Trace element testing in geothermal water is essential for evaluating resource quality, confirming the value of geothermal fluids for drinking mineral water, medical spa, and industrial applications, and controlling potentially toxic elements such as arsenic, mercury, and fluoride. Using advanced techniques such as ICP-MS, ICP-OES, atomic absorption spectrometry, and ion chromatography, accredited third-party laboratories can determine lithium, strontium, bromine, iodine, boron, silica, and heavy metals at microgram to nanogram per liter levels. This article outlines the sampling requirements, analytical methods, applicable standards, testing items, and typical applications of geothermal water trace element analysis.
Technical Overview
Geothermal water is formed through deep circulation of meteoric or connate water in high-temperature reservoirs, where prolonged water–rock interaction under elevated temperature and pressure enriches the fluid with a characteristic suite of dissolved constituents. Beyond the major ions that dominate total dissolved solids, geothermal fluids carry trace elements whose concentrations provide valuable information about reservoir lithology, fluid origin, reservoir temperature, and water–rock equilibrium. Elements such as lithium, rubidium, cesium, strontium, boron, bromine, iodine, and silica often occur at elevated levels in geothermal systems, while toxic elements including arsenic, mercury, antimony, thallium, and fluoride may also be naturally concentrated and require careful evaluation before any beneficial use or discharge.
Trace element analysis of geothermal water is governed by an established framework of national and international standards. Key references include GB 8537 for drinking natural mineral water, GB/T 8538 for standard examination methods of drinking natural mineral water, GB/T 11615 for geothermal resource exploration, the DZ/T 0064 groundwater analysis series, as well as ISO 17294-2, ISO 11885, ISO 15586, ISO 12846, ASTM D5673, and ASTM D1976 for instrumental determination of elements in water. Compliance with these standards ensures that analytical results are technically defensible for resource assessment, product certification, environmental permitting, and regulatory reporting.
Testing Samples
Deep Geothermal Well Fluid
Fluids sampled at the wellhead of deep production wells are typically hot, mineralized, and sometimes gas-rich. Because temperature and pressure changes at the surface cause rapid shifts in pH, redox state, and gas equilibrium, samples must be collected after the well has stabilized, cooled rapidly to ambient temperature, and preserved immediately according to the target analytes. Field parameters such as temperature, pH, conductivity, Eh, and flow rate should be measured at the wellhead at the time of sampling.
Natural Hot Spring and Spa Water
Spring discharges and spa pool water are tested to classify the thermal water type, verify its suitability for balneotherapy or bathing, and monitor hygienic and chemical safety over time. Seasonal variation should be considered when designing sampling programs for spring sources.
Raw Water for Bottled Mineral Water Production
Source water intended for bottling as drinking natural mineral water must be characterized for boundary indicators such as lithium, strontium, zinc, metasilicic acid, selenium, bromide, and iodide, together with a full screen of regulated toxic substances, in order to demonstrate compliance with GB 8537.
Reinjection Water, Tail Water, and Scale Deposits
Wastewater, reinjection streams, and silica or carbonate scale from geothermal facilities are analyzed to assess discharge compliance, reinjection compatibility, corrosion and scaling tendency, and potential environmental impact of trace contaminants such as arsenic and boron.
Sampling and Preservation Requirements
- For dissolved trace metals, filter through 0.45 µm membrane in the field and acidify with high-purity nitric acid to pH below 2; for total recoverable metals, acidify unfiltered samples.
- For mercury, collect in amber glass bottles and stabilize with hydrochloric acid or nitric acid plus potassium dichromate to minimize volatilization and wall adsorption losses.
- For fluoride, bromide, and iodide, collect unacidified samples and store at approximately 4 °C in the dark.
- For soluble silica, use a separate unacidified bottle and analyze promptly to avoid polymerization effects.
- Follow ISO 5667-11 for groundwater sampling techniques and ISO 5667-3 for preservation and handling; maintain full chain-of-custody documentation from field to laboratory.
Testing Items
Testing programs are typically structured around three groups of parameters, selected according to the intended use of the geothermal water and the applicable regulatory criteria.
Characteristic and Beneficial Trace Elements
- Lithium (Li), strontium (Sr), zinc (Zn), and selenium (Se)
- Bromide (Br⁻) and iodide (I⁻)
- Metasilicic acid (H₂SiO₃) and soluble silica (SiO₂)
- Free carbon dioxide (CO₂) and hydrogen sulfide (H₂S)
- Rubidium (Rb), cesium (Cs), boron (B), manganese (Mn), and iron (Fe)
Toxic and Regulated Trace Elements
- Arsenic (As), mercury (Hg), antimony (Sb), and thallium (Tl)
- Cadmium (Cd), lead (Pb), chromium (Cr, including hexavalent chromium), and nickel (Ni)
- Fluoride (F⁻), aluminum (Al), barium (Ba), beryllium (Be), and uranium (U)
- Gross alpha and gross beta radioactivity where radiation screening is required
Supporting Hydrochemical Parameters
- Temperature, pH, electrical conductivity, Eh, and total dissolved solids
- Major cations: potassium, sodium, calcium, magnesium
- Major anions: chloride, sulfate, bicarbonate and carbonate
Testing Methods
Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
ICP-MS is the primary technique for simultaneous ultra-trace determination of metallic elements in geothermal water, covering lithium, rubidium, cesium, strontium, boron, arsenic, antimony, thallium, cadmium, lead, uranium, and many others in a single run. Method detection limits typically range from 0.001 to 0.1 µg/L. Analyses follow ISO 17294-2 and ASTM D5673, using internal standards such as rhodium, indium, and rhenium, and collision or reaction cell technology to resolve polyatomic interferences, notably the argon chloride interference on arsenic-75 in chloride-rich geothermal brines. High-TDS samples are usually diluted and matrix-matched to control signal suppression.
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)
ICP-OES provides robust multi-element determination at low microgram-per-liter to milligram-per-liter levels in accordance with ISO 11885 and ASTM D1976. It is particularly well suited to elements present at higher abundance in geothermal fluids, such as lithium, strontium, boron, barium, and silica, and tolerates high dissolved solids better than mass spectrometric techniques.
Atomic Absorption Spectrometry (AAS)
Flame AAS remains a reliable method for milligram-per-liter level elements such as lithium and strontium. Graphite furnace AAS, following ISO 15586, achieves sub-microgram-per-liter sensitivity for lead, cadmium, and thallium. Hydride generation AAS is used for arsenic, selenium, and antimony (ISO 11969 and ISO 20280), while cold vapor AAS is applied to mercury at detection limits near 0.01 to 0.05 µg/L in accordance with ISO 12846 and ASTM D3223.
Atomic Fluorescence Spectrometry (AFS)
Hydride generation and cold vapor atomic fluorescence spectrometry, as specified in GB/T 8538 and the DZ/T 0064 series, offers excellent sensitivity and low operating cost for arsenic, selenium, antimony, and mercury and is widely used for routine screening of geothermal and groundwater samples.
Ion Chromatography and Molecular Spectrophotometry
Ion chromatography in accordance with ISO 10304 and ASTM D4327 determines fluoride, bromide, and iodide alongside chloride and sulfate with minimal sample preparation. Molecular spectrophotometry is used for soluble silica by the molybdenum blue method and for boron by the curcumin or carmine methods, as specified in GB/T 8538. For advanced studies, speciation analysis of arsenic(III) and arsenic(V) or iron species can be performed by coupling liquid chromatography with ICP-MS.
Quality Assurance and Quality Control
- Multi-point calibration with certified reference solutions and verification with an independent calibration check standard.
- Procedural blanks, laboratory duplicates, and matrix spike recoveries, typically within 80 to 120 percent, to monitor contamination and matrix effects.
- Analysis of certified reference materials or proficiency test samples to confirm accuracy.
- Method detection limit verification and control charts to demonstrate ongoing method performance.
- Accredited third-party laboratories operating under ISO/IEC 17025 quality management systems provide independent, legally recognized results.
Applications
Geothermal resource exploration and reservoir characterization. Trace element ratios involving lithium, rubidium, cesium, boron, and silica serve as geochemical indicators of reservoir temperature, fluid origin, and water–rock interaction, supporting geothermometry and conceptual model development during exploration under GB/T 11615.
Drinking natural mineral water certification. Boundary indicators such as lithium, strontium, zinc, metasilicic acid, selenium, bromide, and iodide determine whether a geothermal or groundwater source qualifies as drinking natural mineral water under GB 8537, with reference thresholds including lithium at or above 0.20 mg/L, strontium at or above 0.20 mg/L, zinc at or above 0.20 mg/L, bromide at or above 1.0 mg/L, iodide at or above 0.20 mg/L, metasilicic acid at or above 25 mg/L, and selenium at or above 0.01 mg/L.
Medical and balneological evaluation. Elevated concentrations of specific components allow classification of thermal waters into recognized therapeutic categories, such as silicon-rich, fluorine-bearing, iodine-bromine, or strontium-type waters, guiding the development of spas, wellness centers, and medical hydrotherapy facilities.
Environmental compliance and impact assessment. Trace element data support discharge permitting, evaluation of arsenic and boron removal requirements, monitoring of groundwater quality around reinjection and disposal operations, and human health and ecological risk assessment.
Operational management of geothermal facilities. Silica, iron, manganese, and barium measurements inform scaling and corrosion control strategies, while consistent monitoring of reinjection water chemistry helps prevent reservoir clogging and maintains long-term field sustainability.
FAQ
Why is trace element testing important for geothermal water?
Trace elements reveal the geological origin and quality of geothermal fluids, determine their economic and therapeutic value, identify naturally occurring contaminants that may pose health or environmental risks, and support regulatory compliance for drinking, bathing, discharge, and reinjection applications.
What is the difference between dissolved and total trace element concentrations?
Dissolved concentrations are measured in samples filtered through 0.45 µm membranes in the field and acidified afterward, representing the mobile fraction in solution. Total recoverable concentrations are determined on unfiltered, acidified samples and include particulate-bound elements. The choice depends on the study objective and the applicable standard.
How should geothermal water samples be collected and preserved?
Samples should be taken at the wellhead after stable flow is established, cooled quickly, filtered and preserved according to each analyte group, stored in appropriate containers, transported cold with chain-of-custody records, and analyzed within the holding times specified in ISO 5667-3 and GB/T 8538.
Which elements determine whether geothermal water qualifies as drinking natural mineral water?
Under GB 8537, one or more boundary indicators, including lithium, strontium, zinc, selenium, iodide, bromide, metasilicic acid, or free carbon dioxide, must reach the specified concentration limits, and all regulated toxic substances must remain within mandatory safety limits.
What detection limits can be achieved?
ICP-MS typically achieves detection limits of 0.001 to 0.1 µg/L for most metals, atomic fluorescence methods reach 0.01 to 0.1 µg/L for arsenic, selenium, and mercury, and ion chromatography determines halides at the 0.01 to 0.1 mg/L level, which is fully adequate for classification and compliance purposes.
How long does a typical trace element analysis take?
Routine single-method determinations are generally completed within five to ten working days after sample receipt, while comprehensive characterization packages covering mineral water certification parameters, toxic element screening, and hydrochemical profiling may require ten to fifteen working days.