MPCA Design Guidance for Large Subsurface Wastewater ...

Wisconsin Department of Commerce Web Site (see Table 4): http://commerce.wi.gov/sb/docs/sb-
PowtsManualAtGrade.pdf
EPA Onsite Wastewater Treatment System Manual:
www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008totaldocument.pdf
3. Preliminary site evaluation (complete Attachment 3)
The next step involves completing a general evaluation of the proposed LSTS site(s). It is recommended
that this preliminary site evaluation be performed not only to determine the suitability of the potential
LSTS site(s), but also to justify the expense of more detailed and time-consuming soils and
hydrogeologic evaluations.
The purpose of the preliminary site evaluation is threefold:
1) to screen the area, or subdivision property, for potentially suitable LSTS sites, dependent on
readily available soils and ground water information
2) to use this information to determine whether a LSTS is a feasible method of wastewater
treatment/disposal for the subdivision development
3) to determine the concerns that may be associated with the proposed LSTS site(s)
The four primary concerns that may be associated with a proposed LSTS site(s) include:
1) Soil suitability (or the soil’s long-term hydraulic acceptance rate).
2) Long-term maintenance of an unsaturated treatment zone.
3) Compliance with the MPCA Nitrogen Policy.
4) Impact of phosphorus to nearby surface waters.
In general, the preliminary site evaluation will consist primarily of a desktop review of readily available
information. It is suggested that the designer seek out and obtain published soils and hydrogeologic
information for the area. If some relevant information is not available, it is recommended that the
designer make conservative assumptions for the data not found based on his/her best professional
judgment.
The end result of the preliminary site evaluation should lead to a conclusion as to whether the more sitespecific
soil and hydrogeologic evaluations should be pursued for a proposed LSTS site. If one or more
of the four concerns listed above appears to be insurmountable, perhaps a different LSTS site or a
different wastewater treatment method should be pursued as opposed to conducting an expensive,
detailed investigation of the current site. If the decision is made to investigate the proposed site further,
it is recommended that the detailed evaluation initially focus on the area of concern that represents the
most environmental consequence.
The preliminary site evaluation may become an iterative process. Based on available information, the
desktop assessment may suggest that evaluation of the proposed LSTS site be discontinued and another
site be pursued. This cycle may repeat itself several times until the most acceptable LSTS location is
found on a given parcel. For this reason, it is recommended that platting of residential subdivisions be
deferred until the ideal LSTS location is found.
The designer should obtain as much of the following general information as possible during the
preliminary site evaluation. It is recommended that this information be evaluated to assess the
significance of the four areas of concern listed above.
County soil survey information (http://websoilsurvey.nrcs.usda.gov/app/)
• soil texture and structure
• soil permeability
• depth to redoximorphic features
Design Guidance for Large Subsurface
Minnesota Pollution Control Agency
Wastewater Treatment Systems April 2010
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