As previously noted in our September posts, PA DEP has convened a BMP Manual Revision Committee. In their notification to attendees, they noted that

… recommended updates may include edits to existing portions, complete omissions, or additions of new material.  All changes much be justified through acceptable principles of engineering or science.  If unable, to do so, subgroups may bring new ideas before the bigger committee for consideration and feedback.

Following is PA DEPs statement on the focus of this  Sub-Group, as presented at the first Committee Meeting:

Much attention has been given to the “loading ratios” approach for sizing infiltration BMPs.  It has been found that this approach is better suited as a planning tool and default for practitioners who do not want to do a more thorough site assessment.  Therefore, this subgroup is tasked with developing clearer and more thorough criteria for sizing infiltration BMPs.  Questions:

Should hydraulic depth be utilized more as limiting factor (to avoid compaction at the air-soil interface)?

Should a risk-based approach to infiltration BMP design be incorporated into the manual?  For instance, should there be a maximum drainage area?  Should underground systems be designed more conservatively than above-ground systems?  What other factors should be considered in the design of infiltration BMPs.  See white paper for discussion on this matter.

Should mandatory pretreatment be incorporated into all infiltration systems (to what level  or particle size - 100 micron?)

Site evaluation and soil infiltration testing:

• Subgroup should reviewt and “beef up” the Soil Testing Protocol for infiltration BMPs
• Entire Appendix C, Protocols 1 and 2 should be evaluated for update.
• Infiltration Range from 0.1 to 10 in/hr;
• infiltration in fill (how long of a consolidation time before an area is no longer considered fill - 5 years?)
• Subgroup should review setbacks from dwellings, septic systems, drinking wells, property lines?  Should topography play a role - hillside hydrology.

Geological Issues (karst, other geologic formation, etc.)

• Should depth to limiting zones be kept at 2 feet.  Should it vary depending on soil permeability?
• When, if at all, should a mounding analysis be required?  Should PA follow NJ’s lead?

Water quality issues (nitrates)  (see Subgroup Topic E)

Infiltration on contaminated sites.  (See Subgroup Topic G)

Should infiltration of storms greater than the 2-year 24-hour event be “specifically” discouraged?   There are some municipalities who require infiltration of the 100-year storm.  Depending on the extent of the project, this can lead to issues in the future.  Many municipalities defer to the PA BMP Manual for technical guidance (MS4 etc.).   Discussion of this topic is important since infiltrating too much water has been identified as a concern.

BMP Manual Revision BMPs, infiltration, PA BMP Manual, planning, water quality