Determination of Arsenic in Farmland Soil: Analytical Methods, Standards, and Applications

Arsenic determination in farmland soil is a core procedure for evaluating agricultural land safety, food-chain risk, and compliance with soil environmental quality standards such as GB 15618-2018. This article provides a comprehensive technical overview of sampling protocols, testing items, analytical methods—including hydride generation atomic fluorescence spectrometry (HG-AFS), ICP-MS, ICP-OES, and XRF screening—and their applications in farmland quality management. Whether you are an environmental engineer, agronomist, or laboratory professional, this guide explains how reliable arsenic data support soil classification, risk control, and remediation decisions.

Technical Overview

Arsenic (As) is a toxic metalloid that occurs naturally in soils at typical background concentrations of 5 to 15 mg/kg, although geological anomalies and anthropogenic activities can elevate concentrations to hazardous levels. In farmland ecosystems, arsenic enters the soil through mining and smelting emissions, historical use of arsenical pesticides, phosphate fertilizers, contaminated irrigation water, coal combustion fallout, and industrial waste discharge. Because inorganic arsenic compounds are classified as Group 1 human carcinogens and are readily accumulated by crops—particularly rice grown under flooded, anaerobic conditions—accurate determination of arsenic in agricultural soil is essential for protecting both human health and food safety.

In soil, arsenic exists primarily as inorganic arsenite As(III) and arsenate As(V), with speciation governed by redox potential and pH. Under the reduced conditions of paddy fields, As(III) becomes more soluble and mobile, increasing phytoavailability. For this reason, regulatory assessment considers not only total concentration but also soil pH, which directly influences the risk screening thresholds established in national standards.

• Regulatory benchmark: GB 15618-2018 (Soil Environmental Quality — Risk Control Standard for Soil Contamination of Agricultural Land) sets risk screening values for arsenic of 20 to 40 mg/kg, varying with land use type (paddy field versus dryland/orchard) and soil pH.
• Monitoring specification: HJ/T 166-2004 defines the technical requirements for soil environmental monitoring, including sampling design, quality control, and data processing.
• Supporting measurement: soil pH is determined according to HJ 962-2018 because the applicable arsenic limit depends on the pH interval of the sampled parcel.

Accredited third-party laboratories routinely perform arsenic determination following nationally and internationally recognized standard methods, ensuring that results are legally defensible for land classification, remediation acceptance, and regulatory reporting.

Testing Samples

Routine Surface Soil Samples

Surface samples represent the plow layer (0–20 cm for dryland; 0–15 to 0–20 cm for paddy fields) where arsenic accumulation and crop root uptake are most significant. Sampling follows a grid-based design in accordance with HJ/T 166-2004. At each grid node, a composite sample is produced by pooling 5 to 20 subsamples collected in an S-shaped or chessboard pattern within a radius of 50 to 100 meters. Typically, 1 to 2 kg of fresh soil is collected per sample using stainless-steel or wooden tools to avoid metallic contamination.

Soil Profile Samples

Profile samples are collected from excavated pits or boreholes at defined depth intervals (for example, 0–20 cm, 20–40 cm, 40–60 cm) to characterize vertical arsenic distribution, distinguish anthropogenic surface enrichment from geogenic background, and assess migration into deeper horizons. Profile sampling is essential for site investigations around mining, smelting, and industrial areas adjacent to farmland.

Special-Purpose Samples

• Rhizosphere soil collected for plant–soil transfer studies and phytoavailability assessment.
• Flooded paddy samples preserved under anoxic conditions when redox-sensitive speciation analysis (As(III)/As(V)) is required.
• Sequential extraction subsamples for arsenic fractionation into exchangeable, Fe/Mn oxide-bound, organic/sulfide-bound, and residual forms.

Sample Preparation and Preservation

Upon arrival at the laboratory, samples are air-dried at room temperature (below 40 °C), homogenized, and sieved through a 2 mm nylon mesh. A representative portion is further ground to pass a 0.149 mm (100-mesh) sieve for total arsenic digestion. Prepared samples are stored in clean polyethylene or glass containers in a dry, contaminant-free environment. For speciation analysis, samples are frozen or freeze-dried immediately to preserve the original arsenic species.

Testing Items

Arsenic determination in farmland soil encompasses several complementary test parameters, each answering a distinct risk-management question:

• Total arsenic (aqua regia extractable): the standard regulatory parameter defined by GB 15618-2018 and measured after HCl–HNO3 extraction; often described as "pseudo-total" arsenic.
• Total arsenic (complete digestion): determined after HF-based digestion (for example, ISO 14869-1) to dissolve the silicate lattice, yielding the true total element content for geochemical studies.
• Available (phytoavailable) arsenic: estimated using selective extractants such as bicarbonate or dilute salt solutions to approximate the fraction accessible to crops.
• Arsenic speciation: quantification of As(III), As(V), monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA), typically by HPLC-ICP-MS.
• Arsenic fractionation: distribution among operationally defined geochemical phases via BCR or Tessier sequential extraction schemes.
• Soil pH: measured concurrently because the applicable risk screening value under GB 15618-2018 is pH-dependent.

Results are reported in mg/kg on a dry-weight basis, together with method detection limits, moisture correction factors, and uncertainty statements where required.

Testing Methods

Hydride Generation Atomic Fluorescence Spectrometry (HG-AFS)

HG-AFS is the reference technique for total arsenic in soil under GB/T 22105.2-2008 and HJ 680-2013. The soil sample is digested with aqua regia (HCl:HNO3, 3:1) using a hot plate, water bath, or microwave system. Pentavalent arsenic in the digest is pre-reduced to As(III) with a thiourea–ascorbic acid mixture, after which arsine (AsH3) is generated with potassium borohydride (KBH4) solution and carried by argon into the atomizer. The method achieves detection limits as low as 0.01 mg/kg with excellent sensitivity, wide linear range, and low interference, making it the workhorse method for farmland soil arsenic in accredited laboratories.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Following microwave-assisted acid digestion (HNO3–HCl, with optional HF addition), ICP-MS (HJ 805-2016) quantifies arsenic at mass 75 with sub-0.05 mg/kg detection limits and simultaneous multi-element capability—ideal for surveys where arsenic, cadmium, lead, chromium, mercury, and other elements must be assessed together. Because the argon plasma combined with chloride from the digest can form the polyatomic interference 40Ar35Cl+ at m/z 75, collision/reaction cell technology (helium KED mode or hydrogen/oxygen reaction chemistry) is applied to ensure accuracy.

Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)

With aqua regia extraction per HJ 803-2016, ICP-OES measures arsenic at the 193.696 nm emission line with detection limits around 1 mg/kg. While less sensitive than HG-AFS and ICP-MS, it offers high throughput and multi-element analysis, and is well suited to contaminated sites where arsenic concentrations are expected to be well above screening values.

Classical Spectrophotometric and Hydride Generation AAS Methods

Legacy methods remain valid references, including silver diethyldithiocarbamate (AgDDC) spectrophotometry per GB/T 17134-1997 and hydride generation atomic absorption spectrometry (US EPA Method 7062, with EPA Method 3050B or 3051A digestion). These approaches are cost-effective but more labor-intensive and are increasingly supplemented by instrumental techniques.

Field Screening by Portable X-Ray Fluorescence (XRF)

Handheld XRF analyzers operating under US EPA Method 6200 provide real-time, non-destructive arsenic screening in the field with detection limits of a few mg/kg—sufficient to distinguish soils near or above the GB 15618-2018 screening values. XRF is invaluable for delineating hotspots and optimizing laboratory sampling plans; however, concentrations approaching or exceeding regulatory thresholds must be confirmed by laboratory digestion and instrumental analysis.

Speciation and Fractionation Analysis

For advanced risk assessment, HPLC coupled with ICP-MS separates and quantifies individual arsenic species in soil pore water and mild extracts, while BCR/Tessier sequential extraction reveals the geochemical binding forms. These analyses identify the labile fraction that drives crop uptake and inform the selection of remediation strategies such as passivation with iron oxides or biochar.

Quality Assurance and Quality Control

• Method blanks, reagent blanks, and digestion blanks carried through the complete analytical procedure.
• Multi-point calibration with correlation coefficients of at least 0.999.
• Certified reference materials (soil CRMs with certified or indicative arsenic values) analyzed with each batch; results within certified uncertainty ranges.
• Matrix spike recoveries typically within 70–130%, and duplicate analyses with relative percent deviation (RPD) within 20–30%, acknowledging natural soil heterogeneity.
• Participation in inter-laboratory proficiency testing schemes by accredited third-party laboratories.

Applications

Arsenic determination in farmland soil underpins a wide range of regulatory, agricultural, and scientific objectives:

• Farmland soil environmental quality surveys and national soil pollution detailed investigations.
• Classification of agricultural land into priority protection, safe utilization, and strict control categories under the Soil Pollution Prevention and Control Law and GB 15618-2018.
• Site investigation and risk assessment around mining, smelting, chemical, and wastewater-irrigated areas.
• Effectiveness evaluation of remediation and passivation projects, including pre- and post-treatment comparisons.
• Origin safety management for agricultural products, including the control of inorganic arsenic in rice and vegetables.
• Monitoring of fertilizer, pesticide, and compost inputs, and of long-term irrigation water quality impacts.
• Environmental impact assessments, land-use conversion reviews, and geochemical baseline mapping research.

FAQ

What is the regulatory limit for arsenic in farmland soil?

Under GB 15618-2018, the risk screening value for arsenic ranges from 20 to 40 mg/kg depending on land use type and soil pH; paddy fields have stricter limits (20–30 mg/kg) than dryland and orchard soils (25–40 mg/kg). Where concentrations exceed the corresponding risk intervention values, the land is classified under strict control and normal agricultural production may be restricted.

Which analytical method provides the best performance?

HG-AFS and ICP-MS offer the lowest detection limits (down to about 0.01 mg/kg) and are the preferred laboratory methods. HG-AFS is highly specific for arsenic, while ICP-MS adds simultaneous multi-element capability, which is advantageous in comprehensive farmland surveys.

Does aqua regia digestion measure true total arsenic?

No. Aqua regia extracts the environmentally reactive arsenic bound to sulfides, organic matter, and Fe/Mn oxides but does not fully dissolve silicate minerals. Regulatory assessment under GB 15618-2018 is based on aqua regia extraction; HF-based digestion (for example, ISO 14869-1) is used when true total content is required for geochemical purposes.

How much soil is needed and how are samples preserved?

Approximately 1 to 2 kg of composite surface soil per sampling point is sufficient for full analysis. Samples are air-dried, ground, and sieved (2 mm for general analysis; 0.149 mm for digestion); speciation samples must be frozen or freeze-dried immediately to prevent species transformation.

How long does arsenic determination take?

Standard laboratory turnaround is typically 5 to 10 working days from sample receipt, including digestion, instrumental analysis, QA/QC review, and report issuance. Urgent projects can often be accelerated by prior arrangement with accredited third-party laboratories.

Can portable XRF replace laboratory testing?

Portable XRF is an excellent screening and delineation tool but is not a substitute for regulatory reporting. Results near or above screening values, or any data intended for compliance decisions, should always be confirmed by laboratory digestion with HG-AFS, ICP-MS, or ICP-OES analysis.