On-Site Testing and Analysis of Total Organic Arsenic in Surface Water

Total organic arsenic in surface water is a key indicator for evaluating arsenic speciation, bioavailability, and ecological risk in rivers, lakes, and reservoirs. On-site testing enables rapid screening and speciation-preserving measurement that avoids the interconversion of arsenic species during sample storage and transport. This article systematically presents the sampling requirements, testing items, field analytical methods, applicable standards, and typical applications of on-site testing and analysis of total organic arsenic in surface water.

Technical Overview

Arsenic in surface water occurs in both inorganic and organic chemical forms. The inorganic species, arsenite As(III) and arsenate As(V), generally dominate and are the most toxic, while organic arsenic species mainly include monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), trimethylarsine oxide (TMAO), and, at lower concentrations in freshwater systems, arsenobetaine and arsenosugars. Organic arsenic compounds are introduced through microbial methylation, biological transformation, certain industrial processes, and historical agricultural uses of organoarsenical chemicals.

Total organic arsenic is generally defined as the sum of all organically bound arsenic species, determined either by direct speciation analysis or operationally as the difference between total arsenic and total inorganic arsenic. Although organic species are often considered less acutely toxic than inorganic arsenic, monitoring the total organic arsenic fraction is essential for understanding arsenic cycling, transformation pathways, and long-term ecological risk in surface water bodies.

On-site (field) testing is particularly valuable for arsenic speciation because arsenic species are chemically unstable: oxidation, reduction, and methylation reactions can continue after sampling, causing significant changes in the As(III)/As(V)/organic arsenic distribution during transport to a fixed laboratory. Field testing provides rapid, near-real-time results for pollution screening, emergency response, and pre-screening before confirmatory laboratory speciation analysis by accredited third-party laboratories.

Testing Samples

Rivers and Streams

Flowing surface water from rivers, streams, and tributaries, including upstream reference points, mid-stream transects, and downstream discharge-affected sections. Composite or depth-integrated samples are collected depending on the monitoring objective.

Lakes and Reservoirs

Standing water bodies used for drinking water sources, irrigation storage, or ecological conservation. Stratified sampling may be required in deep reservoirs where redox conditions vary between the epilimnion and hypolimnion.

Estuarine and Brackish Waters

Mixing zones between freshwater and seawater, where salinity and suspended solids can influence arsenic speciation and interfere with field colorimetric or electrochemical detection.

Agricultural Drains and Irrigation Channels

Water potentially affected by legacy organoarsenical pesticide use, animal feeding operations, or soil methylation processes, where elevated organic arsenic fractions are more likely.

Urban and Recreational Water Bodies

Ponds, landscape lakes, and stormwater receiving waters subject to urban runoff and incidental industrial inputs.

All samples are typically collected in pre-cleaned polyethylene or polypropylene bottles, filtered through 0.45 micrometer membranes on-site when dissolved speciation is required, and handled according to the sampling guidance of the ISO 5667 series (ISO 5667-1, ISO 5667-3, ISO 5667-4, and ISO 5667-6).

Testing Items

The on-site testing program for total organic arsenic in surface water typically includes the following parameters, reported in micrograms per liter (µg/L):

• Total arsenic (after full oxidative digestion of the sample)
• Total inorganic arsenic, comprising As(III) and As(V)
• Monomethylarsonic acid (MMA), the principal methylated organic arsenic species in freshwater
• Dimethylarsinic acid (DMA), commonly associated with biological methylation
• Trimethylarsine oxide (TMAO) and other minor organoarsenic species where method sensitivity permits
• Total organic arsenic, calculated as total arsenic minus total inorganic arsenic, or as the sum of individually quantified organic species
• Supporting field parameters: pH, conductivity, oxidation-reduction potential (ORP), dissolved oxygen, temperature, and turbidity, which are essential for interpreting speciation results

Testing Methods

Sample Collection and On-Site Pretreatment

Samples for speciation are filtered on-site through 0.45 micrometer membranes and kept chilled at approximately 4 degrees Celsius in the dark, with analysis completed as soon as possible, ideally within 24 to 48 hours. Acidification with nitric acid to pH below 2 is used only for total arsenic determination, because acidification can alter arsenic speciation. Where species stabilization is required, chelating or buffering additives validated by the analytical method may be employed.

Rapid Field Screening Techniques

• Arsine-generation test kits based on the Gutzeit principle, in which zinc or borohydride reductants convert arsenic to arsine that develops a color stain on impregnated test paper; these provide semi-quantitative screening, typically in the range of 5 to 100 µg/L
• Portable spectrophotometric kits using silver diethyldithiocarbamate (AgDDC) or molybdenum blue chemistry after digestion, achieving practical quantification limits of roughly 1 to 5 µg/L
• Portable anodic stripping voltammetry (ASV) with gold or gold-film electrodes, offering sub-microgram to low-microgram per liter sensitivity with minimal reagent consumption
• Portable hydride generation accessories coupled to field optical detectors for higher-sensitivity total arsenic measurement

On-Site Determination Strategy for Total Organic Arsenic

Because direct field speciation of individual organic species is difficult, the widely applied operational approach is differential measurement. Total inorganic arsenic is determined by selective hydride generation, in which As(III) is converted to arsine under mildly acidic buffered conditions and total inorganic arsenic is measured after pre-reduction of As(V) with potassium iodide and ascorbic acid. Total arsenic is then measured separately after oxidative digestion with persulfate, permanganate, or ultraviolet-persulfate treatment that converts all organic species to inorganic arsenate. Total organic arsenic is obtained by difference. Method blanks, duplicate measurements, and certified reference materials are used to control bias in the subtraction step.

Laboratory Confirmation and Speciation

Field results are confirmed using high-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) or HPLC coupled with hydride generation atomic fluorescence spectrometry (HPLC-HG-AFS), which separate and quantify MMA, DMA, TMAO, As(III), and As(V) with detection limits at or below 0.1 to 0.5 µg/L. ICP-MS determination of total arsenic follows the general principles of the ISO 17294-2 method.

Applicable Standards

• GB 3838, Environmental Quality Standards for Surface Water, which sets the arsenic limit for Class III surface water at 50 µg/L
• GB/T 5750.6, Standard Examination Methods for Drinking Water, metal parameters, covering AFS, ICP-MS, and silver diethyldithiocarbamate spectrophotometric procedures for arsenic
• ASTM D2972, Standard Test Methods for Arsenic in Water, covering colorimetric, hydride generation, and spectrochemical procedures
• ISO 11969 series, Water Quality, Arsenic, specifying hydride generation atomic absorption and related determination methods
• ISO 5667 series, Water Quality, Sampling, governing sample design, preservation, and handling for rivers, lakes, and reservoirs

Applications

On-site testing and analysis of total organic arsenic in surface water serves a broad range of environmental management purposes:

• Routine surface water quality surveillance and compliance screening against water quality standards for rivers, lakes, and reservoirs
• Emergency response to arsenic-related pollution incidents, spills, or abrupt water quality anomalies requiring same-day results
• Investigation of agricultural areas with legacy organoarsenical pesticide application or intensive livestock operations
• Monitoring of industrial discharge mixing zones and verification of treatment performance at outfalls
• Drinking water source protection and early warning programs for water supply reservoirs
• Scientific research on arsenic biogeochemical cycling, seasonal methylation dynamics, and redox-driven species transformation
• Pre-screening to prioritize samples before cost-intensive laboratory speciation analysis by accredited third-party laboratories

FAQ

What is the difference between total arsenic and total organic arsenic?

Total arsenic includes all arsenic species, inorganic and organic, in the water sample. Total organic arsenic refers only to the organically bound arsenic fraction, principally MMA, DMA, TMAO, and related organoarsenic compounds, determined either by direct speciation or as total arsenic minus inorganic arsenic.

Why is on-site testing preferred for organic arsenic analysis?

Arsenic species can transform through oxidation, reduction, and methylation during sample storage and transport, distorting the original speciation distribution. On-site testing minimizes species conversion, provides immediate results for screening and emergency decisions, and reduces the cost of unnecessary laboratory analysis.

What detection limits can be achieved with field methods?

Semi-quantitative test kits typically reach approximately 5 µg/L, portable spectrophotometric methods achieve about 1 to 5 µg/L, and portable voltammetric instruments can reach roughly 0.5 to 2 µg/L. For lower concentrations and individual species quantification, laboratory HPLC-ICP-MS confirmation is recommended.

How are samples preserved for organic arsenic testing?

For speciation, samples are filtered on-site through 0.45 micrometer membranes, protected from light, chilled to approximately 4 degrees Celsius, and analyzed within 24 to 48 hours. Nitric acid preservation to pH below 2 is reserved for total arsenic samples only, since acidification can change species distribution.

Which regulatory limit applies to arsenic in surface water?

Under GB 3838, the arsenic concentration for Class III surface water, applicable to drinking water sources and general ecological use, must not exceed 50 µg/L. Local or watershed-specific requirements may impose stricter values.

When is laboratory confirmation necessary?

Confirmation is required when field results approach or exceed regulatory limits, when individual organic species must be reported, when concentrations fall below field method detection limits, or when the investigation supports regulatory or legal decision-making.