Clean water is often taken for granted despite growing evidence that it is threatened in many parts of the world by either environmental contamination and/or socioeconomic problems, such as poverty, which often tend to go hand in hand. Arsenic contaminated ground water used for drinking and cooking is commonplace in many parts of Bangladesh. Like other chemical elements known to be poisonous to humans, arsenic is tolerated to some degree, but beyond certain thresholds ingesting arsenic is toxic leading to risk of disease and death.
Arsenic contaminated groundwater currently threatens the health of 70 million people in 61 of 64 districts in Bangladesh. Many people living in districts plagued with arsenic contaminated ground water regularly drink water with concentrations of arsenic far above national and WHO standards. An important study from Prof Peter Atkins and Dr Manzurul Hassan explores how groundwater arsenic concentration varies throughout areas of southwest Bangladesh. Understanding the scale of arsenic contamination, the complex processes that lead to arsenic in groundwater and how arsenic spreads over time is currently needed to reduce arsenic-related health risks. The study reveals a highly uneven spatial pattern of arsenic concentrations that can inform government policy for addressing where high levels of arsenic contamination occur in order to mitigate arsenic poisoning, a health and social hazard. 358 of the 375 tubewells sampled in the study had concentrations of arsenic of at least .05 mg/L and only 17 of the tubewells (4.50 percent) sampled are considered arsenic-safe. This is a large health concern for people living in areas of Bangladesh where the only source of water they have is contaminated with arsenic that is either above or well above the WHO standard (<0.01 mg/L), but also the limit set by the government of Bangladesh (0.05 mg/L).
Using either threshold shows the degree of arsenic contamination tolerated in parts of the country where many people live. In this study only four tubewells were found to meet the WHO permissible limit. The average concentration of arsenic for the areas studied is 0.248 mg/L, 25 times higher than the WHO permissible limit. These concentrations normally varied with depth.
Tubewell depths measured in the study range between 18-200 m. A deep aquifer is normally above 144.5 m, while a shallow aquifer is equal to or less than this depth. The samples were taken using an advanced laboratory method to ensure accuracy (FI-HG-AAS). In the study, these methods have a minimum detection limit of .001 mg/L and are capable of measuring down to .003mg/L which works well for detecting arsenic contamination at or above permissible levels.
The study found arsenic-safe zones in the north, central and south part of the study area in Ghona Union, Satkhira District located in southwest Bangladesh close to the Indian border, but concentrations of arsenic were scattered throughout. Generally, arsenic-safe zones lie in higher elevations, while the most contaminated areas are low-lying and used for agriculture. The reason for varied concentrations of arsenic in different parts of the research site may be due to geological influences such as differences in texture of the aquifers and/or aquifer chemistry. About 46 percent of tubewells are located within 25 m of each other within the study site.
While there are a number of theories as to why there is such large variation in contamination the specific cause is currently unknown. The pattern of arsenic concentration actually ‘varied considerably and unpredictably over a distance of a few metres’, according to the study. Arsenic contamination varied with grain-size distribution, which means the concentration was different depending on whether the sediment below the surface consisted of very fine to medium-sized sand grains. Previous studies have shown a variation in the relationship between grain size and arsenic concentration. This study found arsenic concentration highest in fine to medium size sand grains.
The study area covers only 18 km2, but has a population of 13,287 (recorded in 1991). Arsenic contaminated zones were concentrated primarily in the west and northeast parts of the study area. While contamination zones were found everywhere in the study area, the degree of contamination decreased from west to east. What few safe zones exist are concentrated in the south. Modelling used in the study revealed that less arsenic was found for tube wells at greater depths, but in some cases low levels of arsenic were found in deep aquifers and there is no guarantee that they cannot be contaminated with arsenic in time. Other studies have shown that older wells tend to have a probability of higher arsenic concentrations. A previous study found that only 1 percent of tubewells deeper than 200 m have arsenic above .05mg/L, the WHO standard, but in the study from Hassan and Atkins 75 percent of deep tube wells were above this threshold for arsenic-safe drinking water. While greater depth is still important in terms of accessing arsenic-safe water, arsenic has been found to be wholly absent at depths less than 5 m, particularly in dug wells.
While this study along with others finds that multiple levels of arsenic contamination are present at different depths and are dependent on geological factors, more research is needed on how arsenic moves through ground water systems within different areas of Bangladesh. Whether the variation in local geology is the main cause of the measured differences in arsenic concentration is still an open question. If answered, it can likely assist government, NGOs and communities in addressing the arsenic problem at a much greater scale than at present.
References and Further Reading:
Application of geostatistics with Indicator Kriging for analyzing spatial variability of groundwater arsenic concentrations in Southwest Bangladesh. Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering.
Contamination of drinking-water by arsenic in Bangladesh: a public health emergency. World Health Organization
Hassan MM, & Atkins PJ (2011). Application of geostatistics with Indicator Kriging for analyzing spatial variability of groundwater arsenic concentrations in Southwest Bangladesh. Journal of environmental science and health. Part A, Toxic/hazardous substances & environmental engineering, 46 (11), 1185-96 PMID: 21879851