Design of groundwater monitoring wells

10.3.1 Types of monitoring wells

The installation of a monitoring well or a series of wells should always be preceded by a careful assessment of the purposes and objectives of the monitoring system. The objectives will in many cases dictate the design parameters for the well, including well diameter, well casing and screen materials, well screen length and placement, and well screen slot size and open area. For instance, when the objective is to monitor the extent of three-dimensional contaminant plume, the well screen length should be short enough to conduct sampling of discrete intervals (typically between 0.5 to 2 meters). Moreover, the diameter of the well may need to be large enough to accommodate a monitoring pump of sufficient capacity (inner diameter of at least 2 to 4 inches). The types of monitoring well completions range from single screened interval or open-borehole bedrock wells to more complex multiple-casing or multiple-screen wells Each type of well completion has its applications, advantages and disadvantages. General

recommendations for the application of each well completion type are given by Nielsen (2005) and reported in Table 10.1.

10.3.2 Multi-level groundwater monitoring

Numerous studies have shown that conventional monitoring wells yield composite samples that mask the true vertical distribution of dissolved contaminants in the aquifer (Church and Granato, 1996, Elci, et al., 2003, Nielsen, 2005, Puls and Paul, 1997, Roy and Fouillac, 2004).

Furthermore, the composite samples are strongly biased by numerous factors such as the position and length of the well screens (Figure 10.5), the pumping rate during sampling, and vertical flow in the well (Figure 10.6). Three-dimensional distribution of the dissolved contaminants in groundwater systems appears to be the rule rather than the exception.

Technologies exist now for collecting discrete ground-water samples from multiple depths at monitoring sites. Multi-level groundwater monitoring systems include nests of wells installed in single boreholes and clusters of wells completed to different depths. Various specialized multi-level monitoring systems are commercially available at present.

10.3.3 Design components of monitoring wells.

The completion of a monitoring well may disturb natural groundwater flow and its chemical composition. Such processes like reaction of water with the materials of screen, casing, pumps, etc. (material factor), as well as hydrogeological processes like changing of hydraulic head or contact with atmosphere via monitoring well should be taken into account (Jousma and Roelofsen, 2004, Nielsen, 2005). Some of the processes listed above will act permanently (e.g.

interaction between construction materials of a well and the groundwater) but some may disappear during subsequent exploitation (e.g. presence of drilling fluids)

The design of monitoring wells should also include the materials used for construction. In the case of monitoring wells selected among existing exploitation and observation wells, thorough checking of the construction is necessary. The elements which may lead to non-representative sampling should be eliminated or, as a minimum requirement, described in the sampling protocols. Contamination may be the result of improper material used in well construction and equipment or may be due to materials and processes applied during well drilling and completion. For instance, when the monitoring well is constructed using PVC tubes,

monitoring of numerous organic contaminants will be affected (cf. Table 10.3). Construction of a monitoring well should prevent inflow of water from other aquifer and/or from land surface.

Good practice during design of a monitoring well is mainly attributed to:

• proper localization of screen within the profile,

• suppressing of vertical flow through proper screening of the well,

• usage of non-reactive materials in well completion.

Table 10.1. Recommendation on applications of various well completion types (after Nielsen, 2005).

1. Single-casing, single-screen wells with short well screens

• Monitoring discrete zones (preferential flow pathways, such as sand and gravel lenses in a fine-grained matrix)

• Collecting discrete water-level data (i.e., from a pumping test) 2. Multiple-casing single-screen (telescoping casing) wells

• Monitoring discrete zones beneath confining beds or beneath known or suspected contaminated zones

3. Bedrock completions

a. Single-casing, single-screen (short screen) wells

• Monitoring discrete zones (preferential flow pathways, such as fracture zones or solution chan-nels)

b. Single-casing, single-screen (short screen) wells with surface casing

• Monitoring discrete zones beneath confining beds or beneath known or suspected contaminated zones

• Monitoring the zone immediately beneath unconsolidated overburden c. Open-bedrock boreholes

• Use as a screening tool to monitor thick sequences where only horizontal flow occurs

• Not recommended where vertical gradients are present, or where data from a discrete zone are de-sired

4. Well cluster (multiple single-casing, single-screen (short screen) wells completed at different depths in individual boreholes)

• Monitoring multiple discrete zones (i.e., multiple thin formations in a sequence of alternating coarse-grained and fine-grained materials)

• Monitoring multiple levels in a single thick formation

• Determining vertical gradients

• Evaluating chemical stratification

5. Multiple-screen well (with packers between screened zones)

• Monitoring multiple discrete zones

• Monitoring multiple levels in a single thick formation

• Determining vertical gradients

• Evaluating chemical stratification

• Not recommended where zones of interest are separated by only a few feet

6. Nested wells (multiple single-casing, single-screen wells completed at different depths in a single bore-hole)

• Monitoring multiple discrete zones

• Monitoring multiple levels in a single thick formation

• Not recommended where zones of interest are separated by only a few feet 7. Single-casing, single-screen wells with long well screens

• Use as a screening tool to monitor thick sequences where only horizontal flow occurs

• Not recommended where vertical gradients are present, or where data from a discrete zone are de-sired

8. Multi-level monitoring system

• Monitoring multiple discrete zones

• Monitoring multiple levels in a single thick formation

• Determining vertical gradients

• Evaluating chemical stratification, or measuring small-scale features of contaminant distribution

Figure 10.5. Effect of well screen length on sample concentrations. (a) Three types of

monitoring well completions – single-zone, long-screen well (well “L”); cluster of three wells completed to different depths (wells “M”); and multilevel well (well “N”). (b) Heavy-dashed line shows actual concentration of a dissolved solute in the aquifer. Single-zone, long-screen well (well “L” yields a sample that is a mixture of high concentration of the solute entering the upper portion of the well screens and low concentrations entering the lower portion of the well.

Multilevel monitoring well (well “N”) yields samples that most closely represent the true distribution of the dissolved solute in the aquifer (Nielsen, 2005).

When new monitoring system is being established, it is recommended to install clusters of monitoring wells with relatively short screens (2-4 m – cf. well D in Figure 10.9 – see section 10.5.2). In case of single monitoring well, short screen in a wrong position of the profile is the worst solution (cf. well B in Figure 10.9). A long screen may cause vertical flow through the well from points with slightly higher hydraulic head to lower one (Figure 10.6) and mixing of various groundwater compositions across the vertical direction. Suggested designs of

monitoring wells are given in numerous guidelines. Comprehensive discussion of that problem can be found in Nielsen (1991, 2005).

Materials used for completion of monitoring well should be non-reactive and non-corrosive.

The same concerns all equipment used for monitoring (pumps, tubing etc.). Not in every case completion of a monitoring well is done using non-reactive materials. Some existing wells have construction materials that are reactive. In such cases, it is necessary to know what possible changes in chemical composition of sampled groundwater may occur due to presence of

particular materials in the well (Moore, 2002; Nielsen, 2005). As an example, the recommended materials for completion of monitoring wells and for sampling are listed in Table 10.2 after Remmler (1990) and in Table 10.3 as part of a condensed reference table of compatibility data for 207 organic chemicals (McCaulou et al., 1995).

Common metallic materials are different types of steel, including the most resistant stainless steel which is comparable with teflon. In corrosive environment even stainless steel may be a source of some trace elements such as Fe, Mn, or Cr.

Table 10.2. Recommended materials for monitoring (Remmler, 1990).

Monitoring objective Type of material

Teflon PTFE

Explanation: + suitable; (+) partly suitable; - nonsuitable; ^A assessment per analogiam

Polyvinylchloride (PVC) is a relatively good material when it is certified for contact with potable water. In some countries Pb and Cd compounds have been used as additives for

stabilization of PVC and these metals can be released to water samples (Barcelona et al., 1985).

Even for certified materials one should be aware that maximum permissible concentrations of some contaminants are in ppb level. More data with respect to construction materials is found in Nielsen (1991, 2005). According to this author, negative influence of construction materials is detectable for some trace elements such as As, Pb, Ni and can be avoided by thorough pumping of the well.

Teflon (PTFE) is practically fully resistant to all known contaminants and is the best material for monitoring wells up to 100 m depth (EPA, 1987). Relatively high price may be partly compensated by smaller diameter of wells.