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Bureau Seminar, November 22, 2013

Groundwater in the Ganges-Brahmaputra Delta:
Where Culture and Nature Collide

Link to streaming video: available 11.22.2013 at 8:55am

Peter Knappett
Assistant Professor
Department of Geology and Geophysics
Texas A&M University

Groundwater within unconsolidated deltaic sediments underlying Bangladesh contains high concentrations of naturally occurring arsenic and human-derived fecal contamination. Investigations into the propagation of these two very different groundwater contaminants reveals how culture and nature may interact to negatively impact human health.
Ponds are ubiquitous in rural Bangladesh as villagers build raised foundations to protect their houses from flooding. To evaluate these ponds as point sources of fecal contamination to aquifers, nine transects of monitoring wells were installed radiating away from four latrine ponds. Two ponds were emplaced within medium sand, one was in silt and another was in silty fine sand. E. coli was monitored monthly across a 15 month period. High E. coli concentrations were detected only during the early monsoon and only adjacent to ponds emplaced within medium sand. The ponds were actively filled during the early monsoon to simulate a major rainfall event. This was followed by high frequency monitoring. To understand how multiple rainfall events move FIB further into the aquifer high frequency measurements under a sandy, recently dug pond were modeled over 5 days. Sensitivity to anisotropy suggests the presence of silt layers had limited downward E. coli movement. Reversible attachment to grain surfaces allowed propagation of E. coli up to 10 m laterally. The low water table and frequent rainfalls in the early monsoon create ideal conditions for E. coli movement into aquifers. People could minimize their risk by installing wells further away from these point sources.

Deep aquifers (>100m) are increasingly relied upon for arsenic free drinking water in rural Bangladesh. The long term sustainability of this drinking water source is in question due to the massive depressurization by urban pumping. In places where confining clay layers are absent deep aquifer depressurization will draw young water into the deep aquifer. This water may contain high arsenic from shallow aquifers or, if it comes from rivers, labile organic matter released from organic-rich sediments that could promote the reductive dissolution of iron oxides and therefore release arsenic to groundwater. I explore the impacts of Dhaka pumping on changing recharge sources in the deep drinking water aquifers of Bangladesh. The study area is Araihazar upazila, a rural 150km2 region 20 km east of Dhaka and bounded to the east by the Meghna River. Eighteen pressure transducers monitored hydraulic head across an 8x8km west of the Meghna for one year. Hydraulic gradients trended towards Dhaka throughout the year. During the early monsoon westward gradients positively correlated with short term fluctuations in the level of the Meghna indicating a strong hydraulic connection to the deep aquifer. This effect was muted in the late monsoon and the dry season however. The 2-D shape of the drawdown cone was modeled with Theis and Hantush leaky-aquitard models. The results suggest that Dhaka pumping is creating optimal conditions for enhanced movement of both shallow groundwater and Meghna river water into underlying aquifers. This process may buffer the deep aquifer from depletion, but the long term impact on the deep aquifer water quality may be substantial.


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