Hidden Metals, Clean Crops: First Multi-Matrix Food Safety Scan in Lao Highlands

In the cool, mist-wrapped highlands of Pakxong District in southern Laos, vegetables grow on volcanic soils that have made the region one of the country’s most productive agricultural zones. Yet for all its importance as a vegetable basket, this highland production system has remained largely invisible to systematic food safety monitoring. A new pilot study, published in Environmental Monitoring and Assessment, offers one of the first co-located snapshots of pesticide residues and potentially toxic elements across crops, cultivation soils, and stored irrigation water in the area, and its findings point to a surveillance strategy that may need to focus less on pesticides and more on heavy metals in water and leafy herbs.

The research team, led by Amnuay Wattanakornsiri of Surindra Rajabhat University in Thailand together with colleagues from Pakse Teacher Training College in Champasak Province and several Thai analytical institutions, sampled three monitored sites in Pakxong District between September and November 2024. The timing matters: this window falls within the main growing season, when agrochemical use and irrigation demands are typically at their peak. The researchers collected 45 composite vegetable samples, 15 cultivation soil samples, and 15 stored irrigation water samples, deliberately designing the study so that crop, soil, and water evidence could be compared from the same production locations rather than drawn from scattered, incompatible datasets.

For pesticide screening, the team used four predefined liquid chromatography-tandem mass spectrometry panels, a technique often abbreviated as LC-MS/MS that allows hundreds of chemical compounds to be detected simultaneously at extremely low concentrations. Together the panels covered 213 analyte-method combinations, with some analytes appearing on more than one panel as an internal check. The detection limits were impressively stringent, ranging from 0.004 to 0.005 milligrams per kilogram of fresh weight, meaning that even trace residues well below regulatory thresholds would have been caught. The extraction chemistry followed the widely used QuEChERS approach, a method first published in 2003 that uses acetonitrile extraction and dispersive solid-phase cleanup, alongside a European reference laboratory method based on ethyl acetate extraction.

The headline result from the pesticide side was, in a sense, a non-result: no pesticide target was detected above the applicable limits of detection in any of the 45 vegetable composites. For a region where pesticide use practices are poorly documented and where neighboring countries in the Mekong basin have reported frequent residues in leafy vegetables, this is a striking finding. However, the authors are careful to note an important caveat. Several pyrethroid insecticides that are locally relevant and widely used in Southeast Asian vegetable farming were not included in the target lists, so the absence of detections applies strictly to the compounds that were screened. The clean result is therefore a strong signal about the specific chemicals tested, not a blanket certificate that the produce is residue-free.

The toxic-element picture was more nuanced. Using matrix-appropriate atomic spectrometric methods, the team measured lead, cadmium, arsenic, and total chromium in the vegetables, soils, and stored irrigation water. Acid-digestible cultivation soils, prepared according to the standard EPA 3050B acid digestion procedure, contained lead, arsenic, and total chromium, while cadmium remained below the applicable reporting limits. This pattern is consistent with the geology of the Bolaven Volcanic Field, the basaltic plateau on which Pakxong sits, where chromium and other elements can occur naturally in volcanic-derived soils. The study does not claim to distinguish geogenic sources from agricultural ones, but the soil data provide an essential baseline for future work.

Stored irrigation water emerged as a matrix of particular interest. The water samples contained lead at concentrations between 0.031 and 0.045 milligrams per liter and total chromium between 0.024 and 0.038 milligrams per liter. These are the kinds of numbers that surveillance programs can act on directly, because irrigation water is a controllable exposure pathway. Whether these levels reflect natural background, storage conditions in farm reservoirs, or upstream activities cannot be determined from this pilot, but the consistent detection of both metals across the sampled water sources suggests that water quality deserves a central place in any future monitoring framework for the district.

Among the crops themselves, one vegetable stood out. Coriander, a leafy herb consumed fresh and in large quantities across Lao cuisine, contained all four measured elements: lead at 0.066 to 0.074 milligrams per kilogram fresh weight, cadmium at 0.011 to 0.012, arsenic at 0.011 to 0.012, and total chromium at 0.032 to 0.037. By contrast, occurrence was limited or non-detectable in the two heading Brassica vegetables included in the study, a difference the authors suggest may relate to the contrasting growth habits and uptake physiology of open leafy herbs versus tightly headed vegetables. Leafy herbs with large surface areas and rapid growth cycles are known from studies across Asia and Africa to accumulate metals more readily, and the coriander results align with that broader literature.

The authors are unusually explicit about the limits of what their data can support. With only three sites, a single monitoring period, and no field blanks or field duplicates included in the quality assurance design, the study is descriptive rather than inferential. It identifies patterns that warrant confirmatory surveillance but cannot establish contaminant sources, internal uptake pathways within plants, or cross-matrix transfer from water or soil into edible tissue. The team frames the work as a prioritization exercise: the coriander findings, the lead and chromium in stored irrigation water, and the metal-bearing soils together define where a full-scale monitoring program should concentrate its resources, rather than spreading analytical effort evenly across all crops and matrices.

That framing has practical significance well beyond Laos. Highland vegetable systems across Southeast Asia share similar characteristics: volcanic or upland soils, smallholder production, limited laboratory infrastructure, and growing urban demand for fresh produce. Studies from Cambodia, Vietnam, Thailand, and Ethiopia have documented pesticide residues and heavy metal accumulation in irrigated vegetables, but multi-matrix datasets that link crops to their soils and water sources remain rare, particularly in tropical highland settings. By demonstrating a feasible sampling and analytical workflow, from QuEChERS-based pesticide panels to matrix-specific atomic spectrometry, the Pakxong pilot offers a template that other provinces and countries could adapt at modest cost.

The immediate implications are straightforward. Food safety authorities in Lao PDR now have evidence-based candidates for surveillance prioritization: coriander and similar fresh herbs as indicator crops, stored irrigation water as a key monitoring matrix, and lead, cadmium, arsenic, and chromium as the elements of concern. At the same time, the clean pesticide screen, despite its target-list limitations, suggests that current spraying practices in these three sites may not be leaving detectable residues of the compounds tested, a finding that deserves confirmation with expanded analyte lists, seasonal replication, and proper field quality controls. As the authors conclude, the value of this pilot lies not in definitive answers but in telling a national monitoring program exactly where to look first.

Subject of Research: Pesticide residues and potentially toxic elements in a highland vegetable production system in southern Lao PDR

Article Title: Targeted pesticide screening and multi-matrix toxic-element monitoring in a highland vegetable production system of southern Lao PDR: implications for surveillance prioritization

Article References: Wattanakornsiri, A., Sarma, H., Buasri, W., Khaengkhun, N., Junkhiew, J., Thepsombath, S., Sylaphet, S., Luangsanvang, K., Keomanivong, P., Khambaione, S., Phouvanekham, K., Sansy, B., & Phuengphai, P. (2026). Targeted pesticide screening and multi-matrix toxic-element monitoring in a highland vegetable production system of southern Lao PDR: implications for surveillance prioritization. Environmental Monitoring and Assessment, 198(10), Article 1121. https://doi.org/10.1007/s10661-026-15927-5

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15927-5

Keywords: pesticide residues, heavy metals, Lao PDR, Pakxong District, vegetable safety, irrigation water, coriander, LC-MS/MS, food surveillance, Bolaven Plateau, soil contamination, food safety