Flow and sorption controls of groundwater arsenic in individual boreholes from bedrock aquifers in central Maine, USA

104

Arsenic contamination in groundwater from crystalline basement rocks in West Africa has only been documented in isolated areas and presents a serious health threat in a region already facing multiple challenges related to water quality and scarcity. We present a comprehensive dataset of arsenic concentrations from drinking water wells in rural Burkina Faso (n=1498), of which 14.6% are above 10μg/L. Included in this dataset are 269 new samples from regions where no published water quality data existed. We used multivariate logistic regression with arsenic measurements as calibration data and maps of geology and mineral deposits as independent predictor variables to create arsenic prediction models at concentration thresholds of 5, 10 and 50μg/L. These hazard maps delineate areas vulnerable to groundwater arsenic contamination in Burkina Faso. Bedrock composed of schists and volcanic rocks of the Birimian formation, potentially harbouring arsenic-containing sulphide minerals, has the highest probability of yielding groundwater arsenic concentrations >10μg/L. Combined with population density estimates, the arsenic prediction models indicate that ~560,000 people are potentially exposed to arsenic-contaminated groundwater in Burkina Faso. The same arsenic-bearing geological formations that are positive predictors for elevated arsenic concentrations in Burkina Faso also exist in neighbouring countries such as Mali, Ghana and Ivory Coast. This study's results are thus of transboundary relevance and can act as a trigger for targeted water quality surveys and mitigation efforts.

Section: Arsenic and Lithologymentioning

confidence: 88%

Section: Hydrological and Geological Setting And Its Relevance To Elevated Arsenic Concentrationsmentioning

confidence: 99%

Groundwater arsenic contamination in Burkina Faso, West Africa: Predicting and verifying regions at risk

Bretzler

Lalanne

Nikiema

et al. 2017

104

“…This suggests a more complex mechanism of As mobilization and is consistent with multiple As host minerals changing with progressive metamorphism where As-silicates are proposed as possible host minerals. Secondary Fe or Mn minerals in fractures likely contribute to As distribution in groundwater via pH-influenced desorption or reductive dissolution (Yang et al, this issue). Probably many of these phases are unstable, and also exist as some form of grain coatings but could be derived from the weathering of As-primary minerals in these metamorphic rocks.…”

Section: Discussionmentioning

confidence: 99%

Heterogeneous arsenic enrichment in meta-sedimentary rocks in central Maine, United States

O’Shea

Stransky

Leitheiser

et al. 2015

Self Cite

Arsenic is enriched up to 28 times the average crustal abundance of 4.8 mg kg−1 for meta-sedimentary rocks of two adjacent formations in central Maine, USA where groundwater in the bedrock aquifer frequently contains elevated As levels. The Waterville Formation contains higher arsenic concentrations (mean As 32.9 mg kg−1, median 12.1 mg kg−1, n=36) than the neighboring Vassalboro Group (mean As 19.1 mg kg−1, median 6.0 mg kg−1, n=36). The Waterville Formation is a pelitic meta-sedimentary unit with abundant pyrite either visible or observed by scanning electron microprobe. Concentrations of As and S are strongly correlated (r=0.88, p<0.05) in the low grade phyllite rocks, and arsenic is detected up to 1,944 mg kg−1 in pyrite measured by electron microprobe. In contrast, statistically significant (p<0.05) correlations between concentrations of As and S are absent in the calcareous meta-sediments of the Vassalboro Group, consistent with the absence of arsenic-rich pyrite in the protolith. Metamorphism converts the arsenic-rich pyrite to arsenic-poor pyrrhotite (mean As 1 mg kg−1, n=15) during de-sulfidation reactions: the resulting metamorphic rocks contain arsenic but little or no sulfur indicating that the arsenic is now in new mineral hosts. Secondary weathering products such as iron oxides may host As, yet the geochemical methods employed (oxidative and reductive leaching) do not conclusively indicate that arsenic is associated only with these. Instead, silicate minerals such as biotite and garnet are present in metamorphic zones where arsenic is enriched (up to 130.8 mg kg−1 As) where S is 0%. Redistribution of already variable As in the protolith during metamorphism and contemporary water-rock interaction in the aquifers, all combine to contribute to a spatially heterogeneous groundwater arsenic distribution in bedrock aquifers.

“…It is worth noting that the concentrations of As from all private wells (n=616) are less variable than those from public wells (n=629), suggesting that the frequency of regular monitoring recommended for private wells by government agencies is likely to be sufficient for most wells, with a caveat that water usage or pumping rates do not vary a great deal for individual households. This caveat is because studies, including Yang et al (this issue), found that total (un-filtered and acidified) As concentrations in well water varied considerably when the borehole was pumped over several hours, although the dissolved (filtered and acidified) As concentrations in well water varied little (Bottomley, 1984; Yang et al, this issue). Both studies suggest that As sorbed to Fe precipitates in a borehole can account for differences in total and dissolved well water As concentrations during pumping, with Yang et al (this issue) proposing a conceptual model invoking sorption control of As onto secondary Fe minerals along a groundwater flow path following an oxic-suboxic(-anoxic)-suboxic-oxic gradient from “recharge” to “discharge” into the borehole as a plausible mechanism.…”

Section: Risks From Arsenic In Private Well Water Of Northeast Unitmentioning

confidence: 99%

At the crossroads: Hazard assessment and reduction of health risks from arsenic in private well waters of the northeastern United States and Atlantic Canada

Zheng

Ayotte

2015

Self Cite

This special issue contains 12 papers that report on new understanding of arsenic hydrogeochemistry, performance of household well water treatment systems, and testing and treatment behaviors of well users in several states of the northeastern region of the United States and Nova Scotia, Canada. The responsibility to ensure water safety of private wells falls on well owners. In the U.S., 43 million Americans, mostly from rural areas, use private wells. In order to reduce As exposure in rural populations that rely on private wells for drinking water, risk assessment, which includes estimation of population at risk of exposure to As above the EPA Maximum Contaminant Level, is helpful but insufficient because it does not identify individual households at risk. Persistent optimism bias among well owners against testing and barriers such as cost of treatment mean that a large percentage of the population will not act to reduce their exposure to harmful substances such as As. If households are in areas with known As occurrence, a potentially large percentage of well owners will remain unaware of their exposure. To ensure that everyone, including vulnerable populations such as low income families with children and pregnant women, is not exposed to arsenic in their drinking water, alternative action will be required and warrants further research.