Over 1.3 million Californians rely on unmonitored domestic
wells.
Existing probability estimates of groundwater Mn concentrations, population
estimates, and sociodemographic data were integrated with spatial
data delineating domestic well communities (DWCs) to predict the probability
of high Mn concentrations in extracted groundwater within DWCs in
California’s Central Valley. Additional Mn concentration data
of water delivered by community water systems (CWSs) were used to
estimate Mn in public water supply. We estimate that 0.4% of the DWC
population (2342 users) rely on groundwater with predicted Mn >
300
μg L–1. In CWSs, 2.4% of the population (904
users) served by small CWSs and 0.4% of the population (3072 users)
served by medium CWS relied on drinking water with mean point-of-entry
Mn concentration >300 μg L–1. Small CWSs
were
less likely to report Mn concentrations relative to large CWSs, yet
a higher percentage of small CWSs exceed regulatory standards relative
to larger systems. Modeled calculations do not reveal differences
in estimated Mn concentration between groundwater from current regional
domestic well depth and 33 m deeper. These analyses demonstrate the
need for additional well-monitoring programs that evaluate Mn and
increased access to point-of-use treatment for domestic well users
disproportionately burdened by associated costs of water treatment.
Manganese and arsenic both threaten
groundwater quality globally,
but their chemical behavior leads to both co-contamination and separation
of these contaminants from individual well to regional scales. Here
we tested manganese and arsenic retention under conditions commonly
found within aquifer redox fluctuating and transition zones where
both arsenic and iron phases are present in oxidized forms, but manganese
persists as reduced and soluble Mn(II). Analysis of column aqueous
breakthrough data and characterization of solid-phase products using
X-ray photoelectron (XPS) and absorption spectroscopies (XAS) show
that the addition of bicarbonate increased manganese retention but
decreased arsenic retention, while the presence of manganese and arsenic
together increased both arsenic and manganese retention. In the presence
of O2 arsenic remained oxidized as arsenate under all conditions
measured; however, reduced Mn(II) was oxidized to an average Mn oxidation
state of ∼3 in the absence of arsenate. The presence of arsenate
partially inhibited Mn(II) oxidation likely by blocking ferrihydrite
surfaces needed to catalyze Mn(II) oxidation by O2 and
by stabilizing Mn(II) via ternary complex formation. These results
highlight the interactions between reduced and oxidized contaminants
that can contribute to the co-occurrence or physical separation of
manganese and arsenic in groundwater systems under changing or stratified
redox conditions.
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