Monitoring wells

The following guidance is based on the information provided in Wilson and Haines (2005) that was subsequently incorporated into CIRIA Report C659/665 (Wilson et ah, 2006/2007). It focuses on ground gas rather than vapours. For vapours the number of wells required to characterise a source will be driven by the extent of the chemical contamination of the groimd or soil. 

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Monitoring and control of the production process will be performed by a combination of instrumentation and control equipment plus manual involvement. The level of sophistication of the systems can vary considerably. For example, monitoring well performance can be done in a simple fashion by sending a man to write down and report the tubing head pressures of producing wells on a daily basis, or at the other extreme by using computer assisted operations (CAO) which uses a remote computer-based system to control production on a well by well basis with no physical presence at the wellhead. 

The term aquifer is used to denote an extensive region of saturated material. There are many types of aquifers. The primary distinction between types involves the boundaries that define the aquifer. An unconfined aquifer, also known as a phraetic or water table aquifer, is assumed to have an upper boundary of saturated soil at a pressure of zero gauge, or atmospheric pressure. A confined aquifer has a low permeabiUty upper boundary that maintains the interstitial water within the aquifer at pressures greater than atmospheric. For both types of aquifers, the lower boundary is frequendy a low permeabihty soil or rock formation. Further distinctions exist. An artesian aquifer is a confined aquifer for which the interstitial water pressure is sufficient to allow the aquifer water entering the monitoring well to rise above the local ground surface. Figure 1 identifies the primary types of aquifers. 

Fig. 1. Aquifers and monitoring wells where denotes the well screen and Hthe water-filled space in the monitoring well. (—) denotes the water table level, (—), the potentiometric surface, and ( ) the ground surface. Terms are discussed in text.

Monitoring Well Design for Contaminant Transport Studies... 

It is often important to quantify the contamination of pore fluid in the unsaturated soil 2one, where monitoring wells are ineffective. In this region, suction cup lysimeters are useful (7). These samplers consist of a porous cup, typically ceramic, having two access tubes which are usually Teflon. One access tube provides a pressure-vacuum, the other discharges the sampled fluid to the surface. The porous cup, typically between 2 and 5 cm in diameter, is attached to a PVC sample accumulation chamber. 

Locational considerations include both surficial location and screened interval, ie, the sampling depth. The surficial location is selected based on whether the sample is to represent background quaUty or quaUty at the location of contamination, or potential leak location. In selecting the surficial location, the groundwater flow parameters, velocity and direction, are assumed to be known from other monitoring wells or borings already completed. 

Detailed sampling can include, but is not limited to, the installation of monitoring well networks. After the wells have been installed, aquifer tests are typically performed. Once the aquifer tests are performed and the aquifer characteristics are determined, time series sampling for a given contaminant, or a surrogate, is undertaken. The combined results of these efforts provide the basis for development of a treatment strategy. Modeling can be used as part of this effort to help determine the best technical and most cost-effective techniques to be used at a site. 

The Company and its consultant decided more monitoring wells were needed to better define the situation, and a new drilling program was initiated. 

Defining the plume of the contamination was straightforward by using existing domestic wells and a few monitoring wells. However, determining the nature of the problem beneath the potliner pile and its vicinity was much more difficult. 

A single monitoring well can be designed to accommodate all of these uses and wells are usually designed to collect only those data required by the groundwater monitoring plan. 

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