KE-Mounding Report(full)
Kensington Estates June 2011 Groundwater Mounding Analysis Page 1 GROUNDWATER MOUNDING Report The effect of the underlying groundwater system on the proposed infiltration basin’s ability
to infiltrate stormwater, proposed soil absorption system and its impact on adjacent sites has been estimated by application of Hantush’s Method. The centers of the infiltration basin
and the soil absorption system are approximately 400-feet apart. The analysis was performed by utilizing a computer program that estimates the transient mound height under static water
table conditions. The computer code was written by GeoHydroCycle, Inc. of Newton, Massachusetts and is a two dimensional transient model using Glover’s solution to Hantush’s Method.
Field data was collected for input into the groundwater mounding model. The field work was performed by Berkshire Design Group, Inc of Northampton, MA. A soil textural analysis were
performed to determine the hydraulic conductivity (k) of the most restrictive layer of soil in the area of the proposed infiltration basin based on the Rawls Rate Table provided in the
MADEP Stormwater Handbook: Test Pit # Soil Texture Hydraulic Conductivity, k (ft/day) 1DB(E) Loamy Sand, Medium Sand 16.54 1DB(W) Loamy Sand, Medium Sand 16.54 2DB Loamy Sand, Medium
Sand 16.54 DH#1 Medium Sand 16.54 DH#2 Medium Sand 16.54 DH#3 Medium Sand 16.54 DH#4 Medium Sand 16.54 Table 1 Soil Hydraulic Conductivity Determined from Soil Investigation The following
presents the parameters used to determine the groundwater mounding from the infiltration basin: Infiltration Basin: Estimated Specific Yield or Fillable Porosity: 0.26
Kensington Estates June 2011 Groundwater Mounding Analysis Page 2 Value of specific yield of “Medium Sand”, from Johnson (1967) Volume of basin for recharge: 11,028 ft3 Surface Area
of basin at recharge elevation: 9,318 ft2 Application Rate: (3 )* (10,320 ) 11,028 (3 ) * ( ) 2 3 Days ft ft Days SurfaceArea Volume = = = 0.356 cu.ft./day/sq.ft. Application Rate: 0.356
cu. ft/day/sq.ft. Distance to adjacent property from center of basin: 358.3 ft Calculated time to drain (see attached calculations and Hydrocad): (11,028 ) /(9,318 ) 16.54 /( ) /( )
Re arg ft3 ft2 ft day Volume SurfaceArea ch eRate = = = 13.98 days -1 = 0.072 days Basin dimensions as a rectangle = 142’(length) x 73’(width) Observed Groundwater elevation at basin
location: Not Observed Estimated Seasonal High Ground Water elevation: 139” or >291.22 (Bottom of hole) Calculated mound height: 0.61 feet Shared Septic Soil Absorption Area Estimated
Specific Yield or Fillable Porosity: 0.26 Value of specific yield of “Medium Sand”, from Johnson (1967) Volume of SAS for recharge: 1,235.33 ft3 Surface Area of basin at recharge elevation:
16,564 ft2 Application Rate: (16,564 ) 1,235.33 ( ) 2 3 ftft SurfaceArea Volume = = = 0.0746 cu.ft./day/sq.ft. Application Rate: 0.0746 cu. ft/day/sq.ft. Distance to adjacent property
from center of basin: 358.3 ft
Kensington Estates June 2011 Groundwater Mounding Analysis Page 3 Calculated time to drain (see attached calculations and Hydrocad): (1,235.33 ) /(16,564 ) 16.54 /( ) /( ) Re arg ft3
ft2 ft day Volume SurfaceArea ch eRate = = = 221.8days -1 = 0.005 days Basin dimensions as a rectangle = 202’(length) x 82’(width) Observed Groundwater elevation at SAS location: Not
Observed Estimated Seasonal High Ground Water elevation: 120” or >293.0 (Bottom of hole) Calculated mound height: 0.46 feet As shown on the attached Groundwater Mounding graphs, the
height of the groundwater mound, resulting from proposed recharge is 0.61-feet in the center of the infiltration basin and reduces to 0.01-feet at a horizontal distance of 400-feet.
If we assume ESHGW is at the bottom of the Test Pits holes, as no indication was observed, at Test Pit 2DB the corresponding ESHGW elevation is 291.22. With an additional 0.61’, the
mounded groundwater reaches an elevation of 291.83 and does not reach the bottom of the proposed infiltration basin at elevation 295.0. If the 0.21’ of mounding created by the septic
system is added to this groundwater elevation of 291.83, then a groundwater elevation of 292.04 is attained. The elevation of 292.04 is more than 2-feet below the bottom of the infiltration
basin at 295.0. Groundwater at the observed elevation does not breakout or affect the closest adjacent property when the groundwater elevation is at Estimated Seasonal High Groundwater.
The closest property line is approximately 358.3’ east of the infiltration basins center. Additionally, draw down calculations are provided in the Stormwater Management report and as
stated above a full infiltration bed will be infiltrated within 0.072 days or 1.72 hours. Also, shown on the attached Groundwater Mounding graphs, the groundwater mound height, resulting
from a daily application of 9,240 gallons of effluent, is 0.46’ at the soil absorption system center and reduces to 0.21-feet at a horizontal distance of 400’. If ESHGW is assumed at
the Test Pits holes’ bottoms, as no indication was observed, ESHGW at DH#2 is elevation 293.0. 0.46’ of mounded groundwater reaches an elevation of 293.46 and does not breach the required
4-feet of groundwater separation at the bottom of the proposed SAS (298.33). If the 0.01-feet of mounding created by the infiltration basin is added to groundwater elevation 293.46,
a groundwater elevation of 293.47 is attained. Elevation 293.47 is more than 4-feet below the SAS at 298.33. Mounded groundwater at the observed elevation does not breakout or affect
the closest adjacent property. The closest property line is approximately 69-feet east of the SAS center. Additionally, draw down calculations are provided and as stated above a full
infiltration bed will be infiltrated within 0.005 days or 0.12 hours. Therefore, it is estimated that the effluent introduced into the SAA will infiltrate into the ground over the entire
surface of the trenches during and after a system dosing, allowing the SAS to be treated and infiltrate as designed with no impact on the adjacent property.
Kensington Estates June 2011 Groundwater Mounding Analysis Page 4 In summary, the combined groundwater mounding provided by the infiltration basin and the soil absorption system does
not cause a negative or prohibited impact on either system. The distance from the bottom of the infiltration basin to the estimated, combined, mounded groundwater is 3.4-feet. The distance
from the bottom of the soil absorption system to the estimated, combined, mounded groundwater is 4.6-feet. The required groundwater separation for the infiltration basin is 2-feet and
the require separation for the soil absorption system is 4-feet. It should be noted that groundwater and/or evidence of groundwater was not observed within the test pits, and thus the
bottom elevations of the holes were used to estimate seasonal high groundwater.