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3.4.8 Liquid samples

Contents
3.4.8.1 Sampling locations of surface water and groundwater
3.4.8.2 Sampling of groundwater
3.4.8.3 Sampling of non-aqueous-phase liquids
3.4.8.4 Installation of permanent liquid monitoring points
3.4.8.5 Sampling of sediments
3.4.8.6 Liquid and sediment field sample preparation and preservation
3.4.8.7 Analyses of liquid samples

Characterizing surface or groundwater involves techniques that determine the extent and distribution of contaminants [ISO-2008]. This may be performed by collecting grab samples of the surface or groundwater in a well-mixed zone. At certain sites, it may be necessary to collect stratified water samples to provide information on the vertical distribution of contamination. Sediment sampling should also be performed to assess the relationship between the composition of the suspended sediment and the bedload sediment fractions (i.e., suspended sediments compared to deposited sediments). When judgment sampling is used to find radio-nuclides in sediments, contaminated sediments are more likely to be accumulated on fine-grained deposits found in low-energy environments (e.g., deposited silt on inner curves of streams).
The key considerations for a groundwater characterisation programme are:

  • By identifying of a groundwater contamination the responsible regulatory agency should be contacted, because:
    • Groundwater release criteria and DCGLs should be established by the appropriate regulatory.
    • The default DCGLs for soil may be inappropriate since they are usually based on initially uncontaminated groundwater.
  • Groundwater contamination characterisation programmes should determine the extent and distribution of contaminants, rates and direction of groundwater migration, and the assessment of potential effects of groundwater withdrawal on the migration of groundwater contaminants.
  • Boreholes should be located to provide information on water level and water quality:
    • Up-gradient of any potential sources.
    • In or close to potential source areas.
    • On the down-gradient boundary of the site.
    • As sentinel boreholes and at compliance point.
  • If significant groundwater contamination is detected, further boreholes may be required to define the plume of contaminated water.
  • Hydrogeological testing should be performed to determine the permeability of the rocks/soil and to establish the hydraulic gradients within and across the site.
  • Boreholes should not be completed as long-term monitoring points until the geological and hydro-geological environment is fully understood. In particular:
    • Coordinates of the boreholes/sampling locations should be noted to grid coordinates.
    • The key horizons for contaminant transport should be identified and targeted.
    • Monitoring boreholes should be designed to minimise or prevent vertical flows (‘crossflows’) through the screen and open section.
    • Construction specifications on the monitoring wells should also be provided, including elevation, internal and external dimensions, types of casings, type of screen and its location, borehole diameter, and other necessary information on the wells.
    • The requirements for monitoring and sampling non-aqueous-phase liquids (NAPLs) should be considered.
    • Well construction materials should be compatible with the types and concentrations of contaminants present.
  • Targeted sampling of groundwater is appropriate where the groundwater pathway can be identified with reasonable confidence, i.e., where contaminant sources and groundwater flow directions are known. In this manner, the contaminant plume(s) can be delineated and groundwater quality leaving the site can be monitored.
  • Non-targeted sampling may be appropriate to the earliest stage of an investigation, if there is no information on potential sources of contamination or on the hydro-geological environment.
  • Groundwater background characterisation should be determined by sufficient sampling and analysis of groundwater samples collected from the same aquifer up-gradient of the site. The background samples should not be affected by site operations and should be representative of the quality of the groundwater that would exist if the site had not been contaminated. Consideration should be given to any spatial or temporal variations in the background radionuclide concentrations.

Following completion of the hydro-geological characterisation, long-term monitoring of groundwaters and/or surface waters may be required to:

  • Evaluate environmental liabilities and their development with time.
  • Ensure compliance with regulatory limits (e.g., requisite monitoring: see Section 2.2.2.4).
  • Validate in situ remediation measures (including “natural attenuation”).

In some instances, the requirement for long term monitoring will be established at the start of the site characterisation programme. In other instances, the requirement will only become evident after completion of the site works and evaluation of site data. Where the requirement for long term monitoring is established at the outset of the investigation, the survey design should take account of this.

If long term monitoring is to be undertaken, it is good practice to define and document clearly the objectives of the monitoring before the programme starts. Further, the data from the programme should be subject to regular quality checks and technical assessment, and there should be regular review of the need for continued monitoring. These procedures will ensure that inappropriate data are not collected and that the monitoring programme does not continue beyond the period when it was required.

Good practice procedures for the collection of representative groundwater samples are available and are discussed further in the next alineas. Water abstracted from the boreholes during development and sampling must be managed in accordance with the operating procedures of the site and with national legislation. It may be necessary to treat water prior to disposal onto the ground surface (for example, using activated carbon to remove organic contaminants) or to transport the waste water to a liquid effluent treatment plant (for example, to remove radioactive contamination). Finally, a borehole maintenance programme should be established to ensure that the groundwater sampling points remain fit for purpose.

3.4.8.1 Sampling locations of surface water and groundwater

It is possible that contamination of surface waters and groundwaters may have arisen as a result of operations and activities on the site (see Table 3.1). Consideration should therefore be given to sampling surface waters and groundwaters and to building understanding of the hydrological and hydro-geological environments. Therefore, the sampling of surface and groundwater should be performed in areas of run-off from active operations, at plant outfall locations, both upstream and downstream of the outfall, and any other areas likely to contain residual activity.

The locations of the surface water and groundwater sampling points should take account of factors affecting the temporal and spatial variation in water quality and flows, including:

  • The locations and extents of known or suspected sources of contamination.
  • Surface water and groundwater catchments.
  • Tidal patterns.
  • Seasonal or ephemeral variation in surface water flow.
  • The local and regional groundwater flow pattern at the site (including identification of both horizontal and vertical hydraulic gradients).
  • The hydro-geological properties of the rocks and soils (which, together with information on hydraulic gradients, enables groundwater flow directions and velocities to be estimated).
  • Background water quality.

3.4.8.2 Sampling of groundwater

Groundwater sampling methodologies are described in detail in a number of other guidance documents [IAEA-1999a], [EA-2006], [EPA-1997a]. An outline of the methodology is given below.
Groundwater samples are generally collected by one of two methods:

  • Pump sampling.
  • Bail sampling.

The method used will depend on the feature from which the groundwater sample is being obtained (completed borehole, temporary cased borehole or trial pit) and on issues such as the amount of suspended sediment present and the permeability of the surrounding material. Usual practice is for trial pits to be bail-sampled and for boreholes to be pump-sampled.
Pump sampling is the preferred method of sampling from a borehole because a large volume of water can be withdrawn prior to collecting the sample, ensuring that the sample is representative of the groundwater in the rock mass rather than that in the borehole. It is best practice to withdraw three borehole volumes of groundwater prior to collecting samples, or to carry out in-line monitoring (for electrical conductivity, pH etc) and to sample after measurements have stabilised.

When pump-sampling a borehole on a nuclear-licensed or defense site, adequate provision should be made for disposal of the waste-water generated.

Direct disposal of radioactively contaminated water to ground, or by a surface water body, will not be possible. Similarly, disposal of chemically contaminated water to ground or by a surface water body would require authorisation. Therefore pumping to bowser or to storage containers (drums or IBCs) for disposal via an approved route is recommended.

Use of low-flow pumps which are carefully located in well characterised and designed boreholes can limit the amount of liquid waste generated. These systems are designed not to pump out three borehole volumes, but to directly draw into the borehole, the aquifer water from a flowing horizon. The discharges of the pumps should be monitored for physico-chemical (temperature, conductivity and reduction potential), and samples should only be taken once these parameters have stabilised and indicate aquifer representative water is being taken. Therefore even these pumps will generate some liquid waste.

Radiological monitoring using standard field instruments will typically not detect contamination in water samples, because the radionuclides are typically present at much lower activity concentrations than in soil and may only emit “soft” beta or alpha radiation. Laboratory analysis of groundwater samples for radioactivity is generally required. For example, this is the case for tritium, a “soft” beta emitter, which is a common radioactive contaminant found, as tritiated water, in groundwater in the vicinity of some nuclear-licensed and defense sites, see Section 3.9.2.5. Tritiated water is highly mobile in soils and groundwater. Naturally occurring dissolved radon/radon daughters are also likely to be present.

The selection of suitable sample containers and preservation techniques (typically involving refrigeration or the addition of acid or alkali to prevent precipitation or degradation of the sample) is discussed in existing guidance [EA-2003], [EA-232] and is not considered in detail here.

Exact requirements should be discussed with the analysts, and these may change depending on the method of analysis used and the limit of detection required. All groundwater samples should be filtered (typically to 0.45 µm) in the field prior to addition of the preservative. It is good practice (i) to refrigerate groundwater samples to approximately 4°C after collection and prior to analysis, (ii) to store samples in the dark and (iii) to minimise sample storage time. This is particularly important for analysis of organic compounds, which may otherwise degrade during storage. In practice, refrigeration of large samples (around 5 litres) for radionuclide analysis is impracticable and is not necessary. An illustrative groundwater sample storage and preservation scheme is shown in Table 3.30.

Determinand Container Preservation
All radionuclides except tritium 5 litre HDPE 50 ml HNO3
Tritium 0.5 litre glass None
Metals 1 litre HDPE Hardness, HNO3
Cyanide 0.1 litre HDPE NaOH
Major ions and anions 0.25 litre HDPE None
Non-volatile and semi-volatile organics 1 litre amber glass bottle None
Volatile organics Glass serum vials (sealed with PTFE-faced rubber septum)
None

Table 3.30 Illustrative scheme for storage and preservation of water samples

3.4.8.3 Sampling of non-aqueous-phase liquids

Non-aqueous-phase liquids (NAPL) divide into two types, light NAPL (LNAPL) or dense NAPL (DNAPL). These types are less dense and more dense than water respectively and hence will either sink through or float on the groundwater.

Sampling dense NAPL’s is extremely difficult, primarily because the probability of intersecting a pool of dense NAPL in the base of an aquifer, and having the dense NAPL flow into the borehole, is low. Dense NAPL is usually inferred to be present in an aquifer by, for example, high or increasing dissolved concentrations with depth, or from records of known disposals. The sampling of DNAPL will not be discussed further here. Further information on dense NAPL’s is provided elsewhere (for example, [Burton-1994], [Fetter], [DOE-1990], [EPA-1974].

The sampling of LNAPL may be carried out in a number of ways, provided that the borehole is of suitable design (the screen section of the monitoring point should extend from just above to below the zone of water table fluctuation). The most common and simplest method of sampling is to bail a sample from the surface of the groundwater. The LNAPL sample should be collected before any groundwater purging, and should be carried out in such a way as not to emulsify the free product.

The thickness of LNAPL in the borehole can be determined using an interface probe, although it should be noted that this will probably not reflect the thickness in the aquifer, because of capillary pressure effects.

3.4.8.4 Installation of permanent liquid monitoring points

All of the borehole drilling methods (see Section 3.7.2) may be used for the installation of groundwater or gas monitoring points. The key issues to consider when selecting the drilling technique are:

  • Achieving the project monitoring objectives.
  • Confidence that the drilling technique can achieve the required depth of penetration at the required borehole diameter.
  • Health and safety issues, such as the potential generation of airborne contamination during drilling (for example if air-flush rotary drilling is the selected technique).
  • Any limitations on the use of a flushing medium (e.g., air, foam, water), which may compromise sample quality.
  • Environmental issues, such as spreading of contamination in the ground and control of drilling returns.
  • Speed and cost.

Trial pits may also be used for the installation of shallow monitoring points, by carefully backfilling around the monitoring equipment. However, it should be noted that a large volume of soil would be disturbed and this may affect the results obtained during monitoring.

Details of the design, construction, installation and commissioning of permanent groundwater and gas monitoring points are beyond the scope of this guidance document. Readers should refer to the extensive guidance already available on the subject [Burton-1994], [Digby], [EA-2003], [EA-2006].

3.4.8.5 Sampling of sediments

Information available in literature about this topic will be included in a later edition of EURSSEM.

3.4.8.6 Liquid and sediment field sample preparation and preservation

Liquid samples may need filtering and acidification. Storage at reduced temperatures (i.e., cooling or freezing) to reduce biological activity may be necessary for some samples. Addition of chemical preservatives for specific radionuclides or media may also be required.

Sediment samples, in most protocols, require no field preparation and are not preserved. In some protocols, cooling of soil samples to 4°C is required during shipping and storage of soil samples. This is not a practice normally followed for the radiochemical analysis of soil samples.

When replicate samples are prepared in the field, it is necessary to homogenize the sample prior to separation into replicates. There are standard procedures for homogenizing liquids in the laboratory, but the equipment required for these procedures may not be available in the field. It is preferable to use non-blind replicates where the same laboratory prepares and analyzes the replicates rather than use poorly homogenized or heterogeneous samples to prepare replicate samples.

3.4.8.7 Analyses of liquid samples

The analyses of radio-nuclide concentrations in liquids (e.g., surface and groundwater with or without organic or inorganic constituents, and non-aqueous-phase liquids (NAPL)), should include gross alpha and gross beta assessments, as well as any necessary radio-nuclide-specific analyses. Non-radiological parameters, such as specific conductance, pH, and total organic carbon may be used as surrogate indicators of potential contamination, provided that a specific relationship exists between the radio-nuclide concentration and the level of the indicator (e.g., a linear relationship between pH and the radio-nuclide concentration in water is found to exist, then the pH may be measured such that the radionuclide concentration can be calculated based on the known relationship rather than performing an expensive nuclide-specific analysis).