Groundwater chemical oxygen demand (COD) testing quantifies the oxygen required to chemically oxidize organic matter (and certain reducing inorganic species) dissolved in groundwater, serving as a core indicator of organic contamination for drinking water safety, site investigation, and regulatory compliance. This article summarizes the applicable international and national standards, sampling and preservation requirements, analytical methods, interference control measures, and quality assurance criteria that accredited third-party laboratories follow when determining COD in groundwater samples.
Technical Overview
Chemical oxygen demand (COD) expresses, in milligrams of oxygen per liter (mg/L), the quantity of oxidizable matter in a water sample that can be consumed by a strong chemical oxidant under controlled acidic and thermal conditions. Because groundwater typically carries low levels of dissolved organic matter due to natural filtration through soil and aquifer media, its COD values are usually far lower than those of wastewater or surface runoff. Elevated COD in groundwater signals potential infiltration of sewage, landfill leachate, agricultural runoff, petroleum hydrocarbons, or industrial effluent, and therefore forms part of routine groundwater quality surveillance programs.
Two families of COD-type parameters are relevant to groundwater testing:
- CODCr (dichromate COD): oxidation with potassium dichromate in strongly acidic medium, catalyzed by silver sulfate; the reference method for most water and wastewater matrices.
- CODMn (permanganate index): oxidation with potassium permanganate under milder conditions; internationally recognized as more appropriate for relatively clean water such as groundwater and drinking water, because it offers lower working ranges and gentler digestion.
The principal technical challenge in groundwater COD testing is the very low analyte concentration, which pushes conventional dichromate methods toward their detection limits, combined with potential chloride interference in saline or coastal aquifers. The principal standards organizations' methods applicable to groundwater COD determination include:
- HJ 828-2017 – Water quality – Determination of the chemical oxygen demand – Dichromate method (the current Chinese environmental protection standard replacing GB/T 11914).
- HJ/T 399 – Water quality – Determination of the chemical oxygen demand – Fast digestion–spectrophotometric method.
- HJ/T 70 – Determination of the chemical oxygen demand in high-chloride water – Chloride correction method.
- GB 11892-89 – Water quality – Determination of permanganate index, the preferred method for low-COD groundwater.
- ISO 6060 – Water quality – Determination of the chemical oxygen demand (dichromate reflux).
- ISO 15705 – Water quality – Determination of the chemical oxygen demand index (small-scale sealed-tube method).
- ISO 8467 – Water quality – Determination of permanganate index.
- ASTM D1252 – Standard test methods for chemical oxygen demand (dichromate oxygen demand) of water.
- GB/T 14848-2017 – Standard for groundwater quality, which sets classification limits and drives most groundwater monitoring programs.
Testing Samples
Monitoring Well Groundwater
Samples are collected from dedicated groundwater monitoring wells at industrial facilities, landfills, gas stations, agricultural areas, and background locations. Before sampling, wells must be purged (typically 3–5 well volumes or until field parameters such as pH, conductivity, and temperature stabilize) so that the collected water represents the aquifer formation rather than stagnant casing water. Low-flow purging techniques are recommended for sensitive sites to minimize turbidity, which can artificially bias COD results upward.
Drinking Water Source Wells
Groundwater abstracted for centralized or private drinking water supply is tested to verify conformity with groundwater quality classification requirements and to detect any organic contamination trends. Because concentrations are low, the permanganate index (CODMn) is usually the specified indicator for these matrices, and sensitive methods with correspondingly low detection limits must be selected.
Contaminated Site and Remediation Groundwater
At sites undergoing investigation or remediation (e.g., former industrial plants, fuel depots, solid waste disposal areas), COD is used both as a screening indicator of gross organic contamination and as a performance metric during remediation. High chloride, iron, or sulfide levels frequently accompany contamination at such sites, so interference evaluation is mandatory before results are reported.
Industrial and Construction Dewatering Water
Groundwater pumped from excavations, mines, or dewatering systems is often discharged to surface water bodies or municipal networks, and COD testing of the discharge is commonly required to demonstrate compliance with discharge permits before release.
Testing Items
Depending on the project objective and the applicable groundwater quality standard, the following COD-related items are typically requested from accredited third-party laboratories:
- CODCr – dichromate chemical oxygen demand, for contaminated groundwater, leachate-impacted wells, and dewatering discharges.
- CODMn (permanganate index) – the standard organic indicator for clean groundwater and drinking water sources, with low working ranges suited to trace levels.
- Chloride – essential supporting measurement, because chloride concentration determines whether masking reagents or chloride correction procedures are required for valid CODCr results.
- Supporting organic indicators – BOD5, total organic carbon (TOC), and ammonia nitrogen, which are frequently reported alongside COD to characterize the nature and biodegradability of organic contamination.
- Field parameters – pH, conductivity, dissolved oxygen, oxidation–reduction potential, and turbidity, recorded during sampling to support data interpretation.
Sampling, Preservation and Pretreatment Requirements
Valid COD data begin with correct field procedures. The following requirements are standard practice under HJ 828-2017, ISO 5667 series guidance, and equivalent sampling standards:
- Collect samples into clean glass bottles (wide-mouth, borosilicate) of at least 100 mL capacity; avoid plastic containers that may leach organics.
- Preserve immediately with concentrated sulfuric acid to pH below 2, which inhibers microbial degradation of organic matter.
- Transport and store samples refrigerated at approximately 4 °C in the dark.
- Analyze preserved samples within the holding period specified by the governing method — typically up to 7 days when acidified; unpreserved samples must be analyzed as soon as possible, normally within 24 hours.
- Do not filter the sample unless the monitoring program explicitly defines COD of the dissolved fraction; suspended particulate organic matter is part of the conventional COD measurement.
- Record well depth, purge volume, sampling time, and field parameters on the chain-of-custody documentation to ensure traceability.
Testing Methods
Dichromate Reflux Titration (Reference Method)
The open-reflux dichromate method (HJ 828-2017, ISO 6060, ASTM D1252) is the reference procedure for CODCr. A measured sample portion is mixed with a known excess of potassium dichromate in strongly acidic medium, with silver sulfate as catalyst and mercuric sulfate added to complex chloride. The mixture is refluxed at approximately 146 °C for 2 hours, after which the remaining dichromate is titrated with standardized ferrous ammonium sulfate using ferroin (1,10-phenanthroline) indicator. COD is calculated from the difference in titrant consumption between blank and sample:
COD (mg/L) = 8000 × (V0 − V1) × c / V, where V0 and V1 are the titrant volumes for blank and sample (mL), c is the exact ferrous ammonium sulfate molarity (mol/L), and V is the sample volume (mL).
Under routine conditions the detection limit of the method is on the order of 4 mg/L with a lower reporting bound of roughly 16 mg/L, which means many uncontaminated groundwater samples fall below the quantitative range of this technique.
Closed-Reflux Colorimetric and Rapid Digestion Methods
Sealed-tube digestion followed by spectrophotometric determination (ISO 15705, HJ/T 399, and equivalent EPA colorimetric procedures) digests the sample in pre-dosed reagent vials and quantifies residual dichromate (or generated Cr3+) photometrically. Advantages include small sample volume, lower reagent consumption, batch throughput, and commercially available low-range formulations suitable for cleaner waters. Digestion temperature and time must be strictly controlled (typically 165 °C for about 15–20 minutes in closed vials, or as specified by the validated method), and results must be verified against the reference titration method during method validation.
Permanganate Index Method for Low-Level Groundwater
For groundwater and other relatively clean matrices, the permanganate index (GB 11892-89, ISO 8467) is the method of choice. The sample is boiled with a known excess of potassium permanganate in acidic (or alkaline, for high-chloride water) medium for a fixed time; the consumed permanganate is then determined by back-titration with sodium oxalate or sodium thiosulfate. The method covers low milligram-per-liter ranges and is directly compatible with the classification limits used in groundwater quality standards, making it the standard indicator for drinking water source evaluation.
Chloride Interference and Correction
Chloride is oxidized by dichromate and permanganate, producing a positive bias. Standard countermeasures include:
- Addition of mercuric sulfate to complex chloride in dichromate methods; the reagent dosing specified by the method masks chloride up to approximately 1,000–2,000 mg/L depending on sample volume.
- Application of the chloride correction method (HJ/T 70) or the alkaline permanganate procedure when chloride exceeds the masking capacity, as often occurs in coastal or deep saline aquifers.
- Verification of sample homogeneity and reagent blanks whenever high chloride is suspected, since blank correction becomes significant at low COD levels.
Quality Control and Data Acceptance Criteria
Accredited third-party laboratories operating under ISO/IEC 17025 management systems must demonstrate method performance on every batch. Standard quality control requirements for groundwater COD testing include:
- Reagent blank: a full procedural blank run with each batch; blank consumption of titrant must fall within method control limits.
- Calibration verification: daily standardization of ferrous ammonium sulfate titrant, commonly checked against potassium dichromate standard solution; potassium hydrogen phthalate (KHP) is used as the COD standard material (0.4251 g dried KHP diluted to 1 L corresponds to 500 mg/L COD).
- Parallel duplicates: at least 10% of samples analyzed in duplicate, with relative deviation within method-specified limits (typically below 10% for contaminated samples and consistent with the method's precision statement near the detection limit).
- Spike recovery / certified reference materials: recovery generally within 90–110% for samples at quantifiable concentrations.
- Method detection limit verification: repeated low-level determinations to confirm the laboratory's reporting limit for the groundwater matrix, since values near the method's lower bound carry high relative uncertainty.
- Hazardous waste management: digestion residues containing hexavalent chromium and mercury salts must be collected and disposed of as hazardous chemical waste in accordance with applicable regulations.
Limit Requirements and Result Evaluation
Groundwater COD results are most commonly evaluated against GB/T 14848 (Standard for groundwater quality), which classifies groundwater into five quality classes. For the permanganate index (CODMn, expressed as O2 in mg/L), the indicative classification thresholds are: Class I not exceeding 1.0 mg/L, Class II not exceeding 2.0 mg/L, Class III not exceeding 3.0 mg/L, Class IV not exceeding 10 mg/L, and Class V not exceeding 15 mg/L. Class III is the customary benchmark for water suitable for centralized drinking water supply after conventional treatment, while Class IV water is generally restricted to agricultural and some industrial uses. When results are compared with these limits, the laboratory's reported detection limit must be sufficiently low for meaningful class assignment; reporting "less than 30 mg/L" would obviously be inadequate against a 3.0 mg/L threshold, which is precisely why method selection is critical in groundwater programs.
Applications
- Groundwater quality assessment and classification for regional surveys, water resource evaluation, and drinking water source protection zones.
- Contaminated site investigation under environmental site assessment frameworks, where COD screens for gross organic impacts and guides placement of further targeted analyses.
- Remediation performance monitoring at fuel, solvent, or landfill-impacted sites, tracking the decline of organic load in affected wells over time.
- Landfill and solid waste facility monitoring, where COD trends in perimeter wells provide early warning of leachate migration.
- Discharge compliance verification for construction dewatering, mine drainage, and industrial abstraction released to surface water or sewer networks.
- Regulatory reporting for environmental impact assessments, pollution licenses, and due-diligence audits conducted through accredited third-party laboratories.
FAQ
Why is groundwater COD usually much lower than surface water or wastewater COD?
As water percolates through soil and aquifer material, physical filtration, adsorption, and biodegradation remove most organic matter. Uncontaminated groundwater therefore commonly shows permanganate index values of only a few milligrams per liter or less, whereas domestic wastewater routinely exceeds several hundred milligrams per liter. Any marked COD elevation in groundwater warrants investigation of an upgradient contamination source.
Which COD method should be used for clean groundwater: CODCr or CODMn?
For low-contamination groundwater and drinking water sources, the permanganate index (GB 11892 / ISO 8467) is generally preferred because its working range and detection limit match the low concentrations and the classification thresholds applied to groundwater. The dichromate method (HJ 828-2017 / ISO 6060 / ASTM D1252) is appropriate for contaminated wells, leachate-impacted water, or discharge samples where concentrations comfortably exceed its lower quantitative bound.
How is chloride interference handled in saline groundwater?
Laboratories first measure chloride. Within the masking capacity of the method, mercuric sulfate complexes the chloride so it is not oxidized. Above that capacity, the chloride correction method (HJ/T 70) or the alkaline permanganate variant is applied, and the report must state which procedure was used, because uncorrected chloride causes significant positive bias.
What are the key sampling and holding requirements?
Collect into glass bottles, acidify immediately with sulfuric acid to pH below 2, keep at about 4 °C in the dark, and analyze within the holding time set by the governing method — commonly up to 7 days for acidified samples. Proper well purging before sampling is equally important to guarantee that results represent aquifer water.
What detection limits can laboratories realistically achieve for groundwater?
Conventional dichromate reflux titration typically offers a detection limit near 4 mg/L with a lower reporting bound around 16 mg/L. Closed-tube colorimetric methods in low-range formulations, and especially the permanganate index method, can reliably quantify sub-milligram to few-milligram-per-liter levels, which is why method selection must be aligned with the applicable groundwater quality threshold.
Can COD data be used alone to identify the source of groundwater contamination?
No. COD is a non-specific aggregate indicator. Source identification requires combining it with targeted parameters such as BOD5, TOC, hydrocarbons, volatile organics, chloride, ammonia nitrogen, and stable isotope or tracer studies, together with a sound hydrogeological understanding of flow direction and well positioning.
