In this area, glacial deposits have left deep layers of sand, allowing the spill to penetrate deeply into the soil media.
The plan called for Aztech engineers to implement 5 borings at depths ranging from 70 to 90 feet. A multi channel monitoring tube was installed in each boring allowing the client to monitor up to seven different vertical zones within each borehole. This is a radical new departure from old, cost-intensive practices using traditional well clusters – each well sampling only one vertical zone – and thus requiring multiple boreholes.
The Continuous Multichannel Tubing (CMT) is a narrow and flexible 1.7-inch outside diameter polyethylene tubing segmented into seven channels, allowing groundwater monitoring of up to seven depth-discrete zones if desired. From a single well, one can collect samples from many vertical strata, providing critical understanding of the contaminant distribution and a more accurate three-dimensional assessment of the site.
At the surface collection point, a pump system draws groundwater from the individual channels for testing, providing accurate data on plumes that are often highly stratified.
Each boring was installed using a 4.25-inch hollow stem auger to the maximum sampling depth. The multichannel piping was unrolled from a continuous spool to the correct length, then laid out on a clean surface. The bottom of the channels were stoppered, and then ports were drilled into individual channels at the prescribed sampling depths. These channels were blocked below the ports, and ultrafine mesh screens were fitted over the port openings.
The multichannel piping was then fitted with finned centralizers that position the piping in the center of the hole, while allowing sand and clay to be poured around the pipe, packing and sealing the zones. Sand is used as a communicating media adjacent to the screens, and bentonite clay is then poured in to seal each interval from vertical groundwater migration.
At this site, three ten-foot sections of strata were to be monitored. Two channels on opposite sides of the tube were ported and fitted with screens to allow for continuous vertical monitoring of each prescribed ten foot section of the aquifer. Two channels were used per zone to maintain structural integrity of the tubing and to prevent collapse.
The use of multichannel systems can dramatically reduce costs associated with multilevel monitoring projects. In this case, only one boring was drilled using 4.25-inch hollow stem augers to monitor three different ten-foot zones. A traditional well cluster would have required three separate borings and wells, and thus potentially more effort required for permitting, triple the waste disposal costs, a longer decontamination time, and a larger footprint on the land.
Aztech recommends CMT technology for cost-effective, variable depth monitoring throughout a wide variety of geologies.