Surface Water Sampling

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Exploration Technique: Surface Water 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
Thermal: Water temperature
Surface Water Sampling:
Water sampling is done to characterize the chemical, thermal, or hydrological properties of a surface or subsurface aqueous system.
Other definitions:Wikipedia Reegle

Surface water sampling of hot and cold spring discharges has traditionally been used as a field reconnaissance technique in geothermal exploration to obtain a “first look” into the characteristics 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, 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. Water samples are collected at the surface from springs or geysers to begin to evaluate these processing and thereby assist in siting exploratory and temperature-gradient boreholes during the early stages of prospect evaluation. Water samples may also be collected from surface discharges to facilitate ongoing monitoring of the hydrothermal system during production.

Geochemist sampling a cold seep near the Poncha Hot Springs, CO.

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. Water sampling is a critical aspect of characterizing a geothermal system because the water chemistry, temperature and source can reveal the quality of the resource. Water chemistry is largely controlled by temperature, water-rock interactions, volume of water vs rock, residence time, and contributions from other fluids (mixing), such as cold groundwater, seawater, magmatic fluids, etc.[1] Waters that reach the surface may be over saturated with silica or carbonate at surface conditions and precipitate sinter or travertine, respectively.[2][1] Some geothermal fluids that reach the surface form acid-sulfate springs, generated from rising steam and volatile compounds that condense and mix with an overlying freshwater aquifer, whereupon the H2S in the steam oxidizes to form sulfuric acid.[2][1][3]

Geothermal waters typically range in total dissolved solids (TDS) from a few hundred to > 350,000 parts per million (ppm). [2][1] Liquid dominated reservoirs usually have a composition dominated by Na, K and Cl, but in very saline systems reservoir waters can be Na, K, Ca, and Cl rich.[3][1] Silica and trace element (As, B, Br, and Li) concentrations tend to be high compared to the average meteoric waters, and pH is generally between 6 and 9, although acidic saline liquids can also be associated with geothermal systems (as described above).[2][1]

Field Procedures
Waters can be sampled at the surface from springs, geysers, meteoric-dominated water bodies and discharges, and condensed fumarole steam. There is some variation in the methods and instruments used for taking geothermal water samples that depend primarily on the type of surface feature being sampled and on the type of analysis to be performed on the sample. This is largely due to the fact that water samples are very far out of equilibrium at surface conditions, and so require specialized treatments in the field to prevent precipitation of dissolved constituents and ensure that samples are as representative of reservoir conditions as possible.

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.[4] 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).[5] 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.[6] A synopsis of geochemical sampling and analysis techniques used in geothermal exploration is also provided by Arnorsson et al. (2006).[7]

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 of the sampled discharge is also sometimes measured, either directly or by visual estimation. Sampled waters are typically subjected to chemical and isotopic analyses in order to characterize hydrothermal systems and allow for estimation of reservoir temperatures through the application of various chemical geothermometers. 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
Surface water sampling is best carried out by a qualified hydrologist, geologist, or geochemist familiar with current sampling standards. A practical understanding of how different surface features relate to hydrothermal processes within the geothermal system is also ideal for the purposes of data interpretation, application of various chemical and isotopic geothermometers, and geochemical modeling of the reservoir.
Potential Pitfalls
Geothermal fluid 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.[7] 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 fluids present in the geothermal reservoir at depth, which impacts the results of the fluid analyses and will ultimately affect the results of geothermometric calculations and geochemical modeling.

  1. 1.0 1.1 1.2 1.3 1.4 1.5 Encyclopedia of Volcanoes
  2. 2.0 2.1 2.2 2.3 Chapter 4: Geochemistry
  3. 3.0 3.1 Geothermal Waters: A Source of Energy and Metals
  4. 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.
  5. 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.
  6. Chapter A4: Collection of Water Samples (ver. 2.0)
  7. 7.0 7.1 Sampling and Analysis of Geothermal Fluids

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