The search by SKB (Swedish Nuclear Fuel and Waste Management Co.) for a site to locate the deep geological repository for spent nuclear fuel in Sweden has involved geoscientific investigations at several locations since the 1970s. The objectives were to characterise geologically a bedrock volume as well as its hydrogeology and hydrochemistry. To acquire high-quality hydrogeochemical data, a complete system for groundwater sampling and analysis, as well as for interpretation strategies, has been developed through a continuous process of modification and refinement. Since the largest part of the Swedish bedrock is composed of granitoids, the site investigations had to adapt to the special difficulties of fractured crystalline rocks. This paper discusses the problems with groundwater sampling that are specific to fractured crystalline rocks and describes the solutions adopted and methods developed by SKB since the early 2000s during the site investigations. The methodology described in this paper for the characterisation of deep groundwaters in crystalline rocks is not only applicable in the context of radioactive waste disposal but also useful when sampling groundwaters for any purpose in such rocks. Sampling of groundwaters in fractured rocks at depth, often down to approximately 1,000 m, involves special challenges since the natural conditions of the groundwater are easily disturbed, especially by the initial drilling, but also by every subsequent activity performed in the borehole, including the actual groundwater sampling. The sampling strategy presented in this paper shows that planning of the sampling preferably starts already when the drilling procedure is decided. Each following step is described in detail and includes tracing the drilling fluid, selecting the best borehole sections to sample, procedures for the actual sampling, and selection of analytical protocol; all this with the goal of taking representative samples. Although the evaluation of the sampling uncertainties is not a straightforward procedure, an adequate categorisation routine has been established to classify groundwater samples regarding sample quality, representativeness, and suitability for further interpretations and modelling.
The need for reliable groundwater sampling procedures has been recognised for years. The United States Geological Survey was one of the first institutions to publish rigorous sampling and analytical protocols and procedures [
In crystalline rocks, where groundwater flow is dependent on the fracture systems, the heterogeneous hydrogeological conditions imposed by the fractures and fracture zones result in a very inhomogeneous distribution of groundwater compositions. Therefore, in these environments, in addition to the usual groundwater sampling problems, it is necessary to monitor simultaneously the hydraulic pressure during sampling, in order to establish unambiguously that the groundwater sample represents the fractures intersecting the sealed-off borehole section [
In Sweden and Finland, deep geological disposal of spent nuclear fuel is being planned in fractured rock at approximately 400 to 500 m depth [
One must emphasise that the techniques described herewith are the result of large efforts during a relatively long period of time (since the middle of the 1970s) by several organisations dedicated to the disposal of radioactive wastes around the globe. It is not the purpose of this paper to provide a detailed historical account of the developments in the field of groundwater characterisation in fractured rocks; however, the interested reader may find a short account in the Supplementary Material
Investigations in deep groundwater systems imply borehole drilling followed by logging and sampling activities which, in fractured crystalline rocks, normally result in the mixing of groundwaters from different depths. As a consequence, this can cause a variety of physical processes and chemical reactions that impact the representativity of the water samples.
To avoid disturbances in the system and to obtain as much useful information as possible, drilling and borehole investigations have to be carefully planned to follow a systematic sequence of proven strategies. A close cooperation among different disciplines is also needed during the planning and execution of the field work (and subsequent interpretations). A general investigation sequence is shown in Figure
Schematic overview of a general investigation sequence in a telescopic core-drilled borehole designed for chemical characterisation. The most important hydrochemical sampling methods (Complete Chemical Characterisation, i.e., CCC, and monitoring) are shown in boxes with a darker blue colour and text in bold. GW: groundwaters; Eh: redox potential; T: temperature; EC: electrical conductivity. A more detailed description can be found in Supplementary Material
Drilling is one of the most important activities within the scope of site investigations for a deep repository, and its performance is of particular importance in order to achieve high-quality groundwater samples and representative measurements from the boreholes. The boreholes can be either percussion drilled or core drilled.
Percussion drilling is the faster and cheaper technique used to supplement and increase the number of sampling locations as well as to provide boreholes with diameters between 200 and 250 mm. The technique is restricted to relatively short boreholes in the range of 50 to 300 m. No flushing water is used for the drilling, and no drill cores are obtained since the rock is crushed.
Core drilling is used for deeper boreholes down to around 1,000 m depth or when information from a drill core is required for specific studies like mapping and sampling of fractures, of rock types, and of fracture infills. In the case of the site investigations performed by SKB, flushing water without recirculation is used to cool the drill bit and no-drill mud or lubricants are used to avoid unnecessary contamination. After the late 1980s, most core boreholes drilled by SKB from the ground surface are of the so-called telescopic type: the first 100 m correspond to a wider percussion-drilled borehole, followed by a core-drilled hole with a smaller diameter (76-77 mm). This technique was developed specifically for hydrochemical investigations. It allows efficient gas-lift pumping from the upper percussion-drilled part of the borehole during and after core drilling. This pumping decreases the amount of flushing water and of drilling debris which, otherwise, would be forced into conductive bedrock fractures by the high pressure prevailing during drilling. This type of borehole also allows the installation of standpipes to facilitate groundwater head measurements and sampling during the following long-term hydrochemical monitoring phase (Section
In the case of boreholes drilled from tunnels, the conventional core or percussion drilling techniques are used. The telescopic design is not necessary since there is no need for standpipes or pumping during groundwater sampling. The water from the borehole is discharged during drilling and sampling due to the difference between the pressure in the bedrock and the atmospheric pressure in the tunnel.
Different equipment and investigation methods are in general required for the different borehole types, and they will be indicated in the corresponding sections of this paper. The most extensive hydrochemical investigations are performed in telescopic core boreholes, and their drilling protocol includes the following:
The flushing water is spiked with a tracer (e.g., sodium fluorescein), and it is discharged as return water (i.e., a mixture of flushing water, formation groundwater, and drill cuttings) by gas-lift pumping during drilling. Due to contamination risks, the selection of the flushing water source and the possible impact of the flushing water on the groundwater composition are important issues that are discussed in more detail in Supplementary Material The downhole drilling equipment and the flushing water system require a strict routine of cleanliness (more details can be found in Supplementary Material The percussion drilled part of a telescopic borehole is cased, and the gap between the borehole wall rock and the casing is grouted with cement to prevent groundwater inflow from the upper part to the lower core-drilled borehole part during drilling. Grooves are milled into the borehole wall at certain intervals for length calibration to ensure reliable depth readings. The use of a triple tube system is indispensable for preserving the fracture infillings (in the extracted drill cores) whose study, among other things, will facilitate the correlation of transmissive fractures with the flow log and the BIPS (Borehole Image Processing System, cf. Section
The time delay between drilling and chemical sampling is an important factor affecting the representativity of groundwater samples. Sampling close in time to the completion of the borehole may result in groundwater samples still impacted from the drilling, i.e., flushing water and groundwaters from different depths, introduced by the pressure impact during drilling. However, the problem could be even more serious if a borehole is kept open for some time without packers installed between the different hydraulically conductive fractures. In an inflow area, large volumes of shallow water are likely to intrude from fractures in the upper part of the borehole down to greater depths and mix with deeper groundwaters, and microbial activity and sulfide production could be promoted [
For similar reasons, groundwater sampling should be avoided when activities such as drilling or hydraulic tests are ongoing in the vicinity of the borehole.
The main criteria for the selection of the borehole sections are (1) presence of one or more fractures with a suitable hydraulic transmissivity, (2) appropriate borehole wall conditions (less fractured rock) that allow isolation of the section by inflatable packers, and (3) favourable distribution of the water yielding fractures in the isolated borehole section to facilitate the removal of water in the section prior to sampling (Figure
Schematic drawing of two different situations of the water yielding fractures in an isolated borehole section (modified from [
The selection of water yielding fractures and the isolation of the borehole sections are based on the information provided by the flow logging [
Borehole sections including fractures with moderate hydraulic transmissivities (around
Once the water yielding fracture(s) have been selected, the next important step is to optimise the position of the packers: (1) in order to decrease the required volume of water (Figure
Essential for the sampling procedure is (1) the evaluation of the contribution of flushing water from the drilling, (2) the adequate exchange of water from the borehole sections prior to sampling, and (3) the check of pressure responses in other parts of the borehole (or other boreholes in the vicinity) to exclude short-circuiting effects. Well-documented sampling conditions considering these aspects are important to facilitate the data evaluation during later stages of the site investigation.
The discharge of water prior to sampling is necessary to remove (1) the drilling debris and the remains of flushing water from the drilling and (2) the water initially present in the borehole section (section water). With respect to the flushing water, the calculation of its contribution requires a good homogeneous mixing of the tracer dye in the drilling fluid and frequent analyses of the tracer. The limit for the flushing water content for the best quality data in the SKB site investigations has been set to 2%; however, up to 10% may be considered acceptable if the groundwater data for such samples are used with care. The main problem associated with this task is the often long time needed to reach low enough flushing water content in the samples. With respect to the section water, its removal is needed to exchange the initial mixture of waters present in the borehole section. This water mixture may originate from different fractures at varying depths along the borehole, and in the case of monitored sections, the water may be affected by the stagnant conditions of the isolated section (e.g., microbial activity and corrosion, Section
Finally, three ways to secure adequate sampling conditions are (1) checking the absence of pressure responses in other parts of the sampled borehole or adjacent boreholes, (2) estimation of the sampling-day hydraulic transmissivity (based on flow rate and pressure measurement during sampling) which can be compared with the hydraulic transmissivity values obtained from differential flow logging, and (3) collection of sample series of minimum three samples (if possible) to check their hydrochemical behaviour with time.
The hydrogeochemical investigations conducted by SKB involve groundwater sampling and measurements and analyses of different parameters (chemical and isotopic composition, electrical conductivity, Eh and pH, colloids, dissolved gas, and microbes). Some special topics such as matrix pore water, fracture mineralogy, microbes and gases, and new methods for detailed studies on isotopic zoning in minerals, that require other types of sampling and treatment, have been described thoroughly elsewhere [ Hydrochemical logging (tube sampling) with the purpose of obtaining the composition of the groundwater present along the borehole. A tube consisting of connected 50 m long tube sections is used for the sampling, and each 50 m section constitutes one sample. This type of sampling provides only an approximate characterisation of the depth dependency of the geochemical characteristics of the groundwaters. Comprehensive groundwater characterisation (also known as Long-term hydrochemical monitoring in core and percussion boreholes to study the evolution of the groundwater composition over time (several years; Section
The data obtained with the tube sampling method are mainly used for initial discussion and to allow the comparison with the hydrochemical data obtained later. Even not being suitable for modelling purposes, these data may be useful for the understanding of the borehole hydraulic conditions and its evolution with time. The last two methods, including the special equipment used for sampling of low transmissive fractures, are described in more detail below.
Among all the sampling methods used by SKB, the Complete Chemical Characterisation (CCC), developed during the 1980s [
Outline of the integrated system comprising a carriage/container with downhole equipment (umbilical hose, inflatable packers, pump, downhole measurement probe, and downhole sampling unit) and facilities for lowering and raising this equipment. This system is placed over the borehole (see photo), and the container’s indoor temperature is adjusted to maintain the temperature of the groundwater in the section to be investigated.
The CCC sampling campaigns usually start as soon as the preceding logging activities (Figure
The water is pumped to the ground surface through the downhole equipment and then through the polyamide tube housed in the umbilical hose (Figure
Online regular logging of pH, Eh, dissolved oxygen, electrical conductivity, and groundwater temperature starts as soon as the packers are inflated, and the pumping has started. Once the water is at the surface, sample portions are collected regularly for analysis during pumping (usually for a period of three weeks or until stable Eh readings are obtained).
The sample series of groundwater collected by the CCC method are of the best possible quality and include the most complete set of analyses as well as supporting information. Moreover, the series allow verification of stable conditions and identification and exclusion of single outliers. At the end of the pumping period,
A variation of the CCC method was developed in order to sample low transmissive fractures (
The special sampling unit for low transmissive fractures, to be used with the umbilical hose of the CCC equipment, consists of (a) inflatable packers delimiting a borehole section of a fixed length of one metre (instead of adjustable length of up to 15 m); (b) a dummy, whose surface is coated with Teflon, that is mounted in between the packers to reduce the water volume in the section to 0.3 L (absent in the standard sampling unit); and (c) a single sample container (1.2 L) connected to the sampling section but placed above the upper packer, outside the section. The water is sucked into the evacuated sample container. The filling of water is recorded by a pressure sensor. Once the sampling is finished, the equipment is raised to the ground surface and the water is portioned into bottles and sent for analysis.
The main aim of the hydrogeochemical monitoring is to create a long time series of data to study the evolution of the composition of the groundwaters with time. Apart from obtaining base-line data covering the normal variations, monitoring is essential to study the impacts of the construction and operation of a facility at a later stage.
After completion of the general investigation activities in a borehole (Figure
(a) Installed equipment for pressure, groundwater sampling, and groundwater flow monitoring in a telescopic borehole. A maximum of 10 pressure sections can be installed in a telescopic borehole, of which generally two are equipped for water sampling and circulation of tracers during flow measurements. These circulation sections are connected to three tubes. Two of them connect the section to the two standpipes for pressure measurements and groundwater sampling (the wider pipe), respectively, in the uppermost part of the borehole. The third tube leads all the way to the ground surface for the circulation experiments. (b) Lowering of a pump connected to a minipacker (for isolating the standpipe from the atmosphere) and a 50
Tunnel boreholes are also monitored. The design of the equipment in these boreholes is basically similar although there are no standpipes since no pumping is needed to discharge the groundwater due to the hydrostatic pressure.
Hydrochemical monitoring includes the collection of sample series (minimum of three samples) during continuous pumping/discharge at each sampling occasion. As indicated above (Section
Besides the impact on sensitive constituents, monitored boreholes from the surface are also unsuitable for Eh measurements as the equipment cannot be lowered into the borehole due to the fixed packer system. Additionally, the system with standpipes does not allow completely oxygen-free operation (Figure
Analytical programmes are designed to provide information/data for different purposes: (1) to describe the distribution, age, and geochemical evolution of groundwaters of different origins in the bedrock, (2) to complement the hydrogeological information in order to characterise the flow paths of the water and validate the hydrogeological models and
The analytical protocol for groundwater analyses has included the same basic components and parameters since the beginning of the 1980s: major constituents, nutrient salts, and other anions of lower concentrations, DOC and TOC (dissolved and total organic carbon concentrations, respectively), trace metals, and stable and radioactive isotopes [
Different sample classes are established to define the parameters to be included in the analytical protocol as well as the adequate sampling procedures and sample treatments. The lowest classes include basic measurements and analyses (pH, electrical conductivity, chloride, and alkalinity). An intermediate class includes the main chemical components and some of the isotopes. Sampling according to the highest classes demands trained personnel and specialised equipment since these classes comprise, in addition, components that need online filtering and/or special conservation/treatment of the water sample (trace elements, redox-sensitive components, and additional isotopes).
Analyses that need to be conducted soon after sampling (Fe2+, Fe-tot, NH4, HCO3-, and lab-pH) are conducted onsite. In addition, Cl-, EC, HS-, and ion chromatographic (IC) determinations of SO42-, Br-, and F- are generally performed by SKB but not necessarily at the investigation site. Besides IC, the analyses performed by SKB laboratories are conducted by spectrophotometric, titrimetric, and potentiometric methods (additional methods, such as ICP, are conducted in external laboratories; see Supplementary Material
Reliable and plausible Eh measurements at the very negative range observed for deep groundwaters [
The CCC method includes simultaneous measurements of Eh and pH at depth as well as in a flow-through cell at the ground surface. Three different redox electrodes (platinum, gold, and glassy carbon) measure Eh, and one or two glass electrodes are used for pH at each location (borehole and surface flow-through cells, respectively). Agreeing measurements by the different types of redox electrodes indicate stable conditions and reliable values. The logging continues until the parameters stabilise [
The reference electrode for the Eh and pH measurements is of the Ag/AgCl, double junction type. The downhole reference electrodes and the glass (pH) electrodes are specially designed by SKB to stand high pressures by allowing compression of the electrolyte volume. The electrodes used at the surface are all commercially available. The electrical ground in the probes is galvanically isolated from earth.
With respect to the pH measurements, apart from those performed in the field together with the Eh, pH is also measured in the laboratory at 25°C (batch). The possibility of comparing different measurements to evaluate reliability has been proven to be important.
The chemical data (analytical results and measured values) from different sources are checked in several steps before they are used in interpretations and modelling work.
First screening at the investigation site is important since it is conducted close in time to the sampling and analyses and by personnel familiar with the sampling and analytical performance. This screening involves charge balance calculations, simple consistency checks (Section A further check is performed when the data are entered into SKB’s geoscientific database, mainly to confirm correct entries by signing the quality check for each sample record. Further control is added by plotting large amounts of data in Finally, when the dataset is delivered for hydrochemical interpretation and modelling, the quality of the data is assessed with respect to sample representativity. At this stage, more information is available (a larger dataset, complete isotope data, hydrogeological and geological interpretations, etc.) allowing representativity assessments based on an integrated hydrochemical, geological, and hydrogeological approach (Section
Some basic consistency checks are performed prior to inserting the data in the database. The usual checks are described below:
Comparison of the measured electrical conductivity (EC) with the concentration of the dominating dissolved ion (chloride in most of the Fennoscandian groundwaters) in order to discover outliers. Since the dominating ion contributes the most to the electrical conductivity, the comparison should result in a close to a straight line in the relevant salinity range for the considered deep groundwaters [ Charge imbalance calculations provide verification of reliable major components. The acceptable range is set to ±5%; however, in the case of dilute waters ( Comparison between the values obtained by different analytical methods. In the analytical routine followed by SKB, this applies to iron, sulfate, and uranium (the element and the U-238 activity) which are all routinely determined by two methods that are based on different principles. Bromide concentrations are plotted versus corresponding chloride contents to give a rough check of the plausibility of the bromide concentrations. Some correlation is usually found for the entire dataset also in the case of groundwaters with different origins (marine and nonmarine).
The quality and representativity of groundwater data may be influenced by different factors, for example, contamination from drilling, different sampling methods, the hydraulic conditions in the borehole at the sampling occasion, and the analytical performance. After some initial strategies developed for groundwater data evaluation during the 1980s and 1990s (see the Supplementary Material
The objective of the categorisation is to assess the data quality by grading the set of data corresponding to a sampling occasion from 1 to 5 according to several quality criteria. Of these (1) is the highest quality, while quality (5) is not considered acceptable for modelling purposes (see Supplementary Material
The major reasons for performing this categorisation are to facilitate future interpretation and modelling work by providing well-structured data tables representing quality categorised data and also to guide users on how to select data for their purposes. Additionally, this evaluation is very useful to identify samples unsuitable for general modelling purposes (affected by experimental conditions, grouting, etc.). The first step for the categorisation needs to be a general overview of the dataset to establish the best categorisation criteria.
Once the data have been evaluated and categorised, they are ready to be used in the hydrogeochemical interpretation and modelling. The main objective is to use an integrated framework like the one shown in Figure
Schematic overview of the interpretation and modelling procedure to produce an integrated hydrogeochemical site model/description. Grey frames are used for hydrogeochemical input data. Blue colour indicates geological and hydrogeological methods and data flows, and red boxes indicate descriptive hydrogeochemical interpretation methods, generally performed with specific software.
The methodology developed by SKB for the characterisation of deep groundwaters in crystalline rocks has been based on forty years of experience and of collaboration with other international agencies and research institutions. This paper describes advances and improvements applicable to groundwater sampling, for any purpose, in crystalline rocks. The text also identifies the questions to be considered during data interpretation for the hydrogeochemical characterisation of a crystalline bedrock system.
The sampling protocol emphasises the collection of hydrogeochemical data that accurately represent
Some general points/measures of importance in order to obtain representative groundwater samples of the best possible quality are as follows:
Planning of the hydrochemical investigations at an early stage, i.e., hydrochemical demands need to be considered already when preparing for the drilling of the boreholes. A thoughtful selection of borehole sections based on flow logging and BIPS data as well as specific hydrogeological evaluations to facilitate adequate sampling. Online measurements of Eh and pH, preferably Collection of sample time series to ensure hydrochemical stability. Adequate data evaluation and quality check. Quality categorisation of data to provide guidance on their use for different purposes.
Finally, one of the most relevant issues to consider is the importance and usefulness of close cooperation and integration of hydrogeochemistry with other geoscientific disciplines, such as structural geology, hydrogeology, and geomicrobiology. This collaboration should start already during the planning and execution of the field work (and subsequent interpretations) in order to optimise the quality and the amount of information. The combination of different types of knowledge from all the geoscientific disciplines, ranging from field and laboratory studies to interpretation and modelling work, is the only way to obtain a final coherent and integrated understanding of the system.
The authors declare that they have no conflicts of interest.
This work has been conducted with the support of the Swedish Nuclear Fuel and Waste Management Company (SKB), Stockholm. The experiences and ideas of the SKB personnel and consultants involved in the field and laboratory work at Forsmark and Äspö during the many years of investigations, in combination with talented technical solutions by contracted workshops (Maskinteknik AB in Oskarshamn and Geosigma Geoteknik AB in Uppsala), have resulted in the well-established methods and equipment of today. This research was fully financed by the Swedish Nuclear Fuel and Waste Management Co. (SKB), a company in charge of the safe disposal of the Swedish spent nuclear fuel and other radioactive wastes in deep geologic repositories. It is in SKB’s interest to demonstrate the use of scientifically sound methods during site characterisations.
There is one file with supplementary information associated with this article. Inside that file, 3 supplementary sections are included: Supplementary Material 1 gives a summary of the background on the development of this type of methodology worldwide in the context of radioactive waste disposal. Supplementary Material 2 describes in detail some additional issues related to the drilling and after drilling procedures, especially about the flushing water. Finally, Supplementary Material 3 includes two tables with detailed information on some methodological aspects related to Sections