WORK PLANS - Laserfiche WebLink

Home Browse Search

STANDARD PROCEDURE FOR GROUNDWATER MONITORING WELL CONSTRUCTION FOR WELLS SCREENED ACROSS THE PHREATIC SURFACE UNCONFINED AQUIFERS— HOLLOW STEM AUGER METHOD Groundwater monitoring wells are constructed by inserting or tremming well materials through the annulus of the hollow stem auger. In general, the groundwater monitoring wells are constructed with 10 feet of screen below groundwater and 10 feet above groundwater,for a total screen length of 20 feet. Where shallow groundwater is encountered or perched water dictates otherwise, the screen is adjusted, as appropriate, to maintain a proper seal at the surface (minimum three feet) and to avoid penetrating low permeable horizons or aquicludes. Groundwater wells are installed in accordance with the conditions of the well construction permit issued by the regulatory agency exercising jurisdiction over the project site. Once the borehole has been drilled to the desired depth, approximately six inches of filter sand are tremmied to the bottom of the boring. A groundwater monitoring well consisting of Schedule 40 PVC casing containing 0.020-inch perforations is then inserted through the annulus of the „ hollow stem augers. The well screen is then sandpacked by tremming the appropriate filter sand (Monterey No.3 Sand or equivalent)through the annulus between the casing and augers while slowly retracting the augers. During this operation, the depth of the sand pack in the 1 Q auger is continuously sounded to make sure that the sand remains in the auger annulus during auger retraction to avoid shortcircuiting the well. The sand pack is tremmied to approximately two feet above the screen, at which time pre-development surging is performed to consolidate the sand pack. Additional sand is added as necessary to help assure that the sand pack extends a minimum of two feet above top of screen. Following construction of the sand pack,a two-foot thick, bentonite seal is tremmied over the sand and hydrated in place. The remainder of the borehole is backfilled with neat cement grout. The well head is then capped with a locking cap, and secured with a lock to protect the well from surface water intrusion and vandalism. The well head is further protected from damage with a traffic-rated well box in paved areas or locking steel riser in undeveloped areas. The protective boxes or risers are set in concrete. The details of well construction are recorded on the field logs. Following well construction, the wells are developed in accordance with agency protocols by intermittently surging and bailing the wells. Development is determined to be sufficient once pH, conductivity and temperature stabilize to within 10 percent of the previous two readings. To evaluate groundwater gradient and groundwater elevation, the well heads are surveyed by a professional, licensed surveyor to an assumed or legal bench mark depending on the requirements of the project, in accordance with AB 2886 requirements.