Geology & Vulnerability
Our source of drinking water…
Our drinking water comes from the Lower Portneuf Valley Aquifer, groundwater which is located directly beneath the Cities of Pocatello and Chubbuck. None of our drinking water comes from the Portneuf or Snake Rivers.
Mountain snowpack in the Bannock Range south of Kinport Peak provides the largest component of recharge (replenishment by the absorption or refilling of water) to the aquifer. Snowmelt seeps into the ground and flows by gravity through pore spaces in the rock and soil, eventually ending up in the aquifer beneath the valley floor.
The picture above is an aerial view into the Lower Portneuf Valley from the northwest, looking southeast toward the Portneuf Gap. Scout Mountain appears in the top center of the image. The Simplot Don Plant lies in the lower right hand corner.
Because we live, work, and recreate on top of our only drinking water source, our aquifer is very vulnerable to contamination from human activity.
Geologic history of our aquifer
Approximately 15,000 years ago, Lake Bonneville, the ancestor of the Great Salt Lake, suddenly discharged an immense volume of water to the north through Red Rock Pass near Downey, Idaho. These flood waters continued north through Marsh Valley, turned west near Inkom, passed through the Portneuf Gap, spread out through the valley that is occupied by Pocatello and Chubbuck, and eventually flowed onto the Snake River Plain. The enormous volume of water scoured and eroded the landscape, carrying soil and rock material that was laid down or left behind where water velocity decreased. Deposits of large rounded boulders of basalt, ranging from three feet to ten feet in diameter are characteristic of this flood event, particularly in the Lower Portneuf Valley. Plucked from nearby basalt flows and rounded during several miles of transportation by the flood, these boulders were dropped from the flood waters into deposits up to 300 feet thick.
Our aquifer is made up of those gravels and boulders. In the photo of the old Phil Meador dealership on North Main Street in Pocatello are large boulders. These are the boulders that make up our aquifer. We dig up these boulders whenever we dig up a street in Pocatello. Most aquifers are made up of sand or small gravels (not boulders). Most aquifers move just a few feet a year. In the Portneuf Valley, the water can move the entire length of the valley in one year. It is much easier (faster) for water to flow through boulders with huge spaces between them, than it is for water to flow between the tiny spaces between grains of sand. In an aquifer made up of sand or gravel there is a lot of opportunity for the sand and gravel to come into contact with the moving water and latch onto contaminants in the water. This filtering of contaminants happens much less in our boulder aquifer.
What covers the boulders in our aquifer?
A very thin layer of silt (2- 10 feet) covers our boulder aquifer, and protects our aquifer from contamination. This is not a lot of protection for our aquifer from the things we do on the ground (changing oil, fertilizing, storing chemicals). Whatever we spill on the ground will slowly move through the silt to reach our boulder aquifer. Once it reaches our aquifer contamination can move very fast through our aquifer. The geology of our aquifer impacts vulnerability to contamination.
Beneath the yellow lines.
We are going to cut the earth along the yellow line to look at the geology that lie along the yellow line. Here is what lies beneath the yellow line. In the left is the Portneuf Gap (to the south of the Portneuf Valley). The blue arrows show ground water coming into the aquifer. Much of it comes from the Mink Creek area (large blue area). Some of it comes from ground water south of the Portneuf Gap (separate from the Portneuf River). The yellow unconfined aquifer is filled with the Bonneville boulders that were discussed in a previous picture. In the southern part of the Valley, dark red wells draw water from between those boulders. As the water moves north through the valley it must go over a ‘hill’ of bedrock. This is located at Red Hill. As the water squeezes over this bed rock it then dips down into a northern deeper aquifer that is sealed off beneath layers of clay. Wells in Chubbuck and North Pocatello draw water from these deeper confined aquifers. This water should be better protected, but contamination has been found in wells located in this northern aquifer. We are going to cut the earth along another yellow line (this one running east to west across the southern part of the valley) to look at the geology that lie along the yellow line. Here is what lies beneath the yellow line. The upper gravels in this picture are the Bonneville Flood Gravel (boulders). The silt layer is located above it. You can see how at the Highway Pond we have stripped away the silt layer and dug into the upper gravels, creating a window into our aquifer. The Portneuf Basalt (an ancient lava flow) separates the Bonneville Flood aquifer from the eastern aquifer (which is composed of an older mix of gravel, sand and clay).
The Lower Portneuf Valley Aquifer functions as three aquifers.
In orange is the Southern Aquifer (the main valley aquifer). It is separated from the Northern Aquifer (in red) by the bed rock at Red Hill. To the right of the southern aquifer (in green) is the Eastern Aquifer, which is located east of the basalt cliffs. The red dots are the City municipal wells for Pocatello and Chubbuck.