Background on As mobilization

Typically, As release to groundwater is attributed to the reductive dissolution of As-bearing iron-oxide minerals. However, anthropogenic activities have been blamed to affect natural process. Learn more about the current knowledge.

Schematic of arsenic mobilization — Release of As to groundwater by iron reducing bacteria (right part of the schematic) or by other cycling (left part). A. Nghiem

Biogeochemical mechanisms

Arsenic (As) mobilization and immobilization in the subsurface is dependent upon geochemical reactions. Arsenic is sorbed to oxidized iron (Fe) minerals where its sorption capacity depends on the specific mineral, the speciation of As, and the geochemical environment¹. In reducing aquifers, a well-known process responsible for As release to groundwater is the reductive dissolution of Fe (oxyhydr)oxide minerals to which As is sorbed, coupled to the oxidation of organic matter (OM) by iron reducing bacteria^2-4.

However, besides the often-cited Fe reduction by iron reducing bacteria, other mechanisms may also be responsible for As mobilization in the reducing subsurface, including the sulfur cycle. Following sulfate reduction, sulfide oxidation can be coupled to the reductive dissolution of As-bearing Fe minerals. The available sulfide can also form thioarsenic species, which sorb poorly onto Fe minerals, especially in aquifers that have high oxidized Fe mineral content, thus leading to As mobilization^5-7.

Geology

The geological setting sets the background for the biogeochemical mechanisms: in S/SE Asia, source material originating from mountains supplies large volumes of sediments and As content, combined with Fe minerals, to which As can sorb to and be released from during reductive dissolution². Initial widespread observations indicated that geologically-older, e.g., Pleistocene aquifers are low in As, while younger Holocene aquifers are often enriched in As^8,9. Previous explanations for the distinction between Holocene and older aquifers include differences in OM (more reactive OM in Holocene sediments), the sediment material (differences in crystalline vs. amorphous phases), and the redox conditions (oxic conditions in Pleistocene and older aquifers, reducing in Holocene sediments), and flushing over time leading to lower As in older aquifers^2,10-13.

Hydrology

Flow of groundwater not only controls the distribution of As in the subsurface but also the distribution of DOC and other geochemically significant substrates that regulate groundwater redox state and control soluble As^14,15. Long residence times of groundwater in the subsurface allow for extensive solid-solution interactions and build-up of As in solution, for example, especially in low permeability layers of clay^26,35, while shorter residence times lower As in solution due to increased flushing of contaminated aquifers^11-13. DOC and As, released at another location, can be transported both horizontally or vertically through aquifers under natural flow conditions^18,19. Finally, the various possible recharge sources of an aquifer can also importantly determine whether organic carbon rich sources, such as from ponds^20-22, or high As water, such as from riverbank pore-water reduction^14,23, can infiltrate an aquifer and cause downgradient geochemical and As changes.

References

Although low-As aquifers may become contaminated over time due to natural factors, alarmingly, over the past decade, evidence has linked contamination of low-As aquifers to groundwater pumping^1-3. Throughout S/SE Asia, Pleistocene aquifers that were previously assumed to be low in As have been found to be contaminated due to increased groundwater pumping, and in some cases, by suggested drawdown or infiltration of Holocene groundwater^1,2,4-6.

Other shifts in surface recharge source to aquifers may be impacted by surface water body changes through land use change, for example: OM derived from anthropogenic activities such as sewage⁷ and constructed ponds⁸, leading to increased As concentrations in the shallow groundwater.

Finally, natural advection of dissolved organic carbon and As in aquifers, after being released within the subsurface and then transported both horizontally or vertically through aquifers, is intensified by increased flow rates and changed flow directions due to extensive groundwater abstraction for municipal and irrigation pumping^6,9.