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HazeltinelRoad Runners Rest IllBrinkmann-Woodward Gravel Pits lining Project- Design Technical Memorandum Denver Water August 2003 Section 4 - Seeaaae Anal 4.1 General GE[ performed two-dimensional, steady state seepage analyses to quantitatively evaluate the depth to which the soil-bentonite cutoff wall should be extended into underlying bedrock to meet the Colorado Office of the State Engineer (SEO) leakage criteria for a lined water storage facility. The computer program SEEP/W (GEO-SLOPE, 2001) was used to perform the calculations. SEEP/W is a finite element computer program that uses an iterative mass balance approach to compute the location of the phreatic surface, equipotential contours (lines of constant total water pressure head), and the volume of water flow through a representative cross section. We performed the analysis for the maximum expected seepage conditions, which occur when the pit is empty and the groundwater level outside of the pit is at its highest level. We developed six representative cross sections to estimate the seepage into the combined reservoir created by the Hazeltine/Road Runners Rest IUBrinkmann-Woodward gravel pits. The flow rate (seepage) through each representative section is the flow rate through the cutoff wall above the top of bedrock and the flow rate through bedrock and the cutoff wall below the top of bedrock. The total calculated seepage into the reservoir is the sum of the seepage through each representative cross section times the perimeter length that is applicable for each particular section. 4.2 Allowable Leakage SEO guidelines require That the groundwater leakage rate into an empty below groundwater storage facility be designed to be less than 0.03 ft3/day/ft2 (cubic feet per day per square foot) of perimeter plus 0.0015 ft3/day/ftZ of bottom area (Design Standard), for the pit to be classified as a lined storage facility and not a groundwater well. The SEO also has a Performance Standard that is three times the Design Standard. Monthly water budget accounting is required if the actual groundwater leakage rate into an empty facility is less than the Design Standard. Weekly, or more frequent, water budget accounting is required if the leakage rate into an empty facility is greater than the Design Standard but less than the Performance Standard. If the actual leakage rate into an empty facility is greater than the Performance Standard, corrective repairs to the lining system will be required by the SEO. GEI COnSllltants, Inc. 15 o?osoo?-oi?i i,?-tt~riimru~~~~~ ni