Soil heavy metal determination is an essential laboratory process for quantifying toxic elements such as lead, cadmium, chromium, mercury, and arsenic in soil to support environmental compliance, land redevelopment, and agricultural safety. This technical guide details every step of the procedure, from sampling and pretreatment to acid digestion, instrumental analysis, and quality control, in accordance with recognized GB, ISO, ASTM, and US EPA standards. Use this reference to obtain accurate, reproducible, and defensible heavy metal concentration data for any soil matrix.
Technical Overview
Heavy metals in soil, including arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb), and zinc (Zn), are persistent pollutants that accumulate through industrial discharge, mining, agricultural inputs, and atmospheric deposition. Because they do not degrade and can enter the food chain or groundwater, accurate quantification is a prerequisite for environmental risk assessment, regulatory compliance, and remediation decision-making.
The determination of heavy metals in soil follows a standardized analytical chain: representative sampling, air-drying and particle-size reduction, accurate weighing, acid digestion to bring the target elements into solution, instrumental quantification, and rigorous quality control. The regulatory framework in China is defined by GB 36600-2018 (risk control standard for soil contamination of development land), GB 15618-2018 (risk control standard for agricultural land), and HJ/T 166-2004 (technical specification for soil environmental monitoring), while international equivalents include ISO 11466, ISO 11047, ISO 14869-1, and US EPA Methods 3050B, 3051A, and 3052.
Analytical results are expressed in milligrams per kilogram (mg/kg) on a dry-weight basis, corrected for soil moisture content. Different digestion strategies are selected depending on whether total content (hydrofluoric-acid-based digestion), the environmentally reactive metal pool (aqua regia extraction), or specific chemical species such as hexavalent chromium are required.
Testing Samples
Agricultural and Farmland Soil
Surface soil from the cultivated layer (typically 0–20 cm for dryland and 0–15 cm for paddy fields) is collected from farmland, orchards, vegetable greenhouses, and tea plantations. Results are evaluated against the risk screening values of GB 15618-2018, with particular attention to cadmium and arsenic, which most frequently drive exceedances in agricultural settings.
Construction and Development Land Soil
Soil from residential, commercial, and industrial land parcels is tested against the screening and intervention values of GB 36600-2018. Sampling depth extends from the surface to 0.5 m or deeper intervals in suspected contamination zones. Projects may require only the eight basic items or the full list of 45 controlled parameters depending on the land-use scenario and site history.
Contaminated Site and Industrial Soil
Soil from smelters, electroplating plants, chemical facilities, fuel stations, mining areas, and waste disposal sites is sampled on a dense grid to delineate contamination plumes. These samples support baseline investigations, remediation design, and post-remediation acceptance verification.
Sediment, Sludge, and Amended Soil
River and lake sediments, sewage sludge, composts, and dredged materials intended for land application are analyzed to confirm that heavy metal loading rates remain within applicable control limits before reuse or disposal.
Background and Geochemical Survey Soil
Deep profile samples (subsoil and C-horizon) from undisturbed locations establish natural geochemical baselines, enabling differentiation between geogenic enrichment and anthropogenic contamination.
Testing Items
Testing scope is defined by the applicable risk control standard and the objectives of the investigation. Common scopes include the following:
- Basic eight items under GB 36600-2018: arsenic, cadmium, hexavalent chromium, copper, mercury, nickel, lead, and zinc
- Agricultural screening items under GB 15618-2018: arsenic, cadmium, total chromium, copper, mercury, nickel, lead, and zinc
- Extended items under GB 36600-2018 (45 items in total): beryllium, barium, cobalt, manganese, molybdenum, antimony, selenium, thallium, vanadium, silver, total petroleum hydrocarbons, and others
- Speciation and availability testing: hexavalent chromium by alkaline extraction, DTPA-extractable micronutrient fractions per ISO 14870, and BCR sequential extraction for chemical fractionation studies
- Soil moisture content, determined separately at 105 °C, for dry-weight correction of all reported results
Testing Methods
The complete determination procedure consists of five sequential stages, each of which directly affects data quality. All work should be performed in accredited third-party laboratories or qualified in-house facilities using validated methods and traceable reference materials.
Step 1: Sampling Design and Sample Collection
Sampling locations are established on a systematic grid or by judgmental placement based on site history. At each point, three to five subsamples are collected within a small radius, combined, and reduced by quartering to approximately 500 g to 1 kg of representative soil. Collection tools should be made of wood, bamboo, stainless steel, or uncoated implements; galvanized or brass tools must be avoided to prevent zinc and copper cross-contamination. Samples are transferred into polyethylene or borosilicate glass containers, labeled with unique identifiers, and transported under ambient conditions for most metals, while fresh refrigerated samples analyzed promptly are recommended for hexavalent chromium in accordance with HJ/T 166-2004 guidance.
Step 2: Pretreatment: Air Drying, Grinding, and Sieving
Samples are air-dried at a temperature not exceeding 40 °C in a clean, dust-free room, spread in a thin layer of 2–3 cm and turned regularly. Aggregates are disaggregated with a wooden or agate implement, and visible stones, roots, plant residues, and debris are removed and weighed. The dried sample is ground in an agate mill (metal mills are prohibited for Cr and Ni determinations) and passed through nylon sieves: 2 mm for general analysis, 0.25 mm for selected parameters, and 0.149 mm (100 mesh) for aqua regia digestion methods. The processed sample is homogenized, quartered, and stored in sealed containers for analysis.
Step 3: Weighing and Digestion
An accurately weighed portion of 0.1000 g to 0.5000 g, recorded to 0.0001 g, is transferred into polytetrafluoroethylene microwave vessels or borosilicate digestion tubes. The digestion route is selected according to the target parameter and method requirements:
- Total content by microwave-assisted mixed-acid digestion with nitric acid, hydrofluoric acid, and hydrochloric acid, followed by boric acid addition to complex residual fluoride, per HJ 832-2017, ISO 14869-1, and US EPA Method 3052
- Environmentally reactive metal pool by aqua regia (hydrochloric acid to nitric acid at 3:1) hot-block or microwave digestion, per HJ 781-2016, ISO 11466, and US EPA Methods 3050B and 3051A
- Mercury and arsenic by water-bath aqua regia digestion at 95–100 °C or microwave digestion prior to atomic fluorescence determination, per HJ 702-2014 and HJ 680-2013
- Hexavalent chromium by alkaline extraction with sodium hydroxide and sodium carbonate solution, per HJ 1082-2019 and US EPA Method 3060A
After digestion, vessels are cooled, contents are quantitatively transferred, filtered or centrifuged if necessary, and diluted to a fixed volume (typically 25–50 mL) with ultrapure water containing dilute acid. Reagent blanks are prepared in parallel with every batch.
Step 4: Instrumental Determination
Digested solutions are analyzed against multi-point calibration curves prepared from certified standard solutions. Instrument selection depends on target elements, expected concentration, and required detection limits:
- Inductively coupled plasma mass spectrometry (ICP-MS) per HJ 832-2017, ISO 17294-2, and US EPA Method 6020B for ultra-trace multi-element determination, using indium, rhodium, and rhenium internal standards and collision-cell mode to resolve the argon chloride interference on arsenic at mass 75
- Inductively coupled plasma optical emission spectrometry (ICP-OES) per HJ 781-2016, ISO 22036, and US EPA Method 6010D for simultaneous multi-element determination at milligram-per-kilogram levels
- Flame atomic absorption spectrophotometry per GB/T 17138-1997, GB/T 17137-1997, and GB/T 17139-1997 for copper, zinc, chromium, and nickel
- Graphite furnace atomic absorption spectrophotometry per GB/T 17141-1997, ISO 11047, and US EPA Method 7010 for trace lead and cadmium, using ammonium dihydrogen phosphate or palladium matrix modifiers
- Atomic fluorescence spectrometry per GB/T 22105 and HJ 680-2013, and cold vapor techniques per US EPA Method 7471B, for mercury, arsenic, selenium, and antimony with potassium borohydride reduction
Step 5: Quality Control, Data Processing, and Reporting
Each analytical batch incorporates a full procedural blank, laboratory duplicates with relative deviation typically within 30 percent, matrix spike recoveries within 70 to 130 percent or method-specified limits, and analysis of certified reference soil materials. Calibration verification standards are run every ten samples, and interference checks are performed for spectral overlaps. Final concentrations are calculated from the calibration function, corrected for dilution, sample mass, and moisture content, and reported in mg/kg dry weight. Results are then compared with the applicable GB 36600-2018 or GB 15618-2018 screening and intervention values, with exceedances clearly flagged and interpreted according to land-use class and soil pH.
Applications
Soil heavy metal determination data directly support a wide range of regulatory, commercial, and scientific activities:
- Environmental site assessments and due diligence for land transactions and redevelopment projects
- Detailed agricultural soil pollution surveys and categorization of farmland for safe utilization
- Baseline characterization, process control, and acceptance verification for soil remediation projects
- Compliance monitoring for enterprises listed as key soil pollution sources
- Evaluation of sewage sludge, compost, and dredged sediment prior to land application
- Geochemical baseline surveys, environmental research, and long-term trend monitoring programs
FAQ
What is the difference between total and aqua regia-extractable heavy metals?
Total content is measured after digestion that includes hydrofluoric acid and dissolves the silicate lattice, representing the complete elemental inventory. Aqua regia extraction dissolves the environmentally reactive pool adsorbed to clays, oxides, and organic matter, which is the basis for most regulatory screening values and risk assessments.
How much soil sample is required and how should it be stored?
A minimum of approximately 500 g of sieved soil is recommended for a standard heavy metal panel, with additional quantity reserved for duplicate and confirmation testing. Dried and ground samples for total metal analysis can be stored at room temperature in sealed containers, while hexavalent chromium samples should be kept refrigerated and analyzed promptly.
Why are mercury and arsenic digested separately from other metals?
Mercury and arsenic are volatile and can be lost during prolonged high-temperature open digestion, and both are conventionally quantified by atomic fluorescence spectrometry from dedicated water-bath or microwave digestions per HJ 702-2014 and HJ 680-2013 to preserve recovery and sensitivity.
What detection limits can be achieved?
Typical method detection limits are approximately 0.01 to 0.1 mg/kg for most elements by ICP-MS, around 0.01 mg/kg for cadmium and lead by graphite furnace atomic absorption, and down to approximately 0.002 mg/kg for mercury by cold vapor atomic fluorescence, depending on sample mass, matrix, and instrument conditions.
How long does a complete determination take?
For a routine panel, the full workflow of pretreatment, digestion, instrumental analysis, quality control review, and reporting generally requires seven to fifteen working days after samples arrive at the laboratory, with expedited schedules possible for urgent projects.
How are results compared with regulatory limits?
For development land, results are compared with the screening and intervention values of GB 36600-2018 according to the sensitive or non-sensitive land-use class. For agricultural land, results are compared with the GB 15618-2018 risk screening and control values, which vary with soil pH and crop type for cadmium and several other elements.