Determination of total heavy metals in soil is a fundamental requirement for environmental site assessment, farmland quality monitoring, and contaminated land remediation. This technical article explains the complete analytical workflow—from sample collection and pretreatment to acid digestion and instrumental determination by ICP-MS, ICP-OES, AAS, and AFS—used by accredited third-party laboratories, together with the relevant ISO, GB, and EPA standards, quality control requirements, and practical answers to frequently asked questions.
Technical Overview
Heavy metals are among the most persistent contaminants in soil because they do not degrade biologically, they accumulate over time, and they can transfer into groundwater, crops, and the human food chain. The elements of greatest concern typically include cadmium (Cd), lead (Pb), chromium (Cr), mercury (Hg), arsenic (As), copper (Cu), zinc (Zn), and nickel (Ni). Determining the total content of these elements is the basis for soil pollution surveys, risk screening, land-use compliance, and remediation acceptance.
In analytical practice, "total" heavy metals are defined by the digestion procedure used. Two conventions are widely accepted: the pseudo-total content, obtained by aqua regia extraction as described in ISO 11466 and Chinese standard HJ 803, and the true total content, obtained by complete dissolution of the silicate matrix using hydrofluoric acid (HF) as described in HJ 832 and EPA Method 3052. Most regulatory limit values, including those in GB 15618-2018 (risk screening for agricultural land) and GB 36600-2018 (risk control for development land), are based on aqua regia extraction, which recovers the environmentally relevant fractions bound to sulfides, oxides, carbonates, and organic matter.
All testing described in this article is routinely performed by accredited third-party laboratories operating under ISO/IEC 17025 quality management systems, ensuring that reported data are traceable, defensible, and acceptable to environmental regulators.
Testing Samples
Agricultural and Farmland Soil
Surface samples from the plow layer (typically 0–20 cm) of paddy fields, upland cropland, orchards, and greenhouses. These samples are commonly collected in areas affected by long-term sewage irrigation, fertilizer and manure application, or atmospheric deposition from nearby industrial sources, and are evaluated against GB 15618-2018 screening values.
Construction and Development Land Soil
Soil from former industrial, commercial, and residential parcels scheduled for redevelopment. Stratified samples collected from the surface to several meters depth support classification under GB 36600-2018, which distinguishes Class I (sensitive) land uses such as residential areas and schools from Class II (less sensitive) industrial uses.
Industrial, Mining, and Contaminated Site Soil
Highly heterogeneous samples from smelters, electroplating and chemical plants, fuel storage areas, mining and tailings zones, and landfills. Dense grid-based sampling is required because metal concentrations at such sites can vary by orders of magnitude over short distances.
Sediment and Amended Soil
River and lake sediments, dredged material, and soils amended with sewage sludge, compost, or fly ash, where metal enrichment must be verified before reuse or disposal.
Testing Items
Testing scope is normally organized around the regulated core elements, with extended element lists available for detailed investigations:
- Core eight elements: cadmium (Cd), mercury (Hg), arsenic (As), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn).
- Extended elements: antimony (Sb), beryllium (Be), cobalt (Co), manganese (Mn), molybdenum (Mo), selenium (Se), thallium (Tl), vanadium (V), tin (Sn), silver (Ag), and barium (Ba).
- Hexavalent chromium (Cr(VI)), which requires a dedicated alkaline extraction procedure, for example HJ 1082, because it must not be digested with the total chromium method.
- Supporting parameters such as pH, organic matter, and cation exchange capacity, which influence risk evaluation.
Testing Methods
Sample Collection and Preparation
Representative samples are collected as multi-point composites using wooden, bamboo, or titanium tools to avoid metal contamination. In the laboratory, samples are air-dried at or below 40 °C, stones and plant debris are removed, and the material is ground in an agate mill and passed through a 0.149 mm (100 mesh) nylon sieve. Samples for mercury determination are dried gently and analyzed promptly to prevent volatilization losses. Prepared material is stored in polyethylene or glass containers under chain-of-custody control.
Acid Digestion and Extraction
- Aqua regia extraction (HCl:HNO3 at 3:1, hot-block digestion near 100 °C) per ISO 11466, HJ 803, and HJ 781 — the default approach for regulatory comparability.
- Microwave-assisted digestion with HNO3–HCl–HF in closed PTFE vessels per HJ 832 and EPA Method 3052 — yields true total contents and minimizes losses of volatile Hg and As.
- Four-acid digestion (HNO3–HCl–HF–HClO4) for Pb and Cd determination by graphite furnace AAS, requiring strict perchloric acid safety controls.
- Water-bath aqua regia digestion (GB/T 22105 series) or microwave digestion (HJ 680) prior to atomic fluorescence determination of hydride-forming elements.
- Alkali fusion for refractory matrices when quantitative recovery of silicate-bound elements such as total chromium is required.
Instrumental Determination
- ICP-MS (HJ 803, EPA Method 6020): simultaneous multi-element ultra-trace analysis with internal standards (Rh, In, Re, Bi) and collision/reaction cells to remove the ArCl interference on arsenic-75.
- ICP-OES (HJ 781, EPA Method 6010, ISO 22036): robust determination of Cu, Zn, Cr, Ni, and Pb at mg/kg concentrations.
- Graphite furnace AAS (GB/T 17141): high-sensitivity determination of Pb and Cd with matrix modifiers.
- Flame AAS (GB/T 17138): routine determination of Cu and Zn.
- Atomic fluorescence spectrometry (GB/T 22105 series, HJ 680): excellent sensitivity for Hg, As, Se, Sb, and Bi, reaching sub-microgram-per-kilogram levels for mercury.
- Cold vapor AAS (EPA Method 7471B): an established technique for total mercury.
- Field-portable XRF (EPA Method 6200): rapid on-site screening only; all exceedances must be confirmed by laboratory digestion and instrumental analysis.
Quality Assurance and Quality Control
Each analytical batch includes reagent and full procedural blanks, certified soil reference materials, duplicate samples with relative percent deviations typically within 20 percent, and matrix spikes with recoveries generally between 80 and 120 percent. Multipoint calibration curves with correlation coefficients of at least 0.999 are verified every ten samples. Method detection limits are statistically verified, moisture corrections are applied, and all results are reported on a dry-weight basis.
Applications
Total heavy metal data support a wide range of environmental and regulatory decisions:
- Phase II environmental site assessments and detailed soil pollution investigations.
- Farmland soil quality monitoring and agricultural product food-safety programs.
- Baseline characterization and remediation acceptance testing for contaminated sites.
- Compliance demonstration for land-use change and urban redevelopment projects.
- Environmental impact surveys around mining, smelting, and industrial zones.
- Evaluation of sewage sludge, compost, and phosphatic fertilizer application impacts.
- Soil background value studies and scientific research.
FAQ
What is the difference between total and bioavailable heavy metals?
Total content is measured after aggressive acid digestion of the whole soil matrix, whereas bioavailable (extractable) content is estimated with mild extractants such as buffered DTPA solution per ISO 14870 or dilute calcium chloride. Regulatory screening values are based on total content, while bioavailable fractions are mainly used for agronomic and risk-refinement studies.
Should aqua regia extraction or HF total digestion be used?
Aqua regia extraction is the basis of most regulatory limit values and should be used for compliance testing. HF-based digestion dissolves the silicate lattice and gives true totals, typically 5 to 20 percent higher for lithophile elements such as Cr and Ni, and is preferred for geochemical characterization. The intended data use must be specified before ordering the test.
What detection limits can be achieved?
Indicative method detection limits on a dry-weight basis are approximately Cd 0.01–0.05 mg/kg, Hg 0.002–0.01 mg/kg, As 0.05–0.5 mg/kg, and Pb, Cu, Zn, Cr, and Ni 0.5–2 mg/kg, depending on the instrument, digestion method, and matrix.
How are results evaluated against regulatory limits?
Results are compared with the screening and intervention values of the applicable standard. Under GB 36600-2018, the Class I (sensitive land use) screening values include As 20, Cd 20, Hg 8, Pb 400, Ni 150, Cu 2000, and Cr(VI) 3.0 mg/kg. Agricultural land is evaluated under GB 15618-2018 according to soil pH and land type. Exceedance of a screening value triggers further detailed investigation and human health or ecological risk assessment rather than automatic remediation.
How are samples collected and how long does testing take?
Farmland samples are usually collected as five-point composites per sampling unit, while site investigations follow grid-based designs with stratified depth intervals. Typical laboratory turnaround is five to ten working days after sample receipt, with expedited schedules available for urgent projects.
Can field XRF screening replace laboratory analysis?
No. Portable XRF is a valuable rapid screening tool for delineating contamination, but it has higher detection limits and poorer precision for elements such as Cd, Hg, and As. Regulatory decisions must be supported by validated laboratory digestion methods coupled with ICP-MS, ICP-OES, AAS, or AFS determination.