A New Multi-Level Ground Water Monitoring System Utilizing Multi-Channel Tubing

A new multi-level ground water monitoring system has been developed that uses customextruded flexible 1.6-in. (4.1-cm) outside-diameter (O.D.) multi-channel HDPE tubing (referred to as Continuous Multi-Channel Tubing [CMT]) to monitor as many as seven discrete zones within a single borehole in either overburden or bedrock. Prior to inserting the tubing in the borehole, ports are created that allow ground water to enter six outer pie-shaped channels (nominal diameter = 0.5 in. [1.3 cm]) and a central hexagonal center channel (nominal diameter = 0.4 in. [1 cm]) at different depths, facilitating the measurement of depth-discrete piezometric heads and the collection of depth-discrete ground water samples. Sand packs and annular seals between the various monitored zones can be installed using conventional tremie methods. Alternatively, bentonite packers and pre-packed sand packs have been developed that are attached to the tubing at the ground surface, facilitating precise positioning of annular seals and sand packs. Inflatable rubber packers for permanent or temporary installations in bedrock aquifers are currently undergoing site trials. Hydraulic heads are measured with conventional water level meters or electronic pressure transducers to generate vertical profiles of hydraulic head. Ground water samples are collected using peristaltic pumps, small-diameter bailers, inertial lift pumps, or small-diameter canister samplers. For monitoring hydrophobic organic compounds, the CMT tubing is susceptible to both positive and negative biases caused by sorption, desorption, and diffusion. These biases can be minimized by (1) purging the channels ©Einarson and Cherry, 2001 1 prior to sampling, (2) collecting samples from separate 0.25-inch (0.64 cm) O.D. Teflon sampling tubing inserted to the bottom of each sampling channel, or (3) collecting the samples down-hole using sampling devices positioned next to the intake ports. Currently, more than 1000 CMT multi-level wells have been installed in North America and Europe to depths up to 260 feet (79 m) below ground surface. These wells have been installed in boreholes created in overburden and bedrock using a wide range of drilling equipment including sonic, air rotary, diamond-bit coring, hollow-stem auger, and direct push. This paper presents a discussion of three field trials of the system, demonstrating its versatility and illustrating the type of depth-discrete data that can be collected with the system. Introduction Many investigations have shown that contaminant plumes are typically complex zones that exhibit large variations in concentration over small vertical distances. These variations are caused by spatial and temporal variability of the contaminant sources and heterogeneity of the geologic materials. In sand aquifers, large vertical concentration variability within plumes is enhanced by weak transverse vertical dispersion that preserves the variability over large travel distances (Reinhard et al. 1984; Robertson et al. 1991; van der Kamp et al. 1994). Weak dispersion has been documented during natural gradient tracer experiments in which the tracers were monitored intensively using multi-level depth-discrete samplers (Mackay et al. 1986a; Mackay et al. 1986b; Garabedian et al. 1991; LeBlanc et al. 1994). Conventional monitoring wells are often ineffective for discerning the details of the concentration distribution in plumes and particularly for locating the highest concentration zones because the well screens provide water samples that are a mixture of waters of different composition from various depths (Robbins 1989; Martin-Hayden et al. 1991; Robbins and ©Einarson and Cherry, 2001 2 Martin-Hayden 1991). Nested monitoring wells (i.e., two or more individual wells installed to different depths in the same borehole) can yield depth-discrete samples but their use is discouraged because of the difficulty in installing reliable seals between the different well screens (USEPA, 1986). Clusters of conventional monitoring wells (i.e., closely-spaced wells installed in individual boreholes but completed to different depths) are an alternative to nested wells but commonly do not monitor more than two or three depth intervals because of the economic limitation on the number of wells used in each cluster. To overcome these limitations, multi-level monitoring systems that provide water samples from many depth-discrete levels or ports in a single monitoring hole have been used, such as those described by Pickens et al. (1978), Cherry and Johnson (1982), and Black and Patton (1986). This paper describes a new low-cost permanent multi-level monitoring system that can be used to collect ground water samples and measure hydraulic heads from up to seven discrete zones in one borehole. The system utilizes a single length of custom-extruded flexible tubing, facilitating the installation of reliable annular seals between the monitoring zones using conventional well construction methods where annular materials (e.g., sand and bentonite pellets) are added from the ground surface. Bentonite packers have also been used that allow the entire multi-level well to be constructed above ground and then inserted into a borehole. By using the bentonite packers, seals of exact dimensions and position can be installed. A modification of the system using water-inflated rubber packers for use in rock boreholes is currently undergoing site trials. ©Einarson and Cherry, 2001 3 Materials and Methods Continuous Multi-Channel Tubing (CMT) The key component of the new monitoring system is custom-made, high-density polyethylene (HDPE) tubing. The 1.6-in. (4.1-cm) outside diameter (O.D.) tubing, referred to as Continuous Multi-Channel TubingTM (CMTTM) (patent pending), is extruded with internal partitions, forming seven discrete channels within the larger tube (Figure 1). The honeycomb design creates six outer pie-shaped channels having a nominal inside diameter of approximately 0.5 in. (1.3 cm) and a central hexagonal channel approximately 0.4 in. (1 cm) in diameter. The primary advantage of the new multi-channel tubing over bundles of tubes as described by Cherry et al. (1983) is that there is only one relatively large tube installed in the borehole which simplifies the installation of annular seals placed between the tubing and the borehole wall. The multi-channel tubing can be extruded in lengths currently up to 300 ft. (92 m) and is shipped in 4-ft. (1.2-m) diameter coils (Figure 2). The desired length of tubing, equal to the total depth of the multi-level 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 or other methods (e.g., CPT or geophysical data). Having a continuous length of tubing is a key advantage of the system since it eliminates the need for strong, water-tight joints in the monitoring well. No joints exist because the tubing is one piece. This increases the reliability and reduces the cost of the monitoring system. The tubing is stiff enough to be easily handled, yet light and flexible enough to allow site workers to insert the multi-level well hand-over-hand into the borehole. A small ridge along the outside of one of the channels facilitates identification of specific channels. The collapse and tensile strengths of the tubing have not yet been tested, but are expected to be high due to the internal honeycomb structure of the tubing. ©Einarson and Cherry, 2001 4 Intake Ports and Screens Construction of the intake ports and screens is done before the CMT tubing is inserted into the borehole. Depth-discrete intake ports are created by drilling or cutting 0.38 in. (0.95 cm) holes through the exterior wall of the tubing into each of the channels at the desired depths. Channel 1 ports correspond to the shallowest monitoring interval; channel 2 ports are drilled further down the tubing (i.e. to monitor a deeper zone), and so forth. The central channel, channel 7, is open to the bottom of the multi-level well. In this way, the ports of the various channels are staggered both vertically and around the perimeter of the multi-channel tubing. Typically, each channel is hydraulically connected to only one monitoring interval. However, the well can be constructed with two channels open to the same interval: One channel can be used for measuring water levels; the other for collecting groundwater samples with a dedicated sampling pump. Since two channels are used at each depth, constructing a well this way reduces the number of intervals that can be monitored. For most of the installations performed to date, an intake interval of 4 in. (10 cm) has been created by drilling four holes one inch (2.5 cm) apart. The depth interval of the intake ports can be increased simply by drilling more holes. Well screens are constructed by wrapping synthetic or stainless steel fabric mesh completely around the tubing in the interval containing the ports (Figure 3). The mesh is secured to the tubing using stainless steel clamps. The size of the mesh openings can be selected based on the grain-size distribution of the particular water-bearing zone being monitored. However, a 100 mesh stainless-steel screen having an open area of approximately 0.006 in. (0.15 mm) has been used successfully for most of the installations performed to date. Stagnant water in the tubing below the intake ports is hydraulically isolated by plugging the channels a few inches below each intake port. This has been done by injecting a small amount of a polyethylene sealant into each channel (Figure 3). Polyethylene plugs are also ©Einarson and Cherry, 2001 5 injected into each of the outer six channels at the very bottom of the well. This effectively seals the various channels from just below the intake ports to the bottom of the well. (Pressure tests show that a 1-inch-long plug withstands a pressure differential of over 80 pounds per square inch [552 Kpa] [Solin