Soil arsenic content analysis is a core environmental testing service that quantifies total, leachable, and speciated arsenic in soil and sediment using validated instrumental techniques such as atomic fluorescence spectrometry (AFS), ICP-MS, and hydride generation atomic absorption spectrometry. Accurate arsenic determination supports agricultural land safety, contaminated site investigation, and remediation verification in accordance with national and international standards including GB 36600-2018, GB 15618-2018, and ISO methods. This article outlines the sample types, testing items, analytical procedures, quality control requirements, and typical applications of professional soil arsenic analysis.
Technical Overview
Arsenic (As) is a toxic metalloid that occurs naturally in sulfide-bearing minerals such as arsenopyrite, and it is also widely released into soil through human activities, including mining and smelting, historical use of arsenical pesticides, wood preservatives such as chromated copper arsenate (CCA), coal combustion, and irrigation with arsenic-rich groundwater. Uncontaminated soils typically contain background arsenic levels of approximately 5 to 10 mg/kg, while contaminated sites may exceed several hundred or even several thousand mg/kg.
Inorganic arsenic species, namely arsenite As(III) and arsenate As(V), are classified as Group 1 human carcinogens by the International Agency for Research on Cancer (IARC), and long-term exposure through soil, dust ingestion, or crop uptake is associated with skin lesions, cardiovascular disease, and cancers of the skin, bladder, and lungs. Arsenic mobility in soil is strongly controlled by redox conditions and pH: under flooded, reducing conditions, such as in paddy fields, arsenic is released from iron and manganese oxides and becomes far more bioavailable to rice and other crops.
Regulatory frameworks worldwide have established risk screening and control values for arsenic in soil. In China, GB 36600-2018 (Soil environmental quality - Risk control standard for soil contamination of development land) and GB 15618-2018 (Risk control standard for soil contamination of agricultural land) define the primary benchmarks, while comparable guidance is provided by ISO standards, US EPA regional screening levels, and national regulations in the European Union and other jurisdictions. Accredited third-party laboratories perform soil arsenic analysis to demonstrate compliance with these limits and to support risk assessment and remediation decisions.
Testing Samples
Agricultural Soil
Surface soil samples (typically 0–20 cm for farmland and 0–15 cm for paddy fields) are collected from cultivated land, orchards, vegetable bases, and tea plantations to evaluate arsenic levels against agricultural screening values and to assess food-safety risks from crop uptake, particularly in rice-growing regions.
Construction and Development Land Soil
Soil from residential, commercial, and industrial redevelopment plots is tested under GB 36600-2018, which distinguishes Class I land use (sensitive receptors such as residential areas, schools, and hospitals) from Class II land use (industrial and commercial sites). Stratified sampling at multiple depths is often required during preliminary and detailed site investigations.
Contaminated Site and Industrial Soil
Soil from mining areas, smelters, electroplating facilities, pesticide plants, and former gasworks is analyzed to delineate contamination plumes, characterize waste-impacted material, and verify the effectiveness of remediation actions such as solidification, soil washing, or phytoextraction.
Sediment, Sludge, and Soil Amendments
River and lake sediments, sewage sludge destined for land application, composts, and industrial by-products such as fly ash or phosphate fertilizers are also tested for arsenic, since these materials can act as significant secondary sources of soil contamination.
Testing Items
Soil arsenic analysis programs typically include the following testing items, selected according to the assessment objective and the applicable regulatory standard:
- Total arsenic content in soil and sediment (aqua regia-extractable or total digestion)
- Arsenic speciation analysis: As(III), As(V), monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA)
- Leachable arsenic by toxicity characteristic leaching procedure (US EPA Method 1311, TCLP) or synthetic precipitation leaching procedure (EPA Method 1312, SPLP)
- Bioavailable and bioaccessible arsenic fractions by sequential chemical extraction (BCR or Tessier schemes) or in vitro gastrointestinal assays
- Arsenic distribution with depth and particle-size fractions for contamination profiling
- Soil pH, organic matter, cation exchange capacity, and iron/aluminum oxide content as supporting parameters for mobility and risk assessment
Under GB 36600-2018, the risk screening values for arsenic in soil are 20 mg/kg for Class I land and 60 mg/kg for Class II land, with risk control values of 120 mg/kg and 140 mg/kg respectively. Under GB 15618-2018, agricultural screening values for paddy soil range from approximately 20 to 30 mg/kg depending on soil pH. Exceedance of a screening value does not automatically designate contamination but triggers a detailed risk assessment under the applicable standard.
Testing Methods
Sample Collection and Preparation
Samples are collected following technical specifications such as HJ/T 166-2004 and the ISO 18400 series, typically at 500 g to 1 kg per sampling point, using stainless steel or plastic tools to avoid cross-contamination. In the laboratory, samples are air-dried at room temperature or oven-dried at low temperature, disaggregated, homogenized, and sieved: the fraction below 2 mm is retained for pH and leaching tests, while the fraction below 0.149 mm (100 mesh) is used for total arsenic determination.
Extraction and Digestion
For regulatory arsenic determination, soils are commonly digested with aqua regia (HCl-HNO3) by hot-block or water-bath extraction, consistent with ISO 11466 and HJ 803-2016. Where true total arsenic is required, total decomposition methods employing hydrofluoric acid, such as EPA Method 3052 or ISO 14869-1, are applied. Microwave-assisted digestion (EPA Method 3051A) improves recovery, throughput, and reproducibility for large sample batches.
Instrumental Determination
- Atomic fluorescence spectrometry (AFS): the most widely used technique in China, offering method detection limits at the level of approximately 0.01 mg/kg, specified in GB/T 22105.2-2008 and HJ 680-2013 (microwave digestion AFS)
- ICP-MS (inductively coupled plasma mass spectrometry): multi-element determination with hydride generation or collision-cell modes for arsenic, per HJ 803-2016 and EPA Method 6020B, ideal for large site-investigation datasets
- Hydride generation atomic absorption spectrometry (HG-AAS): a sensitive, cost-effective classical method for arsenic in digests
- ICP-OES (EPA Method 6010D): suitable for higher-concentration contaminated samples where detection limits in the low mg/kg range are sufficient
- Portable and benchtop X-ray fluorescence (XRF, EPA Method 6200): rapid field screening with results in minutes, used to guide sampling before confirmatory laboratory analysis
- Spectrophotometric methods (GB/T 17134-1997, silver diethyldithiocarbamate method): legacy colorimetric techniques still referenced in some regulatory contexts
Speciation and Bioavailability Analysis
Arsenic speciation is performed by high-performance liquid chromatography coupled with ICP-MS (HPLC-ICP-MS), which separates As(III), As(V), MMA, and DMA under carefully controlled extraction conditions that preserve the original species. Bioavailability is evaluated through BCR or Tessier sequential extraction schemes and through in vitro bioaccessibility assays such as the Unified BARGE Method (UBM), which simulate gastrointestinal dissolution of arsenic from ingested soil particles.
Quality Assurance and Quality Control
Accredited third-party laboratories operating under ISO/IEC 17025 validate each analytical batch with method blanks, laboratory duplicates, matrix spikes, certified reference materials (CRMs) such as GBW-series soil standards, and continuing calibration verification. Typical acceptance criteria include spike recoveries of 80–120 percent, relative deviation of duplicates below 20 percent, and calibration correlation coefficients of at least 0.995, ensuring that reported arsenic concentrations are accurate, traceable, and legally defensible.
Applications
Soil arsenic content analysis serves a broad range of regulatory, industrial, and scientific purposes:
- Agricultural land quality surveys and food-safety risk management for rice, vegetables, and other crops prone to arsenic accumulation
- Phase I and Phase II environmental site assessments and soil pollution status surveys for land redevelopment and transactions
- Contamination delineation and remediation design at mining, smelting, and chemical industry sites
- Effectiveness verification after soil remediation and acceptance testing against risk control values
- Environmental impact assessment and compliance monitoring for industrial discharges and solid waste disposal
- Groundwater and ecological risk assessment, since soil arsenic leaching affects underlying aquifers
- Academic research on arsenic biogeochemistry, bioavailability, and exposure pathways
FAQ
What is the detection limit for soil arsenic analysis?
With atomic fluorescence spectrometry and ICP-MS, method detection limits of approximately 0.01 mg/kg or lower are routinely achieved, which is far below typical regulatory screening values. Portable XRF screening generally has practical quantification limits of a few mg/kg and should be confirmed by laboratory digestion and instrumental analysis.
How much soil sample is needed and how should it be preserved?
A minimum of approximately 500 g of soil per sampling point is recommended. Samples are collected in glass or polyethylene containers, protected from contamination, and dried and sieved in the laboratory before analysis. For metals analysis, no refrigeration is strictly required, but chain-of-custody documentation must be maintained.
Why do different digestion methods give different arsenic results?
Aqua regia extracts the environmentally relevant, acid-soluble arsenic fraction and is prescribed by most regulatory standards, whereas total digestion with hydrofluoric acid dissolves the silicate matrix and releases arsenic bound in primary minerals, yielding higher results. The choice of method must match the requirement of the applicable standard.
How long does soil arsenic testing take?
Standard total arsenic analysis is typically completed within 5 to 10 working days after sample receipt. Speciation analysis, bioaccessibility testing, or large site-investigation batches may require additional time for method validation and quality control review.
What happens if arsenic exceeds the screening value?
An exceedance of a risk screening value, such as those in GB 36600-2018 or GB 15618-2018, triggers a detailed site-specific risk assessment that considers exposure pathways, land use, and soil properties. Depending on the outcome, risk control or remediation measures may be required before the land can be safely used.