Questions and answers, December 15, 2016 webinar
Below are questions submitted during the December 15, 2016 webinar on Infiltration. Answers include links and if appropriate, links to additional information.
- Why do you need 3 feet of separation from groundwater for infiltration but in the same pond footprint you can do wet pond construction which is in direct contact with groundwater:
- The Manual states: In general, there is no minimum separation distance required with ponds. Intercepting the groundwater table can help sustain a permanent pool. However, some source water protection requirements may dictate a separation distance or an impervious liner if there is a sensitive underlying aquifer and the bottom material of the pond allows for infiltration.
- How much avg square ft cost for install and maintenance of this example system?
- This table provides general cost information for different infiltration systems. This page was created in the original manual and provides some basic information, although the information is likely dated. For an in-depth discussion of costs for various stormwater management practices, see Weiss et al (2007).
- What kind of vegetation can handle the 6.5' maximum bounce mentioned?
- The 6.5 foot bounce is feasible only in A soils. These soils should drain very fast. Shaw and Schmidt (2003) provides excellent information on vegetation for stormwater management systems. Of particular interest might be the table starting on page 67, Section 2, which shows inundation depths and duration of inundation for different plant species.
- Is there an example scope of work that would describe what a boring consultant is to gather the required documentation necessary for design of SWPPP.
- We're not sure what this question refers to, but the page Understanding and interpreting soils and soil boring reports for infiltration BMPs provides information on what information to collect with soil borings. The critical factor to consider in conducting borings when designing an infiltration BMP is to identify any restrictive layer in the soil profile. A SWPPP could utilize information from the manual in describing how BMP design and investigations will be conducted, or simply reference the manual.
- What is the general thought of infiltration in areas that are in active karst features (less than 50 feet of soil cover) but it is not located in the DWSMA or Wellhead protection area? I have heard the term higher engineering review to determine if infiltration can be done in Karst areas but what does the higher enginnering review actually consist of?
- The Construction Stormwater Permit prohibits infiltration of stormwater runoff “within 1,000 feet up-gradient or 100 feet down-gradient of active karst features unless allowed by a local unit of government with a current MS4 permit”. The question implies that active karst has been identified at a site. Investigation methods in karst areas are discussed here. Higher engineering review is not defined in the permit, nor is it clear what the intent of the higher review is. Presumably a higher engineering review would indicate that there is sufficient confining material between the bottom of the practice and the nearest underlying aquifer. This would be based on the presence of some confining layer of sufficient thickness to attenuate pollutants should the practice fail. This type of analysis should be conducted by a licensed geoscientist or engineer. Additional information may be found at General stormwater management guidelines for karst areas and at the MNDNR's webpage on groundwater sensitivity.
- Does the manual address infiltration in D soils - as required by several watershed districts?
- The Construction Stormwater permit prohibits infiltration in areas of predominately Hydrologic Soil Group D (clay) soils unless allowed by a local unit of government with a current MS4 permit.
- How much avg square ft cost for install and maintenance of this example system?
Is there a minimum offset distance if there is contaminated soil?
Any discussion during design for routing around facility during establishment period?
Are infiltration practices still recommended near areas of high salt loading?
What storm event needs to infiltrate within 48 hours when including the bounce in the drawdown depth?
Was mentioned that mndot grade 2 compost was not recommended for filtration practices due to nutrient leaching. Are there alternative recommendations other than peat moss?
Media passes all tests. How to correct if perc failures occur after placement
At some point will we start focusing on treating different runoff in different ways? Parking lot runoff differently than roof runoff, etc. since they have different treatment needs versus putting all of the runoff together? It seems like we are just creating more dilluted runoff at times. It would also be nice to see more info on routing (bypasses) and not sending more than 1" to the treatment feature by separating quality from rate control.
How long does grade 2 compost leach
Can you talk a little about the use of stormwater-specific geotechnical borings versus interpretation of structural-specific borings (e.g. those for footings and foundations nearby a proposed practice)
Does the MPCA have a recommendation for when infiltration should not be used due to high blow counts/dense soils? What would be considered a high blow count?
Do you recommend vegetating large infiltration basins?
Can you talk more about sub grade infiltration practices?
Many sites we work in have saturated soils within 3' of the existing ground and require mound septic systems. However, the soil borings show a C soil. How are we suppose to provide filtration when this in the case?
Can you point out and discuss the underground systems/
Can you compare annual maintenance costs of infiltration basins vs. subgrade infiltration?
Design: Please clarify the bottom of the infiltration practice. Is it the floor of the basin or the bottom of the planting medium, ripped area, and abutment to the in situ soil.
Are you posting this presentation on line for us to review?
We are in sandy soils with some basins infiltrating at 10+ inches/hr. Is there a reccomended maximum infiltration rate for pollutant removal?