The Construction Stormwater Permit requires a higher level of engineering review for proposed infiltration projects in areas overlying an Emergency Response Area (ERA) where the vulnerability of the DWSMA (Drinking Water Supply Management Area) is classified as moderate, or in areas outside the ERA where the vulnerability of the DWSMA (Drinking Water Supply Management Area) is classified as high or very high. This page provide guidance and recommendations for conducting a higher level of engineering review.
The figure at the right illustrates protection designations used to manage public water supply wells that have wellhead protection plans.
A public water supply well is vulnerable if:
Five classes of vulnerability exist: very low, low, moderate, high, and very high. Within high and very high vulnerability designations, contaminants at the land surface have the potential to move quickly to the underlying aquifer.
There are two conditions in which infiltration is prohibited under the Construction Stormwater General Permit unless a higher level of engineering review is conducted and demonstrates that a functioning treatment system will prevent adverse impacts to groundwater.
Vulnerability assessments for wellhead protection are based on data that was existing at the time of plan development, and are often reliant on stratigraphic information from construction logs for wells and borings constructed for purposes other than characterizing the nature and continuity of confining units. Therefore they should be viewed as a best estimate at the time of plan development, and subject to change during future plan amendments depending on the availability of newer or higher quality data.
In the first condition above, a contaminant will potentially be transported to an underlying aquifer within a moderate time frame (e.g. one year to a decade). The concern is that contaminants reaching an aquifer within the ERA can be transported to a public supply well within a short time (less than one year). It is therefore important to collect additional information about the geologic materials overlying the aquifer or ensure a minimal risk of contaminant exposure in these settings.
In the second condition above, outside of the ERA, the wells most likely to be impacted by infiltration practices are those also completed in the highly vulnerable aquifer and situated within one-year time of travel of the infiltration site. Determinations of ERA are based on sound geologic analysis and modeling, but other receptors beyond the public water supply well are not considered. Thus, private wells in these settings are at risk. Engineering review in this situation entails either conducting more detailed geologic analysis or modeling or conducting a well receptor survey.
Note that aquifer vulnerability for a DWSMA considers only the aquifer in which public water supply wells are located. The vulnerability designation for private wells completed in other aquifers must be determined on a case by case basis. For example, in the schematic to the right, the public supply well is located in an aquifer designated as having low vulnerability. Private wells completed in the same aquifer as the public supply wells have the same aquifer vulnerability designation as the DWSMA for the public well. The private well completed in the shallow aquifer above the confining unit is likely to have very high vulnerability.
Since the CSW permit focuses only on DWSMAs and on the vulnerability designation for DWSMAs, many private wells located in aquifers other than the aquifer for the DWSMA, or private wells located outside of DWSMAs may be at risk from an infiltration practice. It is Highly recommended that receptor surveys be completed for any infiltration practice, including those outside a DWSMA and that engineering reviews be conducted when private wells are likely to be located within a year travel time of the infiltration practice. See the following section for guidance.
The two conditions requiring higher levels of engineering review differ and therefore have different recommendations.
1. Condition 1: Moderate vulnerability overlying an ERA. Because there is a high degree of certainty regarding the boundaries of the ERA, the purpose of the engineering review in this case is to conduct a detailed geologic analysis or provide reasonable assurances that risk of contaminant exposure is limited.
If borings are not utilized, other sources of information may be used, but these should be used with caution. Examples include multiple well boring logs that show similar driller interpretation, or hydrogeologic assessments or studies conducted by professional organizations, such as the United States Geological Survey or Minnesota Pollution Control Agency.
An alternative approach to ensuring protection of a public water supply well is to ensure a minimum risk of contaminants exposure. This includes the following recommendations.
2. Condition 2: High and very high vulnerability outside an ERA. While a geologic assessment can be used in this situation, it is not necessary. If a geologic assessment is conducted, the goal would be to ensure there is sufficient protective material to retard contaminants before reaching the aquifer. The recommendations for conducting a geologic analysis described for condition 1 above can be used.
The primary concern for this condition, however, is to identify or protect other receptors, which typically will be private well owners. This can be achieved by limiting the risk of contaminant exposure, as described above for condition 1. A second option is to conduct a receptor survey. This is a two step process.
Note that these recommendations are relatively conservative and professional geoscientists or engineers may utilize other methods, including modeling.
The types of detailed geological investigations suggested under Condition 1 above could be very beneficial to public water suppliers and the Minnesota Department of Health (MDH) when it comes time to re-assess the vulnerability of the ERA and DWSMA during wellhead protection plan amendment, which occurs every 10-years. If you conduct a higher engineering review, please consider submitting the data and/or reports associated with such investigations to MDH or MPCA. Similarly, if you complete a well-receptor survey under Condition 2 above, please consider providing this information to MDH or MPCA. All wellhead protection plans include an assessment of wells within the DWSMA, and any receptor survey results could be beneficial towards those efforts.
Data, reports, and other information can be submitted using the Comment box at the bottom of this page or send an email to mailto:email@example.com Mike Trojan at the MPCA.
This section will be developed in August 2018.