More confusion over Bangladesh arsenic crisis

Millions of Bangladeshi people risk arsenic poisoning if they drink water drawn from tube wells (see British Geological Survey sued over arsenic, October 2002 Earth Pages News).  Since the disaster first came to light, UNICEF have tested 1.3 million of the estimated 10 million tube wells that are potentially hazardous.  Those deemed safe are painted green, while those which are risky are now red.  Unfortunately, doubts are being cast on the reliability of the commercial test kits that UNICEF use to estimate dissolved arsenic concentrations.  It is claimed that the analytical method have never been validated by controlled field experiments, and also that the minimum level of arsenic that they can detect is ten times higher than the safe level set by the WHO.  A positive contribution to solving the problem is to drill deeper, since it seems as if the condition for release of arsenic from bonding in sedimentary iron minerals is related to bacterial action that creates reducing conditions.  Although deep by comparison with traditional hand-dug wells, the tube wells go down only 50 to 80 metres and do not penetrate the zone in which reducing bacteria survive.

Source:  Pearce, F. & Hecht, J. 2002.  Flawed water tests put millions at risk.  New Scientist, 16 November 2002, p, 4-5.

Seismic bathymetry and Mediterranean debris flows

Tsunamis are an ever present threat in coastal areas, and can be set in motion by submarine debris flows as well as by earthquakes.  As more evidence for ancient tsunamis emerges on coastlines, such as characteristic features in Alaska (seismically induced), the Hawaiian islands and Bahamas (induced by landslips on unstable volcanic islands), and even the east coast of Britain (submarine debris flow off western Norway) their perceived threat has grown.  A team of oceanographers from Spain, Canada, Belgium, Britain and France has re-examined seismic reflection data from the western Mediterranean, to extract detailed topography of the sea floor (Lastras, G. and 6 others 2002.  Seafloor imagery from the BIG’95 debris flow, western Mediterranean.  Geology, v. 30, p. 871-874).  Although the western Mediterranean is seismically quiet, compared with around Italy and Greece, it is floored by products of turbidity flows.  A particularly large example (BIG’95) off the Spanish coast has an estimated volume greater than 26 km3.  Lastras et al. provide exceptional detail of the internal structure and surface shape of this debris flow, which enables them to suggest how it formed.  It coincides with an interface between deep volcanic rocks and a thick cover of soft sediments, along which gradual detachment eventually resulted in a normal fault propagating to the sea floor.  The mechanical instability seems to have been due to rapid deposition from the Ebro river system at a time of low sea level in the Mediterranean around the beginning of the Holocene.  The flow is marked by fluid escape structures, which the authors suggest may have been connected with a rise in bottom-water temperatures.  Is this another example of gas hydrate being involved, as seems likely for the Storegger slide that caused tsunamis along Britain’s east coast (see Collapsing islands, March 2002 Earth Pages News) about 7200 years ago?  The authors do not speculate on that.  However, the detail that they provide about the conditions that culminated in BIG’95 should provide a benchmark for seeking areas prone to such massive and potentially catastrophic events.


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