Determination of Dissolved Oxygen in Groundwater: Sampling, Methods, and Applications

Dissolved oxygen (DO) determination is one of the most important field measurements in groundwater investigation, providing direct insight into redox conditions, recharge pathways, microbial activity, and the natural attenuation of contaminants in aquifers. This article provides a comprehensive technical overview of groundwater dissolved oxygen testing, covering sampling protocols, iodometric and sensor-based analytical methods, relevant standards such as ISO 5813, ISO 5814, ASTM D888 and GB 7489, and practical applications in hydrogeology, water supply engineering and environmental remediation.

Technical Overview

Dissolved oxygen (DO) refers to the molecules of gaseous oxygen (O2) physically dissolved in water and available for chemical and biological reactions. In aquifers, the dissolved oxygen content is controlled by the oxygen carried into the subsurface by infiltrating recharge water and by the continuous consumption of that oxygen through microbial respiration and the oxidation of reduced chemical species such as natural organic matter, iron(II)-bearing minerals, sulfides and methane. Because these consumption processes proceed along the flow path, groundwater is usually oxygen-poor compared with surface water: shallow, rapidly recharged aquifers in coarse, unweathered strata may contain 4 to 8 mg/L, while deep confined aquifers commonly show concentrations below 0.5 mg/L, and many groundwaters are effectively anoxic.

The solubility of oxygen in fresh water at atmospheric pressure is approximately 11.3 mg/L at 10 °C and 9.1 mg/L at 20 °C, decreasing with rising temperature, increasing salinity and decreasing barometric pressure. Field results are therefore commonly reported both as a concentration (mg/L) and as a percentage of saturation. The measured DO concentration is a master variable that defines the redox environment of an aquifer: values above roughly 1–2 mg/L characterize oxic conditions in which aerobic metabolism and oxidative precipitation of iron and manganese dominate, whereas lower values mark the transition through suboxic to anoxic conditions where nitrate reduction, sulfate reduction and methanogenesis become thermodynamically favorable.

Dissolved oxygen is a highly non-conservative parameter. Groundwater is typically oversaturated or undersaturated relative to the atmosphere, so any contact with air during sampling, purging or bottling can change the result by the full magnitude of the true value. Reliable determination therefore depends on controlled sampling techniques such as low-flow purging, flow-through cells or downhole probes, immediate on-site measurement or chemical fixation, and adherence to standardized procedures published by organizations such as ISO, ASTM, EN and the Chinese national standards body (GB).

Testing Samples

Groundwater from Monitoring Wells

Dedicated monitoring wells are the preferred sampling points for DO determination because they allow full control of purge procedures. Low-flow (minimal-drawdown) sampling with purge rates of approximately 0.1–0.5 L/min, following the guidance of ISO 5667-11 and ASTM D4448, is used to draw formation water into the well with minimal aeration or degassing. Field parameters (DO, temperature, pH, conductivity, turbidity, ORP) are monitored during purging, and sampling begins only after they stabilize, typically when successive DO readings vary by no more than approximately 0.2 mg/L or 10% and turbidity remains below 10–20 NTU.

Water Supply and Production Wells

For drinking water, irrigation or industrial supply wells, samples are taken directly at the wellhead from a valve or tap located upstream of any pressure tank, treatment unit, aerator or storage vessel, all of which equilibrate the water with air and falsify the DO result. The line is flushed for several minutes at moderate flow before measurement or bottling, and splashing, cascading and narrow-orifice faucets are avoided.

Springs, Seeps and Shallow Piezometers

Spring water can be measured by immersing a sensor or filling a sample bottle directly at the discharge orifice so that water flows gently into the container without contact with the atmosphere. Shallow piezometers and direct-push profiling points used in site investigations are sampled with peristaltic or bladder pumps at low rates, or with downhole probe sensors deployed in the screened interval.

Sample Containers and Preservation

When the iodometric method is used, samples are collected into 300 mL narrow-neck biochemical oxygen demand (BOD) bottles with tapered ground-glass stoppers. The bottle is allowed to overflow for two to three volumes, the stopper is inserted without entrapping a bubble, and the sample is immediately fixed in the field by adding manganese(II) sulfate solution and alkaline iodide–azide reagent below the meniscus. Properly fixed, dark-stored samples may be transported to the laboratory, but titration is recommended within 8 hours to limit systematic loss. For sensor-based measurements, no container is needed: the determination is performed in a sealed flow-through cell or directly in the well.

Testing Items

The core deliverable of the test is the dissolved oxygen concentration of the groundwater, expressed in milligrams per litre (mg/L), together with the auxiliary measurements needed to interpret it. A typical testing program includes:

• Dissolved oxygen (DO) concentration, mg/L — the primary determination.
• Oxygen saturation index, % — calculated from DO, water temperature, barometric pressure and salinity.
• Water temperature, °C — mandatory for saturation calculation and sensor compensation.
• Barometric pressure — required for calibration and saturation corrections at altitude or depth.
• Simultaneous field parameters: pH, electrical conductivity, oxidation–reduction potential (ORP) and turbidity, used to verify purging stabilization and interpret redox chemistry.
• Supporting laboratory parameters frequently requested with DO: nitrate, nitrite, ammonium, iron(II)/iron(III), manganese, sulfate, sulfide, dissolved organic carbon, chemical oxygen demand (COD) and biochemical oxygen demand (BOD), which allow full redox characterization of the aquifer.

Testing Methods

Iodometric Method (Winkler Titration)

The iodometric method specified in ISO 5813, EN 25813, ASTM D888 and GB 7489 is the classical reference procedure for dissolved oxygen. Manganese(II) sulfate is added to the sample, followed by an alkaline sodium iodide–sodium hydroxide reagent. Dissolved oxygen rapidly oxidizes the precipitated manganese(II) hydroxide to a brown manganese(IV) oxide floc. Upon acidification with sulfuric acid, the oxidized manganese releases iodine in an amount equivalent to the original oxygen content. The liberated iodine is titrated with standardized sodium thiosulfate solution using starch as the endpoint indicator; with 0.025 mol/L thiosulfate and a 300 mL bottle, each millilitre of titrant corresponds to 0.2 mg/L of dissolved oxygen.

• Applicable range: approximately 0.2 to 20 mg/L with normal titration volumes; lower levels can be determined with dilute titrant or photometric endpoint detection.
• Interferences and corrections: nitrite (removed by the azide modification), iron(II) and iron(III) (azide or permanganate modification), suspended solids (alum flocculation), sulfites, thiosulfate and free chlorine.
• Performance: repeatability is typically better than 0.2 mg/L for clean waters, and the method serves as the arbitration procedure against which field sensors are verified.
• Limitations: multi-step wet chemistry, field fixation required, and not suited to continuous monitoring.

Electrochemical Probe Method (Membrane Electrode)

The membrane electrode method standardized in ISO 5814, EN 25814, ASTM D888 and GB 11913 uses a Clark-type polarographic or galvanic cell in which oxygen diffuses through a gas-permeable membrane and is reduced at a cathode. The resulting current is proportional to the oxygen partial pressure and is converted to concentration using temperature, salinity and pressure compensation. Sensors are calibrated in water-saturated air or air-saturated water against the site barometric pressure, with a zero check in sodium sulfite solution.

• Advantages: direct, rapid determination; suitable for continuous in-well profiling and flow-cell monitoring with minimal sample handling.
• Limitations: response depends on sample flow past the membrane; membranes are subject to fouling by the iron and manganese oxyhydroxides and biofilms common in anoxic groundwater; electrolyte and membranes must be replaced periodically; hydrogen sulfide and sulfur dioxide interfere.
• Typical accuracy after calibration: ±0.2 mg/L or ±2% of reading for instruments in good condition.

Optical (Luminescence Quenching) Method

Optical dissolved oxygen sensors, included among the procedures of ASTM D888, measure the quenching of luminescence from an immobilized luminophore (typically a ruthenium or porphyrin complex in a polymer matrix) by molecular oxygen. A blue or green light pulse excites the sensing film, and the lifetime or phase shift of the emitted red light decreases as the oxygen concentration increases, following the Stern–Volmer relationship. Because no oxygen is consumed and no membrane transport is involved, optical sensors exhibit low drift, require little maintenance, are insensitive to sample flow, respond within seconds, and remain accurate at the sub-0.1 mg/L concentrations typical of deep anoxic groundwater. Trace versions of these sensors extend measurement capability down to a few micrograms per litre.

On-Site Measurement, In-Situ Profiling and Quality Control

For groundwater, the measurement location is as important as the analytical method. Best practice combines low-flow purging with a sealed flow-through cell fitted with DO, temperature, conductivity, pH and ORP sensors, so that formation water never contacts the atmosphere. Alternatively, downhole multiparameter sondes are lowered into the screened interval to log DO continuously with depth, which reveals stratification and the position of redox boundaries within the aquifer.

Quality control measures applied by accredited third-party laboratories and field teams include pre- and post-survey calibration verification of sensors against water-saturated air and zero-oxygen solution; periodic cross-checking of sensor results against Winkler titration on the same water; duplicate titrations agreeing within 0.1–0.2 mg/L; reagent blanks; volumetric verification of burettes; documentation of purging stabilization data; and method implementation under an ISO/IEC 17025 quality management system.

Method Selection Guidance

• Regulatory or arbitration purposes, clean or mildly polluted groundwater: iodometric method (ISO 5813 / GB 7489) with the appropriate modification.
• Routine monitoring, continuous logging and low-flow sampling programs: optical or electrochemical sensors in a flow-through cell.
• Deep or anoxic groundwaters with concentrations below 0.5 mg/L: trace-capable optical sensors or photometric iodometric variants.
• Waters with high iron, sulfide, suspended solids or color: field sensor measurement is preferred; wet-chemical methods require the relevant modifications.

Applications

Dissolved oxygen data from groundwater are used across hydrogeology, water supply engineering and environmental management:

• Redox characterization of aquifers: DO defines the boundary between oxic, suboxic and anoxic zones and supports interpretation of iron, manganese, nitrogen, sulfur and carbon speciation along groundwater flow paths.
• Groundwater quality and drinking water assessment: low-oxygen groundwater promotes dissolution of iron, manganese and arsenic and affects taste, color and treatment requirements, while elevated DO informs corrosion management in wells and distribution networks.
• Contaminated site investigation: DO above approximately 1–2 mg/L sustains aerobic biodegradation of petroleum hydrocarbons and BTEX compounds, whereas DO depletion identifies contaminant plumes from landfills, fuel releases and wastewater infiltration and supports monitored natural attenuation assessments.
• Nitrate and agricultural studies: DO controls denitrification, so DO profiles explain nitrate attenuation in shallow aquifers beneath farmland and guide fertilizer and irrigation management.
• Managed aquifer recharge and riverbank filtration: DO is monitored to evaluate oxidative clogging of infiltration surfaces and the redox evolution of recharged water.
• Water resource and geothermal development: DO data guide well-material selection and the assessment of corrosion and scaling potential in production infrastructure.
• National and regional groundwater monitoring networks: DO is a standard indicator parameter in baseline surveys and long-term trend programs supporting groundwater quality classification frameworks.

FAQ

Why must dissolved oxygen in groundwater be measured on site?

Groundwater is usually not in equilibrium with the atmosphere, so pumping, splashing, bottling or even gentle mixing can add or strip oxygen by an amount comparable to the true concentration. Except for samples chemically fixed with the Winkler reagents, DO cannot be preserved in a container. Measurements are therefore performed in a flow-through cell or downhole, or fixed immediately at the wellhead with titration completed as soon as possible, ideally within 8 hours.

What dissolved oxygen concentrations are typical for groundwater?

Values range from near zero to about 8 mg/L. Shallow unconfined aquifers with rapid recharge often contain 2–6 mg/L; deeper or confined aquifers, and groundwaters in organic-rich or reduced formations, commonly contain less than 1 mg/L, and many are fully anoxic. Results must always be interpreted together with temperature and redox-sensitive chemical species.

Which method is more accurate: the Winkler titration or a sensor?

The iodometric method is the internationally recognized reference procedure and, in skilled hands with the correct modification, achieves repeatability better than 0.2 mg/L on clean samples. Modern optical sensors achieve comparable or better accuracy in the field, respond faster, and perform better at the low concentrations typical of groundwater. Good practice verifies sensor readings periodically against a Winkler determination on the same water.

What interferences should be considered?

For the iodometric method, the principal interferences are nitrite, ferrous and ferric iron, suspended solids, sulfite, thiosulfate, free chlorine and readily oxidized organic matter; standard modifications (azide, permanganate, alum flocculation) remove most of them. For electrochemical sensors, hydrogen sulfide, membrane fouling by iron–manganese precipitates and insufficient flow past the membrane are the main concerns; optical sensors are largely immune to these effects but require clean, undamaged sensing films.

Can samples for DO testing be transported to a laboratory?

Only in fixed form. Samples collected in BOD bottles and immediately fixed with manganese(II) sulfate and alkaline iodide–azide reagent, sealed without headspace and stored dark and cool, can be transported, but titration should be completed within about 8 hours. Unfixed samples, and all sensor-based measurements, must be handled on site.

What detection limits and reporting units apply?

Conventional Winkler titration is reliable down to roughly 0.2 mg/L; photometric and micro-titration variants extend this to about 0.05 mg/L. Optical trace sensors can report down to a few micrograms per litre. Results are reported in mg/L, commonly together with the saturation percentage and the measurement temperature.

How is sensor calibration performed in the field?

Sensors are calibrated in water-saturated air or air-saturated water using the site barometric pressure, with a zero point established in sodium sulfite solution. For work at depth or altitude, pressure and salinity compensation is applied. Calibration is verified at the start and end of each field day and cross-checked against a Winkler titration where conditions allow.