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Spatial variation in saturated hydraulic conductivity of sediments at a crude-oil spill site near Bemidji, Minnesota

Water-Resources Investigations Report 98-4053

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Abstract

Saturated hydraulic conductivity of aquifer sediments at a crude-oil spill research site near Bemidji, Minnesota were examined using pneumatically-induced head-difference tests and packer/vacuum system tests. Results from slug tests on 58 wells show that hydraulic conductivity varies both horizontally and vertically in the range from about 10-7 to 10-4 meters per second (m/s), with a median of 7.28 x 10-5 m/s. Hydraulic conductivities of the well-sorted medium to fine sand facies, which contains a majority of the oil plume, range from 1.76 x 10-5 to 9.82 x 10-5 m/s with a median of 5.42 x 10-5 m/s. Hydraulic conductivities of the lower sand and gravel unit, which contains a majority of the plume of dissolved petroleum constituents, range from 4.42 x 10-6 to 5.36 x 10-4 m/s with a median of 2.32 x 10-4 m/s.

The average linear velocity of ground water near the spill site was calculated to examine the effects of advective flow on migration of the plumes of oil and dissolved petroleum constituents. The average linear velocity in the well-sorted medium to fine sand facies during September 1996 was about 11 meters per year (m/year). If we assume that this was the average velocity during the 17-year period since the spill (1979-96), total advective flow of ground water in this facies was about 187 m. During this 17-year period, oil floating on the water table migrated only about 40 m. By comparison, the lower sand and gravel unit had an average linear velocity of about 29 m/year, or about 3 times greater than the velocity in the well-sorted medium to fine sand facies. Based on a 29 m/year velocity, advective flow of ground water in this unit during the 17-year period since the spill was about 493 m; whereas, the plume of dissolved petroleum constituents migrated only about 200 m. These results indicate that spatial variability of hydraulic conductivity and ground-water velocity at the research site likely is a factor affecting the rate of contaminant migration. Additional research is needed to fully evaluate how the contaminant plumes are affected by changes in hydraulic properties of the various lithologic units.

Additional publication details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Spatial variation in saturated hydraulic conductivity of sediments at a crude-oil spill site near Bemidji, Minnesota
Series title:
Water-Resources Investigations Report
Series number:
98-4053
Year Published:
1998
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Mounds View, MN
Contributing office(s):
Minnesota Water Science Center
Description:
iv, 13 p.
Country:
United States
State:
Minnesota
City:
Bemidji
Online Only (Y/N):
N
Additional Online Files (Y/N):
N