Surface Water Mercury and Arsenic Testing: Analytical Methods, Standards, and Quality Control

Mercury and arsenic are among the most toxic trace contaminants in surface water, capable of harming aquatic ecosystems and human health even at nanogram-per-liter to microgram-per-liter concentrations. This technical article explains how surface water mercury and arsenic testing is performed, covering regulatory limits under standards such as GB 3838-2002 and US EPA methods, sample collection and preservation protocols, laboratory analytical techniques, and quality control practices used by accredited third-party laboratories.

Technical Overview

Mercury (Hg) and arsenic (As) are priority pollutants in surface water monitoring programs worldwide. Mercury enters aquatic systems primarily through atmospheric deposition from coal combustion, mining and smelting operations, chlor-alkali production, and improper disposal of mercury-containing products. In water, mercury exists in several chemical forms, including elemental mercury (Hg0), inorganic divalent mercury (Hg2+), and organic species such as methylmercury, which bioaccumulates strongly in aquatic food chains.

Arsenic is a metalloid released into surface water through geothermal activity, weathering of arsenic-bearing minerals, nonferrous metal smelting, pesticide and herbicide use, and industrial effluents. Its toxicity and mobility depend heavily on speciation: inorganic species, particularly arsenite As(III), are considerably more toxic than pentavalent arsenate As(V) and the organic species monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA).

Because both elements are regulated at extremely low concentrations, testing requires contamination-controlled sampling and highly sensitive instrumental techniques. Key regulatory reference values include:

• GB 3838-2002 (Environmental Quality Standards for Surface Water, China): total mercury limits of 0.00005 mg/L for Class I and II waters and 0.0001 mg/L for Class III waters; total arsenic limits of 0.05 mg/L for Class I to III waters.
• WHO drinking-water guidelines: 0.006 mg/L for inorganic mercury and 0.01 mg/L for arsenic, widely used as reference benchmarks for source water protection.
• US EPA human health water quality criteria for arsenic and mercury at sub-microgram-per-liter levels, which drive the use of ultra-trace clean sampling and analysis techniques.

Testing Samples

Rivers, Streams, and Canals

Flowing water bodies are sampled at established cross-sections or monitoring sections, typically at mid-depth and mid-channel away from banks and stagnant zones. Representative composite or grab samples are collected following the technical regulations of GB 12997, GB 12998, and HJ 91.2 to characterize mercury and arsenic levels along the river course, particularly upstream and downstream of industrial or municipal discharge points.

Lakes and Reservoirs

Standing water bodies often exhibit vertical and seasonal stratification that affects trace metal distribution. Samples are taken at multiple depths (surface, mid-depth, and near-bottom) at inlet, center, and outlet stations to capture spatial variability, with reservoirs used as drinking water sources subject to the most stringent testing frequency.

Estuarine and Coastal Waters

Brackish environments where rivers meet the sea require special attention because salinity and suspended particulate matter influence mercury and arsenic partitioning between dissolved and particulate phases. Methods compatible with marine matrices, such as GB 17378.4 for seawater analysis, are applied.

Receiving Waters Near Discharge Points

Surface waters downstream of industrial outfalls, mining areas, landfill leachate zones, and agricultural drainage are sampled to verify compliance with discharge permits and to assess whether receiving water quality objectives for mercury and arsenic are being met.

Testing Items

Typical test items for surface water mercury and arsenic monitoring programs include:

• Total mercury: the sum of all mercury species in an unfiltered, acid-digested sample.
• Dissolved mercury: mercury in the fraction passing a 0.45 micrometer membrane filter.
• Methylmercury: the bioaccumulative organic species of primary ecotoxicological concern.
• Total arsenic: the sum of all arsenic species after complete acid digestion.
• Dissolved arsenic: arsenic in the 0.45 micrometer filtered fraction.
• Arsenic speciation: quantitative separation of As(III), As(V), MMA, and DMA to support toxicity assessment.

For routine compliance monitoring against GB 3838-2002 and comparable international standards, total mercury and total arsenic are the primary reporting parameters, while speciation is added when ecological risk assessment or geochemical studies are required.

Testing Methods

Sample Collection and Preservation

Samples are collected in pre-cleaned borosilicate glass or PTFE-lined containers for mercury and acid-washed polyethylene containers for arsenic, using clean-hands/dirty-hands procedures adapted from US EPA Method 1669 when sub-nanogram-per-liter reporting is needed. Upon collection, samples are acidified with high-purity nitric acid to pH below 2; mercury samples for ultra-trace analysis are additionally stabilized with hydrochloric acid or potassium dichromate per US EPA Method 1631E. Field blanks, equipment blanks, and trip blanks accompany each sampling campaign to verify contamination control. Preserved samples are held at approximately 4 degrees Celsius and analyzed within 28 days for mercury and within 6 months for arsenic, in accordance with HJ 493 and EPA holding time guidance.

Mercury Determination

Several validated instrumental techniques are available, selected according to the required detection limit and regulatory context:

• Cold vapor atomic absorption spectrometry (CVAAS): HJ 597-2011 and US EPA Method 245.1, suitable for routine monitoring with detection limits around 0.01 to 0.1 micrograms per liter.
• Cold vapor atomic fluorescence spectrometry (CVAFS): US EPA Method 1631E and ISO 17852, with bromine monochloride oxidation followed by tin chloride or hydroxylamine reduction, achieving method detection limits of approximately 0.2 to 0.5 nanograms per liter, the technique of choice for GB 3838 Class I and II compliance.
• ICP-MS: US EPA Method 200.8 and HJ 700-2014, offering multi-element capability with gold solution added to stabilize mercury and minimize memory effects.
• Thermal decomposition gold amalgamation AAS: US EPA Method 7473, a direct solids-and-liquids technique requiring no wet chemistry, useful for rapid screening.

Arsenic Determination

Arsenic methods commonly applied by accredited third-party laboratories include:

• Atomic fluorescence spectrometry with hydride generation (HG-AFS): HJ 694-2014, widely used for surface water in China with sub-microgram-per-liter sensitivity.
• Hydride generation atomic absorption spectrometry (HG-AAS): US EPA Method 7062 and ISO 11969, a robust and cost-effective option for trace arsenic.
• ICP-MS: US EPA Method 200.8 and HJ 700-2014, with helium collision mode or reaction cell technology used to resolve the argon chloride (ArCl+) polyatomic interference at mass 75.
• ICP-OES (US EPA Method 200.7) and graphite furnace AAS (US EPA Method 7060) for higher-concentration samples.
• Silver diethyldithiocarbamate spectrophotometry (GB 7485) as a classical colorimetric method for higher-level determinations.

Speciation Analysis

For arsenic speciation, high-performance liquid chromatography coupled with ICP-MS (HPLC-ICP-MS) or ion chromatography-ICP-MS separates As(III), As(V), MMA, and DMA while preserving their original oxidation states. Methylmercury in water is determined by distillation or aqueous ethylation followed by gas chromatographic separation with atomic fluorescence or ICP-MS detection, consistent with US EPA Method 1630 approaches. Speciation samples must be filtered and preserved under cold, dark conditions without strong acidification to avoid species conversion.

Quality Assurance and Quality Control

Reliable results depend on a complete QA/QC system, including multi-point calibration with independent verification standards, method blanks, laboratory control samples, matrix spikes and matrix spike duplicates (typically requiring 75 to 125 percent recovery), duplicate sample analysis, continuing calibration verification every ten samples, and analysis of certified reference materials. Data are reported with full uncertainty information, and control charts track long-term method performance in accordance with ISO/IEC 17025 laboratory competence requirements.

Applications

Surface water mercury and arsenic testing supports a wide range of environmental and regulatory objectives:

• Routine surface water quality monitoring and functional zone compliance assessment against GB 3838-2002 classification limits.
• Environmental impact assessment and post-construction verification for industrial, mining, and infrastructure projects.
• Drinking water source protection and sanitary zone delineation for rivers, lakes, and reservoirs.
• Pollution source apportionment and watershed investigations following accidental releases or historical contamination.
• Ecological and human health risk assessment, including fish tissue and sediment studies where water-column data provide exposure context.
• Remediation performance monitoring at contaminated sites and verification of the effectiveness of mercury or arsenic control measures.
• Academic research on trace metal biogeochemistry, transport, and transformation in aquatic systems.

FAQ

What are the regulatory limits for mercury and arsenic in surface water?

Under GB 3838-2002, total mercury must not exceed 0.0001 mg/L and total arsenic must not exceed 0.05 mg/L for Class III waters intended for drinking water abstraction and general ecological use; stricter limits of 0.00005 mg/L for mercury apply to Class I and II waters. International reference values, such as the WHO guideline of 0.01 mg/L for arsenic in drinking water, are often used to evaluate source water quality.

Which analytical method offers the lowest detection limit for mercury?

Cold vapor atomic fluorescence spectrometry following US EPA Method 1631E or ISO 17852 achieves detection limits near 0.2 nanograms per liter when paired with ultra-clean sampling, making it suitable for verifying compliance with the strictest surface water criteria.

Why is arsenic speciation analysis important?

Different arsenic species differ greatly in toxicity and mobility; inorganic As(III) is substantially more toxic than As(V) or organic species such as DMA. Speciation data enable accurate risk assessment and guide appropriate remediation strategies, whereas total arsenic alone cannot distinguish these forms.

How should samples be preserved before laboratory analysis?

Samples are acidified immediately with high-purity nitric acid to pH below 2 and stored at approximately 4 degrees Celsius. Ultra-trace mercury samples require additional stabilization and clean-sampling protocols to prevent contamination and loss to container walls.

What interferences must be controlled in arsenic testing by ICP-MS?

The dominant interference is the argon chloride polyatomic ion (40Ar35Cl+) at mass 75, which is significant in chloride-rich waters. Collision or reaction cell technology, isotope dilution, and interference correction equations are used to ensure accurate arsenic quantification.

Can one sample be used for both mercury and arsenic determination?

In routine monitoring, a single acidified sample can often support both determinations if container materials and preservation requirements are compatible. For ultra-trace mercury work, however, dedicated glass containers with dichromate or hydrochloric acid stabilization are strongly recommended to guarantee data integrity.