toxic terra

Part nine

In Part 8 (Fall 2022 HydroVisions) of this series, I described fluoride minerals and mineral solubility in some worldwide occurrences, particularly in granitic terranes. However, fluoride also occurs in surface water and groundwater in areas having past and current volcanic activity because fluoride is generally enriched in volcanic gases and geothermal fluids. The most common gases associated with active volcanoes (in general decreasing abundance) are water vapor or steam (H2O) – constituting approximately 50% to 90% of all eruption gases, carbon dioxide (CO2; 1-40%), sulfur dioxide (SO2; 1-25%), hydrogen sulfide (H2S; 1-10%), hydrogen (H2), helium (He), carbon monoxide (CO), and hydrochloric (HCl) and nitric (HNO3) acid (Brunning, 2018). Lesser amounts of other gases may also be emitted, including argon (Ar), fluorine (F), hydrofluoric acid (HF), nitrogen (N), carbonyl sulfide (COS), carbon disulfide (CS2), hydrogen bromide (HBr), ammonia (NH3) and native elements [e.g., sulfur (S), boron (B), and gold (Au) and volatile metals: mercury (Hg) and selenium (Se)]. Generally, H and CO rapidly oxidize to H2O and CO2, respectively, and H2S oxidizes to sulfurous (H2SO3) and/or sulfuric (H2SO4) acids, but volatile sulfur may also result in direct condensates, sublimates, or encrustations as native sulfur deposits. Chlorine monoxide (ClO) may also form when HCl is oxidized and HF can condense in rain or on ash particles (McGee and Gerlach, 1995 and McGee, et al., 1996).

Generally, most volcanic gases rarely reach or impact populated areas in lethal concentrations because of rapid atmospheric oxidation and solubilization in water vapor. One study showed that lethal effects are quite low ~10 km downwind from a volcano. Usually, such hazards are restricted to surrounding volcanic vents and fumaroles and to low spots on the volcanoes’ flanks. In such areas, volcanic gases (e.g., CO2, HF, and SO2) may be harmful to humans, animals, plants and crops, and property (e.g., utility lines, farm equipment, vehicles, and metal objects, which corrode when exposed to gases or acid rain). Exposure to acid gases (e.g., H2SO4, HCl, and HF) can damage eyes, mucous membranes, respiratory systems, and, under extreme conditions, can lead to death. The different volcanic gases affect the earth, people and animals in different ways and health hazards can range from minor to life-threatening, particularly for people with chronic respiratory and/or heart disease. Fluorine gas and HF can condense in rain or on ash particles coating grass, polluting streams and lakes with excess fluorine, eventually impacting groundwater. Animals eating fluorine-tainted, ash-coated grass may be poisoned. Although small amounts of fluorine can be beneficial, excess fluorine causes fluorosis by destroying teeth and bones, including humans when fluorine is leached into domestic water supplies (Hansell, et al. 2006; Motzer, 2014).

One such area is the alkaline volcanic province of the East African Rift Zone (EARZ) or valley, encompassing and extending through Eritrea, Djibouti, Ethiopia, Kenya, Tanzania, Uganda, Rwanda, Burundi and Malawi (Figure 1).

The EARZ contains some of the highest recorded world-wide fluoride concentrations in surface water and in groundwater with fluoride exceeding the World Health Organization’s (WHO) drinking water permissible level of 1.5 mg/L. Because fluoride occurs in hot springs, alkaline lakes, some river systems, and groundwater, it’s routinely consumed by the populous, resulting in severe dental and skeletal fluorosis (Malago, et al., 2017; Motzer, 2017).

The source for most of this anomalous fluoride is attributed to the EARZ hyper-alkaline volcanic rocks, including sodium and calcium rich magmas and intrusions such as nepheline syenites and carbonatite magmas and their associated volcanic ash deposits. Large fluoride concentrations [e.g., in the minerals villiaumite (NaF) and fluorite (CaF2)] are often contained in these melts and their associated volatile fractions with water bodies subsequently accumulating fluoride from rock weathering, leaching of fine-grained and friable volcanic ashes, and from high fluoride in geothermal waters. The weathering of fluoride-containing minerals and other silicates in the lavas and ashes by silicate-hydrolysis reactions commonly results in a sodium-bicarbonate-rich groundwater depleted in calcium and magnesium. Therefore, fluoride can occur in high concentrations because fluorite solubility is not a limiting factor (Brindha and Elango, 2011; Motzer, 2017).

One of the most extensive surveys of EARZ water sources was done in Ethiopia, which has an estimated population (as of December 2022) of ~121.9 million people. From analyses of 1,438 samples of surface water and groundwater, this study found that 24% of all water sources exceeded the WHO permissible level with 50% of deep wells and 90% of hot spring samples exceeding the WHO permissible level. Areas along the Awash River were the most affected where sugar plantations had attracted large settlements. The long-term use of deep groundwater from wells has resulted in high rates of both dental and skeletal fluorosis (Motzer, 2017).

References

Brinha, K and Elango, L., 2011, Fluoride in Groundwater: Causes, Implications and Mitigation Measures, in, S.D. Monroy (editor), Fluoride Properties, Applications and Environmental Management: Nova Publishers, Hauppauge, NY, pp. 111-136.

Brunning, A. 2018, Lava and Volcanic Gases: Periodic Graphics: Chemical & Engineering News: www.cenm.ag/periodicgraphics

Hansell, A.L., Horwell, C.J., and Oppenheimer, C., 2006, The Health Hazards of Volcanoes and Geothermal Areas: Occupational & Environmental Medicine, v. 63, n. 2, pp. 149-156, https://www.ncbi. nlm.nih.gov/pmc/articles/PMC2078062/.

Malago, J., Makoba, E., and Muzuka, A.N.N., 2017, Fluoride Levels in Surface and Groundwater in Africa: A Review: American Journal of Water Science and Engineering, v. 3, n. 1, pp.1-17.

McGee, K.A and Gerlach, T.M., 1995, Volcanic Gas in Volcanic Hazards Fact Sheet: U.S. Geological Survey.

McGee, K.A., Doukas, M.P., Kessler, R., and Gerlach, T.M., 1997, Impacts of Volcanic Gases on Climate, the Environment, and People: U.S. Geological Survey Open-File Report 97-262, Motzer, W.E., 2014, Volcanic Violence – Part 2 (Volcanic Gases): The Vortex, v. LXXV, n. 5, pp.5-6 www.calacs.org

Motzer, W.E., 2017, Toxic Terra – Part 8 (Fluoride): The Vortex, v. LXXIX, n. 1, pp.6-7 www.calacs.org

U.S. Geological Survey, 1999, East Africa Rift Zone map: https:// pubs.usgs.gov/gip/dynamic/East_Africa.html

Figure 1: Schematic showing large, explosive volcanic eruptions that inject water vapor (H2O), CO2, SO2, HCl, HF, and ash (pulverized rock and pumice) to stratospheric heights of 1020 miles (16-32 km) above the Earth’s surface. Most HCl and HF are dissolved in water droplets in the eruption cloud, quickly falling to the surface as acid rain. Source: McGee and Gerlach (1995).

Figure 2: Map of the East Africa Rift showing some historically active volcanoes (red triangles) and the Afar Triangle (shaded at the center). Source: U.S. Geological Survey (1999) https://pubs.usgs.gov/gip/ dynamic/East_Africa.html.