Multilevel Groundwater Monitoring Systems, also referred to as Multi-Depth Groundwater Monitoring Systems, Multilevel Systems (MLSs), or Engineered Nested Wells, are engineered technologies installed in single boreholes above and/or below the water table to obtain data from different depth intervals. The technologies may consist of various pipes, liners, access ports, sampling pumps, pressure sensors, and sealing mechanisms that are installed temporarily or permanently in boreholes drilled into unconsolidated sediments or bedrock.
MLS systems facilitate 1) ongoing measurement and monitoring of depth-discrete water pressures (hydraulic heads) and 2) repeated collection of depth-discrete groundwater samples for chemical testing. Commercial MLS systems are available with as few as three ports (CMT System) to more than 20 ports (MP Westbay and Solinst Waterloo Systems). An essential design element of all MLS systems is that they must prevent hydraulic connection of the various monitored intervals within the wellbore.Einarson, Murray D. 2006. "Multi-Level Ground Water Monitoring." In Practical Handbook of Ground Water Monitoring, edited by D.M. Nielsen, 807-848. CRC Press.
While installed primarily in water-saturated sediments and rock, MLS systems can also be installed in the vadose zone for the collection of depth-discrete soil gas samples. Hybrid MLS systems can be constructed with some ports in the vadose zone and some ports in the saturated zone.Dumble, P., M. Fuller, P. Beck, and P. Sojka. 2006. "Assessing contaminant migration pathways and vertical gradients in a low-permeability aquifer using multilevel borehole systems." Land Contamination &# 38; Reclamation 14 (3):699-712.
History
Prior to the 1970s, collection of discrete groundwater samples from multiple depths in the subsurface required the installation of well clusters or nested wells . Well clusters consist of a closely spaced group of monitoring wells, each well completed to a different depth in individual boreholes. Well clusters were first used in the 1950s at contaminated sites.Cherry, John A., Beth Parker, Murray D. Einarson, Steven Chapman, and Jessica R Meyer. 2015. Chapter 11 Appendix: Overview of Depth-Discrete Multilevel Groundwater Monitoring Technologies: Focus on Groundwater Monitoring in Areas of Oil and Gas Well Stimulation in California. In LLNL Recommendations on Model Criteria For Groundwater Sampling, Testing, and Monitoring of Oil and Gas Development in California. Lawrence Livermore National Laboratory LLNL-TR-669645. Because there is only one well screen in each borehole, there is little risk of vertical connection between zones. The individual wells in the cluster must be installed near one another (e.g., ≤10 ft. apart), so that the head data obtained from them is a result of variations in the vertical head and not horizontal gradients. Also, care must be taken to avoid installing clusters of wells with overlapping screens and sand packs – this may allow vertical movement of contamination between the wells in the presence of vertical hydraulic gradients. Installation of wells clusters can be expensive because of increased drilling costs associated with drilling multiple borings, especially in fractured rock.
Nested wells are wells constructed of two or more well screens and casing assemblies of different lengths installed in a single borehole. The key drawback of nested wells is that it can be difficult to effectively seal the portions of the borehole between the monitored zones. Nested wells were popular in the 1970s but many seal failures occurred. For this reason, nested wells are discouraged or prohibited in many areas.USEPA. 1992. RCRA Ground-Water Monitoring: Draft Technical Guidance.US Army Corps of Engineers. 1998. Monitoring Well Design, Installation, and Documentation at Hazardous, Toxic, and Radioactive Waste Sites.Protection, New Jersey Department of Environmental. 2005. Field Sampling Procedures Manual.Ohio EPA. 2008. Chapter 7 - Monitoring Well Design and Installation.State of Washington. 2008. Washington State Administrative Code (WAC) 173-160-420(3).DTSC. 2014. Well Design and Construction for Monitoring Wells at Contaminated Sites. Sacramento, CA. There are typically three separate monitoring intervals in nested wells, although more monitoring intervals have been constructed in very deep monitoring wells.Hanson, Randall T., M.W. Newhouse, C.M. Wentworth, C.F. Williams, T.E. Noce, and M.J. Bennett. 2002. Santa Clara Valley Water District Multi-Aquifer Monitoring Well Site, Coyote Creek Outdoor Classroom, San Jose, California. USGS. The risk of hydraulically connecting the various monitoring zones is inversely proportional to the thickness of the seals between the monitoring intervals. Thus, shallow nested wells with many monitoring zones are more at risk of hydraulic failure than deep nested wells with fewer monitoring zones.
Because of the limitations of well clusters and nested wells and a desire for monitoring more vertical intervals, researchers at the University of Waterloo (Canada) developed a MLS to collect depth-discrete groundwater samples at a landfill site in Ontario, Canada (Pickens et al. 1978). That system, which contained multiple tubes within an outer PVC pipe, was subsequently commercialized as the Solinst Waterloo system. Further improvements to that system included the addition of packers to hydraulically isolate the monitoring intervals.
In the early 1980s, researchers used multiple gas-drive pumps installed at different depths in boreholes to collect depth-discrete groundwater samples in a fractured rock aquifer.Barvenik, Matthew J., and Richard M. Cadwgan. 1983. "Multilevel Gas-Drive Sampling of Deep Fractured Rock Aquifers in Virginia." Ground Water Monitoring and Remediation Fall:34-40. A commercial version of this gas-drive system, named "Barcad" after its inventors, is available from BESST, Inc.
In the mid-1980s, an MLS consisting of multiple ports separated by blank casings was developed. That system, referred to as the Westbay MP system, utilizes a separate tool lowered into the MLS on a wireline to measure aquifer pressures and collect groundwater samples. The Westbay MP System is commercially available from Nova Metrix.
In the late 1980s, researchers in Israel developed a well insert consisting of multiple diffusion cells that could be inserted into a conventional monitoring well to develop vertical profiles of target solute concentrations.Ronen, Daniel, Mordeckai Magaritz, and Itzhak Levy. 1987. "An in situ multilevel sampler for preventive monitoring and study of hydrochemical profiles in aquifers." Ground Water Monitoring Review 7 (4):69-74. This system is not commercially available.
In the early 1990s, a team from Science and Engineering Associates, Inc. (SEA) with Argonne National Laboratories developed an instrumentation and fluid sampler emplacement technique for in-situ characterization and fluid monitoring in the vadose zone.Williams, Cecilia, William Lowry, David Cremer, and Sandra Daivit Dunn. 1995. SEAMIST In-Situ Instrumentation and Vapor Sampling System Applications in the Sandia MIxed Waste Landfill Integrated Demonstration Program: Final Report. Sandia National Laboratories. Referred to as SEAMIST™, the system was constructed of a flexible liner that was everted into an open borehole. Sand was poured into the everting liner during construction to deploy it to the full depth of the borehole. The sand also acted to keep the liner pressed against the borehole walls. Various sensors and sampling devices could be ported through the liner where they were pressed against the borehole). Mr. Carl Keller acquired the manufacturing rights to the system in 1995 and developed a version that could be deployed below the water table for depth-discrete groundwater monitoring. Referred to as the Water FLUTe (Flexible Liner Underground Technology), the system is commercially available from FLUTe, Inc.
In the late 1990s, Murray Einarson, working for Precision Sampling, Inc. in California, developed a continuous multichannel tubing (CMT) system for monitoring up to seven different zones in the subsurface. Development and testing of the CMT system was the focus of Einarson's MSc thesis at the University of Waterloo.Einarson, Murray D., and John A. Cherry. 2002. "A new multi-level ground water monitoring system utilizing multi-channel tubing." Ground Water Monitoring and Remediation 22 (4):52-65. The CMT system consists of a continuous length of polyethylene tubing that has seven internal channels or lumens. Custom-designed monitoring zones are created on site by cutting ports into the various channels at specific depths. The CMT MLS is commercially available from Solinst Canada.
Advantages
Multilevel systems offer the following advantages over clusters of monitoring wells and nested monitoring wells: Only one casing (or tube) is placed in the borehole. This simplifies the process of installing annular seals between the monitored zones and improves the reliability of the seals. A MLS facilitates the collection of ground-water samples and measurement of hydraulic heads from many more discrete depths than can be obtained using typical well clusters or nests. A MLS facilitates the collection of subsurface head and chemical data over time, which is something that cannot be accomplished using "one-time" profiling tools like CPT or the Waterloo Profiler. A single MLS has a much smaller "footprint" at the ground surface and therefore creates less surface disturbance than a cluster of individual wells. A single MLS is therefore less noticeable and obtrusive than a cluster of wells. The cost of one MLS with several monitoring intervals is generally much less than the cost of an equivalent cluster of wells except in shallow, soft geological deposits where direct push techniques can be used to install many wells quickly. In some areas, permits may be required for each well in a cluster. This can significantly increase the cost of a cluster of monitoring wells. All of the components of multilevel systems are manufactured with stringent QA/QC standards and there are detailed established procedures for MLS installation and for testing the MLS after installation. Each MLS is an engineered system reproducible from site to site. In contrast, nested wells have no standards for the components and the installation procedures typically vary depending on the well installer.
MLSs have the following disadvantages relative to conventional monitoring wells (adapted from Einarson, 2006): Fewer options exist for collecting water samples because conventional sampling pumps generally do not fit down the smaller MLS tubes or are otherwise unsuitable; this means that collection of large volumes of groundwater may be time-consuming and impractical for some types of MLSs; Owing to the specialized nature of the MLS components and monitoring tools, some specialized training or technical assistance may be required, at least initially; Some types of MLSs are more difficult to decommission than conventional or nested wells; and The flow restrictions in the internal components (i.e., the plumbing) of MLSs makes them generally less suitable or versatile for conducting hydraulic tests to measure permeability and storativity.
Installation methods Multilevel Monitoring Systems are typically installed in one of three ways: