Environmental contamination does not necessarily remain at the location where a release occurred.
Depending upon the contaminant and subsurface conditions, releases to soil or groundwater may migrate through the subsurface.
For this reason, groundwater flow direction, topography, geology, soil characteristics, and the location of potential contaminant sources can be important considerations during environmental due diligence.
Groundwater Flow Direction
Groundwater generally moves from areas of higher hydraulic head toward areas of lower hydraulic head.
Where groundwater information is available, the inferred groundwater flow direction can assist in evaluating the potential relationship between an off-site release and the subject property.
For example, if a documented petroleum release is located upgradient of the subject property, additional evaluation may be warranted.
If the release is located hydraulically downgradient, the potential for migration toward the subject property may be reduced.
However, regional groundwater flow should not automatically be assumed to represent site-specific groundwater conditions.
Topography
Surface topography can provide useful information regarding potential drainage and hydrologic conditions.
Topographic information may help identify:
Topography alone should not be used to establish groundwater flow direction, but it can provide supporting information when evaluated with available hydrogeologic data.
- Surface drainage patterns
- Nearby streams
- Low-lying areas
- Potential recharge areas
- Relative elevations
- General site setting
Soil and Geologic Conditions
Subsurface materials can influence contaminant migration.
For example, relatively permeable sand and gravel deposits may provide preferential pathways for groundwater movement.
Fine-grained materials such as clay and silt generally have lower hydraulic conductivity and may restrict the rate of groundwater movement.
However, low-permeability materials do not necessarily eliminate environmental risk.
Contaminants may migrate through:
Therefore, geology must be evaluated in the context of the specific site.
- Fractures
- Sand lenses
- Utility corridors
- Fill materials
- Preferential pathways
- Permeable stratigraphic units
Evaluating an Off-Site Environmental Release
Suppose a documented LUST site is located west of a subject property.
A complete environmental evaluation should consider:
Source: What was released?
Location: Where did the release occur?
Distance: How far is the source from the subject property?
Elevation: How does the source compare in elevation with the subject property?
Groundwater: What is the inferred groundwater flow direction?
Geology: What materials occur between the source and subject property?
Regulatory Status: Has the release been remediated or closed?
Migration: Is there evidence of contaminant migration?
The conclusion should be based on the combination of these factors rather than any single factor.
Why Environmental Judgment Matters
Environmental due diligence involves interpreting multiple sources of information.
A database report may identify a release. A topographic map may indicate relative elevations. Geological information may identify low-permeability materials. Regulatory records may document remediation.
The environmental professional must integrate these lines of evidence to determine whether the identified condition represents an environmental concern to the subject property.
This analytical approach is particularly important when evaluating off-site releases and potential contaminant migration pathways.