Transcript Document

Assessing and Communicating Risk:
A Partnership to Evaluate a Superfund Site
on Leech Lake Tribal Lands
Groundwater Movement and
Contamination
By Cindy Hagley
Minnesota Sea Grant
March 2003
Groundwater
Basics
• 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.
• Groundwater occurs
almost everywhere
within the pore spaces of
saturated rock beneath
the land surface.
Direction of
groundwater
movement
Ground versus Surface Water
• The rate of
Movement
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 waterbearing 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 – 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.
Groundwater Susceptibility
Things to keep in mind:
clay
confined aquifer
• 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.
Treating
Facility Site
Channel
Pump Out
Wells
Containment
Vault
Fish
Hatchery
City Dump
Fox Creek
Pike Bay
Upper Aquifer
Confining Layer
Lower Aquifer
Toward Cass Lake and Pike Bay
Treating
Facility Site
Channel
Pump Out
Wells
Containment
Vault
Fish
Hatchery
City Dump
Company data from wells
suggests west to east flow
Fox Creek
Pike Bay
Company analytical model
suggests NW to SE flow
Pump out wells are
to the
Treating
east of the site Facility Site
Channel
Pump Out
Wells
There are Containment
no sampling wells
to the SE Vault
of the site, so we
Fish
don’t know if contaminants
Hatchery
are moving in that direction
City Dump
Fox Creek
Data from wells suggests west
to east flow
Pike Bay
Analytical model suggests
NW to SE flow
LNAPLs – Light non-aqueous phase liquids
Pump Out
Well
Translation- Floating plumes of
contaminants that don’t mix with water
Toward Cass Lake and Pike Bay
DNAPLs – Dense non-aqueous phase liquids
Pump Out
Well
Translation - Sinking plumes of
contaminants that don’t mix with water
Confining Layer
Toward Cass Lake and Pike Bay
Pump Out
Well
DNAPLs
?
?
?
?
Toward Cass Lake and Pike Bay
?
Groundpenetrating
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
50 feet
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.
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.
Treating
Facility Site
Channel
Pump Out
Wells
Containment
Vault
Fish
Hatchery
City Dump
Fox Creek
Pike Bay