Determination of arsenic content in soil is a fundamental procedure in environmental monitoring, enabling accurate assessment of soil contamination levels, agricultural product safety, and human health risks. This article provides a comprehensive technical overview of soil arsenic analysis, covering sampling and pretreatment procedures, acid digestion protocols, instrumental methods such as hydride generation atomic fluorescence spectrometry (HG-AFS) and inductively coupled plasma mass spectrometry (ICP-MS), applicable national and international standards, and practical applications in farmland evaluation and contaminated site investigation.
Technical Overview
Arsenic (As) is a naturally occurring metalloid and one of the most prevalent inorganic contaminants in soils worldwide. Background concentrations in uncontaminated soils typically range from 2 to 20 mg/kg, with a global average of approximately 5 to 7 mg/kg. Elevated arsenic levels arise from both geogenic sources, such as sulfide mineral deposits and arsenic-rich parent materials, and anthropogenic activities, including mining and smelting operations, historical use of arsenical pesticides and wood preservatives, irrigation with contaminated groundwater, and industrial waste disposal.
In soil, arsenic exists predominantly as inorganic species — arsenite As(III) and arsenate As(V) — which are considerably more toxic than organic arsenic compounds. Its mobility and bioavailability are governed by redox conditions, pH, and the abundance of iron, aluminum, and manganese oxides and organic matter. Because chronic arsenic exposure through the soil–crop–food pathway or incidental dust ingestion is associated with skin, bladder, and lung cancers, accurate quantification of arsenic in soil is essential for environmental risk assessment and land-use management. Regulatory frameworks such as GB 15618-2018 for agricultural land and GB 36600-2018 for development land establish risk screening and control values, making reliable soil arsenic determination indispensable for site owners, regulators, and environmental consultants.
Testing Samples
Soil arsenic determination can be performed on diverse sample types collected from different environmental settings. Sampling design — grid sampling, stratified depth sampling, or composite sampling — directly determines the representativeness of the analytical results.
Agricultural Soil
Topsoil (0–20 cm plow layer) from paddy fields, vegetable plots, orchards, and grain-producing farmland is collected as composite samples of 500 g to 1 kg per unit. Testing supports farmland soil environmental quality classification and safe agricultural production assessment under GB 15618-2018.
Development Land and Industrial Site Soil
Soil from residential, commercial, and industrial parcels, including legacy industrial sites, fuel storage facilities, and smelting or mining areas, is typically sampled at depth intervals such as 0–0.5 m, 0.5–1.5 m, and 1.5–3.0 m. Results are compared against risk screening values in GB 36600-2018 to determine whether further investigation or remediation is required.
Sediment and Dredged Material
River, lake, estuarine, and marine sediments frequently accumulate arsenic due to adsorption onto fine particles and iron oxides. The same digestion and detection techniques apply, and several sediment-specific standard methods are available for consistent evaluation of disposal or reuse options.
Mine Tailings, Sludge, and Soil Amendments
Tailings, sewage sludge, composts, phosphate fertilizers, and other soil amendments are analyzed for arsenic to prevent the introduction of contaminants into agricultural systems through land application.
Testing Items
Core testing items associated with soil arsenic determination include:
- Total arsenic content, reported in mg/kg on a dry-weight basis
- Water-soluble arsenic, extracted with deionized water at a 1:10 soil-to-water ratio
- Available (extractable) arsenic, determined using 0.5 mol/L sodium bicarbonate or DTPA extraction to estimate plant bioavailability
- Arsenic speciation, separating As(III), As(V), monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA) by HPLC-ICP-MS
- Sequential extraction of arsenic fractions bound to exchangeable, Fe/Mn oxide, organic-sulfide, and residual phases
- Supporting parameters such as pH, organic matter, cation exchange capacity, particle size distribution, and iron and aluminum oxide content
Testing Methods
Sample Pretreatment
Samples are air-dried at room temperature below 40 °C or freeze-dried, cleared of plant debris and stones, homogenized, and ground in an agate mill. The material is passed through a 2 mm sieve for extractable arsenic tests and through a 0.149 mm (100 mesh) nylon sieve for total arsenic determination. Processed samples are stored in acid-washed polyethylene or glass containers until analysis.
Acid Digestion and Extraction
- Aqua regia extraction (HCl:HNO3 at 3:1) using a water bath or temperature-controlled hot block, as specified in GB/T 22105.1-2008 and HJ 803-2016; this approach yields a pseudo-total content because silicate minerals are not fully dissolved
- Microwave-assisted digestion with HNO3-HCl-HF in closed vessels, per HJ 680-2013 and HJ 702-2014, achieving complete decomposition of the silicate matrix for true total arsenic
- Hot-plate total digestion with HNO3-HCl-HF-HClO4, a classical total decomposition procedure
- Alternative acid digestion procedures such as US EPA Methods 3050B and 3051A for environmental monitoring programs
Hydride Generation Atomic Fluorescence Spectrometry (HG-AFS)
HG-AFS is the most widely used routine method for soil arsenic in many laboratories. Arsenate in the digest is first pre-reduced to As(III) using a thiourea–ascorbic acid solution. Potassium borohydride in hydrochloric acid medium then converts As(III) to arsine (AsH3), which is swept by argon carrier gas into an argon–hydrogen flame and atomized; atomic fluorescence intensity is measured with an arsenic hollow cathode lamp. The method offers method detection limits of approximately 0.01 to 0.05 mg/kg, a wide linear range, low operating cost, and minimal spectral interference. It is specified in GB/T 22105.1-2008 and HJ 702-2014.
Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
ICP-MS enables simultaneous determination of arsenic together with other heavy metals, with soil detection limits around 0.01 mg/kg. The principal spectral interference, 40Ar35Cl+ at mass 75, is resolved using collision/reaction cell technology (helium kinetic energy discrimination mode) or interference correction equations, while internal standards such as 72Ge or 103Rh compensate for matrix effects and instrument drift. Applicable standards include HJ 803-2016 (aqua regia extraction), HJ 680-2013 (microwave digestion), ISO 17294-2, and US EPA Method 6020B.
Atomic Absorption Spectrometry (AAS)
Graphite furnace atomic absorption spectrometry provides adequate sensitivity for moderately contaminated samples, while hydride generation AAS — in which generated arsine is decomposed in a heated quartz cell — is described in ISO 20280:2009 for arsenic, antimony, and selenium in aqua regia soil extracts. These methods are suitable options where an AAS platform is the primary instrument.
Spectrophotometric and Field Screening Methods
The silver diethyldithiocarbamate (AgDDC) spectrophotometric method (GB/T 17134-1997) remains a legacy technique with lower sensitivity. Portable and handheld X-ray fluorescence (XRF) analyzers offer rapid on-site screening with detection limits of roughly 5 to 10 mg/kg, which is valuable for delineating contaminated zones, although laboratory analysis is still required for confirmatory and regulatory purposes.
Quality Assurance and Quality Control
Valid data generation requires multi-point calibration curves with correlation coefficients of at least 0.999, method blanks, duplicate analyses with relative deviations typically below 20 percent, matrix spike recoveries within 80 to 120 percent, and analysis of certified soil reference materials whose results fall within certified uncertainty ranges. Continuing calibration verification is performed every ten samples, and all results are reported on a dry-weight basis with moisture correction determined at 105 °C. These practices align with the quality management requirements of accredited third-party laboratories operating under ISO/IEC 17025.
Applications
Soil arsenic determination results serve a broad range of regulatory, commercial, and scientific purposes:
- Classification of agricultural land quality and safe production evaluation for grain and vegetable bases under GB 15618-2018
- Risk screening and detailed site investigation of potentially contaminated development land under GB 36600-2018
- Design of remediation strategies and post-remediation acceptance verification
- National and regional soil pollution status surveys and geochemical baseline mapping
- Environmental impact assessment for mining, smelting, and industrial construction projects
- Environmental due diligence for land transactions and brownfield redevelopment
- Compliance monitoring for wastewater irrigation districts and solid waste disposal facilities
- Scientific research on arsenic migration, transformation, and soil–crop transfer behavior
FAQ
What is the detection limit of soil arsenic analysis?
Method detection limits are approximately 0.01 mg/kg for ICP-MS, 0.01 to 0.05 mg/kg for HG-AFS, and about 5 to 10 mg/kg for field XRF screening. These limits are well below typical regulatory screening values, ensuring reliable compliance judgments.
Which digestion method should be selected?
Aqua regia extraction is recommended when results must be compared with regulatory thresholds established on a pseudo-total basis, as in GB 36600-2018. HF-based microwave or hot-plate total digestion is preferred when complete mass balance or geochemical characterization is required. The digestion scheme should always match the referenced analytical standard.
How much soil sample is required for testing?
A field composite sample of 500 g to 1 kg is generally collected per sampling unit. Laboratory analysis consumes approximately 20 to 50 g of the finely ground fraction, with the remainder archived for confirmatory testing.
What is the difference between total and available arsenic?
Total arsenic measures the entire element content after complete or pseudo-total digestion and is used for regulatory comparison. Available arsenic, extracted with water or mild chemical extractants, estimates the fraction potentially taken up by plants and is more relevant for ecological and food-safety risk evaluation.
How are results interpreted against regulatory limits?
Under GB 36600-2018, arsenic risk screening values are 20 mg/kg for first-category land uses and 60 mg/kg for second-category land uses; exceedances trigger detailed risk assessment. For agricultural land under GB 15618-2018, screening values range from about 20 to 40 mg/kg depending on soil pH and crop type.
What is the typical turnaround time for soil arsenic testing?
Routine total arsenic analysis is typically completed within 7 to 10 working days after sample receipt, while speciation analysis and large project batches may require additional time. Expedited services can usually be arranged with accredited third-party laboratories.