Hydrologic Soil Group Knowledge Matrix Refinement Of Soils Parameter For MPCA s Minimum Impact Design Standards Calculator and Stand-Alone Hydrologic Soil Group Determination Funding for this project was provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR)
Existing Resources The Minimum Impact Design Standards (MIDS) Work Group has been working on a stormwater BMP effectiveness calculator since 2009. The focus has been on detailed and accurate estimates of volume and loading reductions expected from a wide variety of existing stormwater BMPs. While the MIDS Calculator is continually updating its estimating capabilities, one of the fundamental inputs, the soils parameter, has potentially been under represented by this calculator. Soils Information used within MIDS Calculator The soil parameter mentioned in the MIDS Calculator is Hydrologic Soil Group (HSG). This parameter is the basis for all volume and loading calculations for each stormwater BMP. Uncertainty around this parameter will limit the validity of any estimates calculated. Therefore it is important that the user understand more about HSGs and how they are determined. There are only two methods mentioned to determine the HSG input parameter in the MIDS Calculator (MPCA-MIDS Work Group, 2013) via the web soil survey or the Minnesota Stormwater Manual (MPCA, 2013). If a user obtains HSG information from the soil survey, they need to understand that this rating is for the natural soil condition not an altered condition as is common in almost every stormwater BMP project. This circumstance limits the reliability of the estimates determined from the calculator. The other option presented by the MIDS Calculator (MPCA-MIDS Work Group, 2013) is to reference the Design Infiltration Rates table (Table 1) from the Minnesota Stormwater Manual (MPCA, 2013). This table correlates soil texture to HSG and infiltration rate. This correlation is simplified and prospectively erroneous due to the numerous assumptions that must be made about the site. Most of the soil/site assumptions that are required for accurate HSG determination do not commonly occur in stormwater BMP projects (e.g. no construction activity). These assumptions are also not mentioned to the user of either the MIDS Calculator (MPCA-MIDS Work Group, 2013) or the MN Stormwater Manual (MPCA, 2013). Soils information is recognized as being important to the estimation process. In a memorandum to the Minimum Impact Design Standards (MIDS) Work Group: Vegetation and Soils (Barr Engineering Company, 2010a), there is a summary of available soils and vegetation datasets from across Minnesota that may provide useful information in predicting a site s runoff characteristics. The discussion provided on using the USDA-NRCS SSURGO (county-level) soil survey information states there is much information contained but mentions little about specific soil properties or how to estimate a Hydrologic Soil Group (HSG) from this information or from field-determined soil properties. This is a necessity for accurate MIDS Calculator results.
In another memo (Barr Engineering Company, 2010b), HSGs are introduced and discussed, but provide little information to the reader on the complete definition or official HSG criteria, which are found within the National Engineering Handbook, Park 630, Chapter 7 (USDA-NRCS, 2009). This memo (Barr Engineering Company, 2010b) also references a table used in the Minnesota Stormwater Manual (MPCA, 2013) to determine the design infiltration rate from a single soil property, soil texture. The infiltration rate of a soil is much too variable to be simply determined by a single soil texture determination. In a MIDS Calculator (MPCA-MIDS Work Group, 2013) that is predicting stormwater BMP impacts in developing/developed areas, it is advisable to provide the calculator s users a more useful tool to assess the appropriate Hydrologic Soil Group (HSG) and potential infiltration rate(s) to be used in the Site Information Section. Hydrologic Soil Groups - Background Hydrologic Soil Groups are defined as the minimum annual steady ponded infiltration rate for a bare soil surface (Soil Survey Staff, 1993). The classification of soils into HSGs is based on the most limiting of hydraulically transmissive horizon/layer in the soil, depth to a hydraulically limiting condition (e.g. bedrock or high clay content) or depth to the watertable. The assignment of a soil to a grouping is based on the overall soil profile, not individual horizons or a single soil property. Beyond these general considerations, numerous soil properties should be assessed. Not only are soil textures to be determined of all the existing soil horizons present, but soil structure shape, bulk density (or soil structure grade) and percent rock fragments. Accurate placement into the HSG cannot be completed without a soil profile description by qualified personnel. Soil Scientists have classified soil map units in soil survey reports into HSGs. These soil survey products are widely available in Minnesota (websoilsurvey.nrcs.usda.gov). This classification should be the foundation for any HSG determination. Note that on-site conditions are not verified, but are representative of the natural soil conditions. Chapter 7 of the National Engineering Handbook (USDA-NRCS, 2009) clearly states that the expertise of these Soil Scientists should be consulted in areas not assigned a HSG or altered site conditions from natural. The Soil Survey Manual (USDA-NRCS, 1993) has guiding criteria for determining HSG from soil properties when soils are in a disturbed state from construction or other activities. The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D). The groups are defined as follows: Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission.
Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission. Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission. Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission. If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that occur naturally in group D are assigned to dual classes. HSGs Applied The HSG Knowledge Matrix provides a standardized decision support system to determine the appropriate HSG from a combination of off-site and field-determined soils information. A hydrologic soil group can be utilized for a number of applications including use in the MPCA MIDS Calculator or sizing a stormwater features using the MPCA s Stormwater Manual (MPCA, 2013). STEP 1: Determine HSG from the Web Soil Survey (WSS) The official soil survey information and documentation now resides online at http://websoilsurvey.nrcs.usda.gov (verified 3/27/13). While users may choose to print off information, this information is subject to change and should be checked online frequently for updates. Online soil surveys still present soil lines on a photographic background indicating the boundaries between different soil types. These maps show the occurrence and distribution of soils and landscapes across the majority of Minnesota. There are numerous soil properties that can be estimated from the WSS in addition to the HSG including soil drainage class, hydric soil, depth to bedrock, etc. that are beyond the scope of this document, but may also prove valuable in proper site and soil documentation and determination for stormwater BMPs. It is important to remember that the WSS predicts the HSG of the soil in a natural state that has not been altered by compaction, construction, etc. To obtain the natural HSGs for a site from the WSS use the following steps:
1. Find parcel - address, state and county, latitude and longitude, PLSS (section, township, range) 2. Outline the desired parcel/area using the Define AOI (Area of Interest) tool. 3. Select the tab for Soil Map near the top of the browser window. 4. You will see aerial photo with soil lines, summary of map units, names, areas in the AOI, and percent of the AOI. Map units denoted such as 401C. 5. List the map units on parcel. 6. Selecting the third tab entitled Soil Data Explorer allows the mapping of selected soil properties over the AOI. 7. Now select the Properties & Qualities Ratings tab 8. Under this tab, there is a suitability rating for Soil Qualities and Features. Select this rating. 9. Now you will see the feature Hydrologic Soil Group. 10. This does not replace determination of specific soil properties, which are required when the site is disturbed or no longer in a native condition. STEP 2: Preliminary modification of the WSS HSG Results A rating of A, B, C, D or A/D, B/D, C/D is the determined HSG for the area selected by a qualified Soil Scientist as reported by the WSS. Each of these ratings has a numerous soil properties at various depths considered (Table 1). At a minimum, the WSS soil descriptions include all horizons to 100cm (39 inches) and include the soil properties of each layer, at minimum, including saturated hydraulic conductivity, depth to water impermeable layer (restrictive soil condition) and depth to high water table (seasonally saturated soil) (USDA-NRCS, 2009). Specifically for those areas with HSGs ratings of A/D, B/D and C/D, they indicate a drained condition/undrained condition rating. Unless the area has a documented, effective and maintained drainage system(s) as the site land-use is proposed to change, the HSGs should be interpreted as a Group D soil. It is very likely that this rating obtained from the WSS is the best-case scenario for the soils of the area. Any modification by urbanization, suburbanization, construction or other disturbance in the past has likely adversely impacted the soils at the site causing compaction, damaging to
soil structure, removal of topsoils, etc. It is important to note that minimal impact design does not remove previous damage to the site and may still impact these sensitive soil properties. STEP 3: Final Determination of the site HSGs Step 3 requires field-based information collected by a qualified professional in describing, interpreting and reporting soils information. Final determination of the HSGs for proper use in the MIDS Calculator can now be completed by completing the HSG - Knowledge Matrix Microsoft Excel spreadsheet. If the HSG has already been determined as a Group D soil, further determinations may not be required. Saturated hydraulic conductivity is not commonly measured as part of a routine soil and site evaluation. If determined in the field, it should be entered into the HSG - Knowledge Matrix, but the HSG Knowledge Matrix can also use field measurements of USDA soil textural classification along with soil structure shape and soil structure grade to place the soil into the appropriate HSG. Depth to high water table is commonly referred to as the seasonally saturated soil and is determined by either physical signs of free water surface, saturated soil conditions or presence of redoximorphic features in the soil. A suggested Stormwater BMP Site Evaluation form is included in Appendix 1 for appropriate assessment of site and soil conditions for use in the HSG Knowledge Matrix spreadsheet. All of the soil information listed in the HSG KM is required for the most accurate hydrologic soil group placement according to USDA-NRCS. However, we recognize some field investigations may not describe all of these soil parameters. The HSG KM does not require a complete soil description to provide a better estimate of HSG than using soil texture or information from the soil survey report alone. The user simply needs to enter the available soil information into the appropriate section of the HSG KM and the spreadsheet will still generate a result. Each soil variable in the HSG KM has a pop-up box that tells users the basic information to enter into this field. For instance, USDA soil texture is determined either in the field by the feel method or in a laboratory and reported. This soil texture class is then entered as a numerical code (1-13) based on a reference table included to the right of the HSG KM. The soil information can continue to be entered with user information contained within each pop-up box. If additional information is needed on a specific soil property, there are numerous soil science publications available to provide further explanation and description of these properties and is beyond the scope of this document. The USDA Soil Survey Manual (1993) is available online and provides a suitable explanation of many soil parameters included in the HSG KM. Another great soil science field reference is the USDA-NRCS Field Book for Describing and Sampling Soils (Schoeneberger et al., 2012).
It is important to note that the HSG KM also educates stormwater professionals as to the many soil parameters that will impact their design. Current stormwater site descriptions do not collect much of the information listed in the HSG KM. This may partially explain permanent stormwater features failing to function as designed. STEP 4: Input into MIDS BMP Calculator (Site Information and Summary) Hydrologic Soil Groups do not change by land-use. In the MIDS Calculator Site Information tab, simply ignore the line under the inputs for Land Cover Forest/Open Space (acres) -- undisturbed, protected forest/open space or reforested land. Enter in the proper areas (acres) of each final HSG, as determined from the HSG KM, in the line titled, Managed Turf (acres) -- disturbed, graded for yards or other turf to be mowed/managed (Figure 1). Proceed to use the MIDS BMP Calculator with the more accurate and realistic soils and site information. Summary There are numerous combinations of soil properties that when combined can provide an accurate HSG. Assumptions made in many existing resources including the MN Stormwater Manual and MIDS Calculator will be violated and will produce inaccurate HSGs and any resultant estimates calculated from them. These results can limit our understanding of stormwater BMP functioning, as data cannot be easily consistently compared due to improper placement of a site into a HSG. The HSG Knowledge Matrix compiles the required soils information to correctly and consistently classify the HSG based on the National Engineering Handbook (USDA-NRCS, 2009) which will aid in higher certainty estimates for volume and load reductions via the MIDS Calculator and other stormwater BMP tools.
Land Cover (acres) A soils B Soils C Soils D Soils Totals (acres) Forest/Open Space (acres) -- undisturbed, protected forest/open space or reforested land 0.00 Managed Turf (acres) -- disturbed, graded for yards or other turf to be mowed/managed 0.00 Impervious Cover (acres) Total: 0.00 Figure 1. Excerpt from Site Information and Summary tab of the MIDS BMP Calculator where HSG information is entered on only the Managed Turf (acres) line.
Table 1 Simplified Hydrologic Soil Group classification table (MPCA, 2013).
Table 2 The soil saturated hydraulic conductivity ranges and other soil property requirements
to properly determine Hydrologic Soil Group (USDA-NRCS, 2009).
References Barr Engineering Company, 2010. Memorandum to MIDS Work Group: Vegetation and Soils. Available online at: http://www.pca.state.mn.us/index.php/view-document.html?gid=14327 Barr Engineering Company, 2010. Memorandum to MIDS Work Group: Regional Hydrologic Metrics Infiltration. Minnesota Pollution Control Agency, MIDS Work Group, 2013. MIDS: Enhancing stormwater management in Minnesota. Minnesota Pollution Control Agency, 2013. Minnesota Stormwater Manual, Overview of Basic Stormwater Concepts. Available online at: http://stormwater.pca.state.mn.us/index.php/main_page Schoeneberger, P.J., D.A. Wysocki, E.C. Benham, and Soil Survey Staff. 2012. Field book for describing and sampling soils, Version 3.0. Natural Resources Conservation Service, National Soil Survey Center, Lincoln, NE. U.S. Department of Agriculture, Natural Resources Conservation Service, 2009. National Engineering Handbook, title 210-VI, Part 630, Chapter 7. Washington, DC. Available online at: http://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/national/water/?&cid=stelprdb1043063. U.S. Department of Agriculture, Natural Resources Conservation Service, 1993. Soil Survey Manual. Agricultural Handbook No. 18, Chapter 3. Washington, DC. Available online at: http://soils.usda.gov/technical/manual/.
Appendix 1 Stormwater BMP Site Evaluation Form Date Time Project Name Location Weather Evaluator s name I am a Designer / Contractor / Inspector / Other LGU(s) Preliminary Information Owner Proposed Infiltration Rate Site History Depth to Infiltration Zone Receiving Water Landscape Position Soil Survey Information Site Evaluation Compaction: Yes/No Surface water in area Test used: Flag / Probe / Auger / Other Floodplain in area Soil observation method: Pit / Push Probe / Bucket Auger / Hand Shovel / Flight Auger / Other Infiltration Tests Soil Description Depth Texture Color Structure Redox Features Hydric? Conclusions: Is there the potential for an underdrain to have an outlet? Yes / No Saturated soils in area Number of soil observations made Describe the variability of site and soil conditions across the proposed BMP area