During environmental site characterization, remediation, and compliance efforts, groundwater monitoring wells have served as the conventional tool-of-choice for accessing groundwater samples. Recently developed direct pushed (DP) technologies provide the means for collecting faster, less expensive groundwater samples when compared to conventionally drilled wells. The most extensive use of these cost-effective technologies, initially used almost exclusively as temporary installations for characterization purposes, have not been widely accepted for long-term monitoring at remedial action sites. For broad acceptance of DP well long-term monitoring applications, comparisons between conventionally drilled wells and DP wells needed to be conducted to validate these innovative approaches.
The purpose of this project was to rigorously compare the results of laboratory analyses conducted on samples obtained from DP wells to those obtained from wells installed utilizing conventional techniques (e.g., hollow-stem auger [HSA] wells). The demonstration consisted of these side-by-side comparisons followed by comprehensive statistical analyses over several years of quarterly monitoring. Five sites (located in New Hampshire, Delaware, California, Massachusetts, and Florida) comprised of various geologic regimes and contaminants of concern were included in this demonstration. Ultimately, the goal of this demonstration was to determine whether DP wells can yield representative data for long-term monitoring applications.
Comparisons between conventional drilled wells and DP wells consisted of evaluation of requirements for mobilization, installation, maintenance, removal, labor requirements for each step, performance metrics based on chemical and hydrogeologic representativeness, training requirements, ease of use considerations, appropriateness of innovative approach (e.g., lithologic restrictions where applicable), pertinent health and safety issues (e.g., less exposure for DP well installations), and overall costs. Hydraulic comparisons comprised of pneumatic slug and conventional aquifer tests were also performed in selected wells.
For the majority of the comparisons conducted during this demonstration project, management decisions were not impacted regardless of whether the well was installed by drilled or direct push methods. Project labor requirements tend to be lower for DP wells since well installation throughput rates are from two to five times higher than for drilled wells. Operator training for both drilled and DP well installation is similar; however, for some DP well installation systems, lower levels of technical expertise are typically required. Many states require licenses for well drillers, yet, the same is not generally true for operators of DP well installation equipment.
In most formations, DP is minimally intrusive and causes less disturbance of the natural formation than conventional drilling techniques, and often employs more mobile push platforms than conventional drilling vehicles. Worker exposure and industrial derived waste (IDW) disposal costs are reduced because little or no potentially contaminated drill cuttings are generated with direct push methods. Since many DP wells have a smaller diameter than traditional drilled wells, purge water volumes, sampling time, and indirect waste disposal costs are reduced for most sampling activities. When coupled with field screening and other site characterizing approaches afforded by direct push sensor and sampling techniques, DP well installations afford expedited, comprehensive plume delineation while establishing infrastructure for long-term monitoring in a single mobilization.
A critical lesson learned stems from the observation that at very close proximities, chemical concentration values can vary significantly. This spatial heterogeneity can impact many other types of comparison projects where adjacent samples are evaluated. Temporal variability is also of significance and should be considered in trend analysis.
According to conservative estimates, cost savings for DP well installations range from approximately 32 to 68 percent. Highest percentage installation savings can be derived when using smaller diameter wells at deeper total depths. Assuming that 500 wells per state would be installed per year, the total anticipated Department of Defense (DoD) savings per year due to DP well installations ranges from approximately $12 million to close to $80 million. Since the majority of DP wells are less than 2 in [5.08 cm] in diameter, the low end DoD cost savings estimate is approximately $20 million per year. Using these conservative estimates, industry savings could exceed $200 million dollars per year with as few as 1300 DP wells per state per year. Life cycle cost savings associated with DP wells is significant, ranging from approximately 17 to 47 percent, and tends to be highest for smaller diameter wells installed to deeper depths
This comprehensive well comparison project constitutes the most thorough and conclusive comparison of conventional drilled wells and direct push wells to-date. With regards to performance, direct push wells performed at a level that warrants their acceptance as long-term monitoring devices for both chemical representativeness and hydraulic assessment categories. As a result of this effort, two American Society of Testing and Materials (ASTM) standards and an Interstate Technology & Regulatory Council (ITRC) Technical Regulatory Guide have been completed and released for government and industry use.