Assessing and Communicating Risk:A Partnership to Evaluate a Superfund Site on Leech Lake Tribal Lands
Groundwater Movement and Contamination
By Cindy HagleyMinnesota Sea Grant
March 2003
Groundwater Basics
• Groundwater occurs almost everywhere within the pore spaces of saturated rock beneath the land surface.
• Groundwater is not like an underground river or lake. In fact, groundwater is more like the water in a sponge, held within the tiny pores.
Direction of groundwater movement
Ground versus Surface Water Movement• The rate of
groundwater flow is very slow compared to surface water.
• Typical groundwater flow rates range from several inches to feet per year.
Groundwater - The Tortoise
Aquifers• Aquifers are water-
bearing layers of rock or sediment that contain usable quantities of water.
• Aquifers are made up of loose materials such as sands and gravel, or bedrock, such as sandstone or fractured granite.
Surface Aquifer
• Unconfined – interacts with surface water
• Influenced by gravity and flows from higher to lower groundwater elevations, much like river water.
• The steeper the slope of the groundwater “table,”
the faster the groundwater will flow.
Confined Aquifer• Pressure, rather than gravity, makes water move in
confined aquifers. Water moves from areas of high to low pressure.
Confining layer
• Confining layer – clay or bedrock
How Wells Influence Groundwater Flow
• Pumping wells create a cone of depression.
• Water flows toward the cone of depression.
• The area affected by the well is called the area of influence.
Pumping Water from Wells• Pumping water from
aquifers can lower groundwater levels.
• Pumping changes groundwater flow patterns.
• For example, water used to flow from groundwater to lake. Now flowing from lake to groundwater.
• Could lead to change in groundwater quality.
How Does Pumping Water from Wells Influence Water Quality?
• Pumping water from a well draws the water table down and can pull in contaminants from the well’s area of influence.
clay
confined aquifer
Groundwater Susceptibility
• Unconfined aquifers with no cover of dense material are susceptible to contamination.
Things to keep in mind:
• Bedrock with large fractures is susceptible, because the fractures provide pathways for contaminants.
• Confined, deep aquifers tend to be better protected than surface aquifers with a dense layer of clay material.
• Wells that connect two aquifers increase the chance of cross contamination between the aquifers.
City Dump
Containment Vault
Fish Hatchery
Treating Facility Site
Pump Out Wells
Pike Bay
Fox Creek
Channel
Upper Aquifer
Lower Aquifer
Confining Layer
Toward Cass Lake and Pike Bay
City Dump
Containment Vault
Fish Hatchery
Treating Facility Site
Pump Out Wells
Pike Bay
Fox Creek
Channel
Company data from wells suggests west to east flow
Company analytical model suggests NW to SE flow
City Dump
Containment Vault
Fish Hatchery
Treating Facility Site
Pump Out Wells
Pike Bay
Fox Creek
Channel
Data from wells suggests west to east flow
Analytical model suggests NW to SE flow
There are no sampling wells to the SE of the site, so we don’t know if contaminants are moving in that direction
Pump out wells are to the east of the site
Pump Out Well
Toward Cass Lake and Pike Bay
LNAPLs – Light non-aqueous phase liquids
Translation- Floating plumes of contaminants that don’t mix with water
Translation - Sinking plumes of contaminants that don’t mix with water
Confining Layer
Pump Out Well
Toward Cass Lake and Pike Bay
DNAPLs – Dense non-aqueous phase liquids
Pump Out Well
Toward Cass Lake and Pike Bay
? ? ?? ?
DNAPLs
50 feet
Ground-penetrating radar survey
(H. Mooers and N. Wattrus – University of MN Duluth)
• Uneven surface of confining layer (variations of ~5 feet in height)
• DNAPLS could be pooling at base of surface aquifer
Groundwater Panel Conclusions
• The confining layer is not continuous and may hold pools of dense contaminants that could be removed.
• There is a risk that contaminants from the surface aquifer could be reaching the deep aquifer.
• Evidence suggests that the pump-out wells may not be extracting all contaminants – some data suggest that a contaminant plume is moving off-site toward Cass Lake/Pike Bay.
Groundwater Panel Conclusions
• There are not enough wells, particularly to the southeast of the site, to properly assess the extent and movement of contaminants.
• Our understanding of the site’s geological character is very poor.
• The site must be better characterized to understand the movement of contaminants –
• additional groundwater wells to the southeast
• more comprehensive analytical model of the site
• additional techniques to better characterize the site, such as Ground Penetrating Radar
• better data quality.
Recommendation
City Dump
Containment Vault
Fish Hatchery
Treating Facility Site
Pump Out Wells
Pike Bay
Fox Creek
Channel
