a water well.

In the Bitterroot, as in much of the west, our water supply comes primarily from snowfall and the snowpack that is formed during the winter months. As the snow melts it feeds the creeks and streams but also infiltrates into the aquifer.

Ravalli County has over twenty thousand wells. The majority of the wells drilled in the valley are for domestic or residential use, 92%.  The next highest use is irrigation at 5%. Though fewer wells are dedicated to irrigation they account for 97% of the daily withdrawals, with domestic wells up taking 1%. Irrigation practices in the Valley can also be a source of groundwater. Though some water is used by the crops and some is lost through evaporation, excess water from irrigation can soak back into the ground and recharge the aquifer.

I spoke with Ginette Abdo, the Groundwater Investigation Program Manager with the Montana Bureau of Mines and Geology (MBMG). MBMG is the equivalent of the State Geologic Survey, they do geologic mapping and characterize groundwater resources in the state of Montana. The Groundwater Investigation Program researches specific hydrogeologic issues to answer area-specific questions. They currently have three active projects in the Bitterroot watershed and one that has been completed – the Stevensville Project.

To address questions of groundwater they start by looking at and defining the aquifer, which is controlled by the geology of the area. Then depending on the question they are trying to address, they collect information from wells to evaluate the groundwater. They might measure groundwater levels to assess flow directions or sample the groundwater to assess water quality.

“The groundwater levels tell us about the timing of aquifer recharge and where the groundwater is discharging. There are a lot of things going on. We look at the physical and chemical characteristics of the water in the aquifers.”

Ginette said that groundwater, like surface water, flows from high to low pressure head, from areas of recharge to areas of discharge. “The groundwater gets recharged by the snowmelt higher in the mountains and then discharges into our river system. So, the Bitterroot River is a big drain, it’s a discharge area for the groundwater.”

Often, MBMG are looking at the connection between groundwater and surface water.  If groundwater levels are higher than surface water in a stream, groundwater will flow into and feed the stream. Similarly, if groundwater levels are lower than surface water in a stream, water from the stream will recharge the aquifer.  “Streams and rivers can gain water from groundwater or lose water to groundwater. We look at that connection … It’s really interesting that this can vary throughout the year.” Ginette said that they looked specifically at the section of the Bitterroot River between Anglers Roost and Woodside Crossing and found that particular reach gains groundwater year-round.

The studies in the Bitterroot are ongoing but you can find out more information on the MBMG website - http://www.mbmg.mtech.edu/WaterEnvironment/GWIP/projects.asp.

When asked about the characteristic of the groundwater in the Bitterroot, Ginette said that the material of the aquifer can make a big difference. On the benches many of the wells are drilled into bedrock, they produce 1-100 gallons per minute and the depth to water can vary widely. “There isn’t a one size fits all even within the same aquifer.”  In the center of the valley it is a shallow aquifer consisting of alluvial deposits from the Bitterroot River, typically sands and gravels, which allows water to move pretty readily, producing 10-1000 gallons per minute.

If your home water supply is a well, you should have your water tested annually. Well testing kits are available at the Ravalli County Environmental Health office at 215 South 4th Suite D in Hamilton. Check out the Bitter Root Water Forum Facebook page for more information and tips on groundwater.

Other great groundwater resources:

MBMG Groundwater Information Center - https://mbmggwic.mtech.edu/

MSU Extension Well Educated Program - https://waterquality.montana.edu/

National Groundwater Awareness Week - https://www.ngwa.org/get-involved/groundwater-awareness-week/groundwater-awareness-week-2021