Substantial contribution of biomethylation to aquifer arsenic cycling

Journal name:
Nature Geoscience
Year published:
Published online

Microbes play a prominent role in transforming arsenic to and from immobile forms in aquifers1. Much of this cycling involves inorganic forms of arsenic2, but microbes can also generate organic forms through methylation3, although this process is often considered insignificant in aquifers4, 5, 6, 7. Here we identify the presence of dimethylarsinate and other methylated arsenic species in an aquifer hosted in volcaniclastic sedimentary rocks. We find that dimethylarsinate is widespread in the aquifer and its concentration correlates strongly with arsenite concentration. We use laboratory incubation experiments and an aquifer injection test to show that aquifer microbes can produce dimethylarsinate at rates of about 0.1% of total dissolved arsenic per day, comparable to rates of dimethylarsinate production in surface environments. Based on these results, we estimate that globally, biomethylation in aquifers has the potential to transform 100 tons of inorganic arsenic to methylated arsenic species per year, compared with the 420–1,250 tons of inorganic arsenic that undergoes biomethylation in soils8. We therefore conclude that biomethylation could contribute significantly to aquifer arsenic cycling. Because biomethylation yields arsine and methylarsines, which are more volatile and prone to diffusion than other arsenic species, we further suggest that biomethylation may serve as a link between surface and subsurface arsenic cycling.

At a glance


  1. Correlation between DMA(V) and arsenite in the bedrock aquifer.
    Figure 1: Correlation between DMA(V) and arsenite in the bedrock aquifer.

    The symbols show the results of the chemical analysis; the line represents the best fit.

  2. Variations with time in arsenic concentrations during aquifer injection test.
    Figure 2: Variations with time in arsenic concentrations during aquifer injection test.

    The symbols show the results of the chemical analysis; the dashed lines show the hypothetical concentrations under the assumption that only groundwater mixing occurred.

  3. Variations in methylarsenical concentration with total arsenic concentration in aquifers contaminated naturally with arsenic.
    Figure 3: Variations in methylarsenical concentration with total arsenic concentration in aquifers contaminated naturally with arsenic.

    Methylarsenicals include monomethylarsonate and dimethylarsinate; in the shaded area, methylarsenicals account for 0.1 to 10% of total arsenic. Open symbols are individual data points from aquifers of the Murshidabad district of West Bengal ( )13; Datong ( )14 and Inner Mongolia ( )15 of China; Taiwan ( )16; Cyprus ( )17; Argentina ( )18; and Florida ( ) 20, New Jersey ( ) 21 and Oregon ( , this study) of the USA. Filled symbols represent average concentrations in the aquifers of Mexico ( )19 and the Nadia district of West Bengal ( )12; error bars show standard deviation.


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Author information


  1. Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA

    • Scott C. Maguffin,
    • Ashley R. Daigle &
    • Qusheng Jin
  2. Department of Geology, Kansas State University, Manhattan, Kansas 66506, USA

    • Matthew F. Kirk
  3. US Geological Survey, Portland, Oregon 97201, USA

    • Stephen R. Hinkle


Q.J. and S.R.H. designed the project. Q.J. compiled previous studies of groundwater methylarsenicals. Q.J. and M.F.K. conducted the field sampling and analysis. S.C.M. carried out the laboratory experiments. Q.J. and A.R.D. carried out the aquifer test. Q.J. wrote the manuscript, with significant input from S.R.H. and M.F.K.

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