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. 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. 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. 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. 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). 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. 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:
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