MPCA Design Guidance for Large Subsurface Wastewater ...

Refer to Attachment 8 for specific information on methods to determine phosphorus attenuation. Also,
submit a work plan that outlines the phosphorus attenuation discussed in the above paragraph to the
MPCA.
9. Hydrogeologic evaluation
The Board of Architecture, Engineering, Land Surveying, Landscape Architecture, Geoscience and
Interior Design (AELSLAGID) requires the use of professional geologists (PG) that are hydrogeologists
for this portion of the review.
Complete a thorough hydrogeologic evaluation of the site and submit the findings of that evaluation to
the MPCA as a report. The MPCA staff should be contacted to discuss the project before beginning the
hydrogeologic evaluation.
The purpose of a hydrogeologic evaluation is to:
1) Estimate the height of the ground water mound above the saturated zone and/or the height that
effluent may perch over an unsaturated restrictive layer. It may be necessary that this
assessment also include a determination of the potential for sideslope break-out (seepage) due to
mounding or perching of effluent. The evaluation of mounding/perching is necessary for all
proposed LSTS systems.
2) Estimate the concentration of nitrate-nitrogen at the LSTS property boundary. This evaluation is
only necessary for LSTS systems that choose Permitting Option 2 (see Section 6 for the
definition of Option 2.)
Information that will be collected and evaluated from the hydrogeologic evaluation includes:
• the direction of ground water flow
• the depth to the static ground water level
• the rate of ground water movement (hydraulic conductivity)
• the hydraulic gradient
• the specific yield
• geologic cross-sectional analysis of the site, including the presence of deeper limiting layers,
water table surface, perched water tables, bedrock, karst, etc.
• characterization of aquifers that may be impacted by the wastewater and whether the impacted
aquifer is connected to a current or future drinking water aquifer
• thickness of the saturated zone
• background ground water quality data
The items above involve hydrogeologic field work operations. This field work involves the drilling of
deep borings at the proposed site followed by an evaluation of the bore holes and excavated material.
The borings, and subsequently installed wells or piezometers, are used to test for the direction of ground
water flow, the static water level, the hydraulic conductivity, the hydraulic gradient, saturated thickness,
and the specific yield.
The sixth bulleted item, geologic cross-sections, involves field work to create boring logs using the
Unified Soil Classification System (USCS) method, and then office work creating two- or threedimensional
profiles. To use the borings to determine ground water quality (including total nitrogen
levels), the LSTS designer will be required to convert some or all of the borings to ground water wells to
collect ambient ground water chemistry samples. This step will only have to be done if Permitting
Option 2 from Section 5 – Nitrate-Nitrogen Treatment is chosen.
If Permitting Option 2 is selected, use this ground water information in the nitrogen model provided in
this section or other contaminant dispersion model. Based on the results of the modeling, some of the
Design Guidance for Large Subsurface
Minnesota Pollution Control Agency
Wastewater Treatment Systems April 2010
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