Bannock County Aquifer Description
The Lower Portneuf Valley Aquifer functions as three aquifers.
Our aquifer can be divided into three distinct parts based on geology. The Northern and Southern Aquifers are separated mid-valley by a ridge of bedrock that outcrops as Red Hill near the Idaho State University Campus. East of the Southern Aquifer is the Eastern Aquifer (shown in green), which is located above the basalt cliffs along South 5th Avenue.The red dots in the diagram are the locations of municipal wells for Pocatello and Chubbuck.
The soil and rock layers beneath the valley floor are composed of the gravel and boulders deposited during the Lake Bonneville Flood. Spaces between the gravel and boulders contain the water that we drink. The photo of the old Phil Meador dealership on North Main Street shows a common sight….large boulders extracted during excavation in the valley. The speed of water movement through an aquifer depends on the size and interconnectedness of spaces or fractures in soil and rock material. Typically, aquifers composed of silt, sand, or small gravels (not boulders) only allow water to move a few feet per year due to the small spaces between grains. In the Lower Portneuf Valley Aquifer, water can move the entire length of the valley in one year because of the much larger spaces between gravel and boulders. Like a coffee filter, the pore spaces in aquifer rock and soil can help remove contaminants and particulate material. Small pores spaces in an aquifer composed of sand or gravel help remove or purify aquifer water moving through it. But if the pore spaces are large, as they are between boulders, contaminants in the aquifer water are less likely to be trapped or slowed down.
What covers the boulders in our aquifer?
A very thin layer of silt (2- 10 feet in thickness) covers our boulder aquifer, which does not provide much protection from contaminants released on the ground surface (residential, business, and agricultural activities).Whatever we spill on the ground will slowly move downward to the aquifer. Once it enters the aquifer, contamination can move and spread very rapidly.
The Aquifer in Cross-Section – “Beneath the yellow lines”
To imagine what the aquifer looks like below the ground, we can pretend to cut the earth from the southeast to the northwest along the yellow line in the picture.
Here is what the earth would look like beneath the yellow line.
Along the left margin of the diagram (also called a cross-section) is the Portneuf Gap on the southeast end of the Portneuf Valley.
The large blue arrow symbolizes underground recharge coming into the aquifer from the Bannock Range, much of it coming from the Mink Creek area. Some recharge (small blue arrows) to the aquifer comes from groundwater movement through the Portneuf Gap (separate from the Portneuf River).
The shallow unconfined aquifer in the southern part of the valley, the Southern Aquifer, is composed of the Lake Bonneville boulder deposits.
As groundwater flows north through the valley, it must go over an underground ‘hill’ of bedrock that outcrops at Red Hill near the Idaho State University campus. Groundwater squeezes over this bed rock high and then dips downward into the northern part of the valley forming the Northern Aquifer. This deeper aquifer is sealed off beneath layers of clay making it a confined aquifer, which should provide protection from contamination seeping downward from the ground surface. Wells in Chubbuck and North Pocatello draw water from this deeper groundwater. However, some contamination has been detected in Chubbuck municipal wells completed in the deeper groundwater body. Some wells in Chubbuck draw from groundwater that is trapped above the confining clay layers. The water trapped on top of the clay layers are called perched groundwater.
To get a different view of the earth under the valley, we can pretend to cut the earth along another yellow line running east to west across the southern part of the valley.
Here is what lies beneath the yellow line that goes from the west bench to the east bench.
The Upper Gravels in this cross-section are the Bonneville Flood Gravels (boulders). The thin silt layer is located above it. The profile of the Highway Pond shows how the silt and gravel layers have been stripped away, creating a window into our aquifer. The Portneuf Basalt, an ancient lava flow, separates the main valley aquifer from the Eastern Aquifer (which is composed of an older and finer grained mix of gravel, sand and clay).
The Highway Pond – A window in our aquifer
When you see water in the Highway Pond and gravel pit, you are seeing the surface of our aquifer exposed. Water constantly moves through the pond from roughly southeast to northwest. Therefore, any contaminant introduced into the pond accidentally or intentionally will eventually enter the municipal water supply. As a risk to the municipal water supply in the valley, the Highway Pond is restricted from use as a recreational or industrial area. The Idaho Department of Environmental Quality (DEQ), the Idaho Department of Lands (DOL), and the Idaho Department of Transportation (ITD) signed a cooperative Memorandum of Understanding in 2001 to ensure protection of the aquifer. At present, the pit is not mined for gravel, vehicular traffic is prohibited, and the pit is slowly being reclaimed with clean excavated material from other locations.