1. Cost effective management of contaminated groundwater is reliant on access to sufficient, good quality, reliable data developed from site characterization programs. The cost to acquire this data can be very expensive, therefore programs need to be well thought through before valuable resources are committed.
Successful programs tend to have a number of common elements, which include but are not limited to:
- Selection of the best affordable expertise to design and implement the program
- Make sure of a conceptual model of the site and revision as new information becomes available. The conceptual model is used as a decision making tool to direct further investigation and to develop a plan to manage contaminated groundwater
- Integration of a variety of characterization tools
- Utilization of a phased approach
One of the tools which has been available for acquiring reliable, good quality groundwater data is the multilevel system. The purpose of this article is to show how a well-planned, phased program employing strategically placed multilevel systems, can provide data of sufficient number and quality, to support monitored natural attenuation as a viable groundwater management option.
2. Multilevel Systems
A multilevel system is a device which allows the monitoring of hydraulic head and/or the sampling of groundwater from more than one zone within a single borehole. The installation of such systems allows the differentiation of the vertical head distribution within an aquifer. Since groundwater flow is a 3-dimensional process, vertical as well as horizontal gradients should be considered, but in many investigations, the vertical component of flow is often overlooked. Such information is increasingly being recognized as crucial for the management and remediation of contaminated groundwater
As an alternative to multilevel systems, the vertical groundwater head distribution can be obtained from several single monitoring well completions within the same general area (well cluster) or by completing a number of single monitoring wells within the same borehole (well nest). See Figure 1.
A number of multilevel groundwater monitoring systems are currently available commercially. The central component of a multilevel system consists of a rigid casing, or a flexible polyethylene sleeve, which fits inside the borehole. The number and location of monitoring ports are determined on the basis of what is known about the site and information obtained from the borehole during drilling (eg core examination, hydraulic testing, borehole logging etc). Ports are hydraulically isolated from one another through the use of packers, bentonite seals or from fluid pressure generated inside the flexible sleeve as it fills with water. Ports are sampled/monitored by means of dedicated samplers and pressure transducers; through tubing which attaches from the port screen up through the casing to the ground surface; or via samplers which travel down the casing, lock into each port and are activated from surface.
Recently there has been considerable interest in the use of multilevels largely because:
- Systems are becoming less expensive and simpler to use
- Improved quality and reliability of data from these systems
- The recognition of the importance of three-dimensional data distribution to manage contaminant plumes and to assess risk to potential receptors
- Fewer boreholes reduces the risk of developing new potential contaminant pathways and reduces the costs associated with the disposal of contaminated soil and groundwater.