the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Aug 2025
Reviews and syntheses: Artisanal small-scale gold mining (ASGM)-derived mercury contamination in agricultural systems: what we know and need to know
Abstract. The escalating global demand for gold has fuelled the rapid expansion of artisanal and small-scale gold mining (ASGM), which has become the largest source of mercury (Hg) emissions worldwide. Here we synthesize current research on the pervasive contamination of agricultural systems by ASGM-derived Hg, identifying the key environmental pathways and subsequent risks to food security. Within these systems, Hg undergoes complex biogeochemical transformations, with the methylation of inorganic Hg into its highly neurotoxic form, methylmercury (MeHg), being a critical process. This is particularly pronounced in rice paddy systems, where microbial activity and favourable redox conditions facilitate Hg methylation, resulting in the bioaccumulation of MeHg in rice grains—a staple food for billions. However, this synthesis reveals that atmospheric uptake is important to total Hg loadings in rice, and more so in tissues of crops grown in unsaturated soils. Indeed, we stress the importance of assessing all potential uptake pathways of Hg in agricultural systems: foliar assimilation from air, uptake from soils/water (particularly MeHg in rice), direct deposition to surfaces, and consumption of contaminated crop tissues (by both humans and livestock/poultry), to delineate the source and ratios of the different pools of Hg within crops and their consumers. A common shortcoming in past studies of ASGM-derived Hg in agricultural systems is that they have commonly overlooked one or more of these uptake pathways. These findings underscore a significant threat to global food chains and human health through the consumption of Hg contaminated produce. Mitigating these risks requires an improved understanding of the quantity of emissions/releases from ASGM, input pathways, and Hg biogeochemical cycling and fate in agricultural landscapes, paving the way for targeted interventions and sustainable management strategies to protect vulnerable communities. We suggest that these goals can be achieved through strategic international and interdisciplinary collaborations, novel and accessible technologies, and care for the dissemination of scientific information to impacted communities.
Competing interests: D.S.M. is a member of the editorial board of the journal Biogeosciences. The authors declare that they have no other conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
(1114 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2025-3847', Jan Gacnik, 17 Sep 2025
- AC2: 'Reply on RC1', David McLagan, 01 Oct 2025
-
RC2: 'Comment on egusphere-2025-3847', Anonymous Referee #2, 18 Sep 2025
General comments:
The review by McLagan et al. is excellently written and covers a novel topic of interest: the role of ASGM in agricultural systems. The manuscript represents a useful resource for readers interested in the available literature on the specific subject and the critical gaps remaining to be investigated, as well as covering the broader aspects of Hg uptake by vegetation. The literature cited is extensive and the review is well-organized. I commend the authors on their work and offer several minor comments and suggestions for improvement.
Specific comments:
- At several points in the manuscript (L464–468, L502–506), the authors suggest a correlation between soil Hg levels and atmospheric Hg0 levels to substitute for the fact that air measurements were not available from certain studies. I am sceptical whether this relationship holds for areas close to ASGM and agricultural activities, as these activities cause deforestation that can lead to release of Hg from soils (more erosion, less Hg0 uptake, and more Hg0 volatilization from soils). For example, see Figure 2B,E from Gerson et al. 2022 (doi:10.1038/s41467-022-27997-3), where you can see that for mining impacted sites there is not necessarily a strong correlation between soil Hg and GEM. There is an array of confounding factors that likely affect the relationship between air GEM and soil Hg concentrations, including the mining techniques, time since deforestation and the soil biogeochemical factors. For example, Carpi et al. (doi:10.1016/j.atmosenv.2014.08.004, 2014) reported that a freshly deforested site showed higher soil Hg concentrations than a pasture site that was deforested 10 years earlier. All this to say that these confounding factors could explain why the air Hg levels, and hence the THg levels in crops, do not always follow the soil concentrations.
- I noticed a couple of instances of discrepancies between years of in-text references vs. bibliography (Zhou and Obrist, 2022 on L393 vs. 2021 in bibliography) and missing references (L406 Yanai et al. 2020). Perhaps worth double checking in case more references were missing from bibliography.
- L157 - Worthwhile mentioning UNEP estimate for releases to land as well?
- L334 - As part of the stomatal vs. non-stomatal discussion, might want to include this recent study which analyzed this issue in a tropical rainforest greenhouse, finding clear evidence of dominance of the stomatal pathway: Denzler et al., doi:10.1021/acs.est.5c05823, 2025
- L420 - A recent study from Tibet discussed the issue of root vs. atmosphere uptake found a relationship with altitude (Wang et al., doi:10.1038/s43247-022-00619-y, 2022), but perhaps less relevant for ASGM, nonpermafrost regions
Technical comments:
Title - minor, but I’ve usually seen ASGM defined with an “and” - Artisanal and small-scale gold mining
L12 - systems (plural?)
L230- is the 60% by mass?
L382 - rephrase to amend unclear “their”
L424 - linked the uptake (missing “to”)
L440 - missing comma before highlighting
L441 - missing “the” before fraction
L467 - check this assumption, does air Hg0 actually correlate well with soil THg?
L490 - delete second at
L517-521 - this sentence meaning is unclear to me (what does “summative” data mean here); potential this additional reference that is not considered should just be deleted for conciseness
Table 1 - Arrazy et al. (2023) studied Indonesian site (also according to text), but listed is Tanzania as location
L555 - a village set up around
L642 - this number is missing a currency
L647 - #orava typo
L855 - please specify: higher soil THg levels
L877 - was briefly discussed
Table 2 - I’m assuming that asterisks mean estimates based on assumptions, but this should be specified in the caption or a table footnote
L1022- comma missing before but
L1071 - isotopes (plural)
L1077-1080 - this sentence is missing a verb
Citation: https://doi.org/10.5194/egusphere-2025-3847-RC2 - AC1: 'Reply on RC2', David McLagan, 01 Oct 2025
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 2,336 | 38 | 18 | 2,392 | 20 | 28 |
- HTML: 2,336
- PDF: 38
- XML: 18
- Total: 2,392
- BibTeX: 20
- EndNote: 28
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
The article “Reviews and syntheses: Artisanal small-scale gold mining (ASGM)-derived mercury contamination in agricultural systems: what we know and need to know” focuses on crops grown at ASGM sites and livestock/poultry as potential sources of human exposure to Hg through ASGM agriculture, while omitting the extensively studied fish, which are covered by other authors. The related literature is reviewed, and relevant data is extracted and synthesized.
General comments:
The review/synthesis is thorough and easy to read. From my side, there are no major comments, which points to the overall good quality of the manuscript. Therefore, only specific comments and technical corrections are outlined below.
Specific comments:
Section 3: This section serves as the opening to the core of the work done by the authors. It would benefit from the inclusion of a “motivation” part for this work, where the authors state related review/synthesis articles and explain how this work is different. This addition would save the reader some exploration and points to other related reviews should they need it.
How did the authors collect the data? Which search tools were used for finding relevant articles? What were the key words used in the search, and did the authors conduct the search with a specific Boolean search query? How were the decisions made regarding the inclusion/exclusion of data for the synthesis? As it stands, the article search/data collection is not traceable and reproducible. I recommend adding an extra short section dedicated to “methods” or “data collection”, similar to e.g. Basu et al. (2018, https://ehp.niehs.nih.gov/doi/10.1289/EHP3904). It is true that the amount of data collected for ASGM agricultural systems is much smaller than in Basu et al. (2018); however, some description would still benefit the article.
Lines 81-83: Solubility is a physical rather than a chemical property. The authors could include chemical properties and list. in the first bracket, an actual chemical property (for example, the formation of different IHg(II) complexes with various inorganic/organic ligands) and then a physical property (here, the current solubility example can be kept).
Lines 88-97: The paragraph starts with an explanation of the global Hg character due to Hg0 properties, and understandably, the story then continues with a focus on Hg0. However, it should not be left out that the emissions are not only in the form of Hg0 but also directly as IHg(II) (g) and IHg(II) on particulates. Currently, it reads as if the only way for Hg to deposit into terrestrial/aquatic environments is through the oxidation of Hg0 to IHg(II) (g) and IHg(II) on particulates, and subsequent deposition. In fact, IHg(II) (g) and IHg(II) on particulates can be emitted directly from the emission source and deposit locally without involvement of redox processes, as I am sure the authors are well aware. This needs to be clarified in the text, if possible.
Lines 166-168: The “note II” on the bottom of the page states, “Note the estimate of primary releases to aquatic systems does not include releases from ASGM activities as the...”. On the other hand, in lines 166-168, numbers appear that estimate total ASGM releases to water and land, which is a bit contradictory to the note. So, the releases to land and water from ASGM were estimated, but water-only releases were not?
Technical corrections:
Line 294: “operate” should likely be “operating”, or “operation”
Line 607: Should be “facilitates oxide regeneration of sulphate … “
Lines 639-643: The paragraph sounds like MeHg is only demethylated in foliage, but later on, a more general “in-planta” demethylation is referred to. Can this be clarified?
Line 647: Is “#orava” a typo? Otherwise, not clear what it should mean
Line 657: “sorbed” should be “sorb”
Line 693: There is a typo at the end of the sentence, “alter estimates.”
Line 848: “… in SE Asia, simply that …” there should be a “and” / “or” after the comma
Line 959: Missing “in” in this part of the sentence: “7.3x higher muscle”
Line 992: “high” should be “higher”
Line 1090: There is a redundant comma in the sentence