Abiotic and biotic factors influencing the mobility of arsenic in groundwater of a through-flow island in the Okavango Delta, Botswana

Mladenov, Natalie; Wolski, Piotr; Hettiarachchi, Ganga M.; Murray-Hudson, Michael; Enriquez, Hersy J.; Damaraju, Sivaramakrishna; Galkaduwa, Madhubhashini B.; McKnight, Diane M.; Masamba, W.R.L.

doi:10.1016/j.jhydrol.2013.09.026

Cited by 53 publications

(19 citation statements)

References 69 publications

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“…In particular, iron levels in the environment are likely to contribute to iron accumulation in tissues. Likewise, contaminants like arsenic can induce inflammatory effects resembling those observed in this study, although at mg/L doses, which are much higher than the μg/L arsenic concentrations previously observed in the Delta [ 51 , 55 ]. Taken together, we conclude that hypoxia is likely the main cause of the observed pathology in Okavango fishes, but we cannot rule out the synergistic effects of other co-stressors.…”

Section: Discussionsupporting

confidence: 63%

Low oxygen: A (tough) way of life for Okavango fishes

et al. 2020

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Botswana’s Okavango Delta is a World Heritage Site and biodiverse wilderness. In 2016–2018, following arrival of the annual flood of rainwater from Angola’s highlands, and using continuous oxygen logging, we documented profound aquatic hypoxia that persisted for 3.5 to 5 months in the river channel. Within these periods, dissolved oxygen rarely exceeded 3 mg/L and dropped below 0.5 mg/L for up to two weeks at a time. Although these dissolved oxygen levels are low enough to qualify parts of the Delta as a dead zone, the region is a biodiversity hotspot, raising the question of how fish survive. In association with the hypoxia, histological samples, collected from native Oreochromis andersonii (threespot tilapia), Coptodon rendalli (redbreast tilapia), and Oreochromis macrochir (greenhead tilapia), exhibited widespread hepatic and splenic inflammation with marked granulocyte infiltration, melanomacrophage aggregates, and ceroid and hemosiderin accumulations. It is likely that direct tissue hypoxia and polycythemia-related iron deposition caused this pathology. We propose that Okavango cichlids respond to extended natural hypoxia by increasing erythrocyte production, but with significant health costs. Our findings highlight seasonal hypoxia as an important recurring stressor, which may limit fishery resilience in the Okavango as concurrent human impacts rise. Moreover, they illustrate how fish might respond to hypoxia elsewhere in the world, where dead zones are becoming more common.

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Section: Discussionsupporting

confidence: 63%

Low oxygen: A (tough) way of life for Okavango fishes

et al. 2020

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“…These conditions likely generated pore waters (in both the muds and underlying channel sands) with P CO2 values that exceeded equilibrium with atmospheric levels of carbon dioxide. In the modern Okavango Delta, saturation indices for siderite have been shown to increase with increasing alkalinity produced by CO 2 degassing (Mladenov et al 2014).…”

Section: Disseminated Iron-oxide Rhombsmentioning

confidence: 99%

The Geologic Context of Wonderstone: A Complex, Outcrop-Scaled Pattern of Iron-Oxide Cement

Burgess

Kettler

Loope

2016

Journal of Sedimentary Research

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Although siderite is a widespread early diagenetic mineral in fluvial systems, it is unstable in oxidizing environments and destroyed in permeable rocks that experience uplift and exhumation. The products of siderite oxidation, however, (mm-to cm-scale rhombs, concretions, and complex bands of iron-oxide cement) are widespread in the rock record of fluvial systems. The fluvial channels of the Shinarump Member of the Chinle Formation in southern Utah and northern Arizona, U.S.A., provide an excellent suite of examples of diagenetic features produced by Triassic and Neogene oxidation of early diagenetic siderite. These diagenetic features also provide direct evidence of the level of the water table during deposition of the Shinarump member. Large, in situ, discoidal concretions containing preserved siderite are present in Shinarump floodplain siltstones. Rip-up clasts derived from the siltstones developed iron-oxide rinds during late-stage, near-surface oxidation. These two structures show that floodplain silts contained abundant organic matter and methanic pore water. Groundwater recharging through these silts carried reducing water through underlying sand bodies and discharged into active channels. Degassing of CO 2 and methanogenesis caused rhombic crystals of siderite to precipitate in channel sands during these wet intervals. Some of this siderite may have been oxidized during dry intervals when groundwater circulation reversed, but most siderite in the channel sands was preserved until the Shinarump was exhumed during the Neogene. As oxygenated near-surface water entered joints in the lithified Shinarump, colonies of iron-oxidizing microbes living in the phreatic zone occupied redox boundaries and used the rhombic crystals of siderite in the sandstone and the spherulitic siderite in transported siltstone intraclasts as their sources of energy and carbon. The ferrous iron released from dissolving siderite within the intraclasts was oxidized at the siltstone-sandstone contact, generating rinded concretions similar to those in the Cretaceous Dakota Formation. Complex banding known as wonderstone was produced in the channel sandstones from oxidation of the rhombic siderite; the pattern is a combination of Liesegang bands and microbially mediated cements. The preserved rhombs are pseudomorphs after siderite crystals that were either oxidized during Triassic dry intervals, or escaped Neogene microbial oxidation in the phreatic zone, only to be oxidized abiotically in the vadose zone. Microbes are likely oxidizing Shinarump siderite a few kilometers down dip of outcrops with exposed wonderstone. At such locations, the Shinarump is in contact with overlying watersaturated Quaternary alluvium.

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“…For example, in Burkina Faso, high-As concentrations were found only in the northern region (Smedley et al 2007;Somé et al 2012;Nzihou et al 2013). In Botswana and Ethiopia, only the Okavango Delta and Rift Valley, respectively, seem be concerned with arsenic pollution (Reimann et al 2003;Huntsman-Mapila et al 2006;Rango et al 2010;Dsikowitzky et al 2013;Mladenov et al 2013;Rango et al 2013). It is certainly clear that the arsenic issue seems to be limited compared to other continents in the world, e.g., Asia or America (Smedley and Kinniburgh 2002), but the arsenic issue in Africa is not well defined.…”

Section: Probable Extent Of As Contamination In Africamentioning

confidence: 99%

Arsenic in African Waters: A Review

Ahoulé

Lalanne

Mendret

et al. 2015

Water Air Soil Pollut

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The review of studies on arsenic in African waters shows that arsenic can be found in high concentrations in both surface water and groundwater. Arsenic concentrations in African groundwater range between 0.02 and 1760 μg L −1 , whilst the level of arsenic in surface water is ranged up to 10,000 μg L −1 . This high level of arsenic in surface water is related to mining operations, agricultural drains, local sediments, disposal, and incineration of municipal and industrial wastes. However, mining activities remain the main source of surface water pollution. They have thereby a strong impact on the concentration of arsenic in the environment. As for groundwater, high levels of arsenic occur in natural conditions. It is due to the presence of iron oxides; sulphide minerals such as pyrite, arsenopyrite, and chalcopyrite; volcanic rocks; and geothermal waters. Few studies in Africa make the link between human health problems and high levels of arsenic in water. Only two articles were found dealing with arsenic remediation. This shows that arsenic, which constitutes a major public health issue in the world, has less interest in Africa although high concentrations of arsenic have been found in both surface water and groundwater in some African countries. Most of the studies carried out on arsenic issues in Africa are dedicated to the characterization and the quantification of the pollution, but studies on the risk to human health and treatment systems are limited. The arsenic issue in Africa needs special attention in order to avoid the problems experienced in some areas mainly in Asia.

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Abiotic and biotic factors influencing the mobility of arsenic in groundwater of a through-flow island in the Okavango Delta, Botswana

Cited by 53 publications

References 69 publications

Low oxygen: A (tough) way of life for Okavango fishes

Low oxygen: A (tough) way of life for Okavango fishes

The Geologic Context of Wonderstone: A Complex, Outcrop-Scaled Pattern of Iron-Oxide Cement

Arsenic in African Waters: A Review

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