Traditional approaches can be either expensive or potentially unreliable. However, feasible solutions have entered the market. White Young Green Environmental (WYGE) recently employed one of these products on a site in south England with favourable results.
Groundwater systems often demonstrate complex hydraulic head, geochemical and contaminant properties as a result of geologic heterogeneity and contaminant chemistry. Under such conditions a robust ground investigation needs to collect high-resolution, depth-
discrete measurements of hydraulic head and groundwater quality before the development of a suitable site model.
Traditionally, gathering such a large set of groundwater data on complex sites has involved installation of a cluster of short-screened wells or piezometers at various depths in close proximity. Clustered wells allow for the requisite high-resolution of vertically distributed water samples and hydraulic head measurements that greatly improve understanding of site conditions. However, clustered wells require a borehole for each well or piezometer, increasing drilling costs. This usually forces a compromise between the investigation budget and resolution of data.
Many site investigations include multiple wells in a single borehole. This nesting technique can allow depth-discrete measurements in the right circumstances. But the quality of nested well hydraulic data can be suspect.
A challenge with nesting is eliminating hydraulic continuity between individual wells, particularly where discrete monitoring points are close together. Successful nested installations require sufficient curing time for the seals above and below each well screen, particularly if more than two wells are in the same borehole. In many cases it can be more cost-effective to drill more holes than to properly emplace several wells in one.
Multi-level water samplers maximise quality of groundwater measurements at various depths in the same hole, while minimising difficulties associated with multiple emplacements. This is achieved via isolated sampling ports at various levels on a single well, which are easier to seal and prevent hydraulic continuity between sampling ports, than in a clustered well.
The three multi-level samplers available in the UK are the Solinst Continuous Multichannel Tubing (CMT), the Solinst Waterloo Multilevel System and the Westbay MP System.
WYGE recently used the Solinst 7 channel CMT, distributed by Waterra (UK). CMT is fabricated from extruded medium density polyethylene and manufactured with either three or seven channels. Each channel acts as an isolated well that is perforated at a desired depth during installation in the field.
The perforated section in each channel is about 80mm long, which is subsequently covered with fine-mesh stainless steel gauze. Fine-grained sand filters around each monitoring port define the response zone, if there is a significant difference in hydraulic conductivity between the filter and the native material.
Bentonite seals placed above and below each sand pack isolate the intervals and prevent hydraulic continuity along the outside of the pipe. Each sand and bentonite layer has a practical minimum length of 0.5m. Therefore, a seven-channel system can monitor, with maximum detail, a minimum vertical span of 7m.
Experience with CMT
Between July and September, WYGE was engaged in a £120,000 intrusive ground investigation of a former landfill in south England as part of a commercial redevelopment. The project’s scope was largely driven by environmental contamination, although geotechnical data was also collected to inform the redevelopment. The site had been intrusively investigated twice previously, where nested wells had monitored groundwater.
Although these showed obvious contamination, reported hydraulic head measurements and plume geometry data was inconclusive. WYGE suspected the nested well installations had been compromised and suggested Solinst 7 channel CMT, as clustered wells were too expensive and nested wells were both expensive and unreliable in the thin strata bands.
The site is about 500m by 250m, generally flat and at grade with the adjacent properties. WYGE installed 19 boreholes; 15 of the borings were advanced to 10m below grade and four were advanced to 25m below grade. The complex geologic setting was 1m of course granular fill over 2m of household black bag landfill, over a 1m layer of a silty relic topsoil, over 1.5m of a course well-sorted gravel, over a 15m thick formation of fine to medium sand interspersed with clay layers 1m to 5m thick, all over chalk.
The strata permeability was extremely varied while household waste suggested potentially high land gas generation. WYGE used from five to seven CMT channels in each borehole to capture the desired level of data resolution.
Where only five of the channels were required, the two unused channels were perforated as duplicate channels for the deeper installations. This avoided unused channels filling with air and creating buoyancy problems during installation.
Once up to speed, it was possible to fabricate seven ports in a customised well in about 30min.
CMT groundwater collection requires smaller equipment, for example, dip meter, than traditional 50mm standpipes as channel openings are smaller. But this was two-edged.
The benefit was that the relatively small-response zone for each port generated small volumes of purge water. However, when collecting water from the low permeability soils, it took longer than anticipated to capture enough volume to satisfy the laboratory requirements for the broad spectrum chemical analysis required.
Although still in early stages of data interpretation with some still being processed, quality appears consistent. This is allowing conceptual model construction with a detailed understanding of groundwater flow characteristic and multiple contaminant plume geometries.
CMT has a material unit cost several times higher than 50mm standpipes, but is not intended as a replacement.
In its proper context CMT is an inexpensive means of collecting detailed vertically distributed data. Had WYGE attempted to gather the same quality of data with nested (if at all feasible) or clustered wells on this project, the estimated time on site would have been about two to four times longer. This means the higher initial CMT cost was saved many times over in the site engineer’s time alone.
Jason Morrissey is senior environmental engineer with White Young Green, Cardiff.