GROUNDWATER MOUNDING FOR SYSTEMS LARGER THAN 2,000 GPD

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GROUNDWATER MOUNDING FOR SYSTEMS LARGER THAN
2,000 GPD

• 2012 MassDEP/ MHOA
• Health Officers Seminars

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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

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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

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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.

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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

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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

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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

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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

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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.

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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

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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

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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 lt0.03
• S G mix 172 16 328
• S G glacial till lt100
• Glacial till lt10

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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

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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

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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

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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

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MODEL RUN 8000 gpd, fine-med sand
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DETERMINING BOTTOM OF BED ELEVATION
106 feet
102 feet
100 feet
Bottom of bed elevation HHGWE mound height
required unsaturated separation
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MOUNDING ANALYSIS SUBMITTAL

• Soil evaluation data
• Test pit and perc test results
• Historical high GW estimate from soil mottling or
  Frimpter Method
• Mounding analysis
• Boring logs
• Well construction details
• Results of any sieve analysis, slug testing, etc.
• List and discussion of mounding input parameters
• Estimate of mounding

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MOUNDING ANALYSIS SUBMITTAL (Continued)

• Minimum bottom of bed elevation
• Historical high groundwater elevation
• Historical high mounded groundwater elevation
• Bottom of bed elevation for the SAS

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METHODS FOR EVALUATING MOUNDING SUBMITTALS

• Use published data to verify that input
  parameters are reasonable and conservative
• Use a web-based mounding calculator to verify
  that the mound height determined by consultant is
  reasonable
• If mounding estimates or parameters appear
  unreasonable discuss your concerns with the
  consultant

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QUESTIONS?

• MassDEP Regional Contacts
• Central Region
• Barbara Kickham [phone]
• Northeast Region
• Criss Stephens [phone]
• Claire Golden [phone]
• Southeast Region
• Kermit Studley [phone]
• Western Region
• (413) 755-