Natural sediment may shield groundwater from arsenic
[DHAKA] Contamination of deep groundwater with arsenic from shallower sources may not be as serious as feared — if pumping deep water is limited to domestic use, a study has found.
Exposure to arsenic-contaminated groundwater has been linked to almost one in every five deaths in Bangladesh, and some 100,000 deep wells have been constructed to pump deeper, cleaner water. Recent modelling studies have suggested that these cleaner water sources are also being contaminated — from shallower water seeping down to replenish deeper wells.
But a study published in Nature Geoscience yesterday (9 October) found that natural adsorption of arsenic by sediment — sand in the aquifers — reduces contamination risk in most areas.
"Deep groundwater in Bangladesh is at risk from contamination by arsenic from shallow groundwater seeping downwards if not carefully managed," Yan Zheng, who co-authored the study while she was a senior scientist at Columbia University, United States, told SciDev.Net. "The risk is higher if deep groundwater is used for irrigation, which consumes a lot more water than [use for] domestic purposes."
Modelling studies have suggested that the contamination of deep groundwater results from shallower water seeping down to replenish pumped deep water. But these studies did not consider the influence of sediment, which can adsorb arsenic, Zheng and her team say.
They tested this adsorption in the field in Bangladesh, and used their results to estimate the vulnerability of deep groundwater to arsenic pollution from shallower water seeping down.
They found that sediment removes around 70 per cent of arsenic within a day, reducing the risk of contamination of deep groundwater in most, but not all areas; and more so when the water is pumped for domestic use only, rather than irrigation. This suggests that current contamination of deep wells is either natural or comes from individual cases of badly designed wells that allow more seepage, Zheng said.
She added that the recommendation for the policymakers "is not to use deep groundwater for irrigation", and to regularly and systematically monitor water quality in the areas identified as more vulnerable to contamination.
Zheng also said that the agricultural sector should urgently look for sources of surface water to use for irrigation instead of groundwater.
Wais Kabir, executive chair of Bangladesh Agricultural Research Council, agreed that irrigation leads to higher risk of arsenic contamination of groundwater and said that Bangladesh needs to "change its food habits" and produce crops that need less irrigation.
S M Ihtishamul Huq, the Department of Public Health Engineering's superintendent engineer, told SciDev.Net: "We have to be more cautious while using groundwater for irrigation where the presence of arsenic is much higher."
He suggested changing crop patterns to reduce dependency on groundwater for irrigation. For example, he said: "We cultivate paddy during the winter using the groundwater irrigation. If we instead produce wheat [in] that period we do not need to irrigate much."
Nature Geoscience doi:10.1038/ngeo1283 (2011)