ARCHIVED REPORTS_XR0006317 - Laserfiche WebLink

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SIERRA STANDARD OPERATING PROCEDURE MONITORING WELL DESIGN AND CONSTRUCTION Where possible, information from published and unpublished reports is reviewed prior to installation of monitoring wells Relevant data includes highest and lowest anticipated ground water elevations, aquifer materials aquifer yield and contaminants expected This information is used to aid the field geologist rather than to predetermine how the wells will be constructed Well construction is based on site spec ftc conditions and is determined in the field after discussion with the senior geologist The wells are screened to monitor the first water-bearing zone encountered If high ground water conditions exist the top of the well screen may be set at static water level or below static water level Twenty feet of well screen will be used in the wells (five feet above static ground water and fifteen feet below static water) unless a five foot clay layer is encountered If a clay layer is encountered, it will be confirmed by sampling The sampling hole into the underlying confining layer will be sealed with bentonite pellets and the well screen will terminate O to I feet into the clay layer When field observations indicate that low permeability materials are acting as an aquitard to prevent movement of contaminants less screen may be used Monitoring wells are constructed with flush-threaded, 2-inch or 4-inch diameter, slotted PVC, stainless steel or teflon well screen and PVC, stainless steel or teflon blank casing The sand pack is sized to retain 90% of the aquifer material based on the observations of the field geologist The sand is placed into the annular space around the well screen to approximately 2 ft above the top of the well screen If high ground water conditions exist, the sand may be placed 0 to 1 ft above the top of the well screen Two feet of bentonite pellets are used to separate the sand from the sanitary surface seal (grout) If high groundwater conditions exist 1/2 ft of bentonite maybe used to separate the sand from the sanitary surface seal The grout (Portland cement with approximately 3-5%bentonite powder) is poured into the annular space above the bentonite pellets If the surface seal is greater than 5 feet thick, grout consisting of cement mixed with 3-5% bentonite powder will be tremied or pumped unto the annular space above the bentonite pellets to prevent the infiltration of surface water into the well