Screening for arsenic contamination

Millions of people in Bangladesh and West Bengal have unwittingly drunk groundwater that is contaminated with arsenic as a result of natural processes for up to 20 years. They are potential victims of the greatest mass poisoning in human history. Dreadful as the possible fate awaiting them might be – they may develop various cancers – discovery and ten years of research into their problems has alerted geoscientists to the hazard of environments like those in which they live. That arsenic poses great dangers is common knowledge, but until unmistakable signs of arsenic poisoning appeared there (black wart- and mole-like skin lesions), the hazard was thought to be restricted to former mining areas where oxidation of iron sulfides released the traces of arsenic locked within those minerals. From studies in West Bengal and Bangladesh has emerged a cause that was completely unexpected: it involves one of the commonest minerals at the Earth’s surface, goethite or FeOOH. This yellow-brown colorant of many sediments has the remarkable property of being able to adsorb or ‘mop-up’ a large range of elements dissolved in water with which it comes into contact. Among these is arsenic. In the oxidising conditions that sponsor the formation of goethite as a coating on sedimentary grains the mineral actually prevents a great deal of natural, geochemical pollution. Yet, exposed to reducing conditions, commonly developed when buried organic material begins to rot, goethite may dissolve and release its potentially toxic load into groundwater. This is precisely the source of arsenic at levels more than 100 times the safe level in some wells on the Ganges-Brahmaputra plains. The story does not stop there, however.

When sea level stood about 130 m lower than now, at the last glacial maximum, rivers rising in the Himalaya cut deep valleys in the coastal areas. As sea-levels rose these rapidly filled with new sediments, most of which were stained with goethite. But they were interbedded with thick organic-rich peats that formed during periods of slow sea-level rise. It is the peats and more finely dispersed vegetable matter that caused the reduction and solution of goethite, and thus the arsenic that it carried. Especially high arsenic levels develop in sediments derived from specific areas in the Himalaya. So a suite of conditions conducive to arsenic hazard have emerged from unravelling the tragedy of the northern plains of the Indian subcontinent. It is possible to use that suite as a means of predicting other risky areas, one of the first to be revealed being in the Red River delta of northern Vietnam: the population of Hanoi is at risk from well water drawn from the Red River sands and gravels. Systematic computer screening of known geology, topography and soil conditions in Southeast Asia is beginning to throw up other problematic areas (Winkel, L. et al. 2008. Predicting groundwater arsenic contamination in Southeast Asia from surface parameters. Nature Geoscience, v. 1, p. 536-542) where concentrations of arsenic in drinking water are highly likely to exceed the maximum recommended level of 10 μg l-1 (parts per billion). The pilot study highlights the known areas, but also the deltas of Mekong River in Cambodia and southern Vietnam, the Irrawaddy in Burma (Myanmar) and the Chao Phraya basin of Thailand. Hopefully, geochemical testing will reveal in details which wells are at risk and which are not, in these three regions: it would be easy to reject perfectly safe groundwater that often occurs close to contaminated areas, as found in Bangladesh, without careful testing. The implicated mineral, goethite, is itself a cheap and abundant means of remediation if contaminated water is passed through goethite-rich filters. But the large areas at risk in SE Asia, together with others discovered by epidemiologists in northwestern India, the Indus plains of Pakistan and in Mongolia, create a chilling scenario for many other populous, sediment-rich areas elsewhere. Winkel et al’s approach surely needs to be refined and applied globally.

See also: Polizzotto, M.L. et al. 2008. Near-surface wetland sediments as a source of arsenic release to ground water in Asia. Nature, v. 454, p. 505-508. Harvey, C.F 2008. Poisoned waters traced to source. Nature, v. 454, p. 415-416.

Cause of Javan mud volcano

Since May 2006 the largely urban Sidoarjo area of eastern Java has been plagued by continuous eruption of hot mud and steam from a vent that suddenly appeared. Around 7 km2 have been buried by up to 20 m of noxious mud, giving a total emission of about 0.05 km3 at a rate of 100 thousand m3 per day. Although nobody has been killed, the mud volcano is an economic and social disaster, 30 thousand people having been displaced. The area is one of active petroleum exploration, and locals blame a blow out from a nearby gas exploration well, though scientists and the exploration company point to the eruption having begun a couple of days after a magnitude 6.3 earthquake in the area around the capital Yogyakarta, 250 km away. If the latter, economic losses may be difficult to recover from insurers; if the former, there will be a rare old furore. So, a thorough evaluation of what the cause may have been is welcome (Tingay, M. et al. 2008. Triggering of the Lusi mud volcano: Earthquake versus drilling initiation. Geology, v. 36, p. 639-642). Being a mix of Australian, German and British geologists, the authors have no axe to grind. They consider that seismic influence was highly unlikely, in this case, although many mud volcanoes have formed close to earthquake epicentres in other areas. On the other hand, the well that was being drilled at the time suffered a loss of drilling mud shortly before the volcano began to erupt, suggesting escape to fractures at depth around the well. Moreover, the hole was not cased at depth. The most likely trigger was creating a passageway up the well for high-pressure fluids to escape from the 3 km deep target limestone sequence into shallower unconsolidated clays. They were liquefied and escaped as a lateral blow out

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s