WorldWide Drilling Resource

The Importance of Groundwater Monitoring and Recovery Wells Part 2 Submitted by Wes McCall, P.G. Geoprobe ® Environmental Geologist Where Should the Wells Be Placed? One of the most basic challenges to overcome is simply where the well should be placed and what part of the formation should be screened to achieve my monitoring or recovery goals. When working in unconsolidated formations, the hydraulic profiling tool (HPT) provides logs of electrical conductivity and relative permeability to define the local hydrostratigraphy. If volatile organic com- pounds (VOC) such as trichloroethylene (TCE) are the primary contaminants of concern, logging with the Geoprobe ® membrane interface probe can provide logs of con- taminant level and distribution. Cross sections or 3-D plots of the Geoprobe ® hydraulic pro- filing tool and Membrane Interface Probe (MIP) logs can be used to accurately select the locations and depths for well placement and screen intervals. When setting wells in saturated sand formations, it can be difficult to control for- mation heave. Heave or “blow-in” can occur when the center bit is pulled out of the throat of the augers or when the expendable point is knocked out of a direct push (DP) installed casing. In these situations, a pressure (P) imbalance between the formation outside the casing (high P) and the open bore of the casing (low P) causes the rapid flow of saturated sand, etc. into the open casing or auger string. This can also happen if the DP casing or augers are retracted too quickly after the well screen and risers are in- serted in the well bore. If this happens, the well can become bridged inside the casing or augers and it may be difficult, if not impossi- ble, to retract the augers or casing without pulling the well out of the ground. When formation conditions are such that heave is possible, clean water must be added to the casing or augers to balance the pressure inside the casing with the pressure outside in the formation. The water level inside the casing should be maintained at or above the static water level of the formation to prevent f orma t i on heave and l oss o f t he bore/well. Once well construction is complet- ed, the clean water added to the well is purged out during development and water quality parameters are monitored to determine which representative for- mation water is obtained for sampling. Let’s discuss the proper equipment for the job next month! Phillip Ricker with Geo Lab in Dacula, Georgia, advancing 8.25-inch hollow stem augers with a Geoprobe ® 7822DT for a 4-inch PVC recovery well installation. According to Lee Shaw, Geoprobe ® customer service, “Installing 2-inch prepacks with direct push methods is a game changer!” 19 WorldWide Drilling Resource ® AUGUST 2016