26.4 Groundwater sampling
COMMENTARY ON 26.4
Groundwater samples are generally recovered and tested for the following reasons:
- construction related, typically as part of the assessment of the aggressiveness for buried concrete or corrosive action on metals; or
- geoenvironmental for investigating the presence of groundwater contamination.
Water samples collected from trial pits and boreholes at the time of their formation are unlikely to provide as reliable a representation of groundwater quality as those from permanent installations. However, such samples can provide some preliminary information which assists in the design of a subsequent groundwater monitoring programme.
Groundwater sampling should be carried out in accordance with BS EN ISO 22475-1 or BS ISO 5667-11. Samples should be collected using a method appropriate to the data quality objectives, depth to groundwater, well performance, sample requirements and the parameters to be determined.
NOTE 1 Sampling of water from discrete depths within a borehole might be required. This can be achieved by the use of packer seals to isolate the required sections, the use of samplers capable of taking samples from predetermined depths or the installation of multiport instruments from which to sample. Common methods for taking samples include (disposable) bailers, passive sample collectors, inertia pumps, submersible pumps and low-flow techniques including peristaltic and bladder pumps. The most appropriate technique is a site-specific decision. Where a permanent sampling pump is installed, samples of groundwater can be readily collected at intervals over a period of time, for example, to identify gradual changes in groundwater quality.
Samples of groundwater should be analysed for pH, dissolved oxygen, temperature and conductivity in the field. Other parameters, for example nitrite, may also be determined in the field. The advice of the analytical laboratory should be obtained, and the laboratory should be informed of any field results.
Water level and depth of well measurements should be taken after sampling in order to avoid disturbance of the water column. The datum from which groundwater levels and borehole depths are measured should be recorded (e.g. ground level, top of the casing).
Where it is necessary to obtain samples of pore water in the unsaturated zone, special equipment, such as a piezometer with a ceramic or plastic tip, should be installed. Care should be taken to avoid the installation penetrating the saturated zone. Alternatively, a large undisturbed soil sample may be collected and the pore water removed by filtration, or by using a diaphragm or centrifuge.
Where practicable, groundwater should be characterized using data from a series of sampling operations. For example, a number of samples should be taken over a relatively short period, and then less frequently over a longer period. The periods between sampling events should be dependent on the findings of the earlier sampling operations. In addition, the sampling frequency should be based on the temporal and spatial variations in the quality of the groundwater and its flow.
NOTE 2 Changes in the quality of groundwater are usually much more gradual in time and space than those in surface waters. In some aquifers, factors producing seasonal variations in quality could exist. Groundwater levels vary during the year in response to a variety of factors, including changing weather conditions and plant transpiration rates. Obtaining a full picture of annual variations requires monitoring over 12 months and could require several years. Groundwater levels can also fluctuate during the day due to tidal influences. The timing of sampling can be adapted to take into consideration known or expected fluctuations in groundwater levels (for example, due to tidal influence), flow directions, etc. Continuous monitoring of pH, temperature and electrical conductivity can provide a useful means of monitoring the need to increase or decrease the sampling frequency. In cases where there has been a considerable change in any of the parameters, it is advisable to consider extending the range of parameters being monitored.
26.4.2 Sampling during excavation and drilling
Water samples should normally be obtained as excavations are dug or boreholes are drilled. They can be used for screening for the presence of groundwater contamination and to establish whether it is necessary to install monitoring wells. However, caution should be applied when considering the analytical data from such samples, since the ground disturbance caused by digging can affect the composition of the water sample.
Care should be taken to ensure that any groundwater samples that are taken from boreholes during drilling are representative of the water-bearing stratum and have not been altered by water entering the borehole from other strata, or by contact with any water or drilling fluid used to advance the borehole. The depth and method of sampling, as well as the subsequent storage and handling of samples, can influence the results of analyses undertaken on groundwater samples.
When groundwater samples are to be taken from a stratum that has been in contact with drilling fluids or tools, all water-bearing strata from higher levels should first be sealed off by borehole casings. As far as possible, all the water in the borehole should be removed by bailing or pumping and the sample taken from water that collects by seepage.
NOTE The water can contain a substantial amount of suspended particles that require field filtration or settlement before analysis. To overcome this, a larger volume of water may be taken to compensate for the volume of material removed by settlement or filtration.
26.4.3 Sampling from installations
Some investigations need the use of permanent monitoring wells from which groundwater samples can be taken at various times; in these circumstances, the monitoring wells should be constructed, installed and developed in accordance with Clause 52.
One of the most important aspects of sampling is to ensure that the sample is as representative as possible of the in-situ conditions. Purging should immediately precede any sampling, to remove stagnant water; water within a monitoring well that has not been recently purged is not always representative of water in the surrounding strata for a variety of reasons, including oxidation and loss of volatiles.
The impact of purging should be assessed alongside the benefits of improved sample integrity taking into account the purpose of the sampling exercise, data quality objectives and well recharge/performance when deciding on the most appropriate purging and sampling technique.
NOTE 1 There might be circumstances (e.g. when volatile contaminants are present) when purging is not appropriate. In such cases, micro-purging (see Note 4) may be considered. In addition, or alternatively, samples of pre- and post-purge water may be collected during the early stages of an investigation to compare results. This information can then be used to optimize subsequent sampling.
Purging should be undertaken at a flow rate less than that used for development of the well and greater than that proposed for sampling. The volume of water to be purged should vary according to the monitoring well type, its construction and the hydrogeological conditions. The purge volume is dependent on the design of the monitoring point, e.g. the diameter and depth of the water column. The water level should, therefore, always be measured prior to purging. Additionally, measurement of the water level during purging can give an indication of the drawdown.
To ensure that purging has been effective, monitoring of chemical parameters, such as electrical conductivity (EC), pH, temperature, redox potential (Eh), dissolved oxygen (DO) turbidity and specific parameters, should be carried out during the purging operation. A flow cell may be used for measurement of these parameters. As a minimum, EC should be measured. When using micro-purging techniques, purging should continue until successive readings of conductivity, pH and temperature have stabilized.
NOTE 2 Guidance on groundwater purging volumes and strategies is provided in BS ISO 5667-11.
NOTE 3 If the well purges dry prior to sufficient water being removed, it might need to be allowed to recharge before later sampling.
The method of disposal of the purged water should be selected carefully as the purged water might not be suitable for re-introduction into the borehole or to be allowed to drain to ground or the local drainage system. The use of low-flow purging or micro-purging may be used to reduce disposal volumes.
NOTE 4 Low-flow purging or micro-purging is where the water column above the pump intake is not disturbed and water is drawn locally at a very low flow rate. Purging by this means may be carried out using a non-displacement pump (such as a bladder pump) at a flow rate that minimizes drawdown to the system. Typical flow rates at the pump intake for both low-flow purging and sampling are in the order of 0,1 L/min to 0,5 L/min, depending upon the site-specific hydrogeology.
Micro-purging should be carried out using dedicated pumps, as passing a pump through the water column causes mixing and disturbance. Bailers, grab samplers and inertial pumps should not be used for micro-purging.
26.5 Sample containers, storage and transport
Any sample container used should be "inert", i.e. should not cause contamination of the sample, should not absorb any sample components (for example, organic compounds) and should not allow losses of volatile components. Except where there are other recommendations (see Note 1 and BS 10175), about 1 L of water should be collected in a clean PET (polyethylene terephthalate), polyethylene, polypropylene or glass bottle, which should be rinsed three times with the water being sampled before filling. Containers should be completely filled with the water so as to minimize contact with the air.
The samples should be transported and stored in the dark, at 4 °C to 6 °C and tested as soon as possible after sampling to minimize any potential for chemical and biological changes before examination, and in any case within 24 hours for time-dependent analytes such as COD and BOD.
NOTE 1 More stringent requirements might apply in certain circumstances, particularly when accurate or extensive chemical testing is to be undertaken in order to investigate possible chemical contamination. Additional requirements could include special sampling techniques, multiple samples in different sample containers with different fixing agents, duplicate sampling, and special sample handling procedures.
NOTE 2 More information on appropriate sampling procedures, preservatives and containers for waters is given in BS ISO 5667-1 and BS ISO 5667-3.