Geothermal Well Underground Hot Water Testing and Inspection: A Comprehensive Technical Guide

Geothermal well underground hot water testing is an essential quality control and resource evaluation process for geothermal development projects, spa and balneotherapy facilities, and district heating systems. This technical guide explains the sampling procedures, hydrochemical parameters, analytical methods, and applicable international and national standards (ISO, ASTM, GB, DZ) used by accredited third-party laboratories to assess geothermal water quality, scaling and corrosion potential, reservoir characteristics, and therapeutic value.

Technical Overview

Underground hot water produced from geothermal wells is a complex hydrothermal fluid whose chemical composition is shaped by reservoir temperature, host rock lithology, water-rock interaction, mixing with shallow groundwater, and residence time in the aquifer. Systematic inspection of geothermal well water serves multiple purposes: characterizing and classifying the geothermal resource, evaluating its suitability for heating, bathing, agriculture, or balneotherapy, predicting scaling and corrosion in surface facilities, assessing reinjection compatibility, and demonstrating environmental compliance.

According to widely applied national specifications such as GB/T 11615 (Geothermal Resources Geological Exploration), groundwater with a temperature at or above 25 °C recovered from a well is generally regarded as geothermal water, and is commonly classified as warm (25–40 °C), warm-hot (40–60 °C), hot (60–90 °C), or super-hot (above 90 °C). A full hydrochemical inspection typically combines field measurements at the wellhead with comprehensive laboratory analysis, following sampling and analytical standards including ISO 5667, ISO 17294, ASTM D series methods, and the DZ/T 0064 series for groundwater analysis.

Objectives of Geothermal Water Inspection

• Determination of physical properties, major ion chemistry, and total dissolved solids for water type classification
• Identification of therapeutic (balneotherapeutic) components such as fluoride, metasilicic acid, hydrogen sulfide, radon, lithium, strontium, bromide, and iodide
• Assessment of trace metals and potential contaminants for health and environmental evaluation
• Estimation of reservoir temperature using chemical and isotope geothermometers
• Evaluation of calcium carbonate and silica scaling potential and corrosivity for system design
• Verification of water quality stability over time for well field operation and reinjection management

Testing Samples

Artesian (Self-Flowing) Well Water

For flowing artesian geothermal wells, samples are collected directly from the wellhead discharge pipe after an adequate purging period that ensures the sample represents formation water rather than fluid stored in the wellbore. Temperature, flow rate, and pressure conditions are recorded at the moment of sampling.

Pumped Production Well Water

For pumped wells, the well is purged until field parameters (temperature, pH, electrical conductivity) stabilize, typically after removing three to five wellbore volumes. Samples are taken from a sampling valve upstream of any treatment, filtration, or aeration equipment.

Reinjection and Monitoring Well Water

Water destined for reinjection, as well as samples from observation and monitoring wells, is collected to evaluate chemical compatibility between reinjected fluid and reservoir water, to detect clogging risks, and to track long-term changes in the geothermal reservoir.

Downhole and Production Test Samples

During exploration drilling and production tests, samples may be recovered from specific depth intervals using downhole samplers or from surface separators and steam-water separation equipment. These samples are preserved and transported under controlled conditions, typically cooled to about 4 °C, protected from light, and delivered to the laboratory within the specified holding times per ISO 5667-3.

Testing Items

Physical and Field Parameters

• Wellhead and downhole temperature, flow rate, and wellhead pressure
• pH value, electrical conductivity, redox potential (Eh), and salinity
• Total dissolved solids (TDS), suspended solids, turbidity, color, odor, and transparency
• Total hardness, total alkalinity, and density

Major Ion Chemistry

• Cations: potassium (K⁺), sodium (Na⁺), calcium (Ca²⁺), magnesium (Mg²⁺), ammonium (NH₄⁺), total iron, ferrous iron (Fe²⁺), aluminum, and manganese
• Anions: chloride (Cl⁻), sulfate (SO₄²⁻), bicarbonate (HCO₃⁻), carbonate (CO₃²⁻), nitrate (NO₃⁻), nitrite (NO₂⁻), and fluoride (F⁻)
• Results are used to classify the water type (for example, chloride-sodium type or bicarbonate type) using Piper or Shukarev classification diagrams

Characteristic and Balneotherapeutic Components

• Metasilicic acid (H₂SiO₃) and dissolved silica (SiO₂)
• Fluoride, hydrogen sulfide (H₂S), and total sulfide
• Lithium, strontium, rubidium, cesium, barium, bromide, and iodide
• Boric acid (H₃BO₃) and boron
• Radon (²²²Rn) and total radioactivity indicators (gross alpha, gross beta)
• Under commonly applied specifications such as GB/T 13727, metasilicic acid at or above 25–50 mg/L, fluoride at or above 2 mg/L, TDS at or above 1000 mg/L, and water temperature at or above 34 °C are frequently used as thresholds for naming therapeutic thermal mineral waters

Trace Elements and Heavy Metals

• Arsenic, mercury, lead, cadmium, chromium (including hexavalent chromium), copper, zinc, selenium, nickel, and antimony
• These data support sanitary evaluation, discharge compliance, and corrosion source diagnosis

Dissolved Gases

• Free carbon dioxide (CO₂), dissolved oxygen (O₂), and total carbon dioxide
• Hydrogen sulfide (H₂S), methane (CH₄), nitrogen (N₂), and helium where relevant
• Radon and thoron in gas phase for high-temperature or gas-rich geothermal systems

Isotopic and Geochemical Indices

• Stable isotopes δ²H and δ¹⁸O to determine recharge origin and water-rock exchange
• Tritium (³H) and carbon-14 for groundwater age dating and circulation evaluation
• Silica (quartz and chalcedony), Na-K, K-Mg, and Na-K-Ca geothermometers for reservoir temperature estimation
• Langelier Saturation Index (LSI), Ryznar index, and Larson corrosion index for scaling and corrosivity assessment

Microbiological Parameters (for bathing and spa applications)

• Total bacterial count, total coliform, fecal coliform, and Pseudomonas aeruginosa
• Legionella pneumophila in distributed hot water systems and bathing facilities

Testing Methods

Inspection is performed in two stages: rapid on-site measurement of unstable parameters and comprehensive instrumental analysis in the laboratory. Sampling quality assurance follows ISO 5667-1, ISO 5667-3, and ISO 5667-11 (groundwater sampling), together with national technical regulations for sampling and preservation. Separate aliquots are typically prepared: unfiltered samples for anions and organic-free analysis, filtered and acidified samples for metals, and special containers for silica, sulfide, and gas analyses.

On-Site (In-Situ) Measurements

• Temperature, pH (ISO 10523 / ASTM D1293), electrical conductivity (ISO 7888 / ASTM D1125), Eh, and turbidity (ISO 7027) are measured immediately at the wellhead
• Free CO₂, dissolved oxygen (ASTM D888), hydrogen sulfide, and total alkalinity are titrated on site because degassing and pressure loss rapidly alter their values

Laboratory Instrumental Analysis

• Ion chromatography for fluoride, chloride, sulfate, nitrate, and nitrite (ISO 10304-1, ASTM D4327)
• Inductively coupled plasma optical emission spectrometry (ICP-OES) for major and minor cations (ISO 11885)
• Inductively coupled plasma mass spectrometry (ICP-MS) for trace elements and heavy metals (ISO 17294-2)
• Flame and graphite furnace atomic absorption spectrometry for sodium, potassium (ASTM D3561), calcium and magnesium (ASTM D511), iron (ASTM D1068), mercury by cold vapor AAS (ASTM D3223), and arsenic by hydride generation (ASTM D2972)
• EDTA complexometric titration for hardness and calcium (ISO 6058, ISO 6059); argentometric titration for chloride (ASTM D512, ISO 9297); turbidimetric or gravimetric methods for sulfate (ASTM D516)
• UV-Visible spectrophotometry for silica (ASTM D859), fluoride (ASTM D1179), ammonium, and nitrite
• Liquid scintillation counting for radon and tritium; low-background alpha/beta counting for gross radioactivity
• Isotope ratio mass spectrometry (IRMS) for δ²H and δ¹⁸O; accelerator or conventional radiometric methods for carbon-14
• Gravimetric determination of total dissolved solids and suspended solids after filtration and drying
• National groundwater analysis method series (DZ/T 0064) and mineral water testing methods (GB/T 8538) are applied where specified for regulatory acceptance

Applications

• Geothermal resource exploration, grading, and reserve evaluation, including reservoir temperature estimation and water type classification under GB/T 11615
• Certification of therapeutic thermal mineral water for spas, sanatoriums, and bathing developments under GB/T 13727 naming criteria
• Design and operation of geothermal district heating systems, including heat exchanger and pipeline material selection based on corrosion and scaling indices
• Reinjection compatibility studies, clogging risk assessment, and sustainable reservoir management
• Agricultural greenhouse heating, soil warming, and aquaculture water suitability evaluation
• Environmental impact assessment, wastewater discharge compliance, and groundwater protection monitoring
• Sanitary and hygienic control of bathing facilities, including Legionella risk management in hot water distribution networks
• Long-term well field monitoring to detect reservoir cooling, mixing, or depletion trends

FAQ

How often should geothermal well water be tested?

A full hydrochemical analysis is required during the exploration and commissioning stage and is typically repeated at least once per year during operation. Field parameters such as temperature, pH, and conductivity should be monitored monthly or quarterly, while microbiological testing for bathing facilities follows local sanitary regulations, often on a monthly or quarterly basis.

Why must some parameters be measured on site rather than in the laboratory?

Geothermal water is strongly affected by pressure and temperature changes at the wellhead. Carbon dioxide degassing shifts pH and can trigger calcium carbonate precipitation, while oxygen ingress alters redox-sensitive species such as iron, sulfide, and hydrogen sulfide. Measuring pH, Eh, alkalinity, dissolved gases, and temperature immediately at the wellhead preserves the representativeness of the sample.

What is the difference between a full analysis and a simplified analysis?

A simplified (routine) analysis covers field parameters, major ions, and selected indicators and is used for regular monitoring. A full analysis additionally includes trace elements, characteristic components, gases, and isotopes, and is performed at the exploration stage, for therapeutic water certification, or when significant changes in water chemistry are observed.

How is reservoir temperature estimated from water chemistry?

Chemical geothermometers rely on temperature-dependent equilibrium reactions. Dissolved silica (quartz or chalcedony geothermometer), sodium-potassium and potassium-magnesium ratios, and Na-K-Ca relations are combined to estimate the temperature of the deep reservoir, which is often higher than the measured wellhead temperature. Results are cross-checked with isotope data and downhole temperature logs.

How is scaling and corrosion potential evaluated?

Laboratory data are used to calculate the Langelier Saturation Index and Ryznar index for calcium carbonate scaling, to assess silica saturation with respect to amorphous silica, and to compute the Larson ratio from chloride, sulfate, and alkalinity for corrosion tendency. Dissolved oxygen, free CO₂, H₂S, and TDS further inform material selection and inhibitor programs for surface equipment.