2.2 Advantages of Multilevels Over Well Clusters and Well Nests
Multilevel systems have several advantages over well clusters or nested wells.
Individual wells of a well cluster will each require their own borehole and well materials. Material costs as well as costs for the disposal of contaminated soil and groundwater for well clusters can be significantly greater compared with the costs associated with a multilevel system installed in a single borehole.
With nested wells, the orientation of the individual monitor casing is not easily controlled. This can create a non-uniform annulus, which can lead to bridging of backfilled material, short-circuiting between sampling zones, and a breach in the integrity of hydraulic seals. Multilevel systems fitted with centralizers positions the system in the centre of the borehole, providing a uniform annulus for backfilling or inflating packers and ensuring a competent seal.
2.3 Sampling and Instrumentation
A number of devices are available for sampling groundwater from multilevel systems. These include: inertial, bladder, double valve and peristaltic pumps, bailers, and specialized discrete samplers. Continual development of these products has led to miniaturization and lower cost, which makes dedication of these samplers more affordable.
The purging of a multilevel system prior to sampling generates a smaller volume of groundwater per monitoring point than larger diameter monitor wells because of the smaller volume of water in the sampling tube or channel. Multilevel systems are also amenable to low flow sampling techniques due to the lower flow rates of multilevel sampling devices.
Small diameter water level meters capable of entering 10mm diameter tubing are available for use in multilevel systems for manual measurements of water level. Alternatively, pressure transducers can be dedicated to multilevel ports for long term automated monitoring of water levels.
Multilevel systems can provide the necessary detailed data to characterize contaminant plumes for (i) assessing risk to potential receptors and (ii) characterizing the plume for the purpose of designing a remedial action plan.
Einarson and Mackay (2001) describe how transects of multilevels installed across a contaminant plume orthogonal to the groundwater flow direction can be used to estimate the contaminant mass flux towards a receptor such as a water supply well (see Figure 2). Under certain assumptions, the estimated contaminant mass flux can be used to back calculate a worst case contaminant concentration at the receptor.
A program to remediate contaminated groundwater should be designed with the benefit of a reliable conceptual model which depicts the source area, contaminant transport pathways and potential receptors.