Soil Heavy Metal Arsenic and Mercury Analysis: Methods, Standards, and Applications

Soil heavy metal arsenic and mercury analysis is a core environmental testing service used to quantify two of the most toxic regulated contaminants in agricultural land, industrial sites, and remediation projects. Accredited third-party laboratories apply validated digestion procedures and highly sensitive techniques such as atomic fluorescence spectrometry (AFS), ICP-MS, and cold vapor methods to detect arsenic and mercury at trace levels in soil. This article explains the analytical principles, applicable standards, evaluation criteria, typical sample types, and common applications of arsenic and mercury analysis in soil.

Technical Overview

Arsenic (As) and mercury (Hg) are among the most strictly regulated potentially toxic elements in soil environmental management worldwide. Both contaminants are persistent and non-degradable: they accumulate in surface soil, transfer into crops and groundwater, and threaten human health through the food chain, direct ingestion, and inhalation exposure pathways. Arsenic, a metalloid existing mainly as trivalent As(III) and pentavalent As(V) species, is a confirmed human carcinogen, while mercury can be methylated by microorganisms in flooded paddy soil to form methylmercury, a highly bioavailable and neurotoxic compound that concentrates in rice grain.

Typical anthropogenic sources include mining and non-ferrous metal smelting, coal combustion, historical application of arsenical pesticides and herbicides, sewage irrigation, phosphate fertilizer impurities, electroplating and chemical manufacturing discharges, and atmospheric deposition. Because natural background concentrations are generally low (commonly 5 to 15 mg/kg for arsenic and 0.01 to 0.3 mg/kg for mercury in unpolluted soils), moderate anthropogenic inputs are sufficient to exceed regulatory thresholds and trigger risk management requirements.

In routine practice, total arsenic and total mercury are determined by accredited third-party laboratories and evaluated against soil environmental quality standards, such as GB 15618-2018 for agricultural land and GB 36600-2018 for development land in China, as well as ISO and US EPA methods for international projects.

Testing Samples

Agricultural and Farmland Soil

Samples from paddy fields, vegetable production bases, orchards, and grain-producing areas are analyzed to assess crop safety and soil environmental quality. Paddy soil receives particular attention because flooded, reducing conditions promote arsenic release and mercury methylation, increasing plant uptake risk.

Development Land and Brownfield Soil

Soil from former industrial plants, gasworks, chemical and metallurgical sites, fuel stations, and landfills is tested during site investigation and prior to redevelopment. Results are screened against risk control values to determine whether further risk assessment, remediation, or risk control measures are required before land transfer or construction.

Mining and Smelting Site Soil

Soil surrounding tailings ponds, mine dumps, and smelters often shows strong spatial heterogeneity and elevated arsenic and mercury levels. Systematic grid sampling combined with dense laboratory verification is used to delineate contamination plumes.

Background and Monitoring Samples

Reference samples from unpolluted areas, nature reserves, and long-term monitoring grids establish local geochemical baselines and support environmental impact assessments and regional soil pollution surveys.

Testing Items

Standard testing portfolios for soil arsenic and mercury analysis include the following items:

• Total arsenic (As) in soil
• Total mercury (Hg) in soil
• Water-soluble, acid-extractable, and bioavailable fractions of arsenic and mercury
• Speciation analysis: As(III), As(V), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA); inorganic mercury and methylmercury (MeHg)
• Sequential extraction (BCR or Tessier schemes) to characterize geochemical binding forms and mobility
• Supporting parameters: pH, organic matter, cation exchange capacity (CEC), redox potential, and particle size distribution

Speciation and fractionation tests are optional add-ons that provide deeper insight into mobility, bioavailability, and ecotoxicological risk, and are especially valuable for remediation feasibility studies.

Testing Methods

Laboratory determination of arsenic and mercury in soil combines robust sample preparation with highly sensitive instrumental quantification. Method selection depends on project requirements, regulatory context, and target reporting limits.

Sample Preparation and Digestion

Field samples are air-dried at room temperature (below 40 °C), homogenized, ground, and sieved through a 100-mesh (0.149 mm) nylon sieve. Common digestion schemes include water-bath aqua regia extraction (GB/T 22105.1-2008), microwave-assisted digestion with nitric-hydrochloric acid mixtures (HJ 680-2013, HJ 702-2014, US EPA Method 3051A), and hot-plate digestion (US EPA Method 3050B). For mercury, an oxidizing environment and strict temperature control are maintained to prevent volatilization losses.

Atomic Fluorescence Spectrometry (AFS)

AFS is the primary technique in Chinese national standards for these two elements. Arsenic in the digest is pre-reduced with thiourea–ascorbic acid, then converted to arsine by potassium borohydride hydride generation; mercury is measured by cold vapor generation. Typical method detection limits reach approximately 0.01 mg/kg for arsenic and 0.002 mg/kg for mercury in soil, meeting all common regulatory screening thresholds.

ICP-MS

Inductively coupled plasma mass spectrometry (ISO 17294-2, US EPA Method 6020B, HJ 803-2016) enables simultaneous multi-element determination, which is efficient when the full suite of eight regulated heavy metals is required. Collision or reaction cell technology is used to resolve the argon chloride (40Ar35Cl+) polyatomic interference on arsenic at mass 75, and gold or stabilizing agents are often added to control mercury memory effects.

Cold Vapor Atomic Absorption and Direct Mercury Analysis

Mercury can also be determined by cold vapor atomic absorption spectrometry (US EPA Method 7471B, ISO 16772) or by direct thermal decomposition–gold amalgamation analyzers (US EPA Method 7473). Direct mercury analysis requires no acid digestion, eliminates volatilization loss, and delivers results within minutes, making it ideal for low-level mercury determination and high-throughput screening.

Field Screening by Portable XRF

Portable X-ray fluorescence analyzers (US EPA Method 6200) provide rapid on-site screening of arsenic at concentration ranges relevant to screening values. XRF data are suitable for delineating sampling boundaries but must be confirmed by laboratory analysis before compliance decisions.

Quality Assurance and Quality Control

Validated quality control measures include method blanks, laboratory duplicates, matrix spikes with recoveries typically between 80% and 120%, certified reference materials (such as GBW-series soil standards), continuing calibration verification, and periodic method detection limit studies, ensuring defensible and reproducible data.

Standards and Evaluation Criteria

Sampling and analysis are conducted in accordance with HJ/T 166-2004 for soil environmental monitoring and the HJ 25 series of technical guidelines for site investigation and risk assessment. Under GB 15618-2018 for agricultural land, arsenic screening values range from 20 to 30 mg/kg for paddy soil and 25 to 40 mg/kg for dryland, while mercury screening values range from 0.5 to 1.0 mg/kg for paddy soil and 1.3 to 3.4 mg/kg for dryland, depending on soil pH.

Under GB 36600-2018 for development land, arsenic screening values are 20 mg/kg for first-class (sensitive) land use and 60 mg/kg for second-class land use, and mercury screening values are 8 mg/kg and 38 mg/kg respectively. Concentrations below the screening value generally allow normal land use; concentrations between screening and control values trigger detailed risk assessment; and exceedance of control values requires risk intervention, remediation, or restrictive land-use control.

Applications

Arsenic and mercury analysis in soil supports a wide range of regulatory, commercial, and scientific objectives:

• Classification of agricultural land into priority protection, safe utilization, and strict control categories
• Phase II environmental site investigations and human health risk assessments for contaminated land
• Remediation design input, remediation effect evaluation, and project completion acceptance
• Environmental impact assessment baselines and compliance monitoring for key industrial enterprises
• Due diligence for land transfer, mergers and acquisitions, and brownfield redevelopment
• Geochemical mapping, soil background surveys, and academic research on element cycling
• Food safety support, including arsenic and mercury source tracing in grain and vegetable production areas

FAQ

Why are arsenic and mercury usually analyzed together?

Both elements form gaseous hydrides or cold vapors, so they are efficiently measured on the same atomic fluorescence spectrometer using a shared aqua regia digestion, and they appear together in the same national standard methods (GB/T 22105.1-2008, HJ 680-2013), which improves cost efficiency.

What detection limits can be achieved?

With AFS, method detection limits of approximately 0.01 mg/kg for arsenic and 0.002 mg/kg for mercury are routinely achieved, far below the strictest regulatory screening values, ensuring reliable compliance judgments.

How should soil samples be collected and preserved for mercury analysis?

Samples should be collected with stainless steel or non-contaminating tools following HJ/T 166-2004 or HJ 25.2 requirements, stored in sealed glass or polyethylene containers away from light and heat, and delivered promptly, because mercury is volatile and speciation can change during storage. Speciation analysis requires field-moist, chilled or frozen handling.

Can portable XRF results be used for regulatory compliance?

No. XRF is a screening tool for delineating contamination in the field. Regulatory decisions and remediation acceptance must be based on laboratory analysis by validated methods in accredited third-party laboratories.

How long does the analysis take?

Standard turnaround is typically 7 to 10 working days from sample receipt, depending on sample volume and whether speciation or sequential extraction is required; expedited services can often be arranged.

What happens if results exceed the screening values?

Exceedance does not automatically mean unusable land. A detailed site investigation and human health risk assessment are conducted under the HJ 25 series; depending on the outcome, options include targeted remediation, engineering controls, adjustment of land use, or long-term monitoring.