Groundwater Sampling

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Exploration Technique: Groundwater Sampling

Exploration Technique Information
Exploration Group: Field Techniques
Exploration Sub Group: Field Sampling
Parent Exploration Technique: Water Sampling
Information Provided by Technique
Hydrological: Water composition and source of fluids. Determination of mixing ratios between different fluid end-members. Determination of fluid recharge rates and residence times.
Thermal: Water temperature.
Groundwater Sampling:
Groundwater sampling is done to characterize the chemical, thermal, or hydrological properties of subsurface aqueous systems. Groundwater sampling is typically performed to monitor water quality, study recharge and flow in groundwater systems, and evaluate degree of mixing in between meteoric water and thermal fluids in geothermal reservoirs.
Other definitions:Wikipedia Reegle

Groundwater and reservoir fluid sampling are routinely used in geothermal exploration to provide accurate evaluations of the geothermal system under investigation. Geothermal waters develop a unique chemical signature through a series of potentially complex processes relating to reservoir rock interactions, groundwater recharge, fluid mixing ratios, and phase transitions. All of these factors influence the chemistry of thermal fluids derived from the original fluids that recharge the dynamic hydrothermal system. Data obtained through analysis of reservoir fluids and groundwater samples collected from monitoring wells can also be integrated into regional-scale hydrologic and thermal gradient models that can assist in identifying the source of thermal fluids in the geothermal system.

Analyst testing a sample of condensed steam at the Olkaria geothermal power plant in Kenya’s East African Rift Valley. Photo taken by Roberto Schmidt of Getty Images, featured on theGrio Inspiration website.[1]

Use in Geothermal Exploration
Water sampling is routinely used in geothermal exploration and monitoring to characterize the chemical composition of the fluid, measure the temperature, or conduct isotope studies to derive the provenance of thermal waters. Groundwater sampling is a critical aspect of characterizing a geothermal system because the water chemistry, temperature, and source can reveal the quality of the resource. Groundwater chemistry is largely controlled by temperature, water-rock interactions, volume of water vs rock, residence time, and contributions from various fluid sources (mixing), such as seawater, magmatic fluids, etc.[2] The dissolved silica and trace element (As, B, Br, and Li) contents of meteoric waters tends to be low compared to those of typical geothermal fluids.[3][2] However, interaction with reservoir rocks may cause groundwater to become oversaturated with silica or carbonate, leading to precipitation of sinter or travertine at surface conditions, respectively.[3][2]

Field Procedures
Waters can be collected from groundwater aquifers using a variety of downhole and wellhead sampling techniques. Relatively simple procedures for sampling groundwater from a well utilize bailers or airlifting techniques. While these methods typically lead to aeration and flashing of thermal fluids as they encounter lower pressures during ascent, they are generally adequate for groundwater well sampling, as the temperature difference between cool groundwater aquifers and the surface is significantly lower.[4]

Various types and sizes of commercially available reusable bailers commonly used for well water sampling. Image featured on the Geotechnical Services website product page.

There is also some variation in the field treatment methods used during collection of groundwater samples that depend primarily on the analytical techniques to be applied later in the lab. Water samples collected at atmospheric conditions experience increasing disequilibrium from subsurface conditions as a result of cooling, decompression, boiling/flashing, volatilization of dissolved constituents, and water-rock interactions during ascent. To minimize the effects of these processes, specialized field treatments are used to obtain representative groundwater samples.

Waters sampled for chemical analysis are stored in brimful polyethylene bottles with Polyseal caps following filtration from a large syringe attached to a filter holder containing 0.8 um filter paper.[5] Each individual sample consists of 10-500 mL of filtered water, depending on the requirements of lab analytical techniques to be applied later. Duplicates are taken at each sample point and then treated in the field in preparation for chemical analyses. A set of duplicates sampled from a single surface discharge might included a bottle of unacidified (untreated) water for anion analysis, a bottle of water acidified dropwise with dilute HCl to pH <2 for cation analysis, a bottle containing sampled water diluted with deionized water (between 1:5 and 1:10 ratio) for measurement of silica content.

Samples to be used for isotopic analysis are collected in glass bottles filled to the brim with raw (unfiltered) water and sealed with a Polyseal cap. As with standard compound and major/trace elemental analyses, analysis for isotopes of different elements requires specialized treatment of the sample in the field. For example, tritium analysis requires a significant volume of water (up to 500 mL), whereas analysis for stable isotopes that are present in greater abundance in natural samples requires less water to be sampled by a full order of magnitude (approximately 30 mL).[6] In order to analyze the 13C content of dissolved HCO3, the water sample must be treated with NH4OH and then saturated with SrCl2. For analysis of the 18O content of dissolved SO4, the water sample is treated with formaldehyde.

For a detailed description of modern water sampling techniques, methods, and instrumentation, consult Chapter A4 of the National Field Manual for the Collection of Water-Quality Data, published online by the U.S. Geological Survey.[7] A synopsis of geochemical sampling and analysis techniques used in geothermal exploration is also provided by Arnorsson et al. (2006).[8]

Data Access and Acquisition
Temperature and pH data are typically measured in the field at the time of sampling, and are recorded in conjunction with the sampling coordinates. The flow rate is also typically measured at the wellhead gauge for flowing wells. Sampled waters are typically subjected to chemical and isotopic analyses in order to estimate mixing ratios and recharge of the hydrothermal system. Data from these analyses can also provide useful information regarding the source of thermal fluids and help to constrain the age of the hydrothermal system.
Best Practices
Groundwater sampling is best carried out by a qualified hydrologist, geologist, or geochemist familiar with current sampling standards. While groundwater sampling is relatively simple compared to in-situ thermal fluid sampling, a practical understanding of how various processes can affect the bulk chemistry of sampled waters as they are brought to the surface ensures that appropriate sampling procedures are used to obtain a sample that is representative of subsurface conditions. A working understanding of fluid recharge and mixing in geothermal systems is also ideal for the purposes of data interpretation, application of various chemical and isotopic geothermometers, and geochemical modeling of the reservoir.
Potential Pitfalls
Water sampling techniques are designed to prevent concentrations of dissolved species from changing via reactions that occur as samples cool, or through exposure of samples to the atmosphere.[8] While the intensity of these effects is significantly reduced for colder meteoric waters, detailed characterization of hydrologic regimes requires precise analysis of end-member fluids that recharge the geothermal system. Failure to adhere to proper sampling procedures and treatment practices can result in mineral precipitation in sample vials and/or re-equilibration of the sample at surface conditions, both of which disturb the chemical composition of the fluid. These processes shift the fluid chemistry of the sample away from that of waters present in the groundwater aquifers that may receive chemical inputs from geothermal fluids at depth, which impacts the results of the fluid analyses and will ultimately affect the results of geothermometric calculations and geochemical modeling.

Additional References

  1. theGrio. Kenya Becoming a Geothermal Powerhouse [Internet]. 05/21/2012. theGrio. [updated 2012/05/21;cited 2013/10/10]. Available from:
  2. 2.0 2.1 2.2 Encyclopedia of Volcanoes
  3. 3.0 3.1 Chapter 4: Geochemistry
  4. Michael L. Sorey,Gene A. Suemnicht,Neil C. Sturchio,Gregg A. Nordquist. 12/1991. New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits. Journal of Volcanology and Geothermal Research. 48(3-4):229-263.
  5. Fraser E. Goff,Tamsin McCormick,Pat E. Trujillo Jr,Dale A. Counce,Charles O. Grigsby. 1982. Geochemical Data for 95 Thermal and Nonthermal Waters of the Valles Caldera - Southern Jemez Mountains Region, New Mexico. Los Alamos, NM: Los Alamos National Laboratory, NM. Report No.: LA-9367-OBES.
  6. John A. Musgrave,Fraser E. Goff,Lisa Shevenell,Patricio E. Trujillo Jr,Dale Counce,Gary Luedemann,Sammy Garcia,Bert Dennis,Jeffrey B. Hulen,Cathy Janik,Francisco A. Tomei. 1989. Selected Data from Continental Scientific Drilling Core Holes VC-1 and VC-2A, Valles Caldera, New Mexico. Los Alamos, NM: Los Alamos National Laboratory, NM. Report No.: Report No. unavailable.
  7. Chapter A4: Collection of Water Samples (ver. 2.0)
  8. 8.0 8.1 Sampling and Analysis of Geothermal Fluids

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