Trace Arsenic Determination in Surface Water: Experimental Methods, Standards, and Laboratory Testin

Trace arsenic determination in surface water is a critical analytical procedure for evaluating water quality, protecting aquatic ecosystems, and safeguarding public health. This guide explains the experimental principles, sampling protocols, standard methods such as HJ 694-2014 and HJ 700-2014, and quality control practices used by accredited third-party laboratories to quantify arsenic at microgram-per-liter levels. Whether you monitor rivers, lakes, reservoirs, or estuaries, understanding these methods ensures accurate and compliant water quality data.

Technical Overview

Arsenic is a naturally occurring metalloid widely distributed in the Earth's crust, entering surface waters through geological weathering, mining drainage, smelting operations, agrochemical application, geothermal activity, and industrial effluent discharge. In natural waters it exists primarily as inorganic species, arsenite As(III) and arsenate As(V), both of which are toxic, with trivalent arsenic generally regarded as the more hazardous form. Chronic exposure through contaminated water is associated with skin lesions, cardiovascular disease, peripheral neuropathy, and cancers of the skin, bladder, and lungs, which is why arsenic is classified as a Group 1 human carcinogen by the World Health Organization's International Agency for Research on Cancer.

Because arsenic concentrations in unpolluted surface waters are typically at the low microgram-per-liter level, highly sensitive analytical techniques are essential. The governing Chinese environmental benchmark, GB 3838-2002 (Environmental Quality Standards for Surface Water), sets a total arsenic limit of 0.05 mg/L for Class I to Class III waters and 0.10 mg/L for Class IV and Class V waters, while the WHO drinking water guideline value is 0.01 mg/L. Reliable determination at these levels depends on hydride generation atomic fluorescence spectrometry (HG-AFS), inductively coupled plasma mass spectrometry (ICP-MS), or graphite furnace atomic absorption spectrometry (GFAAS), supported by rigorous sampling, preservation, and quality assurance procedures.

Testing Samples

Trace arsenic determination is applicable to a wide range of natural surface water matrices. Typical sample categories submitted to accredited third-party laboratories include:

Rivers and Streams

Flowing water bodies that receive catchment runoff, mining-influenced drainage, and industrial or municipal effluent. Sampling is usually performed at designated cross-sections, with grab or depth-integrated samples collected in acid-cleaned polyethylene or glass bottles and acidified on site.

Lakes and Reservoirs

Stationary or stratified water bodies used for drinking water supply, irrigation, aquaculture, and recreation. Thermal stratification, seasonal turnover, and sediment resuspension influence the distribution of arsenic between dissolved and particulate fractions, so sampling depth must be documented.

Estuaries and Transitional Waters

Brackish waters where salinity gradients affect arsenic speciation and can introduce chloride-related matrix effects. For these samples, method selection and interference management, such as collision-cell correction in ICP-MS, are particularly important.

Ponds, Wetlands, and Geogenically Affected Waters

Small or slow-moving water bodies in mineralized, geothermal, or sulfide-rich geological regions where elevated naturally occurring arsenic may be present, as well as wetlands and ponds near industrial sites requiring baseline or surveillance monitoring.

Testing Items

The core compliance parameter is total arsenic, with optional speciation and fractionation analyses available for research and diagnostic purposes:

• Total arsenic (unfiltered, acid-digested sample) — the regulatory parameter under GB 3838-2002.
• Dissolved arsenic — determined on samples filtered through 0.45 micrometer membranes prior to acidification.
• Particulate arsenic — calculated as the difference between total and dissolved fractions, indicating sediment-bound transport.
• Arsenic speciation (As(III), As(V), monomethylarsonic acid, dimethylarsinic acid) — performed by high-performance liquid chromatography coupled with ICP-MS, with samples preserved using EDTA and analyzed promptly to maintain speciation integrity.
• Supporting parameters — pH, electrical conductivity, turbidity, suspended solids, and oxidation-reduction potential to aid geochemical interpretation of the arsenic data.

Testing Methods

Several standard methods are suitable for trace arsenic determination in surface water, and the choice depends on the required detection limit, matrix salinity, and laboratory instrumentation.

Hydride Generation Atomic Fluorescence Spectrometry (HG-AFS, HJ 694-2014)

This is the most widely used routine method for surface water monitoring. The sample is pre-reduced with a thiourea–ascorbic acid mixture in hydrochloric acid medium to convert all As(V) to As(III). Potassium borohydride then generates arsine gas, which is swept by argon into a heated quartz atomizer; arsenic atoms excited by a hollow cathode lamp emit characteristic fluorescence proportional to concentration. The method offers method detection limits typically in the range of 0.1 to 0.3 micrograms per liter, wide linear range, low reagent cost, and strong resistance to matrix interference, making it ideal for freshwater compliance testing.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS, HJ 700-2014 and ISO 17294-2)

After preservation with nitric acid, samples are aspirated directly into the plasma and arsenic is quantified at mass 75. Internal standards, such as germanium, rhodium, or indium, correct for matrix suppression and instrumental drift, while collision-reaction cell technology or correction equations resolve the argon chloride (40Ar35Cl+) polyatomic interference. ICP-MS achieves sub-microgram-per-liter detection limits and simultaneously quantifies dozens of other trace elements, which is valuable for comprehensive water quality characterization and estuarine samples after appropriate dilution.

Graphite Furnace Atomic Absorption Spectrometry (GFAAS)

Electrothermal atomization with matrix modifiers such as palladium or nickel nitrate allows direct determination of arsenic with detection limits on the order of 1 microgram per liter. Pyrolysis and atomization temperatures are optimized to suppress chloride and phosphate backgrounds. GFAAS remains a dependable option where AFS or ICP-MS instrumentation is unavailable.

Spectrophotometric Silver Diethyldithiocarbamate Method (GB 7485)

In this classical colorimetric approach, arsine generated from the acidified sample is absorbed into a silver diethyldithiocarbamate solution, forming a red complex measured spectrophotometrically at approximately 530 nanometers. With a detection limit of about 0.007 mg/L, it is suitable for moderately contaminated waters and cross-checking, although it has largely been superseded by instrumental techniques for trace-level work.

Sampling, Preservation, and Quality Assurance

Samples are collected following HJ 493-2009 and the ISO 5667 series using acid-washed containers, filtered in the field when dissolved arsenic is required, and preserved with high-purity nitric acid to pH below 2. Preserved samples are held at low temperature and analyzed within 14 days. Quality control measures include field and reagent blanks, calibration with at least five standard points (correlation coefficient of 0.999 or better), duplicate analyses, matrix spike recoveries typically within 80 to 120 percent, certified reference materials, and periodic method detection limit verification, ensuring data are defensible for regulatory reporting.

Applications

Trace arsenic determination in surface water supports a broad spectrum of environmental management and scientific objectives:

• Routine surface water quality monitoring and functional zone classification against GB 3838-2002.
• Drinking water source protection zone assessment and safety evaluation of raw water intakes.
• Environmental impact assessment and completion acceptance monitoring for construction and industrial projects.
• Pollution incident investigation, early warning, and source tracing along river systems.
• Evaluation of remediation performance, such as constructed wetlands, iron-based adsorption, and sediment capping projects.
• Geochemical cycling research, ecological risk assessment, and long-term trend analysis under national monitoring programs.

FAQ

What is the detection limit for arsenic in surface water testing?

Hydride generation atomic fluorescence spectrometry typically achieves detection limits of 0.1 to 0.3 micrograms per liter, ICP-MS can reach 0.05 to 0.5 micrograms per liter depending on the matrix, and the spectrophotometric method reaches approximately 7 micrograms per liter. All of these are sufficient to verify compliance with the 50 microgram per liter GB 3838-2002 limit for Class I to III surface waters.

How should surface water samples for arsenic be collected and preserved?

Collect samples in acid-cleaned polyethylene or glass bottles following HJ 493-2009, filter through 0.45 micrometer membranes in the field if dissolved arsenic is required, and immediately acidify with high-purity nitric acid to pH below 2. Store samples cold and analyze within 14 days.

Which method is better, HG-AFS or ICP-MS?

HG-AFS is cost-effective, highly sensitive, and the standard routine choice for freshwater arsenic monitoring in accordance with HJ 694-2014. ICP-MS under HJ 700-2014 provides multi-element capability in a single run and superior matrix tolerance for saline waters, but requires higher instrument investment and careful control of the argon chloride interference.

What is the difference between total arsenic and dissolved arsenic?

Total arsenic is measured on an unfiltered, acid-digested sample and includes both dissolved arsenic and arsenic bound to suspended particles; it is the regulatory compliance parameter. Dissolved arsenic is measured on a 0.45 micrometer filtered sample and better reflects bioavailable arsenic and mobility in the water column.

Can arsenic speciation be determined in surface water?

Yes. Coupling liquid chromatography with ICP-MS allows separation and quantification of As(III), As(V), and organoarsenic species. Speciation requires special preservation with EDTA, refrigerated storage, and rapid analysis to prevent species transformation between sampling and measurement.

How long does a trace arsenic test take?

For routine total arsenic analysis of properly preserved samples, an accredited third-party laboratory typically reports results within five to ten working days, including pre-treatment, instrumental analysis, quality control review, and issuance of a formal test report.