Investigation techniques

Well-executed field work is the key to obtaining reliable and good quality data needed for an understanding of the basic geology and hydrogeology of the site and of the types of contaminants and their behaviour in the subsurface. Techniques that can be used to accomplish the objectives of

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the investigation include, among others, site reconnaissance, drilling, borehole and well testing, groundwater sampling, and field measurements and tests. This chapter provides only an overview of the available techniques. For details, the reader is referred to standard texts, such as Soil, vadose zone, and ground-water contamination (Boulding, 1995), Groundwater and wells (Driscoll, 1986), or Principles of contaminant hydrogeology(Palmer, 1996), from which this overview was prepared.

(i) Site reconnaissance

As a precursor to more expensive techniques, a quick and relatively inexpensive reconnaissance can help obtain preliminary data needed for the location of test boreholes, monitoring wells, and sampling points. Several approaches can be used: geological mapping, geophysical survey, soil vapour sampling, and shallow groundwater sampling.

Geological mapping indicates consolidated and unconsolidated materials in the area being investigated. The maps show the rock types and the distribution of geological structures. Geo-logical mapping can also be used for a specific purpose; for example, for the identification of geological materials that may contain potentially contaminating substances.

Geophysical surveys can help investigators characterise the site conditions and contam-ination (Zohdy et al., 1974). Geophysical techniques are rapid and non-disruptive to the site, and less expensive and less time-consuming than drilling. The following five techniques are com-monly used in groundwater investigations: ground-penetrating radar (GPR), electromagnetic conductivity (EM), electrical resistivity (ER), seismic refraction (SR), and magnetometry (MM) (Table 4.4).

New geophysical exploration techniques are being introduced all the time. For example, a relatively new technique is the nuclear magnetic resonance (NMR), also referred to as magnetic resonance sounding (MRS), which is used to determine water saturation directly with depth in unconfined or semi-confined aquifers to depths of approximately 100 m (Roy and Lubczynski, 2000).

Soil vapour sampling is one of the ‘newer’ reconnaissance techniques used to locate and sample volatile contaminants (Boulding, 1995). It is accomplished by pushing a thin-wall, small-diameter tube into the subsurface (generally 1.5 to 3 m, and up to 6 m), above the water table, using a one-way drive point. Once the desired depth is reached, the tube is pulled back slightly

TABLE4.4 Application of surface geophysical techniques in groundwater investigations

Application Geophysical technique

GPR EM ER SR MM

Changes in stratigraphy and lithology • •

•

Depth to bedrock

•

^•

Fracture detection •

•

^•

Karst features (cavities, sinkholes, etc.)

•

^•

Depth to water table

•

^•

Detection of plumes •

•

Location of buried objects (drums, UST, etc.) • •

•

Location of buried pipes

•

^{• • •}

Location of abandoned wells •

•

Note:

•

primary technique, • secondary technique.

and the vapor in the tube is evacuated to induce soil vapour to fill the tube. A sample is drawn into a syringe to transfer it to a portable gas analyser or to a laboratory.

Shallow groundwater sampling is a powerful tool for rapid assessment, especially in difficult locations, such as in roads or alleys or in buildings. Essentially, it is a variant of the soil vapour technique. A thin-wall tube is pushed to the water table and slightly below it. A sample is collected and analysed either in the field or in a laboratory. Although deep penetration is possible, the recommended maximum depth is about 5 to 8 m depending on sediment type (Palmer, 1996).

(ii) Drilling

A number of drilling techniques are available for subsurface exploration and monitoring. There is no universal drilling technique applicable for all subsurface conditions and well installations. In most cases, the drilling contractor is best qualified to select the particular drilling procedure for a given set of parameters. However, the investigator should have a strong working understanding of the drilling techniques in order to stay within given time and budget, especially if unforeseen problems occur. Basically, three techniques are commonly used: auger drilling, rotary drilling, and cable-tool drilling (Driscoll, 1986). Their principal characteristics are summarised in Table 4.5. The most common are auger drilling (for shallow holes) and rotary drilling (for deeper holes).

Although cable- tool rigs are used less commonly in developed countries, they are still widely used in many parts of the world.

(iii) Borehole and well testing

Geophysical borehole logging is usually carried out after collecting the soil and rock samples for determining lithology. The logs aid in filling the gaps between intervals of sampling and in revealing fine details of the subsurface strata. Geophysical logs also can be used to estimate hydrogeological characteristics. Most of the geophysical logs are done in the open uncased borehole. Only rarely is a single logging method used. Multiple logs provide more information than individual logs, and many logs require other logs for interpretation. The logs are usually made as pen-and-ink strip charts. This gives a continuous record, which is most useful. Inter-pretation of logs requires considerable training and skill. From the many borehole logging methods available, six are of major use in groundwater investigations (Table 4.6).

Hydraulic conductivity of a formation is an important aquifer parameter needed in many contamination studies. Normally, hydraulic conductivity can be determined by a pumping test, when the water level is measured prior to starting the pump and its drawdown during the pumping. In wells drilled into formations with low permeability or equipped with small-diameter casing, a so-called slug testcan be performed. During the test, the response of water levels in wells

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TABLE4.5 Principal test drilling techniques (Source:Driscoll, 1986)

Drilling Common Geological Formation sampling Drilling

technique depth (m) material capability speed

Auger 15–45 Unconsolidated Reliable Rapid

drilling samples penetration

Rotary drilling 100+ All types Representative samples Fairly fast

(hydraulic or air) relatively difficult penetration

to collect

Cable-tool 20–100 Unconsolidated, Good samples Slow

(percussion) caving formations, penetration

drilling hard rock

to adding or removing a volume of water is measured. The response can be interpreted in terms of hydraulic conductivity, using various methods, such as Bouwer-Rice (1976), Cooper-Bredehoeft-Papadopulos (1967), Hvorslev (1951), and Nguyen-Pinder (1984).

Pumping tests to determine the capacity of an aquifer to yield water usually are not performed in contamination studies. However, they do produce more accurate results than the slug or bailer test methods. If they are needed, their description can be found in groundwater hydrology textbooks (e.g. Driscoll, 1986; Fetter, 1994).

(iv) Groundwater sampling

Groundwater sampling is one of the most important tasks performed during a contamination investigation. Well sampling provides the geochemical and contaminant chemistry information for the problem under consideration. Together, the borehole log and the water quality information provide the two basic sources of observed subsurface data.

There are two fundamental considerations that are common to most groundwater quality sampling programs: 1) the establishment of individual sampling points (i.e. in space and time) and 2) the elements of the water sampling protocol (Barcelona et al., 1987).

The placement and number of sampling points will depend on the complexity of the hydrogeological setting and the degree of spatial and temporal detail needed to meet the infor-mation needs. It is better to start with fewer points and a low sampling frequency. The placement and number of sampling points can be phased to gradually increase the scale of the monitoring program.

Usually, a sampling plan and procedures, commonly called the sampling protocol, are prepared prior to field investigations. A sampling protocol may be given by government guidance documents or it may be written specifically for a given investigation type or contaminant type.

Generally, the sampling protocol would address:

•

water level measurements made prior to sampling;

•

well purging (the volume of stagnant water that should be removed from the well or borehole);

TABLE4.6 Major borehole geophysical methods used in groundwater investigations and their application (Source:Boulding, 1995; Palmer, 1996)

Logging method Hydrogeological application

Resistivity (normal or single-point) Generally determine the character and thickness of the various strata;

identify porous (sand) sediment; indicate water quality and possible contamination.

Spontaneous potential (SP) Distinguish clay/shale and sand/sandstone lithology or fresh and brackish water.

Natural gamma A qualitative guide for stratigraphic correlation and permeability or (in both uncased and cased holes) for estimating rock type.

Caliper Measure the borehole diameter; determine casing depth; locate cavities; indicate joints in carbonate aquifers.

Flow meter Determine source and movement of water in a well (especially, fractures producing water and zones of high permeability);

locate intervals of leakage in artesian wells; trace casing leaks or plugged screen.

Fluid temperature Trace the movement of the water injected into an aquifer and the dispersion, dilution, and movement of contaminants.

•

sample collection, preservation, and handling;

•

quality assurance and control;

•

sample storage and transport;

•

chain-of-custody record.

Representative sampling is the result of the execution of a carefully planned sampling protocol, well-trained staff, and good field-laboratory communication. A representative water sample may be defined as: ‘a minimally disturbed sample taken after proper well purging, which will allow the determination of the chemical constituents of interest at predetermined levels of accuracy and precision’ (Barcelona et al., 1987).

A wide variety of devices and installations are available for the sampling of groundwater (Weaver, 1992). They can be broadly classified as portable well samplers (used in permanently installed and screened wells) and in-situ samplers, which do not require cased monitoring wells (Boulding, 1995). Portable samplers include either suction devices or non-suction devices (Table 4.7). Schematic diagrams of the most commonly used portable samplers are shown on Fig. 4.5.

In-situ sampling probes are relatively new devices, which are either portable, used for rapid collection of samples without the installation of permanent wells, or left in place for continued monitoring. In monitoring wells, dedicated (left-in-place) sampling equipment can be used to minimise the potential for cross-contamination of samples between wells as well as the amount of equipment cleaning needed between wells. Dedicated sampling equipment is installed in a well and is used only for sampling that well. The initial purchase of dedicated sampling equipment for each well is more expensive than buying one portable equipment for an entire site. However, the time and money spent moving and cleaning equipment would be greatly reduced.

(v) Other characterisation methods

Field investigations include a variety of other measurements and tests performed in the field, such as water level measurements, tracer tests, fracture characterisation, and fracture traces.

A field investigation normally begins with measuring water levels in the monitoring wells and existing wells in the area. In addition to recording water levels prior to well purging and sample collection, the measurements also provide the basic data for a potentiometric map. The depth to water is typically measured manually with a steel tape, electric probe, or pressure

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TABLE4.7 Commonly used groundwater sampling devices (Source:Boulding, 1995; Lindorff et al., 1987) Min. well Max. Sample Potential for Ease of

diameter sampling delivery chemical operating and Sampling device (cm) depth (m) rate (l/s) alteration maintenance

Suction devices

Peristaltic pump 1.3 8 0.15–125 Slight-moderate Easy

Centrifugal pump 2.5 5 15–400 Moderate-high Moderately

difficult

Non-suction devices

Bailer 1.3 Unlimited Variable Slight-moderate Easy

Bladder pump 3.8 120–300 8–50 Minimum-slight Easy

Gas displacement pump 2.5 90 2–160 Moderate-high Easy

Syringe sampler 3.8 Unlimited 3 Minimum-slight Easy

Electric submersible 5.1 60 8–25 Slight-moderate Moderately

pump (helical rotor) difficult

Note: Schematics of sampling devices are included in Fig. 4.5.

FIGURE4.5 Schematic diagrams of the most commonly used sampling devices (Source:Lindorff et al., 1987)

inducer. The air-line method is useful in pumped wells where water turbulence may preclude using a more precise method.

Tracer tests can be used to measure or estimate the various hydrogeological parameters, most commonly the direction and velocity of flow and dispersion, or to identify sources, velocity, and direction of movement of contaminants. The variety of tracer tests is almost infinite, con-sidering the various combinations of tracer types, geological settings, hydrogeological conditions, and injection and sampling methods (Käss, 1998). No one ideal tracer has been found. Because of the complexity of natural environment and a wide variety of tracers, the selection and use of tracers is almost as much an art as it is a science (Barcelona et al., 1987). The purpose and practical constraints of a planned tracer test must be specified prior to the actual planning of the test. The more common types of tracer tests include single-well technique (injection/withdrawal and borehole dilution) and two-well technique (one testing for uniform flow and another for radial flow). The most commonly used tracers can be grouped into six general categories (Boulding, 1995): ions, dyes, gases, isotopes, water temperature, and solid particles.

In fractured rock terranes, the first step usually involves determining the spacing, orien-tation, depth, and length of fractures. The methods employed to gain this information depend on the surface or outcrop expression of fractures, type of material, depth of unconsolidated material over the aquifer, and fracture orientation (vertical and horizontal). Besides manual measurements, azimuthal seismic survey is another possible method for detecting fracture orientation. Fracture trace and lineament analysis using air photos or satellite images is a useful starting point for identifying possible areas of concentration and preferential direction of groundwater flow. In favourable conditions, fracture expressions may be visible even during field reconnaissance. For example, during periods of dry weather, in croplands underlain by shallow bedrock, crop growth is more vigorous over fractures filled with fine-grained soil, which holds more moisture than surrounding rocks. A basic understanding of a site’s tectonic history and fracture orientation would contribute to better understanding of potential contaminant pathways.

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