Soil Nickel Content Testing: Analytical Methods, Standards, and Applications

Soil nickel content testing is a fundamental procedure in environmental site assessment, agricultural land quality management, and heavy metal pollution control programs worldwide. By accurately quantifying total and bioavailable nickel in soil using standardized digestion and instrumental techniques such as ICP-MS, ICP-OES, and atomic absorption spectrometry, regulators, landowners, and environmental consultants can compare measured concentrations against risk screening values such as those in GB 36600-2018 and GB 15618-2018. This article provides a comprehensive technical overview of soil nickel sampling, sample preparation, analytical methods, quality control, and typical applications.

Technical Overview

Nickel (Ni, atomic number 28) is a naturally occurring transition metal found in virtually all soils, primarily derived from the weathering of parent rocks. In unpolluted soils, nickel background concentrations typically range from 5 to 50 mg/kg, with a global median of approximately 20–30 mg/kg. Soils developed on ultramafic and serpentine parent materials can naturally contain several hundred to several thousand mg/kg of nickel, which is an important consideration when interpreting analytical results. Nickel exists in soil mainly in the divalent oxidation state (Ni2+), adsorbed onto iron and manganese oxides, clay minerals, and organic matter, and its mobility is strongly influenced by soil pH, redox conditions, and cation exchange capacity.

Anthropogenic sources of soil nickel contamination include nickel mining and smelting, stainless steel and alloy manufacturing, electroplating facilities, battery production, fossil fuel combustion, petroleum refining, phosphatic fertilizer application, biosolid (sewage sludge) amendments, and atmospheric deposition near industrial emission sources. While nickel is an essential ultratrace element for certain plants and microorganisms, elevated concentrations can cause phytotoxicity, reduce crop yields, and pose human health risks. Certain nickel compounds are classified by the International Agency for Research on Cancer (IARC) as Group 1 human carcinogens via inhalation, and nickel is also one of the most common causes of allergic contact dermatitis worldwide.

Regulatory frameworks worldwide establish risk-based thresholds for nickel in soil. In China, GB 36600-2018 (Soil Environmental Quality — Risk Control Standard for Soil Contamination of Development Land) sets nickel screening values of 150 mg/kg for Class I (sensitive) land use and 900 mg/kg for Class II land use, while GB 15618-2018 (Risk Control Standard for Soil Contamination of Agricultural Land) specifies screening values ranging from 60 to 190 mg/kg depending on soil pH and land use type. Comparable guidance is provided by ISO, EN, ASTM, and US EPA risk assessment frameworks. Reliable soil nickel content testing by accredited third-party laboratories is therefore essential for determining whether a site complies with these standards and whether further investigation, risk assessment, or remediation is required.

Testing Samples

Surface Soil Samples

Surface soil samples, generally collected from the 0–20 cm depth interval, are the most common matrix for nickel testing. They reflect recent deposition, agricultural inputs, and shallow contamination sources. Sampling is typically performed on a grid basis following technical specifications such as HJ/T 166-2004, with multiple subsamples combined into a representative composite sample.

Subsurface Soil and Profile Samples

Subsurface samples are collected from boreholes or soil pits at stratified depth intervals, commonly down to 6 meters or to the groundwater table, to characterize vertical contaminant distribution and identify deeper sources such as buried waste, historic fill, or leaking pipelines. These samples are critical for development land investigations conducted under HJ 25.1 and HJ 25.2.

Agricultural Soil Samples

For farmland monitoring, samples are taken from the plow layer (0–20 cm, or 0–15 cm for paddy fields) and, for orchards, from the 0–40 cm root zone. Sampling points should avoid fertilizer bands, field edges, and drainage ditches to ensure representativeness of the cultivated area.

Industrial and Contaminated Site Samples

Samples from active or legacy industrial sites — including electroplating plants, smelters, metal finishing workshops, and storage or drum-handling areas — are collected using judgmental (targeted) sampling around suspected source areas, supplemented by grid sampling across the broader site.

Sediment, Sludge, and Stockpiled Soil Samples

Dredged sediments, biosolids, and excavated soil stockpiles can also be analyzed for nickel to support waste classification, disposal decisions, or verification of remediation outcomes.

Testing Items

Soil nickel testing services typically cover the following analytical items, depending on the purpose of the investigation:

• Total nickel content (mg/kg, dry weight basis)
• Nickel speciation via BCR sequential extraction (acid-soluble, reducible, oxidizable, and residual fractions)
• DTPA-extractable (phytoavailable) nickel
• Water-soluble and CaCl2-extractable nickel
• Leachable nickel by TCLP (EPA Method 1311) or SPLP (EPA Method 1312) procedures
• Multi-element panels including cadmium, lead, chromium, arsenic, mercury, copper, and zinc, together with supporting parameters such as pH, organic matter, and cation exchange capacity

Testing Methods

Sample Collection and Preparation

Because stainless steel contains nickel, contact between the soil sample and stainless steel tools, sieves, or containers should be minimized; non-metallic (nylon or PTFE) sieves, wooden or bamboo spatulas, and polyethylene containers are preferred. Samples are air-dried at room temperature (below 40 °C), disaggregated, homogenized, and quartered. For total nickel determination, the analytical portion is ground to pass a 0.149 mm (100-mesh) sieve using an agate mill.

Acid Digestion Techniques

Aqua regia extraction (HCl:HNO3, 3:1) — performed by hot-plate digestion per EPA Method 3050B and ISO 11466, or by microwave-assisted extraction per EPA Method 3051A, ISO 12914, and HJ 832-2017 — dissolves the environmentally reactive fraction and is widely used for regulatory compliance testing. For true total nickel content, including nickel bound in silicate lattices, complete dissolution using hydrofluoric acid mixtures (HF–HNO3–HClO4) is required in accordance with ISO 14869-1. The choice of digestion method must match the regulatory basis of the applicable standard, since aqua regia may under-recover nickel in mineral-rich or serpentine-derived soils.

Flame Atomic Absorption Spectrometry (FAAS)

FAAS is a robust, cost-effective technique specified in GB/T 17139-1997, HJ 491-2019, ISO 11047, and EPA Method 7000B. Nickel is measured in an air–acetylene flame, commonly at the 232.0 nm resonance line. Method detection limits are typically on the order of 1–5 mg/kg, making FAAS well suited for moderately to heavily contaminated soils but less suitable for precise background-level studies.

Graphite Furnace Atomic Absorption Spectrometry (GFAAS)

GFAAS, also covered by ISO 11047, improves sensitivity by two to three orders of magnitude, achieving detection limits near 0.05–0.1 mg/kg. It is appropriate when nickel concentrations are expected to be at or below background levels, although matrix modifiers and background correction are required to control interferences.

Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)

ICP-OES (HJ 803-2016, ISO 11885, EPA Method 6010D) offers simultaneous multi-element determination with a wide linear dynamic range and detection limits for nickel in soil of roughly 0.01–0.1 mg/kg, depending on digestion dilution factors. It is the workhorse method for multi-metal site investigations and agricultural soil surveys.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

ICP-MS (EPA Method 6020B, ISO 17294-2) provides the highest sensitivity, with detection limits at sub-µg/kg levels, and is ideal for background surveys, geochemical baseline studies, and ultra-trace regulatory work. Common isotopes include 60Ni and 62Ni, with collision-cell or reaction-cell technology used to resolve polyatomic interferences.

Field Portable XRF Screening

Portable X-ray fluorescence (pXRF) analyzers operated per EPA Method 6200 enable rapid on-site screening of nickel, typically with detection limits of 20–50 mg/kg in soil. XRF results are suitable for delineating contamination plumes and optimizing sampling locations, but a representative subset of samples (commonly 10–20%) should be confirmed by laboratory digestion and instrumental analysis.

Quality Assurance and Quality Control

Accredited third-party laboratories apply a full suite of QC measures to guarantee data defensibility:

• Method blanks and reagent blanks below method detection limits
• Laboratory duplicates with relative percent deviation (RPD) typically ≤ 20%
• Matrix spike recoveries within 80–120%
• Analysis of certified soil reference materials (CRMs) within certified uncertainty ranges
• Continuing calibration verification within ±10% of true values
• Interference check standards for ICP techniques and internal standards for ICP-MS
• Results reported on a dry-weight basis with measured moisture content

Applications

Soil nickel content testing supports a broad range of environmental, agricultural, and industrial objectives:

• Environmental site assessments and phase investigations for development land
• Detailed soil pollution surveys and human health/ecological risk assessments
• Mandatory self-monitoring programs for key regulated industrial enterprises
• Agricultural land classification, safe utilization zoning, and farmland product safety evaluation
• Investigation of legacy contamination at mining, smelting, and metal processing sites
• Verification and acceptance testing following soil remediation projects
• Due diligence for brownfield redevelopment and land transactions
• Soil background surveys and environmental geochemical mapping
• Scientific research on nickel speciation, mobility, bioavailability, and plant uptake

FAQ

What is the difference between total nickel and aqua regia-extractable nickel?

Aqua regia extraction recovers nickel from most anthropogenic and geogenic reactive phases but does not fully dissolve silicate minerals, so results may be lower than the true total in soils rich in resistant minerals. HF-based total dissolution measures all nickel, including that locked in the crystal lattices of primary and secondary silicates. Compliance testing should follow the digestion method specified by the governing standard.

Which analytical method should I choose?

For contaminated sites with expected concentrations well above 10–20 mg/kg, FAAS or ICP-OES is usually sufficient and economical. For background-level studies, multi-element surveys, or very low reporting limits, ICP-MS is preferred. Field XRF is an excellent low-cost screening tool when used with laboratory confirmation.

How are results interpreted against regulatory standards?

Measured nickel concentrations are compared with the applicable risk screening values — for example, 150 mg/kg (Class I) or 900 mg/kg (Class II) under GB 36600-2018 for development land, or 60–190 mg/kg under GB 15618-2018 for agricultural land depending on pH. Exceedances of screening values trigger detailed investigation and risk assessment rather than automatic remediation.

Does high nickel always indicate pollution?

No. Soils formed on ultramafic and serpentine rocks naturally contain elevated nickel. Comparison with local soil background values, consideration of geology, and speciation analysis help distinguish geogenic enrichment from anthropogenic contamination.

How much sample is needed and what is the typical turnaround time?

A minimum of 500 g of soil per sample is generally recommended. Standard turnaround for nickel determination at accredited third-party laboratories is typically 5–10 working days after sample receipt, with expedited service often available.

Why is sample preparation critical for nickel analysis?

Nickel contamination can be introduced from stainless steel equipment, and poor homogenization leads to unrepresentative results. Strict protocols — non-metallic tools, air-drying, fine grinding to 100 mesh, and coning-and-quartering — are essential for accurate and reproducible measurements.