Determination of Heavy Metal Content in Site Soil: Standards, Analytical Methods, and Testing Workfl

Determination of heavy metal content in site soil is a core component of environmental site assessment, delivering the quantitative data needed to evaluate contamination risks, verify regulatory compliance, and support land redevelopment decisions. By combining standardized sampling protocols with validated analytical techniques such as ICP-MS, atomic absorption spectrometry, and atomic fluorescence spectrometry, accredited third-party laboratories can accurately quantify metals including lead, cadmium, arsenic, mercury, chromium, copper, nickel, and zinc in accordance with GB 36600-2018, GB 15618-2018, and international standards. This article explains the complete testing workflow, from sampling design and sample pretreatment to digestion, instrumental determination, and quality control.

Technical Overview

Site soil heavy metal determination refers to the quantitative measurement of toxic metal and metalloid concentrations in soil at development sites, industrial land, and farmland, with results reported in milligrams per kilogram on a dry-weight basis. Heavy metals such as lead (Pb), cadmium (Cd), arsenic (As), mercury (Hg), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn) are persistent, non-degradable contaminants that can enter the human body through direct ingestion, inhalation of dust, and dermal contact, and can also leach into groundwater. Reliable analytical data therefore underpins every stage of soil environmental management, from preliminary site investigation through risk assessment, remediation design, and post-remediation verification.

In China, the principal regulatory drivers are GB 36600-2018, the risk control standard for soil contamination of development land, which defines risk screening values and risk control values for Class I (sensitive) and Class II (non-sensitive) land uses, and GB 15618-2018, the corresponding standard for agricultural land, whose screening values vary with soil pH and crop type. Site investigation, risk assessment, and remediation effect evaluation follow the HJ 25 series of technical guidelines, while general soil monitoring practice is governed by HJ/T 166-2004, the technical specification for soil environmental monitoring. Internationally, comparable frameworks include ISO 11466 and ISO 11047 for aqua regia extraction and atomic absorption determination, as well as ASTM E1903 for Phase II environmental site assessment.

A typical determination workflow comprises four linked steps: (1) stratified sampling across the site according to a designed grid; (2) laboratory pretreatment including air-drying, grinding, and sieving; (3) acid digestion or extraction to bring the metals into solution; and (4) instrumental quantification using spectrometric techniques matched to the target elements and expected concentration ranges. Each step is supported by documented quality assurance and quality control measures, and testing should be performed by accredited third-party laboratories holding ISO/IEC 17025 accreditation.

Testing Samples

Surface Soil Samples

Surface soil, generally collected from 0 to 0.5 meters below ground level, represents the zone of most direct human contact and is the primary medium for screening-level assessments of GB 36600-2018 basic items. Surface samples are collected using non-metallic or stainless-steel tools to avoid cross-contamination, with typical sample masses of 500 grams to 1 kilogram per point.

Subsurface and Deep Soil Samples

Subsurface samples are obtained from boreholes advanced by direct-push rigs, hollow-stem augers, or rotary drilling, commonly at depth intervals such as 0.5–1.5 m, 1.5–3 m, and 3–6 m, with additional samples taken at lithological changes, stained or odorous zones, and just above the groundwater table. Vertical profiling identifies buried contamination sources and characterizes the three-dimensional extent of metal plumes.

Stockpiled, Backfill, and Imported Soil

Excavated stockpiles, engineered backfill, and imported clean soil are sampled discretely or in composites to verify that concentrations meet remediation targets or acceptance criteria before reuse or disposal. Remediation validation sampling follows the layout defined in the remediation effect evaluation plan.

Agricultural and Background Soil Samples

Farmland samples are collected from the plow layer (0–20 cm) and subsoil for assessment against GB 15618-2018, while local background samples from uncontaminated reference locations help distinguish anthropogenic contamination from natural geochemical baseline levels.

Testing Items

The seven inorganic basic items required by the screening value table of GB 36600-2018 form the default parameter list for development land investigations:

• Arsenic (As)
• Cadmium (Cd)
• Hexavalent chromium (Cr(VI))
• Copper (Cu)
• Lead (Pb)
• Mercury (Hg)
• Nickel (Ni)

Depending on site history and the risk assessment outcome, the extended inorganic item list may be added:

• Antimony (Sb), beryllium (Be), cobalt (Co), thallium (Tl), and vanadium (V)
• Zinc (Zn), manganese (Mn), barium (Ba), molybdenum (Mo), selenium (Se), and silver (Ag)

For agricultural land under GB 15618-2018, the screening items are cadmium, mercury, arsenic, lead, chromium, copper, nickel, and zinc, evaluated against pH-dependent thresholds. Supporting parameters are essential for data interpretation and risk modeling and typically include soil pH (HJ 962-2018), dry matter and water content (HJ 613-2011), organic matter, and cation exchange capacity. Chromium speciation analysis, distinguishing hexavalent chromium from total chromium, is particularly important because Cr(VI) is far more toxic and mobile and carries a much stricter screening value.

Testing Methods

Sampling Design and Field Protocols

Sampling layouts follow HJ 25.1-2019 for site environmental investigation and HJ/T 166-2004 for general soil monitoring. Preliminary investigations commonly apply systematic grids of approximately 40 m × 40 m over suspected contamination areas, refined to denser grids such as 20 m × 20 m during detailed investigation where exceedances are found. Judgmental sampling targets obvious source areas such as storage tanks, electroplating lines, waste discharge points, and smelter stacks. Field QA measures include dedicated sampling tools cleaned between locations, chain-of-custody documentation, field duplicates at a frequency of at least 10 percent, and cold-chain transport at approximately 4 °C to the laboratory.

Sample Preparation and Pretreatment

On receipt, samples are logged, air-dried at room temperature or freeze-dried for mercury-sensitive projects, crushed, homogenized, and quartered. Grinding is performed with agate or zirconia mills to prevent metal introduction, and the test portion is sieved through a 2 mm sieve for general analysis and through a 0.149 mm (100 mesh) sieve for metal digestion subsamples. Because all results are reported on a dry-weight basis, moisture content is determined in parallel according to HJ 613-2011.

Acid Digestion and Extraction

The choice of digestion procedure defines what fraction of the metal is measured and must match the applicable standard:

• Aqua regia extraction (hydrochloric acid and nitric acid, 3:1) by hot-block or microwave heating, per ISO 11466, EPA Method 3051A, and the acid systems used in the GB/T 17138 and GB/T 17141 method series, recovers environmentally relevant, acid-soluble metals and is the basis of most regulatory screening comparisons.
• Total digestion with hydrofluoric acid, nitric acid, and perchloric acid, per ISO 14869-1, EPA Method 3052, or the microwave procedure of HJ 680-2013, dissolves silicate minerals and is used when true total element content is required.
• Alkaline digestion with sodium hydroxide and sodium carbonate, per EPA Method 3060A and the corresponding HJ procedure, preserves chromium oxidation state and is mandatory before Cr(VI) determination; acidic digestion is not acceptable for this analyte.

Instrumental Determination

Instrument selection depends on target elements, expected concentrations, and required detection limits:

• Flame atomic absorption spectrometry (FAAS) for copper, zinc, lead, nickel, and total chromium at mg/kg levels, following GB/T 17138, GB/T 17139, and GB/T 17137, as well as ISO 11047.
• Graphite furnace atomic absorption spectrometry (GFAAS) for trace lead and cadmium at sub-mg/kg levels, following GB/T 17141 and the EPA 7000B series.
• Inductively coupled plasma optical emission spectrometry (ICP-OES) for simultaneous multi-element determination at moderate to high concentrations, per EPA Method 6010D and ISO 22036.
• Inductively coupled plasma mass spectrometry (ICP-MS) for ultra-trace multi-element analysis of cadmium, thallium, beryllium, and other extended items, with microwave digestion per HJ 680-2013 and EPA Method 6020B, achieving reporting limits down to approximately 0.01 mg/kg for many elements.
• Atomic fluorescence spectrometry (AFS) for mercury, arsenic, antimony, selenium, and bismuth, following GB/T 22105 and HJ 702-2014, with mercury detection limits in the low microgram-per-kilogram range.
• Cold vapor or direct thermal-decomposition mercury analyzers (EPA Methods 7471B and 7473) as alternatives for total mercury, and ultraviolet-visible spectrophotometry (EPA Method 7196A) or flame AAS after alkaline digestion for hexavalent chromium.

Field Screening with Portable XRF

Portable X-ray fluorescence analyzers operated under EPA Method 6200 provide real-time, non-destructive in-situ screening of metals such as lead, chromium, copper, zinc, and arsenic, enabling adaptive sampling and rapid delineation of hotspots. However, XRF is a screening tool: its sensitivity for cadmium and mercury is generally insufficient for comparison against strict screening values, so a percentage of XRF locations must always be confirmed by laboratory analysis with documented correlation.

Quality Assurance and Quality Control

Validated datasets from accredited third-party laboratories incorporate full analytical batch control, including reagent blanks and method blanks, laboratory duplicates at a minimum 10 percent frequency, matrix spikes and spike duplicates, certified reference soils or standard materials traceable to national metrology institutes, initial and continuing calibration verification, and interference checks for ICP techniques. Method detection limits are verified annually and must sit well below the applicable GB 36600-2018 or GB 15618-2018 screening values. Final reports present results on a dry-weight basis with flags for exceedances, supporting straightforward comparison against Class I or Class II land-use screening values and risk control values.

Applications

Heavy metal determination in site soil supports a wide range of environmental, regulatory, and commercial objectives:

• Brownfield redevelopment and change of land use, where soil data confirm whether a site meets Class I or Class II screening values before residential or commercial construction.
• Industrial enterprise relocation, requiring soil pollution status investigation reports for regulatory closure under national soil pollution prevention legislation.
• Mining, smelting, electroplating, tannery, and chemical plant legacy sites, where multi-metal and speciation data guide remediation design.
• Agricultural land quality and food safety programs, evaluating farmland against GB 15618-2018 and supporting safe utilization categorization.
• Remediation effect evaluation under HJ 25.5-2018, verifying that excavated, stabilized, or washed soils meet remediation targets.
• Environmental due diligence in mergers, acquisitions, and financing, following frameworks such as ASTM E1903 Phase II assessments.
• Scientific research and regional soil pollution surveys, including geochemical baseline mapping and background level studies.

FAQ

What is the difference between aqua regia extraction and total digestion?

Aqua regia extraction recovers the acid-soluble, environmentally relevant fraction of metals and is the basis of most regulatory screening comparisons in the GB system, while total digestion with hydrofluoric acid dissolves the silicate lattice and measures true total element content. For risk assessment against GB 36600-2018 or GB 15618-2018, aqua regia-based standard methods are generally specified unless the evaluation explicitly requires total content.

How are sampling points and depths determined?

Sampling density and depth follow HJ 25.1-2019, based on historical land use, contaminant characteristics, and the conceptual site model. Systematic grids of roughly 40 m × 40 m are typical for preliminary investigation, refined during detailed investigation, with surface samples from 0 to 0.5 m and subsurface samples collected at regular depth intervals, at lithological transitions, and above the water table.

Can portable XRF replace laboratory analysis?

No. XRF is an excellent rapid screening and delineation tool, but laboratory methods such as ICP-MS, GFAAS, and AFS offer far lower detection limits and better accuracy, which is essential for cadmium, mercury, and arsenic at screening-value concentrations. Regulatory submissions always require confirmatory laboratory data.

Which regulatory limits apply to my site?

Development land is evaluated against GB 36600-2018 screening values according to the planned land-use class (sensitive uses such as residential and schools, or less sensitive uses such as industrial and commercial), while farmland is evaluated against GB 15618-2018 thresholds that depend on soil pH and crop type. Exceedance of screening values triggers detailed investigation and risk assessment under HJ 25.3, and exceedance of control values generally requires risk control or remediation.

How are results reported?

Results are expressed in mg/kg on a dry-weight basis, with moisture correction performed per HJ 613-2011. Reports include sampling coordinates and depths, analytical methods and detection limits, complete QA/QC evidence, and direct comparison tables against applicable screening and control values.

What is the typical project timeline?

A routine program including sampling, transport, pretreatment, digestion, instrumental analysis, and reporting generally takes two to four weeks, depending on the number of sampling points, parameter list, and whether hexavalent chromium speciation or urgent screening under accelerated turnaround is required.