Definition of pollution sensitivity
The Minnesota Department of Natural Resources (DNR) defines an area as sensitive if natural geologic factors create a significant risk of groundwater degradation through the migration of waterborne contaminants (MS § 103H.101).
Migration of contaminants dissolved in water through unsaturated and saturated sediments is affected by many things, including biological degradation, oxidizing or reducing condition and contaminant density. General assumptions include:
- Contaminants move conservatively with water.
- Flow paths are vertical.
- Permeability of the sediment is the controlling factor.
Pollution sensitivity of the near-surface materials
The near-surface sensitivity assessment estimates the time required for water to travel from the land surface; through unsaturated sediment, and finally to the water table. The water table is assumed to be at a 10’ depth. Transmission rates are based on the soil type and the texture of surficial geologic units as mapped in the county atlas series. The travel time varies from hours to approximately a year.
Pollution sensitivity of buried aquifers and the bedrock surface
The pollution sensitivity of buried sand and gravel aquifers and of the first buried bedrock surface represents the approximate time it takes for water to move from land surface to the target (residence time). Groundwater chemistry is used to support hypotheses relating geologic factors to travel time. Dye traces, naturally occurring chemicals, and other human-introduced chemicals are used to date groundwater and better understand flow paths and residence times.
Five relative classes of geologic sensitivity are based on overlapping time of travel ranges (Very High, High, Medium, Low, and Very Low). The pollution sensitivity is inversely proportional to the time of travel.
- In areas of higher sensitivity contaminants may reach the groundwater within hours to months.
- In areas of lower sensitivity there is time for a surface contamination source to be investigated, and possibly corrected, before serious groundwater pollution develops.
Relatively high sensitivity does not mean that water quality has been or will be degraded. If there are no contaminant sources, pollution will not occur. Low sensitivity does not guarantee protection. Leakage from an unsealed well for example, may bypass the natural protection, allowing contamination to directly enter an aquifer.
|Time of Travel Criteria|
Geologic sensitivity ratings are based on the time required for water at or near the surface to travel vertically to the water table or other groundwater of interest. Longer travel times imply a lower sensitivity to pollution. Dye trace, tritium, and carbon-14 studies can indicate the relative ages of groundwater.
DNR (Minnesota Department of Natural Resources) 2014, Procedure for determining near-surface pollution sensitivity maps, v. 2.1., accessible at https://files.dnr.state.mn.us/waters/groundwater_section/mapping/
Geologic Sensitivity Workgroup, 1991, Criteria and guidelines for assessing geologic sensitivity of ground water resources in Minnesota: Minnesota Department of Natural Resources, Division of Waters, St. Paul, MN, 122 p. Original printed document scanned December 2003. A few pages contain minor clarifications of the original text. Accessible at https://files.dnr.state.mn.us/waters/groundwater_section/mapping/
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