Determination of Heavy Metals in Paddy Field Soil: Sampling, Analytical Methods, and Risk Evaluation

Heavy metal determination in paddy field soil is essential for evaluating the food safety risk of rice production systems, where elements such as cadmium, lead, arsenic, and mercury can accumulate through irrigation, atmospheric deposition, and agricultural inputs. This article provides a comprehensive technical guide covering sampling strategies, regulated testing items, digestion and instrumental methods such as ICP-MS, atomic absorption spectrometry, and atomic fluorescence spectrometry, quality control requirements, and risk evaluation against the screening values of GB 15618-2018.

Technical Overview

Paddy fields are a uniquely sensitive receiving environment for heavy metal pollution. Rice is cultivated under flooded, anaerobic conditions in which redox potential and pH fluctuate dramatically during alternate wetting and drying cycles, directly controlling the solubility, speciation, and bioavailability of cadmium (Cd), arsenic (As), mercury (Hg), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn). Because rice is an efficient accumulator of cadmium in particular, soil-borne metals can transfer into the grain and enter the human diet, making paddy soil testing a critical control point for food safety.

Contamination of paddy soils originates from multiple pathways, including irrigation with mine-impacted or industrial water, atmospheric deposition from smelting and coal combustion, long-term application of phosphate fertilizers and manures containing trace metals, and the historic use of metal-based pesticides. Once accumulated, most heavy metals persist in the plow layer for decades, so reliable quantitative determination is the foundation for land classification, safe-utilization management, and remediation decision-making.

In practice, heavy metal determination in paddy soil is governed by risk-based standards rather than simple background comparison. Under the framework of GB 15618-2018 (Soil environmental quality — Risk control standard for soil contamination of agricultural land), measured total concentrations are compared against pH-dependent risk screening values and risk intervention values, and confirmatory analysis is typically performed by accredited third-party laboratories using validated spectrometric methods.

Testing Samples

Plow-Layer Composite Samples (0–20 cm)

The plow layer is the primary sampling target because it corresponds to the root zone of rice and the layer most affected by tillage, fertilizer application, and irrigation inputs. Samples are collected on a systematic grid, typically 5 to 20 individual core subsamples are combined into one composite sample of at least 1 kg. Collection should follow recognized soil sampling guidance such as the ISO 18400 series, with stainless steel or plastic tools to avoid introducing zinc, copper, or chromium from galvanized or brass equipment. Each point is recorded with GPS coordinates, and the composite is reduced by coning and quartering before transport under chain-of-custody procedures.

Deep Profile and Subsurface Samples (20–60 cm)

Where vertical migration is suspected, for example near electroplating sites, mine tailings, or old industrial drainage channels, profile samples are taken by horizon to 40 cm or deeper. Subsurface data distinguish surface deposition from deep geogenic enrichment and support source apportionment. A clean wooden or plastic spatula should be used to scrape the exposed core face before each horizon is sampled to prevent cross-contamination between layers.

Rhizosphere and Root-Zone Samples

Rhizosphere soil, obtained by gently shaking soil adhering to rice roots at tillering or harvest, reflects the chemical environment actually experienced by the plant. These samples are often analyzed in parallel with total metal determination for DTPA- or CaCl2-extractable fractions, soil pH, redox potential, and organic matter to interpret bioavailability and predict grain uptake risk.

Verification and Long-Term Monitoring Samples

For remediation projects and long-term monitoring networks, paired baseline and post-treatment samples are collected from identical grid coordinates, with splits archived in polyethylene bottles at room temperature in the dark. Archiving allows retrospective re-analysis when more sensitive instrumentation or new regulated elements are introduced.

Testing Items

The core testing panel for paddy field soil consists of the eight regulated inorganic indicators plus pH, because the GB 15618-2018 screening values are stratified by soil pH. All values are reported in mg/kg on a dry-weight basis.

• pH ≤ 5.5: Cd 0.3, Hg 0.5, As 30, Pb 80, Cr 200, Cu 50, Ni 60, Zn 200 (mg/kg)
• 5.5 < pH ≤ 6.5: Cd 0.4, Hg 0.5, As 30, Pb 100, Cr 150, Cu 50, Ni 70, Zn 200 (mg/kg)
• 6.5 < pH ≤ 7.5: Cd 0.6, Hg 0.6, As 25, Pb 140, Cr 200, Cu 100, Ni 100, Zn 250 (mg/kg)
• pH > 7.5: Cd 0.8, Hg 0.7, As 20, Pb 240, Cr 250, Cu 100, Ni 190, Zn 300 (mg/kg)
• Risk intervention values (all pH ranges): Cd 1.5, Hg 3.0, As 150, Pb 400 (mg/kg)

Concentrations below the screening value indicate generally acceptable risk for rice production, while exceedances trigger detailed investigation and, above the intervention values, strict risk control or land-use restriction. Depending on the site history, the panel is frequently extended to include selenium (Se), thallium (Tl), antimony (Sb), cobalt (Co), vanadium (V), manganese (Mn), and beryllium (Be). Supporting parameters, including organic matter, cation exchange capacity, and redox-active iron fractions, are commonly measured to strengthen the interpretation of metal mobility.

Testing Methods

Sample Preparation

Field samples are air-dried at room temperature below 40 °C in a dust-free environment, following the pretreatment principles of ISO 11464. Plant residues, root fragments, and stones are removed by hand picking. The dried sample is crushed with an agate mill, passed through a 2 mm sieve for pH and extractable analyses, and further ground to below 0.149 mm (100 mesh) for total heavy metal determination. Mechanical grinding with chromium-steel or brass components must be avoided to prevent trace contamination.

Digestion and Extraction

For regulatory total-concentration comparisons, 0.2 to 0.5 g of prepared soil is digested using one of the following approaches:

• Aqua regia digestion (HCl:HNO3 3:1) per ISO 11466, suitable for Cd, Pb, Cr, Cu, Ni, Zn and widely applied for GB 15618 compliance analysis
• Four-acid total digestion (HCl–HNO3–HF–HClO4) per ISO 14869-1 or microwave-assisted acid digestion, which dissolves silicate lattices and recovers near-total element content
• Microwave-assisted aqua regia extraction for volatile hydride-forming elements such as Hg, As, Se, Sb, and Bi, which minimizes volatilization losses
• Single extraction with DTPA or dilute CaCl2 for plant-available fraction assessment in bioavailability studies

Instrumental Determination

The selection of instruments balances sensitivity requirements, element scope, and throughput. Standard reference methods include:

• Graphite furnace atomic absorption spectrometry (GFAAS) for Cd and Pb, per GB/T 17141, with detection limits at the 0.01 mg/kg level
• Atomic fluorescence spectrometry (AFS) for Hg and As, per GB/T 22105, reaching sub-microgram per kilogram detection limits
• Flame atomic absorption spectrometry (FAAS) for Cu and Zn (GB/T 17138), Cr (GB/T 17137), and Ni (GB/T 17139), and multi-element AAS determination per ISO 11047
• Inductively coupled plasma optical emission spectrometry (ICP-OES) per ISO 11885 for simultaneous multi-element determination at mg/kg concentrations
• Inductively coupled plasma mass spectrometry (ICP-MS) per ISO 17294-2 for ultra-trace, multi-element quantification with internal standard correction
• Portable and benchtop energy-dispersive X-ray fluorescence (XRF) spectrometry for rapid in-situ screening, with all exceedances confirmed by laboratory digestion methods

Soil pH is measured potentiometrically in a 1:2.5 soil-to-water suspension per ISO 10390, since pH directly determines which screening value applies.

Quality Assurance and Quality Control

Defensible data require a full QA/QC protocol embedded in every batch: method blanks to detect reagent or digestion contamination; duplicate analyses of at least 10 percent of samples with relative percent difference typically within 20 to 30 percent; certified reference soils analyzed with each batch and recovered within the certified uncertainty; matrix spike recoveries generally between 85 and 115 percent (70 to 130 percent for volatile elements such as Hg and As); multi-point calibration with independent verification standards every 10 samples; and documented method detection limits below one-tenth of the applicable screening values. Participating laboratories should hold accreditation to ISO/IEC 17025 for the specific soil matrix and analyte combinations.

Applications

Heavy metal determination in paddy soil directly supports a wide range of regulatory, agronomic, and commercial objectives:

• Detailed investigation and verification of exceedances found during agricultural land soil pollution surveys and soil environment monitoring networks
• Zoning of rice-producing areas into priority protection, safe utilization, and strict risk control categories for land management decisions
• Evaluation of remediation measures such as lime and biochar passivation, apatite amendment, water management with prolonged flooding, and low-accumulating rice cultivar selection
• Acceptance testing and long-term effectiveness verification after soil remediation projects, using before-and-after comparisons at fixed sampling points
• Environmental impact assessment and compliance monitoring for mining, smelting, electroplating, and wastewater irrigation areas adjacent to rice paddies
• Quality screening of phosphatic fertilizers, composts, amendments, and irrigation water that repeatedly load metals into paddy fields
• Origin environment certification, geographical indication verification, and food safety traceability programs for rice brands
• Scientific research including geochemical baseline mapping, pollution source apportionment with isotopic fingerprinting, and bioavailability modeling

FAQ

Why is cadmium the highest-priority metal in paddy fields?

Rice is one of the most efficient cadmium accumulators among staple crops because cadmium shares uptake pathways with zinc and manganese. Under the aerobic periods of alternate wetting and drying, soil redox potential rises and cadmium solubility increases sharply, driving root uptake and grain accumulation even when soil concentrations are only moderately elevated. For this reason, cadmium has the lowest screening value among the regulated metals and is almost always included even in minimal testing programs.

What is the difference between total and extractable heavy metal determination?

Total concentration analysis, after aqua regia or four-acid digestion, is the regulatory basis for comparison with GB 15618-2018 screening values. Extractable analysis, using DTPA or dilute calcium chloride, estimates the plant-available fraction and correlates better with actual grain uptake. Compliance programs rely on total concentrations, while remediation design and bioavailability research typically combine both.

How many samples are needed to characterize a paddy field?

For general surveys, a grid of roughly 100 m × 100 m to 400 m × 400 m is typical depending on land area and expected heterogeneity, with each grid cell represented by one composite of 5 to 20 subsamples. Detailed investigation of identified exceedances uses denser grids, and cadastral parcels suspected of uneven contamination require point-by-point sampling rather than compositing.

Should I choose ICP-MS or atomic absorption spectrometry?

ICP-MS offers the widest element coverage, the lowest detection limits, and the best throughput for multi-element surveys, making it the preferred platform for large monitoring programs. GFAAS remains a robust and cost-effective choice for cadmium and lead alone, and AFS delivers unmatched sensitivity for mercury and arsenic at low cost. Many laboratories combine AFS for Hg and As with ICP-MS for the remaining elements in a single reporting package.

How should results exceeding the screening value be handled?

A single exceedance does not immediately indicate unusable land. The response is phased: confirmatory sampling with tighter grid spacing, combined analysis of rice grain from the same plots, assessment of irrigation water and fertilizer inputs, and evaluation of soil pH and organic matter. Depending on the outcome, management shifts to safe-utilization measures such as liming, water regime optimization, variety substitution, or, above intervention values, planting structure adjustment and strict risk control.

How often should paddy soil be re-tested?

General monitoring plots are commonly re-sampled on a three-to-five-year cycle, whereas high-risk zones near industrial or mining sources, remediation demonstration plots, and areas applying metal-bearing amendments should be monitored annually. Any remediation project requires paired baseline, mid-term, and post-treatment sampling to demonstrate effectiveness against the applicable screening values.