Questions and answers, December 15, 2016 webinar 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 2013 Construction Stormwater Permit prohibited 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 2018 CSW permit no longer allows the option of infiltration under these conditions. 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. Meeting the 1 inch volume requirement and the 48 hour drawdown requirement on D soils would require a very large infiltration practice. The manual provides management guidelines for sites with D soils.
• 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).
• Is there a minimum offset distance if there is contaminated soil?
  • While there is no specific separation distance between an infiltration BMP and contaminated soil or groundwater, we developed a page for the manual to address this issue. See this section in the manual.
• Any discussion during design for routing around facility during establishment period?
  • This is not specifically discussed on the construction page for infiltration practices. It is REQUIRED that infiltration systems not be excavated to final grade until the contributing drainage area has been constructed and fully stabilized. This section provides some information on erosion and sediment control during construction.
• Are infiltration practices still recommended near areas of high salt loading?
  • The manual does not discuss infiltration in areas of high road salt application. Areas of salt storage are discussed and may be considered stormwater hotspots. This is a topic that deserves some attention and it may be appropriate to designate transportation areas with high road salt application as stormwater hotspots. Specific guidance needs to be developed, however.
• What storm event needs to infiltrate within 48 hours when including the bounce in the drawdown depth?
  • The Construction Stormwater permit requires that Permittee(s) must design the project so that the water quality volume of one (1) inch of runoff from the new impervious surfaces created by the project is retained on site (i.e. infiltration or other volume reduction practices) and not discharged to a surface water. This volume is considered to be captured instantaneously by the BMP and must be infiltrated within 48 hours. Note that all water delivered to the BMP must be removed from the BMP within the drawdown time, either through infiltration or through infiltration and outflow through an overflow pipe.
• 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?
  • The manual does not recommend that specific composts be used or not used. To reduce the likelihood of exporting phosphorus from BMPs that have an underdrain, the P content in the media must be less than 30 mg/kg per Mehlich III (or equivalent) test. This would equate to a media with about 5 percent organic matter with a P concentration of about 0.06 percent in the organic matter. We agree that the P content of different sources of organic matter will likely vary and that over time, less P is likely to leach from the media. We are hoping to eventually have more specific information about media and different sources of organic matter.
• Media passes all tests. How to correct if perc failures occur after placement
  • It is difficult to provide a specific answer. It is essential to determine the cause of the failure. The manual unfortunately does not currently provide good information on remediating practices that are performing poorly. The University of Minnesota has developed a page that provides useful information on this topic.
• 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.
  • If the focus is on water quantity (volume), then the source of the runoff is less important provided we are not contaminating groundwater or overloading the treatment practice with pollutants. For water quality purposes, this comment raises an important point. If the goal is to meet water quality goals, it will be most efficient to implement practices where pollutant loads are greatest. Information on this page may be helpful in identifying those areas, although actual water quality monitoring would be preferred.
• How long does grade 2 compost leach
  • We can't specifically answer this question, but the issue of P leaching was examined in setting P credits for green roofs. A technical document produced by the contractor provides some insight into this issue. See link. For green roofs, it appears that they leach P for the first 4 to 10 years after construction and then may begin retaining P.
• 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)
  • Fundamentally the methods are similar, but for designing infiltration practices, it becomes more important to characterize the entire profile. Thus, nearly complete recovery in samples is essential. The focus on borings will be determining the vertical thickness of confining layers and the horizontal extent of these confining layers. This is one reason why in situ infiltration tests are recommended rather than borings.
• 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?
  • We looked for information on this and found a few papers that might be worth exploring. See 1, 2, 3, 4. In general, changes in the number of blow counts with depth provides a qualitative indication of the presence of a confining layer.
• Do you recommend vegetating large infiltration basins?
  • Yes. Vegetation has multiple benefits, including protecting the practice, providing additional retention of water and pollutants, and providing habitat and aesthetic value.
• Can you talk more about sub grade infiltration practices?
  • Not sure what this question is asking. Infiltration basins, trenches, dry wells, and underground practices are discussed together in the manual. When there are differences between the practices they are pointed out. Examples include information tables.
• 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?
  • Infiltration is prohibited in the Construction Stormwater permit if the seasonal high water table is within 3 feet of the bottom of the practice. See the definition of saturated soil. An underdrain system is recommended in these situations. The permit requires filtration systems with less than three feet of separation to include in impermeable liner (Part III.D.1.h).
• Can you point out and discuss the underground systems/
  • Infiltration basins, trenches, dry wells, and underground practices are discussed together in the manual. When there are differences between the practices they are pointed out. Examples include information tables.
• Can you compare annual maintenance costs of infiltration basins vs. subgrade infiltration?
  • The manual does not provide specific information on cost for different infiltration practices. This paper may provide some useful information. Also see 1, 2 (Table 6-10), and 3.
• 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.
  • If engineered media are utilized, the bottom of the practice is the base of the engineered media. If native soils are used, the base would be the depth to which excavation occurred. This diagram may be useful and includes an example where soils were ripped to alleviate compaction.
• Are you posting this presentation on line for us to review?
  • Yes. See link
• We are in sandy soils with some basins infiltrating at 10+ inches/hr. Is there a reccomended maximum infiltration rate for pollutant removal?
  • The Construction Stormwater permit prohibits infiltration in areas where soil infiltration rates are more than 8.3 inches per hour unless soils are amended to slow the infiltration rate below 8.3 inches per hour or as allowed by a local unit of government with a current MS4 permit. The reason for this prohibition is the likelihood that pollutants will not be attenuated by the practice. We have looked for information in the literature to support this and for guidance on how to modify soils that have excessive infiltration rates, but we have been unable to find good information on the topic. Amending with compost and/or clay is a likely mechanism for reducing infiltration rates, but we cannot yet quantify these effects.