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.|
- Field Sampling
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. 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). 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. A synopsis of geochemical sampling and analysis techniques used in geothermal exploration is also provided by Arnorsson et al. (2006).
- theGrio. Kenya Becoming a Geothermal Powerhouse [Internet]. 05/21/2012. theGrio. [updated 2012/05/21;cited 2013/10/10]. Available from: http://thegrio.com/2012/05/21/kenya-becoming-a-geothermal-powerhouse/
- Encyclopedia of Volcanoes
- Chapter 4: Geochemistry
- 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.
- 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.
- 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.
- Chapter A4: Collection of Water Samples (ver. 2.0)
- Sampling and Analysis of Geothermal Fluids