MTBE Diving Plumes

$EPA NRMRL

\ NATIONAL RISK MANAGEMENT RESEARCH LABORATORY
www.epa.gov/nrmrl GROUND WATER AND ECOSYSTEMS RESTORATION RESEARCH

MTBE Diving Plumes

Introduction

There are approximately 400,000 confirmed releases of gasoline
from underground storage tanks in the United States. At least
200,000 contain methyl tertian butyl ether (MTBE) at significant
concentrations. The impact from these releases on ground water
are monitored with shallow wells surrounding the releases.

Because there are so many spills and because monitoring wells
are expensive, there is a strong economic incentive to keep the
network of monitoring wells simple and small.

Occasionally, MTBE plumes will move below the bottom of the
screen of conventional monitoring wells and escape detection.

This is called plume diving. If the MTBE plume escapes detection,
it can contaminate public water supply wells.

Background

Because MTBE does not sorb to aquifer material and because there is no biological degradation in many plumes,
there is an opportunity for MTBE plumes to be very long. The longer the plume, the more likely that it will dive
below the screen of conventional monitoring wells. Plumes can dive deeper into an aquifer by a number of
mechanisms. Plumes can be buried by recharge of precipitation, moving down to the aquifer from the surface.
Plumes tend to flow around clay layers and through sand or gravel layers. If the layers that control flow of ground
water dive into the aquifer, so will the plume.

Objectives

• Document the phenomenon of plume diving at real gasoline spills

• Determine the properties of the aquifer that caused the plume diving

• Develop or evaluate tools that can be used to predict plume diving, and identify the best depth intervals for
monitoring wells in order to detect diving plumes

• Develop criteria that can be used to recognize MTBE plumes with no natural biodegradation
Approach

Detailed case studies were conducted on MTBE plumes at East Alton, Illinois; Spring Green, Wisconsin; and
Milford, Michigan. Each of these plumes impacted a municipal water supply well. Plume diving through burial by
recharge was estimated, using an EPA application available on the Web. Plume diving caused by structure of the
aquifer was estimated by determining the vertical distribution of clay and sand layers, and the vertical distribution
of hydraulic conductivity. The geochemistry of the receiving ground water and of the plume was characterized in
order to recognize conditions where MTBE did not degrade.

Screened Interval Conventional UST Well

Point of Discharge

-X

2000 3000 -W00 5000 6000
Distance (feet)

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Accomplishments

At all three sites, the MTBE plumes dived deeply into the aquifer. Diving of the plume at Spring Green, Wisconsin,
could be predicted from aquifer recharge, using the plume diving calculator. Visit: EPA Online Tools for Site
Assessment Calculation (http://www.epa.gov/extrmurl/learn2model/part-two/onsite/diving .htm).

Plumes at the East Alton, Illinois, and the Milford, Michigan, sites followed layers of sand and were constrained by
layers of clay in the aquifer. The plume was found at the contact between a deep layer of sand and the layer of clay
above it. Clay and silt often have high electrical conductivity and sand and gravel often have low conductivity. A
simple tool recently available on the market can determine the electrical conductivity of aquifer material to depths
of 100 feet or greater at reasonable cost. The tool was used at the East Alton site to map the clay and sand layers
and predict the most likely interval to harbor the plume. Water samples were collect in discrete vertical intervals.
The highest concentrations of MTBE were in the intervals that were predicted from the electrical conductivity
measurements.

In most aquifers, bacteria will degrade MTBE readily if oxygen is available. At all three sites, the MTBE plume
was devoid of oxygen. At the East Alton, Illinois, site, the uncontaminated water in the aquifer was also devoid of
oxygen.

Future Tasks

An EPA report on plume diving is in preparation.

Wilson, J.T., R.R. Ross, and S. Acree. (Submitted for publication). "Site Characterization to Determine the
Influence of Stratigraphy on a Diving Plume of MTBE in a Municipal Well Field." Ground Water Monitoring and
Remediation.

Investigators

Collaborators

Illinois Environmental Protection Agency

Michigan Department of Environmental Quality

New York Department of Environmental
Conservation

Steven Acree
John Wilson

U.S. EPA Region 5

Wisconsin Department of Natural Resources

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