Vertical Distribution Detection of Heavy Metals in Soil: Depth-Discrete Sampling, Analytical Methods

Vertical distribution detection of heavy metals in soil is a core technique in environmental site investigation that characterizes how contaminants such as cadmium, lead, arsenic, chromium, and mercury vary with depth in the soil profile. By collecting depth-discrete samples from the surface down to several meters and analyzing them with spectroscopic and spectrometric methods, this approach reveals the true three-dimensional extent of contamination, supports source identification, and provides the decisive input for remediation design such as excavation depth determination. This article explains the technical principles, sampling strategy, target analytes, analytical methods, quality assurance requirements, and typical applications of vertical heavy metal profiling in soil.

Technical Overview

Vertical distribution detection of heavy metals in soil refers to the systematic collection and chemical analysis of soil samples taken at successive, documented depth intervals within a single borehole or soil pit, in order to construct a concentration-versus-depth profile for one or more heavy metals. Unlike surface soil surveys, which only capture the topmost layer where many contaminants accumulate, vertical profiling answers three questions that surface data cannot: how deep the contamination extends, whether the concentration peak lies at the surface or at depth, and whether the contaminant plume has reached environmentally sensitive strata or groundwater.

Heavy metals rarely distribute themselves uniformly with depth. Their vertical patterns are governed by the contamination pathway, soil physicochemistry, and time. Characteristic signatures include:

• Surface enrichment with exponential decline — typical of atmospheric deposition from smelting, traffic, or coal combustion, where lead, cadmium, mercury, and arsenic accumulate in the topsoil and organic-rich A horizon.
• Subsurface concentration peaks — indicative of buried waste, historical fill material, leaking underground tanks, or leachate from an intermediate source.
• Deep penetration and elongated plumes — associated with mobile species such as hexavalent chromium, or with acidic, low-organic-matter, coarse-textured soils that weakly adsorb cadmium and zinc.
• Uniform background-level concentrations — observed below the contaminated zone, confirming the vertical extent has been bounded by the investigation.

Vertical profiling is therefore an essential component of contaminated land assessment under frameworks such as GB 36600-2018 (risk control standard for development land) and GB 15618-2018 (risk control standard for agricultural land) in China, as well as international sampling standards of the ISO 18400 series and ASTM drilling and sampling guides. It converts a two-dimensional surface map into a defensible three-dimensional contamination model used for human health and ecological risk assessment.

Testing Samples

Because the objective is to resolve concentration gradients, samples must be strictly depth-discrete. Composite mixing of soil from different depths is not permitted, as it destroys the very information the study seeks. Typical sample categories are described below.

Surface Soil Samples (0–20 cm or 0–50 cm)

The uppermost interval usually exhibits the highest concentrations for deposition-driven contamination. For agricultural assessments, the plow layer (approximately 0–20 cm) is collected with stainless steel or bamboo tools to avoid metal cross-contamination. At industrial and construction-land sites, the surface interval is commonly extended to 0–50 cm. Multiple near-surface points may be composited laterally within the same depth interval to characterize an exposure unit, but never vertically.

Subsurface Soil Samples (20 cm to 3 m)

Subsurface samples are recovered from boreholes advanced with direct-push, hollow-stem auger, or percussion drilling equipment. Standard practice is to sample at fixed vertical intervals — for example 0.5 m increments in the first 1.5 m and 1 m increments below that — or, where visible stratigraphy or contamination indicators (discoloration, odor, staining, photoionization detector readings) are present, to take additional judgmental samples at the apparent interface of interest.

Deep Soil and Core Samples (Below 3 m)

Where contamination is known or suspected to be deep, continuous soil cores are advanced to 6 m, 10 m, or deeper until two consecutive intervals return concentrations at or below local background values. Undisturbed cores collected in plastic or stainless liners preserve the stratigraphic sequence, allow precise interval sub-sampling, and support parallel geotechnical or lithological logging. Sample recovery is recorded for every interval to verify depth accuracy.

Horizon-Based (Stratigraphic) Samples

In soil science and background studies, samples are instead divided by genetic horizons — the A (topsoil), B (subsoil), and C (parent material) horizons. Comparing metal concentrations among horizons distinguishes anthropogenic enrichment in the A horizon from the geogenic baseline of the C horizon, and identifies clay-enriched B horizons that may act as adsorption barriers to downward migration.

Testing Items

The core analyte package mirrors the eight regulated heavy metals and metalloids of GB 36600-2018 and GB 15618-2018, and is typically expanded based on site history:

• Priority eight metals: cadmium (Cd), mercury (Hg), arsenic (As), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn).
• Speciation-sensitive species: hexavalent chromium (Cr(VI)), which is far more toxic and mobile than total chromium and is a listed screening item for development land.
• Supplementary metals: antimony (Sb), selenium (Se), beryllium (Be), thallium (Tl), cobalt (Co), vanadium (V), molybdenum (Mo), and manganese (Mn), added where smelting, electroplating, mining, or chemical manufacturing histories indicate.
• Supporting physicochemical parameters: pH, soil organic matter, cation exchange capacity, redox potential, moisture content, and particle size distribution. These parameters control metal adsorption and mobility and are essential for interpreting why the vertical profile takes the shape it does.
• Optional chemical speciation: sequential extraction (for example, the BCR three-step procedure) partitioning each metal into acid-soluble, reducible, oxidizable, and residual fractions to assess bioavailability and migration risk at selected depths.

Testing Methods

Depth-Discrete Sampling and Field QA/QC

Boreholes are located on a gridded or judgmental basis according to the site conceptual model. Drilling and sample recovery follow recognized procedures such as ASTM D6169 for the selection of soil sampling devices and ASTM D6640 for the collection and handling of cores from core-barrel samplers, together with the sampling strategies of the ISO 18400 series. Sampling equipment is decontaminated between intervals, samples are transferred to certified containers, labels record the top and bottom depth of every interval, and full chain-of-custody documentation is maintained in line with ASTM D4840. Field quality assurance includes field duplicates at a typical frequency of at least 10%, equipment rinsate blanks, and field-portable X-ray fluorescence (XRF) screening, performed in accordance with ASTM D8064, to guide real-time deepening or densification of boreholes.

Sample Preparation and Digestion

In the laboratory, samples are air-dried or freeze-dried at low temperature, disaggregated, and sieved — typically through a 2 mm sieve, with an aliquot ground to pass a 0.149 mm (100 mesh) nylon sieve. Mercury and volatile-sensitive samples are stored cold and prepared without heat. Total-content determination uses:

• Microwave-assisted digestion with hydrofluoric acid mixtures (ISO 12914) for true total metal content, dissolving silicate lattices.
• Aqua regia hot-plate or microwave digestion for the environmentally relevant pseudo-total content, the basis of most regulatory screening comparisons.
• Alkaline digestion followed by diphenylcarbazide UV-Vis spectrophotometry for hexavalent chromium, avoiding the reduction of Cr(VI) that occurs in acidic media.

Instrumental Determination

Digests are measured by complementary instrumental techniques selected according to expected concentration and required detection limit:

• Inductively coupled plasma mass spectrometry (ICP-MS) — ultra-trace multi-element determination for Cd, Pb, Ni, Cu, Zn, Sb, Tl, Be, and similar elements, offering the lowest detection limits for deep background intervals.
• Inductively coupled plasma optical emission spectrometry (ICP-OES) — robust multi-element determination for moderate-to-high concentrations of Cr, Cu, Zn, Mn, and Ni.
• Graphite furnace atomic absorption spectrometry (GF-AAS) — highly sensitive single-element determination of Pb and Cd, consistent with ISO 11047 and GB/T 17141.
• Atomic fluorescence spectrometry (AFS) — the preferred technique for trace Hg and As after hydride generation or thermal reduction, aligned with GB/T 22105 and the principles of ISO 16772 and ISO 20280.
• Cold vapor atomic absorption and direct thermal-decomposition mercury analyzers — for total mercury with minimal sample handling.

Laboratory QC includes method blanks, laboratory control samples, matrix spikes, certified reference materials, and calibration verification standards at defined frequencies, with accepted recovery and precision limits stipulated by the applicable standard methods.

Data Presentation and Profile Interpretation

Results are reported as milligrams of metal per kilogram of dry soil against the sampled depth, and plotted as concentration-depth curves for each borehole and each element. Interpretation compares each interval against the applicable risk screening values of GB 36600-2018 or GB 15618-2018, the local soil background range, and the shape of the profile itself. The decisive deliverable is the identified lower boundary of contamination at each location — the depth at which all target metals fall below screening values and approach background — which directly defines the remediation volume, excavation depth, and verification sampling plan.

Applications

Vertical distribution detection of heavy metals in soil underpins a wide range of environmental, agricultural, and industrial activities:

• Contaminated site investigation and brownfield redevelopment: delineating the vertical extent of lead, arsenic, cadmium, and Cr(VI) contamination at former industrial plots to support risk assessment and remediation design before land-use change.
• Remediation design and excavation control: determining precise excavation depths, avoiding over-excavation of clean soil, and providing the baseline for post-remediation verification sampling.
• Agricultural soil pollution surveys: evaluating whether heavy metals in farmland are confined to the plow layer or have migrated toward the root zone and shallow groundwater, guiding safe planting and soil amendment decisions.
• Mining, smelting, and tailings-affected land: tracing deep migration of thallium, cadmium, and arsenic along fracture zones and permeable strata surrounding ore-processing facilities.
• Landfill and legacy waste sites: detecting subsurface concentration peaks produced by leachate migration from buried deposits and defining the depth of contaminated fill.
• Groundwater and vadose zone risk assessment: using soil profile concentrations as the source term for leaching models and contaminant transport predictions.
• Scientific research: reconstructing deposition history, establishing soil geochemical baselines, and validating vertical migration models under different pH and redox conditions.

FAQ

What depth intervals are typically used for vertical profile sampling?

Common practice is 0–50 cm at the surface, followed by 0.5 m intervals to approximately 3 m and 1 m intervals below that, supplemented by judgmental samples at visible stratigraphic changes or contamination indicators. For agricultural surveys, the top 0–20 cm plow layer is sampled separately. Tighter intervals are used near the identified contamination boundary to define the vertical extent with confidence.

How deep should boreholes be advanced?

Drilling should continue until at least two consecutive depth intervals return concentrations below the applicable risk screening values and consistent with local background, or until uncontaminated parent material and the water table are adequately characterized. At sites with deep sources, boreholes of 6 m to 10 m or more may be required.

Can samples from different depths be mixed into one sample?

No. Vertical compositing destroys the depth information that is the purpose of the study. Each depth interval must be collected, containerized, and analyzed as a discrete sample. Lateral compositing within the same depth interval is acceptable for exposure-unit characterization but not for vertical distribution assessment.

Can field-portable XRF replace laboratory analysis?

Portable XRF is a valuable screening tool for guiding borehole depth decisions in real time, and its performance is defined in standards such as ASTM D8064. However, regulatory decisions and remediation boundaries must be confirmed by laboratory digestion and ICP-MS, ICP-OES, AAS, or AFS analysis, because XRF detection limits for elements such as cadmium, mercury, and arsenic in typical soils may not reliably meet screening thresholds.

How are vertical distribution results interpreted?

Each interval is compared against the screening values of GB 36600-2018 or GB 15618-2018 and against local background data. Concentration peaks at the surface point to deposition sources; peaks at depth point to buried sources or leachate migration; a monotonic decline to background defines the lower contamination boundary. Supporting pH and organic matter data explain the observed mobility of each metal.

What is the difference between total content and speciation analysis?

Total (or aqua-regia extractable) content quantifies the overall metal burden of each depth interval and is the basis of regulatory screening. Speciation analysis, such as BCR sequential extraction, fractionates the metal into exchangeable, reducible, oxidizable, and residual forms, revealing which portion is bioavailable and mobile — critical when assessing whether deep contamination will continue to migrate toward groundwater.

What quality assurance measures apply to vertical profiling?

Key measures include documented depth control and core recovery, decontaminated sampling equipment, certified sample containers, complete chain of custody per ASTM D4840, at least 10% field duplicates, equipment blanks, and full laboratory QC (blanks, spikes, duplicates, certified reference materials). Combined, these ensure that concentration differences between depths reflect real geochemical variation rather than sampling or analytical artifacts.