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GROUNDWATER MOUNDING FOR SYSTEMS LARGER THAN 2,000 GPD

REGULATORY CITATIONS. 310 CMR 15.212(2): DEPTH TO GROUNDWATER 310 CMR 15.240(12): SOIL ABSORPTION SYSTEMS For systems with a design flow of 2,000 gpd or greater, the separation to high groundwater as required by 310 CMR 15.212(1) shall be calculated after adding the effect of groundwater mo

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GROUNDWATER MOUNDING FOR SYSTEMS LARGER THAN 2,000 GPD

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    1. GROUNDWATER MOUNDING FOR SYSTEMS LARGER THAN 2,000 GPD 2012 MassDEP/ MHOA Health Officers Seminars

    2. REGULATORY CITATIONS 310 CMR 15.212(2): DEPTH TO GROUNDWATER 310 CMR 15.240(12): SOIL ABSORPTION SYSTEMS For systems with a design flow of 2,000 gpd or greater, the separation to high groundwater as required by 310 CMR 15.212(1) shall be calculated after adding the effect of groundwater mounding to the high groundwater elevation as determined pursuant to 310 CMR 15.103(3) 310 CMR 15.103(3) outlines methods to determine historical high groundwater elevations .”.”

    3. WHAT IS GROUNDWATER MOUNDING? The rise in height of the water table caused by the addition of sewage effluent emanating from a soil absorption system

    4. WHY THE NEED FOR A MOUNDING ANALYSIS? To insure that adequate separation exists between the bottom of the proposed SAS and the mounded historical high water table. Revisions to Title 5 now permit the use of sites with Perc rates between 30 to 60 minutes/inch for new construction.

    5. USING THE MOUNDING ANALYSIS Determine the historical high groundwater elevation Calculate the height of the mound Add the calculated mound to the HHGWE to determine the final mounded groundwater elevation Add the required unsaturated separation (4 or 5 feet) Establish the minimum bottom of bed elevation

    6. DATA NEEDED TO CALCULATE MOUNDING Historical high groundwater elevation Initial saturated thickness of aquifer Porosity or specific yield of material Average hydraulic conductivity of material Recharge rate (based upon design flow) Duration of application Length and width of proposed SAS

    7. HISTORICAL HIGH GROUNDWATER ELEVATION Determined by use of soil evaluation criteria outlined in 310 CMR 15.103 (3) Redox concentrations/depletions (mottling) or the Frimpter Method Elevation based upon a surveyed datum

    8. SATURATED THICKNESS Typically determined using data obtained from a boring or a monitoring well Calculated by subtracting the depth to the historical high water table from the depth to bedrock or the top of a confining layer Where bedrock is not encountered estimate saturated thickness by assuming that bottom of boring is total depth

    9. POROSITY AND SPECIFIC YIELD POROSITY: loosely defined as the percentage of void space existing in a certain media EFFECTIVE POROSITY: loosely defined as the percentage of void space that allows water to flow through a certain media. This drainable volume is also known as SPECIFIC YIELD.

    10. SPECIFIC YIELD VALUES (%) Coarse gravel 0.23 Medium gravel 0.24 Fine gravel 0.25 Coarse sand 0.27 Medium sand 0.28 Fine sand 0.23 Silt 0.08 Clay 0.03 . .

    11. HYDRAULIC CONDUCTIVITY DEFINITION: A measure of the ability of an aquifer to transmit groundwater. HOW DETERMINED: Slug tests – preferred method Sieve analysis – acceptable alternative Grain size estimation – only used as a check Note: Perc test should not be used to determine hydraulic conductivity

    12. HYDRAULIC CONDUCTIVITY VALUES (FT/DAY) MATERIAL AVERAGE RANGE Fine gravel 1476 1181 - 3280 Medium gravel 886 689 - 1181 Coarse gravel 492 328 – 689 Coarse sand 148 65 - 328 Medium sand 39 16 - 65 Fine sand 8 3 - 16 Silt 0.3 0.03 – 3 Clay 0.0007 <0.03 S & G mix 172 16 – 328 S & G glacial till <100 Glacial till <10

    13. RECHARGE RATE, SAS DIMENSIONS AND DURATION Recharge rate (ft/day): Rate = Q/(L x W) Q = design flow in cubic feet/day L = length of SAS in feet W = width of SAS in feet Duration (days): Duration of Application should be modeled at 90 – 180 days

    14. REQUIRED FIELD WORK Test pits/perc tests Determination of historical high groundwater Determination of long term application rate Sizing of SAS Borings/monitoring wells Depth to water table (for Frimpter analysis) Saturated thickness Hydraulic conductivity testing Slug tests Collection of samples for sieve analysis

    15. THE MOUNDING ANALYSIS Performed by a PE or a hydrogeologic consultant Mounding is typically estimated by the use of a computer model. Two categories of models: Analytical models Numerical models For most discharges between 2,000 and 10,000 gpd analytical models are sufficient

    16. WEB-BASED MOUNDING PROGRAM http://www.aqtesolv.com/forum/rmound.asp Groundwater mounding calculator developed by Glenn M. Duffield, HydroSOLVE, Inc. Easy to use analytical web-based program Good program to check project proponents work

    17. MODEL RUN – 8000 gpd, fine-med sand

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