WorldWide Drilling Resource

The Importance of Groundwater Monitoring and Recovery Wells Part 1 Submitted by Wes McCall, P.G. Geoprobe ® Environmental Geologist One of the most important lessons we have learned in the environmental industry over the last 20 years is monitoring wells are for monitoring, not for site characterization. Often, contamination in an aquifer is focused and moving along relatively narrow migration pathways, sometimes less than two feet in width. By regulatory design and purpose, monitoring wells usually have screen intervals roughly 10-20 feet in length, sometimes longer. This means monitoring wells are generally not effective at accurately locating these narrow zones of high contamination due to dil- ution across the long screens. Use of monitoring wells for site characterization in the past has led to the improper design of systems trying to remediate large volumes of an aquifer (at significant cost) where contaminants are actually below the action level. These improperly designed remediation systems don’t target the narrow zones of high contamination where they would be most effective; so, contamination will persist for much longer than it should and of course, the cost will be much higher than needed to ac- complish the desired effect. Use of direct push groundwater profiling methods and logging methods such as Membrane Interface Probe (MIP), hydraulic profiling tool (HPT), or a combi- nation of the two Membrane-Interface Probe and Hydraulic Profiling Tool (MiHpt), can provide high-resolution characterization. This will help guide the design of monitoring and remediation systems so they target the zones of high-level contamination and accomplish the desired effect at a lower cost. Monitoring Wells are for Monitoring…Not for Site Characterization The purpose of monitoring wells is to let us know when contaminants have been released to the subsurface at concentrations which may pose a significant risk to human health or the environment. Long screened monitoring wells are not properly designed to help us understand contaminant distribution or migration; other tools and methods are used for that purpose. The primary purpose of recovery wells is to recover either free product, light nonaqueous phase liquids (LNAPL) or dense nonaqueous phase liquids (DNAPL), as well as elevated levels of contaminants dissolved in the groundwater. Properly placed and designed recovery wells en- able us to recover free product and remove hazardous contaminants from aquifers, protecting groundwater resources and drinking water supplies. Next month, we’ll cover where these wells should be placed. Huss Drilling in Dade City, Florida, uses a Geoprobe ® rotary sonic rig to drive 6.5-inch casing to start a 2-inch monitoring well inside a warehouse building. They performed continuous soil sampling, and obtained groundwater samples by installing temporary screens and retracting the sample casing at specific intervals. Over 30 mon- itoring wells, installed to nearly 100 feet below ground surface, were completed. Marc Natali, field supervisor and drill operator for C.S. Drilling in Naperville, Illinois. Everybody wants to save the earth; nobody wants to help mom do the dishes. P.J. O’Rourke 8 JULY 2016 WorldWide Drilling Resource ®