This paper reviews technologies utilised for groundwater monitoring programs applicable mainly to shallow, unconfined aquifers; especially the monitoring well design, drilling, pumping, purging and sampling aspects. The roles of program objectives, sampling protocol, analysis and Quality Assurance are also examined.
Throughout the world groundwater monitoring is an important component of catchment management. Groundwater monitoring programs broadly serve four roles:
some or all of which may be incorporated into the objectives of the program. This provides indicators of groundwater health and monitors the effects of human activity within the catchment; facilitating the formulation of appropriate catchment management policies.
This paper examines groundwater monitoring technologies applicable mainly to shallow, unconfined aquifers (see Figure 1) and includes:
QA is examined as an integral part of any monitoring program (as important as any specific technology) and the need to incorporate QA throughout every phase of a program.
Every environmental monitoring program must have clearly defined objectives for proper planning and execution and to gauge its success; an integral part of QA. A groundwater monitoring program is governed by cost limitations and objectives often have to be revised in consultation with end-users.
A general objective of groundwater monitoring is to obtain a true indication of the physico-chemical and biological constitution of the water within the aquifer. It is important that:
Ongoing monitoring is to ensure that any degradation of water quality is detected at the earliest possible time. Any follow-up investigation should identify the source and establish a remedial program to clean-up the aquifer and prevent danger from the contamination.
Specific objectives are generally based on either (Keith 1990):
A fundamental requirement for groundwater monitoring is access to the water within the aquifer through monitoring wells; this is a critical component of the monitoring program.
A monitoring program should permit installation of new wells; existing wells are often poorly located and are generally designed for maximum yield rather than selectivity. The number of monitoring wells required and their spatial distribution (ie in transects or arrays) will depend on the hydrologic complexity of the catchment and the objectives of the program. Changes in the placement and number of required wells often evolves in the light of collected data or variations in program objectives; thus should be a normal part of program evolution.
Design of monitoring wells requires knowledge of the site’s hydrogeology and subterranean geochemistry; their construction demands a higher level of care than normally applied to production wells. In construction, care needs to be taken to minimise cross-contamination of aquifers with loosened topsoil – particularly in areas where soil contamination is likely (Keith 1990); practically, there will always be some trace contamination of the aquifer from surface and near surface layers (Smith et al. 1988). Design criteria include:
The simplest, narrow diameter monitoring wells capable of accommodating the purging and sampling equipment are preferred for routine monitoring as these will least disturb the environment and minimise the purging volume required. With the general availability of small diameter (50 mm), submersible pumps capable of 30 metre lift, 50 mm (ID) monitoring wells have become a standard in monitoring well technology (Barcelona et al. 1988).
The occurrence of vertical flow between permeable strata within an aquifer system may lead to mixing of sample water from multiple zones. To limit such flow monitoring wells should be designed and constructed to either monitor one discrete stratum or have segregated screens to enable multi-level monitoring (Kent and Payne 1988).
Selection of an appropriate drilling technology is important in achieving the desired quality of groundwater samples. The main governing criteria in descending order of importance are (Barcelona et al. 1988):
Appendix A summarises a selection of drilling techniques commonly used in the water-well industry with their advantages and disadvantages in relation to monitoring well construction.
Hand augering, although the slowest and most labour-intensive of methods, is the least disturbing environmentally (augers range from 50 to 100 mm) and can be used in locations inaccessible to machinery. Hand augering provides the best form of stratigraphy logging with resolution of ultra-thin sedimentary lenses down to the millimetre thickness level. It is limited to approximately 7 metres depth and cannot penetrate indurated or consolidated substrates (such as ferruginised, indurated sandstone or calcrete). It is highly suited to investigations in:
Drive point installations which comprise a pointed steel driving point attached to a robust steel pipe; the whole being manually driven into the ground, the sampling system placed inside the steel pipe which is subsequently withdrawn allowing the soil to collapse onto the sampling structure. This system is suited to shallow aquifers and has been used successfully down to 8 metres in sand (Stites and Chambers 1991).
Cable-tool drilling is one of the oldest and simplest methods used in the water well industry. The penetration rate is slow compared to other methods but recovery of core samples is excellent and the equipment required is simple and relatively inexpensive.
Hollow-stem and solid-stem augering are ideal mechanical techniques for well construction due to the absence of drilling fluids and minimal disturbance of the substrate; they are limited to approximately 40 metres depth and will not penetrate consolidated rock. Hollow-stem augers are particularly valuable in that they facilitate continuous collection of geologic core samples and in non-cohesive soils (i.e. those prone to caving-in) the monitoring well can be assembled and installed through the hollow core prior to removal of the auger.
Air-rotary drilling is the other method of choice for well construction since it uses compressed air (forced down the centre of the drilling stem) to convey cuttings to the surface along the periphery of the borehole introducing minimal contaminants into the well. Being suitable for all geological formations, it is often used where hollow-stem augers cannot penetrate consolidated substrates.
Well casing and screen materials contaminate the collected sample. The materials should have minimum chemical impact on the samples and retain their structural integrity within the subterranean environment for the life of the monitoring program. Barcelona et al. (1988) rank the commonly used materials from best to worst in the following order:
Many monitoring wells are constructed from rigid PVC; absorption of organic compounds by this PVC is low, typically1 less than 1 ng/cm2. Note however, that flexible PVC is quite different; added plasticisers (including phthalate esters) leach into water. U.S. EPA recommends exclusive use of stainless steel or polytetrafluoroethylene (PTFE or Teflon™ in preference to PVC (Kent and Payne 1988) but this does impose a significant cost penalty. Where the cheapest materials are used from cost constraints, additional tests and QA samples may be required to identify any bias introduced by those materials.
1Units for this parameter are usually given in absorption per unit of exposed area. Note that larger diameter tubes tend to have a smaller effect.
Well development is an important phase of commissioning (or recommissioning) a monitoring well and is often overlooked in the planning stage. The drilling of a well produces a sleave of finely ground materials (fines) around the borehole which (when wet) form an occlusive mud cake around the well casing. This limits the hydraulic conductivity into the screened section and must be removed to enable effective sampling, allow an adequate flow rate into the well and provide water samples reasonably free from suspended solids (to reduce subsequent filtering).
The underlying principle of each technique is to generate repetitive and shock flow reversal through the mud cake thereby breaking it up and flushing the fines into the well for removal. The end-point is variable from clear water delivery to a consistent minimum content of suspended solids.
Monitoring changes in groundwater requires an understanding of the physical and chemical characteristics of the aquifer system. This includes the hydrologic and geochemical characteristics of the aquifer and usually involves a desk survey of all known information from previous test drilling; in the absence of such information additional test drilling will be required. Ideally the knowledge will include (Kent and Payne 1988):
Valuable hydrologic information is gained by recording the water table depth prior to purging or sample collection which may reflect seasonal variation and other perturbations (recharge or draw-down events). Recording the level at the end of (but during) the purging cycle is also valuable for assessment of changes in hydraulic conductivity; applicable only to dormant (ie non-producing) monitoring wells or production wells that can be shut down long enough to allow recharge to fill the cone of depression. All this information should be recorded as part of the sampling log and form part of the sampling protocol.
Due to the rapid degassing and evaporation of water at reduced pressure, vacuum lift pumping is limited to 8 metres depth (Blake 1989); beyond which pump suction reduces the pressure within the well to levels comparable to the vapour pressure of the groundwater. For greater depths, down-the-well (submersible) pumping is required with sufficient head and capacity to lift the water to the surface.
Whenever water is stagnant in the well casing for extended periods of time (ie two hours or more) it has had the opportunity to react with the well casing material, exchange gasses with the atmosphere and suffer microbial activity. It is well accepted (Davis and Barber 1994) that, due to this chemical alteration and vertical cross contamination from different strata within the well, stagnant water must always be removed prior to sample collection to obtain a truly representative sample. Purging and sampling should preferably take place at the same level in the well; this is normally achievable using the same pump. Additionally, the equipment should always be lowered to the same level in the well for consistency.
Hydraulic effects during pumping (such as turbulent flow) may cause sample alteration; hence a sampling protocol should include a purging and sampling routine for each monitoring well (Kent and Payne 1988):
The purging and sampling cycles therefore require a variable-delivery type pump or two separate pumps.
Suggested purging volumes range from:
Excessive purging not only wastes time but causes undue disturbance of the environment (both to the aquifer and the surface where the purge water is released). The U.S. Geological Survey recommends pumping the well until temperature, pH and conductivity are constant; i.e. they should be stable over two successive well volumes. Since pH is particularly sensitive to CO2 loss, in-line measurements provide more accurate results than grab samples (Barcelona et al. 1988). The extent of well purging will vary with the hydraulic properties of the aquifer system under investigation and may vary with time as aquifer properties are altered; thus all purging activities should be routinely documented as part of the sampling protocol.
A sampling program is designed to meet specific objectives in light of the hydrologic and geochemical characteristics of the aquifer. Most importantly, a sampling protocol must be written. This is a detailed description of all the procedures to be followed in the collection, handling, packaging, preservation, transportation, storage and documentation of all samples. The sampling protocol must list all equipment and information needed for sampling (Keith 1990):
A properly designed sampling protocol (and strict adherence to it) is the best guarantee of obtaining samples that accurately represent the aquifer. Expensive ‘high technology’ sampling and monitoring equipment are no substitute for a properly trained, dedicated team of sampling personnel who meticulously follow a proven sampling protocol (Barcelona et al. 1988).
“Groundwater is a very complex matrix” (Smith et al. 1988) but despite its variability in quality, it has some unique properties:
Such groundwater cannot be aerobic. Hence a representative groundwater sample usually should not contain particulate matter and should be protected from air (as far as possible) to prevent oxidation. Pressure filtration (which avoids degassing) under a nitrogen blanket through a 0.45 (m polyvinylidene fluoride or polytetrafluoroethylene (Teflon™ medium is recommended (Smith et al. 1988).
Six types of installation commonly used for groundwater monitoring are (Cherry et al. 1983):
It is recommended that all pipes are fitted with an end-cap at the bottom to prevent sediment from filling the pipe under the hydraulic action of the groundwater seeping into the pipe. Similarly, all slotted, perforated or screened sections of pipes (including single hole sampling apertures) should be covered with fibreglass cloth, nylon screen or stainless steel screen; this can be omitted where screened sections are in coarse formations or in a gravel pack.
Water-table standpipes (Standpipes) usually comprise PVC pipes with slots or perforations along the lowest 3 to 6 metres of pipe. Standpipes are useful in the preliminary investigative stages to establish both water table depth and fluctuation; particularly valuable for unexplored sites. Design of the monitoring transects can proceed based on this information. Average or typical water quality can be monitored via standpipes (eg. where production wells are to be established) but special sampling techniques are required to resolve vertical contaminant concentration profiles where thin or stratified plumes are involved. Figure 2 depicts a typical standpipe installation.
Standpipe piezometers (piezometers) usually comprise PVC pipes with slots or perforations along the lowest 0.3 to 0.6 metres of pipe. Piezometers are useful throughout all investigative stages to:
Piezometers may be installed singly or nested together in the same borehole reducing drilling costs and environmental disturbance; Figure 3 depicts such an installation.
Auger-head suction samplers were designed during the investigations into the Borden aquifer in Canada (Cherry et al. 1983) because the standpipes and piezometers in-situ could not resolve the vertical distribution of contaminants. The design criteria for the auger-head sampler were:
The drilling technique obviously could not use drilling fluids thus the hollow-stem auger method was selected. As depicted in Figure 4 the device comprises a porous, cone-shaped brass point which is fixed to the leading end of the auger and is protected by the cutting head. At the desired depth a water sample is drawn (by vacuum) up to the surface via the ‘TYGON™’ tubing; this limits its depth of operation to 8 metres and precludes some types of sampling such as for Volatile Organic Compounds (VOCs) or where outgassing may change the sample chemistry.
Multilevel point samplers – vacuum or suction samplers were developed in 1976 by Pickens et al. (1978) for testing dispersion in non-cohesive sand or gravel aquifers. Figure 5 depicts this type of sampler which comprises a series of single sampling ports positioned at the required level in the casing wall; each port has its own sampling tube. Up to 30 sampling points can normally be accommodated within a well – mainly restricted by the ID of the casing and sampling tube diameter. The use of suction limits this system to 8 metre depth.
Multilevel point samplers – pressure or positive displacement samplers are similar to the suction type sampler except that some form of pump or positive displacement device is provided down-the-well (see Figure 6); a consequence is each sampling port now requires two tubes – one for the sample flow and one to drive the pumping device. Further discussion of these devices follows in the next section – Sampling Devices.
Bundle piezometers as depicted in Figure 7 comprise a number of small diameter piezometers installed around the outside of a central pipe; each piezometer has a small screened length placed at the required depth. These devices are inexpensive, can be assembled on-site and installed through the centre of hollow-stem augers. Suction is normally used for sampling due to the small piezometer diameter; automatically limiting the operational depth to 8 metres. Greater operating depths can be achieved through use of narrow tube bailers or larger size piezometers and submersible pumps.
Sampling devices in common use include:
Suction-lift pumps are usually electrically or engine driven centrifugal pumps and operated at ground (surface) level for convenience and ease of maintenance. These need ‘priming’ i.e. they require a continuous column of water from the sampling level up into the pump body – provision of water for this purpose may contaminate the aquifer being examined. Note, suction-lift pumps employ a strong negative (gauge) pressure that can cause degassing of the sample
Electrical submersible pumps are operated down-the-well at the sampling level; generally self-priming and require connection to an electrical power source. Both portable and fixed installations of submersible pumps are used for sampling; Figure 8 depicts a typical slim format submersible pump.
Positive-displacement pumps (such as bladder pumps or gas-driven samplers) are also operated down-the-well at the sampling level and require a source of compressed gas for operation; typically air or nitrogen. Figures 9 to 11 depict typical gas-lift equipment. It is noteworthy that some gas-displacement pumps can cause gas stripping of both carbon dioxide (which changes the sample pH) and Voces (Kent and Payne 1988).
Bailers, comprise various ‘bucket’ type devices that are lowered into the well, fill with water at the required level and are brought to the surface for sample collection; as depicted in Figures 12 and 13. They are commonly used for small diameter, shallow wells that cannot accommodate a submersible pump and are suitable for both purging and sampling. Bailers are inexpensive, portable, easy to operate and maintain, however their disadvantages include (Kent and Payne 1988):
Multi-level samplers are only limited by human ingenuity; they include the bailers and nested wells (previously covered). The following are some additional specialised techniques in current use.
Packer systems are an active non-permanent technique for sampling wells with long screened sections. The system is lowered to the desired depth, the packers are inflated to hydraulically isolate the sampling pump within the casing and (after purging) a sample is collected (see Figure 14). The packers can then be deflated and repositioned to another level or withdrawn at the end of sampling.
A variation of the packer system is shown in Figure 15. Here multiple sampling ports are created by the required number of packers and a travelling sampler (the sonde) is used to obtain the respective samples. Although removable, this type of system is more of a permanent installation.
Separation pumping is a three-pump technique where the two main pumps (located at the top and bottom) create a water flow divide or stagnation point at a given depth within the well (i.e. a point with no upward or downward flow). The depth of this point is adjustable by varying the pumping rates of the two main pumps (see Figure 16). The precise height of the divide can be determined by a heat-pulse flow detector and the sampling pump is placed at this point. The essential components in this system are the main pumps which must have accurately variable pump rates. The system is portable and only limited by casing diameter (i.e. must be sufficiently large to accommodate the pumps).
Baffle systems use a packer with a penetrating inner tube (baffle) and a main pump above (see Figure 17). The principle is to achieve perfectly radial, horizontal flow into the well around the baffle from which a sample can be collected; sample pumping rates are kept low to avoid disturbing this horizontal flow . Repositioning the packer and baffle enables sampling at different levels. The system is suitable for portable applications.
Multi-port sock samplers are elongated packer elements (socks) of elastomeric material that are inserted into the well and inflated with either gas or water to hydraulically isolate the sampling ports (see Figure 18). Each port normally has its own sampling pump and tubing (all contained within the sock) hence requires a relatively large well (more than 80 mm – Lerner and Teutsch 1995). Sock samplers can be used in both cased wells and uncased boreholes, and may be used as a permanent or portable monitoring system.
Experience has shown that the major source of error in the measurement of groundwater quality is due to the variability in the sampling of groundwater (Smith et al. 1988); this underscores the importance of the sampling protocol and its strict observance. Sampling should always proceed from upgradient wells (which are normally least contaminated and most closely reflect background water quality) down into the catchment or contaminated area; this minimises cross-contamination. Between sampling points, common use equipment needs to be washed using laboratory (i.e. phosphate-free) detergent and distilled (as opposed to de-ionised) water followed by an acid rinse (0.1 N HCl) or solvent rinse (hexane or methanol) followed by a triple rinse in distilled water.
Following the purge cycle, the first samples to be collected should be for the volatile constituents, TOC, TOX, and those requiring field filtration or field measurement. Then the large volume samples for extractable organic compounds, total metals and nutrient anion determinations should be collected, treated and stored for dispatch to the laboratory (Barcelona et al. 1988).
It is important to establish the sample volume to be collected; unfortunately there is no simple guide to determine that sample. A minimum volume can be established from the minimum volume required for chemical analysis and the number of replicates desired; Lerner and Teutsch (1995) suggest one litre as a suitable standard appropriate to most investigations.
Absorption of air emissions into samples may be a problem in the vicinity of certain industries (eg petrochemical or food processing plants). Under these circumstances a number of field blanks should be collected to establish the background absorption in that area. Such blanks consist of distilled water, passed between two glass sample bottles six times at the sampling location immediately prior to the actual sample collection (Kent and Payne 1988).
The greatest risk to sample integrity lies in sorption of target analytes by flexible components such as tubing, pump bladders, gaskets and seals. A summary of recommendations is provided in the Table 1 below; comments on the ‘hard’ materials from the construction materials for wells section are equally applicable here.
|Recommended for most monitoring work, particularly for detailed organic analytical schemes. The material least likely to introduce significant bias or imprecision. The easiest material to clean in order to prevent cross-contamination.|
|Strongly recommended for corrosive high dissolved solids solutions. Less likely to introduce significant bias into analytical results than polymer formulations (PVC) or other flexible materials with the exception of Teflon™|
|PVC (flexible)||Not recommended for detailed organic analytical schemes. Plasticisers and stabilisers make up a sizeable percentage of the material by weight as long as it remains flexible. Documented interferences are likely with several priority pollutant classes.|
(medical grade only)
|Flexible elastomeric materials for gaskets, O-rings, bladder, and tubing applications. Performance expected to be a function of exposure type and the order of chemical resistance is as shown. Recommended only where a more suitable material is not available for the specific use. Actual controlled exposure trials may be useful in assessing the potential for analytical bias.|
To prevent samples changing physically, chemically or biologically during transport and storage they are generally refrigerated or preserved by the addition of acid or alkaline solutions. Common problems are (Kent and Payne 1988):
Sample analysis is a critical part of groundwater monitoring – it is expensive and thus needs to be minimised, however it must be:
QA is as much part of the monitoring process as any technology or technique. Since failure at any stage of groundwater monitoring activities can impair the effectiveness of the program, it is imperative that an overall process is in-place to ensure every task is executed correctly. This produces reliability of results for users and avoids loss of (sometimes unrecoverable) data – maximising the cost-effectiveness of the program.
A crucial decision that must be made at the planning stage and documented into the sampling protocol, is the number and type of quality control samples or standards (controls) to be taken. These will be determined by the nature of the errors to be assessed (both random and systematic) and the accuracy desired in their assessment (Keith 1990).
There are basically two types of controls which are used to determine whether:
Controls for the analytical procedure comprise check standards and laboratory controls.
Controls for specific analytes comprise field spike samples and background (control site) samples.
This paper reflects on the spectrum of technologies encompassed in groundwater monitoring. A monitoring program not only involves different disciplines and technologies, but also a wide range of personnel from field assistants to research scientists. In order to achieve the goals of groundwater monitoring and ultimately fulfil its role in the catchment management process, the monitoring program itself requires careful management; Quality Assurance (QA) offers a soundly structured approach to achieve this. QA forces precise definition of objectives from the outset enabling levels of accuracy and precision for every phase of the program to be determined with corresponding selection of the most appropriate technology. This type of approach ensures that the program will be cost-effective and (most importantly) that the results obtained will best serve the needs of the end-user. Proper catchment management can only be achieved if the groundwater monitoring program provides reliable results from which decisions can be confidently made.
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Advantages and disadvantages of Selected Drilling Methods for Monitoring Well Construction (modified, after Barcelona et al. 1988 and Keely and Boateng 1987)
A hand operated implement with a hollow, shaped cutting head (usually between 50 and 100mm in diameter) with some 1.5 metres of shaft which can be extended by (detaching the handle) in 1.5 metre increments.
A manually (or mechanically) installed well; a steel pipe (typically 30 to 50 mm ID) with a suitable driving point is driven into the ground to the desired depth. The sampling system is installed within the steel pipe, the latter is withdrawn leaving the sediment to close-in around the sampling tube/s.
Successive 1.5 metre lengths of spiral-shaped drill stem are rotated into the ground to create the borehole; cuttings are brought to the surface by the turning action of the auger.
Washing action of water forced out of the bottom of the drill rod clears the borehole to allow penetration; cuttings are brought tot he surface by water flowing up the outside of the drill rod.
The borehole is created by dropping a heavy ‘string’ of drill tools down the hole, crushing/fracturing the materials at the bottom. Cuttings are removed occasionally by bailer. Generally, casing is driven just ahead of the bottom of the hole; hole diameter usually exceeds 150 mm.
Rotating bit breaks formation; cuttings are brought to the surface by a circulating fluid (drilling ‘mud’ – often a bentonite slurry). Mud is forced down the interior of the drill stem, out the bit, and back up the annulus between the drill stem and hole wall. Cuttings are removed by settling in a ‘mud pit’ at the surface and mud is recirculated back down the drill stem.
Similar to hydraulic rotary except that the drilling fluid is circulated down the borehole outside the drill stem and is pumped up the inside, just the reverse of the normal rotary method. Water is used as the drilling fluid, rather than mud, and the hole is kept open by the hydrostatic pressure of the water standing in the borehole.
Very similar to hydraulic rotary, the main difference being that air is used as the primary drilling fluid as opposed to mud or water. In uncohesive, unconsolidated formations, a temporary casing is often driven as the borehole is drilled to minimise problems with cave-ins.
Air rotary with reciprocating hammer connected to the bit to fracture rock.