TY - JOUR
T1 - Overlapping redox zones control arsenic pollution in Pleistocene multi-layer aquifers, the Po Plain (Italy)
AU - Rotiroti, Marco
AU - Bonomi, Tullia
AU - Sacchi, Elisa
AU - McArthur, John M.
AU - Jakobsen, Rasmus
AU - Sciarra, Alessandra
AU - Etiope, Giuseppe
AU - Zanotti, Chiara
AU - Nava, Veronica
AU - Fumagalli, Letizia
AU - Leoni, Barbara
N1 - Funding Information:
This work was supported by Fondazione Cariplo [grant number 2014-1282 ] and through the scientific collaboration no. 2018-CONV25-0024 between University of Milano-Bicocca and INGV .
Publisher Copyright:
© 2020 Elsevier B.V.
PY - 2021/3/1
Y1 - 2021/3/1
N2 - Understanding the factors that control As concentrations in groundwater is vital for supplying safe groundwater in regions with As-polluted aquifers. Despite much research, mainly addressing Holocene aquifers hosting young (<100 yrs) groundwater, the source, transport, and fate of As in Pleistocene aquifers with fossil (>12,000 yrs) groundwaters are not yet fully understood and so are assessed here through an evaluation of the redox properties of the system in a type locality, the Po Plain (Italy). Analyses of redox-sensitive species and major ions on 22 groundwater samples from the Pleistocene arsenic-affected aquifer in the Po Plain shows that groundwater concentrations of As are controlled by the simultaneous operation of several terminal electron accepters. Organic matter, present as peat, is abundant in the aquifer, allowing groundwater to reach a quasi-steady-state of highly reducing conditions close to thermodynamic equilibrium. In this system, simultaneous reduction of Fe-oxide and sulfate results in low concentrations of As (median 7 μg/L) whereas As reaches higher concentrations (median of 82 μg/L) during simultaneous methanogenesis and Fe-reduction. The position of well-screens is an additional controlling factor on groundwater As: short screens that overlap confining aquitards generate higher As concentrations than long screens placed away from them. A conceptual model for groundwater As, applicable worldwide in other Pleistocene aquifers with reducible Fe-oxides and abundant organic matter is proposed: As may have two concentration peaks, the first after prolonged Fe-oxide reduction and until sulfate reduction takes place, the second during simultaneous Fe-reduction and methanogenesis.
AB - Understanding the factors that control As concentrations in groundwater is vital for supplying safe groundwater in regions with As-polluted aquifers. Despite much research, mainly addressing Holocene aquifers hosting young (<100 yrs) groundwater, the source, transport, and fate of As in Pleistocene aquifers with fossil (>12,000 yrs) groundwaters are not yet fully understood and so are assessed here through an evaluation of the redox properties of the system in a type locality, the Po Plain (Italy). Analyses of redox-sensitive species and major ions on 22 groundwater samples from the Pleistocene arsenic-affected aquifer in the Po Plain shows that groundwater concentrations of As are controlled by the simultaneous operation of several terminal electron accepters. Organic matter, present as peat, is abundant in the aquifer, allowing groundwater to reach a quasi-steady-state of highly reducing conditions close to thermodynamic equilibrium. In this system, simultaneous reduction of Fe-oxide and sulfate results in low concentrations of As (median 7 μg/L) whereas As reaches higher concentrations (median of 82 μg/L) during simultaneous methanogenesis and Fe-reduction. The position of well-screens is an additional controlling factor on groundwater As: short screens that overlap confining aquitards generate higher As concentrations than long screens placed away from them. A conceptual model for groundwater As, applicable worldwide in other Pleistocene aquifers with reducible Fe-oxides and abundant organic matter is proposed: As may have two concentration peaks, the first after prolonged Fe-oxide reduction and until sulfate reduction takes place, the second during simultaneous Fe-reduction and methanogenesis.
KW - Groundwater quality
KW - Iron
KW - Methanogenesis
KW - Peat
KW - Sulfate
KW - TEAPs
UR - http://www.scopus.com/inward/record.url?scp=85096869348&partnerID=8YFLogxK
U2 - 10.1016/j.scitotenv.2020.143646
DO - 10.1016/j.scitotenv.2020.143646
M3 - Article
C2 - 33257069
AN - SCOPUS:85096869348
SN - 0048-9697
VL - 758
JO - Science of the Total Environment
JF - Science of the Total Environment
M1 - 143646
ER -