Assessing%20and%20Communicating%20Risk:%20A%20Partnership%20to%20Evaluate%20a%20Superfund%20Site%20on%20Leech%20Lake%20Tribal%20Lands

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Assessing and Communicating RiskA Partnership
to Evaluate a Superfund Site on Leech Lake
Tribal Lands

• Groundwater Movement and Contamination

By Cindy Hagley Minnesota Sea Grant March 2003
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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
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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.

sw
GW
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Groundwater - The Tortoise
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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.

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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.

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Confined Aquifer

• Pressure, rather than gravity, makes water move
  in confined aquifers. Water moves from areas of
  high to low pressure.

• Confining layer clay or bedrock

Confining layer
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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.

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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.

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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 wells
  area of influence.

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Groundwater Susceptibility
Things to keep in mind

• Unconfined aquifers with no cover of dense
  material are susceptible to contamination.

• 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.

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Treating Facility Site
Channel
Pump Out Wells
Containment Vault
Fish Hatchery
City Dump
Pike Bay
Fox Creek
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Upper Aquifer
Confining Layer
Lower Aquifer
Toward Cass Lake and Pike Bay
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Company data from wells suggests west to east flow
Company analytical model suggests NW to SE flow
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Data from wells suggests west to east flow
Analytical model suggests NW to SE flow
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Translation- Floating plumes of contaminants that
dont mix with water
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DNAPLs Dense non-aqueous phase liquids
Translation - Sinking plumes of contaminants that
dont mix with water
Confining Layer
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Ground-penetrating radar survey (H. Mooers and
N. Wattrus University of MN Duluth)

• DNAPLS could be pooling at base of surface aquifer

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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.

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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 sites geological
  character is very poor.

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Recommendation

• 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.

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Treating Facility Site
Channel
Pump Out Wells
Containment Vault
Fish Hatchery
City Dump
Pike Bay
Fox Creek