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Multi-Level Groundwater Monitoring Systems (MLSs)
Multi-Level Groundwater Monitoring Systems, also referred to as "Multi-Depth Groundwater Monitoring Systems," "Multi-Level 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 transducers, 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 (piezometric 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 System) and 24 ports (Waterloo System). An essential design element of all MLS systems is that they must prevent hydraulic connection of the various monitored intervals within the wellbore.

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.

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. 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. There are typically three separate monitoring intervals in nested wells, although more monitoring intervals have been constructed in very deep monitoring wells. 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; and is still available, today. Further work by Solinst, allowed the ability to isolate each multilevel zone by adding packers to the Waterloo system.

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. A commercial version of this gas-drive system, named "BARCAD" after its inventors, is available from BESST, Inc.

In the mid 1980s, a MLS comprised 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. 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. 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). Dr. 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. The CMT system consists 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 of MLS Technologies Over Other Methods
Multilevel systems offer the following advantages over clusters of monitoring wells and nested 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.
 * 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.

Dedicated multilevel systems also have some disadvantages, including the following:


 * Fewer options exist for sampling dedicated multilevel systems than for conventional monitoring wells. This is due to the design of the wells and the relatively small diameter of sampling tubes installed inside the multilevel wells. Several small-diameter pumps have been developed, however, to facilitate collection of ground-water samples from small-diameter wells and tubing.
 * Owing to the specialized nature of some of the components or monitoring tools used in multilevel systems, some training or technical assistance is generally recommended, at least for first-time installers of the systems.
 * It may be more difficult to decommission specialized multilevel monitoring systems than conventional single-interval PVC monitoring wells.

Installations in Open Boreholes
Boreholes drilled into bedrock or silt and clay deposits usually stay open after the hole is drilled and the drill string has been removed. Multilevel wells can therefore be constructed directly inside of the open boreholes. Oftentimes, it is not necessary to have a drilling rig on site during the construction of the multilevel well if the multilevel well casing can be lowered into the borehole by hand or using a winch.

Because the boreholes stay open, however, the annular space between the well casing and the boreholes must be sealed to prevent vertical flow of ground water between the various monitored zones. With some multilevel systems (e.g., Westbay MP, Solinst Waterloo), expandable, inflatable rubber, poly- urethane, or Viton packers can be used to seal the annular space between the monitored zones. The annular space can also be sealed by backfilling the annulus with alternating lifts of sand (at the depths of the intake ports) and clay or cement (in the intervals between the various intake ports). Finally, the novel design of the Water FLUTe system using a continuous liner also seals the borehole between the sampling ports.

Installations in Unconsolidated, Collapsing Sedimentary Deposits
Unlike boreholes drilled into competent bedrock, most boreholes drilled in unconsolidated deposits will not stay open when drilling has been completed and the drill rods are removed. Consequently, some method of keeping the borehole open is necessary while the multilevel well casing is inserted and the well constructed. One way to accomplish this is by advancing steel drive casing as the borehole is drilled. The steel drive casing is left in the borehole while the well casing is inserted, and is then pulled back incrementally as the multilevel well is constructed. If the formation will collapse completely around the multilevel well casing, it is usually not necessary to install annular seals between the monitored zones because the collapsing sand restores the original permeability of the formation. If the formation will not collapse completely around the multi-level well casing, however, gaps can exist in the annular space, allowing vertical flow of ground water between different monitoring zones. In this case, alternating layers of sand and bentonite or cement must be emplaced by backfilling as the steel drive casing is withdrawn from the borehole. Drilling methods that employ driven casing include dual-tube direct push (ref), air-rotary casing advance, and rotasonic. Rotasonic drilling (also referred to simply as sonic drilling) is ideal for installing multilevel monitoring wells because (1) steel drive casing is advanced as drilling progresses; (2) continuous cores are routinely collected (logs of the cores can then be used to design the multilevel wells); and (3) the rate of penetration is usually high.

Installations in multi-screened wells
A popular way to install the dedicated MLSsystems is inside of multiscreened wells instead of directly in boreholes (Fig ___). With this type of installation, the multilevel monitoring system is installed inside a steel or PVC well that has been constructed with short screens at multiple depths. The depths of the well screens correspond to the depths of the ports in the multilevel monitoring system. This adds another step to the well installation process (i.e., first installing a multiscreened well), but has several advantages. First, installing conventional steel or PVC wells is straightforward and routine for most drilling contractors. Hence, it is not necessary that the drilling contractor have expertise in installing multilevel monitoring systems. Once the multiscreened wells have been installed and developed, the drilling contractor's job is done, and the multilevel systems can often be installed by field technicians, often at a lower cost. Secondly, the various monitoring zones can be developed using standard well development equipment and procedures before the multilevel monitoring systems are installed in the wells. Finally, installing multilevel systems inside multiscreened wells may simplify the task of decommissioning the wells once they are no longer needed. Most of the multilevel systems can be constructed so that they can be easily removed from the multi-screened wells. Then, the multi-screened wells can be pressure-grouted or drilled out using standard well decommissioning procedures.

Westbay MP System
Nova Metrix currently produces the Westbay MP system, a modular instrumentation system for multilevel ground-water monitoring. The MP system consists of two parts: 1) the casing system and 2) portable probes and tools that provide a compatible data acquisition system.

The Westbay MP casing system (Figure __) is designed to allow the monitoring of multiple discrete levels in a single borehole. One single string of water-tight Westbay casing is installed in the borehole. Each level or monitoring zone has valved couplings to provide a selective, controlled connection between the ground water outside the casing and instruments inside the casing. Westbay packers or backfill are used to seal the borehole between monitoring zones to prevent vertical flow of ground water and maintain the natural distribution of fluid pressures and chemistry. The Westbay system can be installed in either open boreholes or cased wells with multiple screens.

Data are obtained using one or more wireline probes with sensors that are lowered inside the casing to each monitoring zone. The probes locate and open the valved ports to measure fluid pressure, collect fluid samples or perform hydraulic response tests. Multiple probes can be connected in series to provide continuous multilevel data. Laptop computers or smartphones interface with the probes and collect data at the surface or from remote locations.

The design of the MP system results in no limit in the number of zones that can be completed in one borehole, apart from the physical ability to fit the length of the components in the well. The user can have materials on site ahead of time as it is not necessary to know the precise size of the borehole or the desired location of seals and monitoring zones before the equipment is shipped. Pressure measurements are made under shut-in conditions, making the system responsive to pressure changes. Ground-water samples are collected at formation pressure without repeated purging.

The Westbay system has been in use since 1978 and has been installed in a variety of geologic environments ranging from soft seabed sediments to unconsolidated alluvial deposits, to highly fractured bedrock. Depths of installation varied from 100 ft (30 m) to greater than 7,000 ft (1200 m).

Solinst Waterloo System
The Solinst Waterloo Multilevel ground-water monitoring system is a modular multilevel monitoring system manufactured by Solinst Canada, Ltd. to collect groundwater data from multiple depths within a single drilled borehole. Originally developed by researchers at the University of Waterloo, it consists of a series of monitoring ports positioned at specific intervals along 2-in. Schedule 80 PVC casing (Figure ___). The ports are typically isolated in the borehole either by in-line packers (permanent), or by alternating layers of sand and bentonite backfilled from the surface. The Solinst Waterloo Multilevel system can also be installed inside multi-screened wells.

The ports and packers are connected to the 2-in. Schedule 80 PVC casing with a special water-tight joint. Monitoring ports are constructed of stainless steel or PVC and have the same water-tight joint to connect with the other system components. Water is added to the inside of the 2-in. PVC casing to overcome buoyancy during installation and to expand permanent packers.

Each monitoring port has either a single or dual stem. Each stem is connected to either: an open tube that runs inside the 2-in. PVC casing to the ground surface; (2) a double valve pump; (3) a bladder pump; or (4) a pressure transducer. Pressure transducers can be connected to a data logger for continuous recording of water levels. If open tubes are connected to the port stems, samples can be obtained from inside the tubes using a peristaltic pump, an inertial-lift (i.e., check-valve) pump, or a double-valve gas-drive (positive displacement) pump. Water levels can also be measured in the open tubes using small- diameter water-level meters, or pressure transducers. Because each port is plumbed to some type of monitoring device, contact between ground water entering the ports and water added to the inside of the 2-in. PVC casing is prevented. If a single stem is used, only one monitoring device can be used per monitored zone. If dual stems are used, two devices (e.g., a bladder pump and pressure transducer) can be used per zone.

Depending on the monitoring options chosen, the number of zones that can be monitored typically ranges from three to eight, although systems with as many as 15 sampling ports have been installed. Systems installed in fractured rock formations are typically installed in 3- or 4-in.-diameter core holes. A wellhead that facilitates simultaneous purging and sampling of all monitored zones is available.

Solinst CMT System
The Solinst continuous multichannel tubing (CMT) system is a multilevel soil gas &/or ground-water monitoring system that uses custom-extruded flexible 1.7-in. OD multichannel MDPE tubing to monitor as many as seven discrete zones within a single borehole in either unconsolidated sedimentary deposits or bedrock (Figure ____. Prior to inserting the tubing in the borehole, ports are created that allow either soil gas or, ground water to enter six outer pie-shaped channels and a central hexagonal center channel at different depths, facilitating the measurement of depth-discrete piezometric heads and the collection of depth-discrete ground-water samples. Well screens are constructed by wrapping synthetic or stainless steel fabric mesh completely around the tubing in the interval containing the ports. The mesh is secured to the tubing using stainless steel clamps. The Solinst CMT system is described in detail by Einarson & Cherry.

The multichannel tubing can be extruded in lengths up to 300 ft and is shipped in 4-ft-diameter coils. The desired length of tubing, equal to the total depth of the multilevel well, is cut from a coil, and the well is built at the job site based on the hydrogeologic data obtained from the exploratory boring.

Sand packs and annular seals between the various monitored zones can be installed by backfilling the borehole with alternating layers of sand and bentonite.

A small (1.1 in.) diameter three-channel CMT system has also been developed for installation with DP sampling equipment. Sand pack and bentonite cartridges are available for the three-channel CMT system.

Water FLUTe System
The Water FLUTe (Flexible Liner Underground Technology) is a multilevel ground-water monitoring system that uses a flexible impermeable liner of polyurethane-coated nylon fabric to isolate up to 20 discrete intervals in a single borehole. The system comes in various sizes and can monitor boreholes from 2 to 20 in. in diameter (most installations are in 4- to 10-in. diameter boreholes). The system is custom-made at the factory to the customer's specifications. Sampling ports are created in the liner at the specified depths and small-diameter tubing (0.17 and 0.5 in. OD) is connected to the sampling ports. Pressure transducers and cables (if used) are also installed at the appropriate positions in the liner. The system is pressure tested to 300 psi at the factory. The system is shipped to the job site on a reel and is lowered to the bottom of the borehole by spooling the liner, sampling tubes, transducer cables, etc. off the reel. The system is shipped "inside out" which facilitates everting the liner and tubes into the borehole. Once the liner is everted, the sampling tubes and cables are inside the liner. The force required to evert the liner comes from hydrostatic pressure that is created by filling the liner with water at the ground surface. Ground water in the borehole is either displaced by the liner or can be pumped out during the installation. The borehole is sealed over its entire length by the pressurized liner. The system is removable by reversing the installation procedure, and may be installed in open boreholes or multi-screened wells. The Water FLUTe Sytem is described in more detail by Cherry, Parker and Keller.

Samples are collected by applying gas pressure to the sampling tubes, which forces the ground-water sample to the surface. Two check valves are installed in each of the sampling tubes. One of the check valves prevents the water sample from being forced back out of the sampling port when the pressure is applied. The second check valve prevents the ground-water sample from falling back down the sampling line between pressure applications.

Depth-to-water measurements can be made inside the sampling tubing using small- diameter water-level meters. Optional dedicated pressure transducers facilitate continuous, long-term pressure monitoring.

Complimentary hydrogeologic insights can be gained during and after deployment of Water FLUTe systems. Monitoring the rate of groundwater displacement during deployment yields vertical profiles of aquifer transmissivity. Also, information regarding the natural groundwater flow conditions in fractured bedrock can be gained by performing detailed temperature surveys inside of lined boreholes.

Decommissioning MLS Systems
To decommission a MLS system in either open rock holes or in multi-screen casing, the MLS must either be removed to allow the hole to be grouted up, or the MLS must be designed such that, when left in place, it can be fully sealed by grouting (i.e. a "grouted-in-place" system). For backfilled installations, removal of the MLS can be difficult, so the MLS systems are typically grouted in place. Alternatively, the MLS must be drilled out or over-drilled for removal and then the over-drilled hole grouted up.

Installation of MLS systems in multi-screened cased wells simplifies the decommissioning process for all systems, which is one reason why such installations are becoming popular. Having a smooth, low-friction, consistent inside-diameter pipe surrounding the MLS system simplifies removal of the MLS components. The multi-screened well casing can then be decommissioned using standard methods such as pressure grouting or by drilling out the casing.

Specific guidance and protocol for decommissioning the various commercial MLS technologies is available from the system manufacturers.

Overburden and rock
All of the commercially available multilevel systems can be installed in unconsolidated overburden, typically using the backfill method. With the FLUTe system, however, the annular seals are created by the liner that is pressed against the borehole wall via pressure applied by water, air or sand that fills the liner.

All of the systems can be installed in bedrock. With the exception of FLUTe, annular seals can be installed via the backfill method. Expandable, inflatable packers are also available for the Solinst Waterloo and Westbay systems. Seals for FLUTe MLS systems are created by the FLUTe liner as discussed above. FLUTe systems have been primarily installed in bedrock coreholes, often as part of investigations that incorporate elements of the Discrete Fracture Network (DFN) approach.

Depth ranges
All of the engineered MLS technologies can be installed at shallow depths, but only the Westbay MLS system can be installed to depths up to several thousand feet. The Solinst CMT system is the most common shallow MLS system. There have been more than 5,000 installations of CMT systems worldwide to depths typically less than 200 feet (https://www.solinst.com/products/multilevel-systems-and-remediation/403-cmt-multilevel-system/); with isolated installations to 500 feet using 3 Channel CMT. Solinst Waterloo systems are typically installed to depths up to 1000 feet. Water FLUTe and Westbay systems have been installed to depths greater than 500 feet, although there have been more deep Westbay systems installed that FLUTe systems. The deepest engineered MLS system is a Westbay system with 11 monitoring ports that was installed to a depth of 7,126 feet in Illinois.

Contaminated site assessment and remediation
All of the MLS systems have strong applications in assessing contaminated sites. The MLS wells can be installed along sampling transects to provide economical, high-resolution insights into the contaminant distribution and flux rates. Identification of plume cores is very important for designing effective in situ remediation systems such as permeable reactive barriers (PRBs). Transects of MLS systems also provide robust data for performance monitoring of in situ remediation. MLS systems also provide information regarding vertical hydraulic gradients. Strong vertical gradients in a head profile can be used to identify aquitards that constitute strong barriers to vertical contaminant migration.

Geotechnical
Depth-dependent hydraulic head data is very important for geotechnical studies. Some of the earliest applications of vertical head profiles were in support of geotechnical studies of rock stability (e.g., Patton 1983).

Water resources
There is a strong need for depth-discrete head and water quality data in water resources studies and planning. Variations in heads with depth can identify effective aquitards that can impede recharge to deep aquifers. Vertical head profiles provide important calibration points for groundwater models. Depth-discrete groundwater quality data is also very useful for optimization of well construction and pumping programs to extract groundwater of acceptable quality. One of the biggest deployments of Westbay MLS systems was to support management of a multi-layer aquifer in Orange County, California. Westbay MLS well are also used to optimize groundwater quality and extract for a water district in the Mojave Desert of California.

Further Reading and External Links
https://www.solinst.com/products/multilevel-systems-and-remediation/

http://www.westbay.com/

http://www.flut.com/WaterFLUTe/water_method.html

https://www.solinst.com/resources/cmt/discretezone.pdf

http://www.itrcweb.org/DNAPL-ISC_tools-selection/Content/Resources/DNAPLPDF.pdf

https://clu-in.org/download/contaminantfocus/dnapl/Treatment_Technologies/DNAPL-ER-200318-GR.pdf

http://www.waterboards.ca.gov/water_issues/programs/groundwater/sb4/docs/llnl_recommendations_report.pdf