Article provided by Andrew Tranter, Associate Technical Director at RSK

Turbidity is defined as the measure of the relative clarity of a liquid.  It is caused by the presence of organic and inorganic particulates from local sediments/rocks as well as microbial organisms that have been picked up/transported within the groundwater (Ref 1).

Turbidity is routinely monitored at groundwater abstraction wells (as required by the Drinking Water Inspectorate) for potable water supplies as an indicator for microbial organisms, particularly cryptosporidium. The shutdown of the public water supply at Brixham in May 2024 due cryptosporidium illustrates how severely water companies can be impacted by cryptosporidium.   Also, high turbidity levels may negatively impact on the water treatment process.  However, turbidity measurements cannot distinguish between microbial or mineral particulates, and therefore activities that could lead to an increase of mineral particles within the groundwater, such as piling, are undesirable.

In the first instance, a piling risk assessment should be undertaken in line with the Environment Agency’s updated guidance (Ref 2) (which has just been published) if piling operations have the potential to impact on a groundwater abstraction well (e.g. the site is located within Zone 1 of a Source Protection Zone (SPZ)).  A number of measures may be considered to mitigate risk, for example using a different piling technique or altering pile depth.  However, the Environment Agency will often request that groundwater monitoring is carried out when the site is located within Zone 1 of a SPZ, including the measurement of turbidity to confirm that the pilling operations have not adversely impacted the aquifer and abstraction well.

The AGS previously published ‘Assessment and Mitigation of Turbidity Risks from Piling’, dated July 2023 which focussed more on turbidity risk assessment but should be read in conjunction with this article.  There is currently no UK guidance on how turbidity should be monitored during piling operations, and therefore a discussion on the various instruments available, and important considerations in relation to the sampling methodology, is provided below.

Instruments for Monitoring Turbidity

Turbidity is measured using instruments that detect the amount of light scattered by particulates. High levels of scattered light correspond with higher levels of particulates and turbidity.  A range of light sources can be used depending on the technique, such as natural light (i.e. Secchi discs used for surface water measurements); tungsten lamps (i.e. used in bench top meters) and light emitting diodes (LED), which are often utilised within modern instruments used for groundwater sampling.  The results can be expressed in different units which are dependent on the technique, the most common are nephelometric turbidity units (NTU) or formazine nephelometric units (FNU), which are considered to be equivalent (Ref 3).

Instruments used for measuring turbidity within groundwater can be split into three broad categories: laboratory bench top meters, field portable meters, and dedicated probes attached to water quality devices.  A comparison of advantages and disadvantages of each technique are provided within Table 1, below.

Table 1: Comparison of different techniques.

Whilst undertaking sampling during piling operations the use of a water quality meter is considered to be the most suitable technique as it provides an immediate result of turbidity that allows rapid assessment of the condition of the aquifer.  A water quality meter can also measure other useful water quality parameters such as dissolved oxygen, electrical conductivity, redox, and pH.  Some water quality meters can also be linked to telemetry for continuous monitoring.

However, before selecting a water quality meter the manufacturer’s specification should be checked to ensure that the turbidity probe is not affected by ambient light or drift from temperature differences when using a flow cell during sampling. On the most sensitive sites, field readings should be cross-checked against laboratory data to confirm suitability of the method.

Borehole Construction

To ensure that the sampling is representative of the actual conditions within the aquifer, it is critical that careful consideration is given to the design and construction of the monitoring wells. Design objectives should be clearly stated as part of the piling risk assessment for the site, which are based upon the development of a conceptual site model (CSM) of realistic contaminant source-pathway-receptor linkages.

At least one well should be positioned hydraulically up-gradient, and two down-gradient of the site. If pragmatic within the site constraints, a stand-off from the monitoring well and the area subject to piling is beneficial to reduce the potential for the monitoring well to pick up localised disturbance of the soils during piling operations.

Furthermore, keeping a record of the position of the piling rig during piling operations is also useful to aid the interpretation of results.

The slotted section of the monitoring wells (i.e. response zone) should target the water body/depth of interest, including the full pile depth within the water body as defined by the CSM.  To prevent borehole installations from being clogged by fine sediments, a granular annulus is placed around the response zone. For fine sediments of less than 2mm it be also be necessary to wrap a geotextile membrane around the pipe.  However, the membrane should have a pore size of not less than 85 microns so that the target particles can pass through it (majority of suspended particles are <10 microns, and particles >100 microns are unlikely to stay suspended in groundwater (Ref 6). If the strata is fractured rock then geotextile membrane should not be used.  A bentonite seal is required above and below the response zone to prevent mixing from other strata/bodies of water.

Prior to sampling, the monitoring well will need to be thoroughly ‘developed’ in accordance with BS ISO 5667-11 (Ref 5) to remove any drilling fluids/sediment, and allow the surrounding granular filter to settle.  Once ‘developed’ the monitoring well should be left for the conditions to return to equilibrium with the surrounding groundwater, which can take several weeks depending on the surrounding stratum.

Following development and prior to sampling, monitoring wells require purging to remove any stagnant water and ensure that the sample is representative of the aquifer.  The quantity of water purged is dependent on the well construction and hydrogeological conditions.

Sampling method

A number of different techniques have been developed to collect groundwater samples.  Those that require the removal of significant amounts of water during purging and/or agitate the groundwater are not ideally suited for providing a rapid assessment of turbidity as they are likely to cause a temporary increase in the amount of sediment within the monitoring well, e.g. using bailers or HDPE pipe with foot valves.

The low flow/micro-purging technique, as outlined within BS ISO 5667-11, is considered to be the most suitable as it minimises the amount of disturbance to the groundwater, and can target specific depths of interest as defined by the CSM.  The technique involves removing a small volume of water at low flow rate to cause minimal disturbance to the aquifer. The tubing inlet should be placed within the response zone of the well. The monitoring equipment should be kept clean and calibrated in line with the manufacturer’s standards.

The number and frequency of monitoring rounds should be agreed in advance with the Environment Agency/Local Water Authority, and split into three phases: baseline (to characterise the initial condition of the groundwater); during piling (to assess any impact during piling operations); post-piling (to confirm there has been no longer term impact on the aquifer).  It is essential that the condition of the aquifer is well characterised prior to piling operations, and takes into account any seasonal fluctuations in the groundwater levels that may affect turbidity.  Therefore, more than one visit (often multiple visits) will be required to confirm the baseline turbidity concentrations. The frequency of monitoring during the operational phase will be determined by the risk assessment, in lower risk settings a daily reading may be sufficient, whereas in fractured rock close to the abstraction well then real-time monitoring may be required.

Conclusions

In summary, turbidity is an important water quality indicator used by water authorities to determine suitability of groundwater during abstraction.  Monitoring of turbidity is therefore typically required by the Environment Agency where a pilling risk assessment indicates there is a potential risk to the abstraction well (usually when the site is located within Zone 1 of a SPZ).  Turbidity results can be impacted by a wide range of factors, and therefore the following must be considered to ensure that they are reflective of the actual conditions with the aquifer.

• Meters that can provide rapid on-site testing, and are rugged enough to survive the harsh conditions of a construction site, are preferable to ensure that any issues can be highlighted and acted upon in a timely manner. These need to be kept clean and calibrated in line with manufacture recommendations.
• It is critical that boreholes are carefully constructed and ‘developed’ to minimise the amount of turbidity in the groundwater caused by the disturbance of the surrounding soils during drilling/sampling so that it is not attributed to the piling operations. The borehole should then be left to settle before monitoring starts, ideally for several weeks.
• At least one well should be located hydraulically up-gradient and at least two down-gradient of the site so that the impact on the aquifer can be determined during piling operations. If practical (which is often not the case) a stand-off between the location of the monitoring wells and area subject to piling is beneficial to reduce any localised impact from piling on the well.
• Baseline monitoring should be carried out prior to piling operations in order to characterise the turbidity concentration within the aquifer. To provide confidence in the results and assess any possible variation (e.g. seasonal), often multiple visits will be required.
• The low flow/micro purging technique is considered to be the most suitable method for monitoring turbidity as it minimises disturbance to the aquifer. However, other techniques can be used with appropriate justification.

Ref 1 US Geological Society www.usgs.gov/labs/national-water-quality-laboratory/science/science-topics/turbidity.

Ref 2 CLAIRE, 2025, Piling and Penetrative Ground Improvement Methods on Land Affected by Contamination: Guidance on Pollution Prevention.

Ref 3  Word Health Organisation (WHO), 2017, Water Quality and Health Review of Turbidity: Information for regulators and water supplier.

Ref 4 British Standard, first published 2015, Code of Practice for Ground Investigations, ref BS5930+A1:2020

Ref 5 International Standards Organisation (ISO), 2009, Water Quality- Sampling Part 11: Guidance on Sampling of Groundwaters, ref ISO 5667-11:2009.

Ref 6 Burris et al, 2020.  Tunnelling, Chalk and turbidity: conceptual model of risk to groundwater public water supplies.  P. Burris, C. D. Speed, A. E. Saich, S. Hughes, S. Cole and M. Banks.  Quarterly Journal of Engineering Geology and Hydrogeology