Soil Arsenic Content Testing: Analytical Methods, Standards, and Technical Requirements

Soil arsenic content testing quantifies the concentration of arsenic in soil samples using validated analytical techniques such as hydride generation atomic fluorescence spectrometry, ICP-MS, and X-ray fluorescence screening. Accurate arsenic determination is essential for farmland safety evaluation, construction land risk screening, contaminated site assessment, and remediation verification. This article explains sampling protocols, digestion procedures, instrumental methods, regulatory limits, and quality control requirements for reliable soil arsenic analysis.

Technical Overview

Arsenic (As) is a toxic metalloid and one of the most tightly regulated inorganic contaminants in soil worldwide. It occurs naturally through the weathering of arsenic-bearing minerals such as arsenopyrite, but elevated concentrations in soil are most frequently associated with anthropogenic activities, including mining and smelting, coal combustion, historical use of arsenical pesticides and wood preservatives, industrial wastewater discharge, and irrigation with contaminated water. Uncontaminated soils typically contain between 1 and 15 mg/kg of arsenic, with a global average of approximately 5 to 10 mg/kg.

Chronic exposure to arsenic through soil ingestion, dust inhalation, drinking water, and food chain transfer is associated with skin lesions, cardiovascular effects, and several forms of cancer, and arsenic is classified as a Group 1 human carcinogen by the International Agency for Research on Cancer (IARC). Because arsenic is non-degradable, contaminated soil remains a long-term risk source unless it is remediated or managed. Consequently, soil arsenic content testing is a core requirement in environmental impact assessments, land redevelopment projects, agricultural safety programs, and remediation performance verification. In China, the key regulatory drivers are GB 36600-2018 (risk control standard for soil contamination of development land) and GB 15618-2018 (risk control standard for soil contamination of agricultural land), supported by internationally recognized methods published by ISO, ASTM, EN, and US EPA method series.

Testing Samples

Agricultural and Farmland Soil

Samples from paddy fields, vegetable plots, orchards, and other cultivated land are tested to determine whether arsenic concentrations comply with GB 15618-2018, which specifies pH-dependent screening values of approximately 20 to 40 mg/kg depending on land use type and soil acidity. Paddy soils deserve particular attention because flooded, reducing conditions can mobilize arsenic and increase uptake into rice grains.

Development and Construction Land Soil

Soil from residential, commercial, recreational, and industrial parcels is screened against GB 36600-2018, which sets arsenic screening values of 20 mg/kg for sensitive land uses such as residential areas, schools, and hospitals, and 60 mg/kg for less sensitive uses such as industrial and commercial sites. Exceedance of screening values triggers detailed risk assessment or intervention values.

Contaminated Site and Brownfield Soil

Soil from former industrial facilities, mining areas, smelters, pesticide plants, and wood treatment sites is tested to delineate contamination plumes, classify excavated soil, and verify that post-remediation concentrations meet cleanup targets. Both in-situ and ex-situ samples are commonly submitted.

Sediments, Tailings, and Solid Waste

River and lake sediments, mine tailings, slag, and solid waste intended for co-disposal or landfilling are analyzed to evaluate leaching potential and disposal classification, often in combination with leaching toxicity tests.

Testing Items

Soil arsenic testing services typically cover the following analytical items:

• Total arsenic content, determined after complete or near-complete decomposition of the soil matrix using hydrofluoric acid-based digestion or fusion techniques.
• Aqua regia extractable arsenic, the basis of most routine regulatory determinations in China (HJ 803-2016, ISO 11466) and Europe, which dissolves environmentally relevant arsenic fractions.
• Water-soluble and leachable arsenic, evaluated through water extraction or leaching procedures such as US EPA Method 1311 (TCLP) and US EPA Method 1312 (SPLP).
• Arsenic speciation, distinguishing highly mobile and toxic trivalent arsenic As(III) from pentavalent arsenic As(V), performed by HPLC coupled with ICP-MS.
• Chemical fractionation by sequential extraction (BCR protocol) to quantify the exchangeable, reducible, oxidizable, and residual fractions and assess long-term mobility.
• Simultaneous multi-element screening alongside cadmium, lead, mercury, chromium, copper, nickel, and zinc, which is cost-effective because most instrumental methods quantify many elements in a single run.

Testing Methods

Sampling and Sample Preparation

Sampling follows HJ/T 166-2004, the technical specification for soil environmental monitoring, or international equivalents. Surface samples are generally collected from 0 to 20 cm for agricultural land, while site investigations collect samples by stratum to the maximum investigation depth, commonly 0 to 6 m or deeper. Samples of at least 500 g are collected using stainless steel or non-contaminating tools, transported in polyethylene or glass containers, and protected from cross-contamination. In the laboratory, samples are air-dried or freeze-dried at low temperature, homogenized, and sieved through 2 mm and 0.149 mm (100 mesh) nylon sieves in preparation for digestion.

Digestion Procedures

Because arsenic is a trace element bound within silicate lattices, sulfides, and iron-manganese oxides, digestion chemistry directly controls result comparability. Common approaches include:

• Microwave-assisted digestion with hydrofluoric acid, nitric acid, and hydrochloric acid for total arsenic, consistent with ISO 12914 and US EPA Method 3051A principles.
• Hot-plate multi-acid digestion (HNO3-HCl-HF-HClO4) for total decomposition.
• Aqua regia reflux extraction (HCl-HNO3) per HJ 803-2016 and ISO 11466, used for most regulatory reporting of extractable arsenic.

Hydride Generation Atomic Fluorescence Spectrometry (HG-AFS)

Atomic fluorescence spectrometry is the reference technique in China for soil arsenic and is specified in HJ 680-2013, which combines microwave digestion with AFS determination. Pentavalent arsenic in the digest is first pre-reduced to As(III) using a thiourea-ascorbic acid solution, after which potassium borohydride in alkaline carrier solution generates arsine gas that is swept into an argon-hydrogen flame for atomization and fluorescence detection. The method offers excellent sensitivity, with method detection limits typically around 0.01 mg/kg, wide linear range, low reagent consumption, and minimal spectral interference, making it highly suitable for trace-level arsenic in soil.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

ICP-MS, following US EPA Method 6020B and ISO 17294-2 measurement principles, is the preferred technique when multi-element data are required from a single digestion. Because monoisotopic arsenic at mass 75 suffers polyatomic interference from argonium chloride (40Ar35Cl+), helium collision mode or correction equations are applied. Detection limits in solid samples are commonly below 0.1 mg/kg. ICP-MS is also the detector of choice for arsenic speciation when coupled to high-performance liquid chromatography.

Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)

ICP-OES per US EPA Method 6010D and ISO 22036 provides robust throughput for samples with moderate to elevated arsenic concentrations, such as mining soils and tailings, although its detection limits are generally inferior to those of AFS and ICP-MS for trace-level work.

Hydride Generation Atomic Absorption Spectrometry (HG-AAS)

US EPA Method 7062 describes the determination of arsenic by hydride generation followed by quartz furnace atomic absorption, historically significant and still used in laboratories equipped with AAS platforms. Detection limits of approximately 0.1 mg/kg are adequate for most screening applications.

Field X-Ray Fluorescence (XRF) Screening

Portable energy-dispersive XRF analyzers operating per US EPA Method 6200, and laboratory wavelength-dispersive XRF per HJ 780-2015, enable rapid on-site screening with results available in minutes. With detection limits for arsenic typically in the range of 2 to 10 mg/kg, XRF is ideal for delineating hotspots and optimizing sampling grids, while confirmatory analysis of a statistical subset of samples by AFS or ICP-MS remains standard practice.

Quality Assurance and Quality Control

Reliable data require a documented QA/QC system covering the complete analytical chain:

• Method blanks, laboratory control samples, and calibration verification standards run at defined frequencies.
• Matrix spikes and parallel duplicate samples with relative deviation acceptance criteria, typically within 20 to 30 percent.
• Certified reference materials (CRMs) of matched soil matrix to verify digestion recovery and accuracy.
• Statistically derived method detection limits and reporting limits, with results reported on a dry-weight basis (mg/kg dw).
• Testing performed by accredited third-party laboratories holding ISO/IEC 17025 accreditation and, where applicable, national environmental monitoring qualification for the cited standards.

Applications

Soil arsenic content testing supports a wide range of regulatory, industrial, and scientific objectives:

• Environmental site investigation and risk assessment for land redevelopment, parcel transfers, and change of land use.
• Agricultural soil safety evaluation, farmland soil environment quality classification, and safe utilization category management.
• Remediation design, engineering acceptance, and post-remediation verification for soil washing, solidification, thermal treatment, and phytoextraction projects.
• Compliance monitoring around nonferrous mining, smelting, chemical, and power generation facilities.
• Groundwater and drinking water source protection studies, since soil arsenic is a key source of aquifer contamination.
• Ecological risk assessment of sediments and agricultural product safety studies linking soil loads to crop uptake.
• Academic research, geochemical baseline mapping, and litigation support in contamination disputes.

FAQ

What is the difference between total arsenic and extractable arsenic?

Total arsenic measures all arsenic including the fraction locked in silicate minerals, requiring hydrofluoric acid digestion, whereas aqua regia extractable arsenic measures the acid-soluble, environmentally relevant fraction. Regulatory standards in China and Europe are generally based on aqua regia extraction, so result comparability depends on matching the digestion protocol to the applicable standard.

Which detection limits can be expected?

Typical method detection limits are approximately 0.01 mg/kg for hydride generation AFS, below 0.1 mg/kg for ICP-MS, around 0.1 to 1 mg/kg for ICP-OES, and 2 to 10 mg/kg for field XRF screening. All of these are well below the strictest screening value of 20 mg/kg in GB 36600-2018.

How much sample is required and how should it be preserved?

A minimum of 500 g of soil per sample is recommended for heavy metal analysis. Samples should be collected with clean non-metallic tools, stored in sealed polyethylene or glass containers, kept cool and dark during transport, and processed within the holding times specified in the governing sampling standard.

How long does soil arsenic testing take?

Standard turnaround for routine single-element determination is typically 5 to 10 working days from laboratory receipt, while multi-element packages, speciation analysis, or projects requiring extensive sampling campaigns may require additional coordination time.

When is XRF screening sufficient?

XRF is appropriate for rapid hotspot delineation, excavation control, and screening-level site characterization, provided a representative percentage of samples, commonly at least 10 percent, is confirmed by laboratory AFS or ICP-MS analysis. Regulatory acceptance decisions should rely on laboratory confirmatory data.

How are results interpreted against standards?

Results are compared with the applicable screening and intervention values of GB 15618-2018 for agricultural land or GB 36600-2018 for development land, or with site-specific risk-based cleanup targets derived from exposure modeling. Concentrations above screening values do not automatically require remediation but trigger detailed risk assessment and, where necessary, risk control or remediation measures.