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