Total dissolved solids (TDS) determination is one of the most fundamental physicochemical tests for evaluating groundwater quality, and it plays a critical role in drinking water safety assessment, hydrogeological surveys, and irrigation suitability classification. This article provides a comprehensive technical overview of groundwater TDS testing, covering gravimetric, conductivity-based, and ion-summation methods, applicable standards such as GB/T 5750.4, GB/T 14848, and ASTM D5907, quality control requirements, and typical application scenarios.
Technical Overview
Total dissolved solids (TDS) refers to the total mass of dissolved constituents—primarily inorganic salts such as calcium, magnesium, sodium, potassium, bicarbonate, carbonate, sulfate, chloride, and nitrate, together with small amounts of dissolved organic matter—contained in a unit volume of water. In practice, TDS is operationally defined as the residue remaining after a water sample is filtered through a 0.45 µm membrane filter and evaporated to dryness at a specified temperature (105°C or 180°C), with results expressed in milligrams per liter (mg/L).
For groundwater, TDS is a master indicator that reflects the overall mineralization of the aquifer. It is influenced by the mineralogy of the host rocks, residence time, recharge conditions, and anthropogenic inputs such as agricultural runoff, industrial discharge, landfill leachate, and seawater intrusion. Elevated TDS affects water palatability, causes scaling in pipes and boilers, accelerates corrosion, reduces the suitability of water for irrigation, and may signal contamination of the aquifer.
TDS determination for groundwater is typically performed in accordance with the following reference standards and guidelines:
- GB/T 5750.4 — Standards for drinking water quality examination methods, total dissolved solids by the gravimetric method.
- GB/T 14848 — Groundwater quality standard, which defines TDS classification limits for Classes I through V.
- GB 5749 — Standards for drinking water quality, which sets a TDS limit of 1,000 mg/L for centralized drinking water supplies.
- ASTM D5907 — Standard test methods for filterable (dissolved) solids in water.
- Standard Methods for the Examination of Water and Wastewater, Method 2540 C — Total dissolved solids dried at 180°C.
- US EPA Method 160.1 — Residue, filterable, gravimetric, dried at 180°C.
- ISO/IEC 17025 — General requirements for the competence of testing and calibration laboratories.
Under GB/T 14848, groundwater is classified according to TDS as follows: Class I ≤ 300 mg/L, Class II ≤ 500 mg/L, Class III ≤ 1,000 mg/L, Class IV ≤ 2,000 mg/L, and Class V greater than 2,000 mg/L. Groundwater used for centralized drinking water supply should generally comply with Class III requirements. For comparison, the US EPA secondary drinking water standard recommends a TDS level of 500 mg/L, and the World Health Organization notes that palatability of drinking water deteriorates significantly above approximately 1,000 mg/L.
Testing Samples
TDS testing applies to a wide range of groundwater sample types collected from different hydrogeological settings and end-use scenarios. Representative sampling and proper preservation are essential to obtain valid results.
Drinking Water Supply Well Samples
Raw groundwater from municipal supply wells, rural centralized water sources, and private domestic wells, tested to verify compliance with drinking water quality standards before and after treatment.
Environmental Monitoring Well Samples
Groundwater collected from purpose-drilled monitoring wells at industrial sites, landfills, gas stations, contaminated sites, and mining areas, used to track changes in mineralization and detect contamination plumes or seawater intrusion.
Agricultural and Irrigation Well Samples
Groundwater from irrigation wells, evaluated for salinity hazard and suitability for crop production, particularly in arid and semi-arid regions where soil salinization is a concern.
Geothermal and Mineral Water Samples
Deep-circulating geothermal water, thermal springs, and natural mineral water sources, for which TDS is a key parameter in resource characterization and product classification.
Industrial Source Water Samples
Groundwater used as boiler feed water, cooling water, or process water, where dissolved solids directly affect scaling, corrosion, and treatment requirements.
Brackish and Hypersaline Groundwater Samples
High-mineralization groundwater from coastal, confined, or deep aquifers, including feed water for desalination projects, where TDS may range from several thousand to tens of thousands of mg/L.
Samples are generally collected in clean glass or polyethylene containers after purging the well and stabilizing field parameters. Filtration through a 0.45 µm membrane is performed as soon as practical, samples are stored at 4°C without chemical preservation, and analysis is completed within seven days in accordance with standard hold-time requirements.
Testing Items
The core testing item is total dissolved solids (TDS) in groundwater, expressed in mg/L. To strengthen data interpretation, TDS testing is frequently combined with the following supporting parameters:
- Electrical conductivity (EC), used as a field screening indicator and for cross-verification of gravimetric results.
- Total solids (TS) and total suspended solids (TSS), to distinguish dissolved from particulate matter.
- Salinity, for classification of brackish and saline groundwater.
- Major ion composition, including calcium, magnesium, sodium, potassium, bicarbonate, carbonate, sulfate, chloride, and nitrate, for hydrochemical characterization and ion-balance validation.
- Water hardness and pH, which are closely correlated with dissolved mineral content.
Testing Methods
Gravimetric Method (Reference Method)
The gravimetric method is the standard arbitration technique for TDS determination and is specified in GB/T 5750.4, ASTM D5907, and Standard Methods 2540 C. The procedure is as follows:
- Filter a well-mixed sample through a pre-rinsed 0.45 µm membrane filter to remove suspended solids.
- Measure a suitable aliquot (typically 100 mL, adjusted according to the expected TDS concentration) into a clean evaporating dish that has been pre-dried, cooled in a desiccator, and weighed to the nearest 0.1 mg.
- Evaporate the sample to near dryness on a water bath or in a drying oven at approximately 100°C.
- Dry the residue in an oven at 105 ± 2°C (or 180 ± 2°C, depending on the specified protocol) for two hours.
- Cool in a desiccator and weigh; repeat the drying, cooling, and weighing cycle until consecutive weighings differ by no more than 0.4 mg (constant weight).
- Calculate the result using the formula: TDS (mg/L) = (m1 − m0) × 10⁶ / V, where m0 is the mass of the empty dish in grams, m1 is the mass of the dish plus dried residue in grams, and V is the sample volume in milliliters.
Drying at 180°C is preferred when a more accurate representation of true dissolved ion content is required, because it removes mechanically occluded water and converts bicarbonate to carbonate with the loss of carbon dioxide and water. Drying at 105°C retains crystallization water in certain salts and yields slightly higher results; the selected temperature must always be reported with the result.
Electrical Conductivity Conversion Method
Dissolved ions carry electric current, so EC measured with a conductivity meter can be used to estimate TDS using an empirical conversion factor: TDS (mg/L) ≈ EC (µS/cm) × k, where k typically ranges from 0.55 to 0.76 depending on the ionic composition of the water. A factor of 0.55 to 0.64 is commonly applied to sodium-chloride-dominated groundwater. This method is rapid, inexpensive, and suitable for field screening, well profiling, and continuous online monitoring, but it is an estimation and is not accepted as a substitute for the gravimetric method in compliance testing.
Ion Summation Method
In hydrogeological investigations, TDS may be calculated as the sum of the quantified concentrations of the major dissolved cations and anions. The method provides a useful cross-check of the gravimetric result, and agreement between the total dissolved cation and anion equivalents within 5% is generally considered acceptable analytical quality. Deviations beyond this threshold indicate analytical error or the presence of unmeasured constituents.
Online and Automated Monitoring
For long-term groundwater monitoring networks, desalination plants, and wellhead treatment facilities, online TDS analyzers based on conductivity sensors with automatic temperature compensation and data telemetry provide continuous, real-time measurements, with periodic gravimetric verification performed by accredited third-party laboratories.
Quality Control and Assurance
Reliable TDS data require rigorous quality control, including method blanks, duplicate samples (relative percent difference typically within 5%), balance calibration with certified reference weights, preconditioned and pre-weighed dishes, strict adherence to constant-weight criteria, controlled desiccator conditions, and participation in proficiency testing. Laboratories holding ISO/IEC 17025 accreditation demonstrate technical competence in performing these determinations.
Applications
TDS determination for groundwater supports a broad spectrum of scientific, regulatory, and engineering purposes:
- Drinking water safety assessment for municipal, rural, and private water supplies, including verification of compliance with GB 5749 and international drinking water guidelines.
- Groundwater quality classification and zoning under GB/T 14848 for regional hydrogeological surveys and water resource planning.
- Irrigation water suitability evaluation, including salinity hazard assessment for crop and soil management.
- Industrial water management, including boiler feed water, cooling water, and process water quality control and scaling risk assessment.
- Geothermal and mineral water resource characterization and product development.
- Environmental monitoring of landfills, contaminated sites, mining districts, and coastal aquifers affected by seawater intrusion.
- Performance monitoring of water treatment and desalination systems, including reverse osmosis and electrodialysis processes.
- Baseline and impact studies for environmental impact assessment and groundwater remediation projects.
FAQ
What is the difference between TDS, total solids, and suspended solids?
Total solids (TS) represent all residue left after evaporating an unfiltered sample; suspended solids (TSS) are the particulate matter retained on a 0.45 µm filter; and TDS is the residue from the filtrate that passes through the filter. In other words, TS is approximately equal to TDS plus TSS.
Why are two drying temperatures, 105°C and 180°C, specified?
Drying at 105°C retains water of crystallization in certain salts, while drying at 180°C removes occluded water and thermally decomposes bicarbonate to carbonate, yielding a result closer to the true sum of dissolved ions. The two values can differ by several percent, particularly in bicarbonate-rich groundwater, so the drying temperature must be clearly stated in the test report.
What TDS level is acceptable in drinking water?
The national standard for drinking water in China sets a TDS limit of 1,000 mg/L, and groundwater used for centralized supply should meet Class III requirements of GB/T 14848 (≤ 1,000 mg/L). Water with TDS below 600 mg/L is generally rated as good in palatability, while levels above 1,000 mg/L noticeably impair taste.
How should groundwater samples be collected and preserved for TDS testing?
Wells should be purged until field parameters stabilize before sampling. Samples are collected in clean, rinsed glass or plastic containers after filtration through a 0.45 µm membrane, stored at 4°C without chemical preservatives, and analyzed as soon as possible, preferably within seven days.
Can a conductivity meter replace the gravimetric method?
No. Conductivity-based TDS values are convenient estimates suitable for screening and continuous monitoring, but the conversion factor varies with water composition. Regulatory compliance and reportable results must be based on the gravimetric method.
How is TDS related to hardness and salinity?
Hardness refers only to the polyvalent cations, mainly calcium and magnesium, and is therefore a subset of TDS. Salinity describes the total concentration of dissolved mineral salts and is closely related to TDS; groundwater with TDS between 1,000 and 10,000 mg/L is commonly classified as brackish, and above 10,000 mg/L as saline.
