Arsenic mobilization in the low-flow regime of groundwaters in Bangladesh
Cornelia Härtig (08/2008)
Support: Britta Planer-Friedrich
publication: Freiberg Online Geoscience FOG Vol.20
The research project was run in cooperation of the University in Freiberg (Germany) with Dhaka University (Bangladesh) and founded by the DFG. The extensive arsenic contamination of groundwaters in Bangladesh is a severe health hazard for Bengali people. But yet, mechanisms behind arsenic release to groundwater are not well understood. The field investigation included a regularly monitoring of the groundwater chemistry at seven observation wells. The focus was on temporal and depth-dependent variations of total arsenic and its species between September until December 2007. Competition for sorption sites between arsenic and phosphorus was investigated with by an injection spike test. Water tables were measured at the observation wells regularly. At about 40 irrigation wells water tables were leveled and pumping tests were carried out to get a better understanding of the hydraulical conditions at Titas and their possible influence on arsenic cycling.
Two other master theses preceded this thesis within the project, investigating the sediment properties (Lissner, 2008) as well as spatial and temporal water chemistry variations between February throughout July 2007 (Steinborn, 2008).
Diploma thesis Jörg Steinborn (2008):
Diploma thesis Heidi Lissner (2008):
Summary of the Diploma Thesis
Investigations regarding hydraulical properties of the subsurface and a two-weekly monitoring of the hydrochemistry at seven piezometers at Titas/Daudkandi, Bangladesh, were carried out between September throughout December 2007 in the framework of a project founded by the German Research Foundation (DFG). The subsequent data analysis moreover includes results of sedimentary analyses and monitoring data of two preceding surveys executed between February throughout July 2007 at the Titas's test site wells.
At Titas, a shallow grey reduced sandy aquifer down to 28 m depth is separated by a huge clayrich grey reduced aquitard, from a leaky to confined deeper aquifer below 81 m depth, which showed partially reddish-brown oxidized sandy sediments. Total Arsenic concentrations detected with ICP-MS exceeded WHO guideline values (10 μg As/L) in all seven piezometers, but varied distinctly with depth. Arsenite, Arsenate, and even Monothioarsenate were detected with AECICP-MS, whereas the more mobile and most toxic Arsenite represented the predominant species in all depths. Highest As values were detected in the deeper shallow aquifer wells near to and in a sand lens encased within a peat-bearing aquitard layer. The occurrence of high dissolved Fe and Mn concentrations, and significant positive correlations of As with Fe, P, DOC, TIC, H2CO3, NH4+ over depth in the Ca-HCO3-type groundwaters of the five shallow aquifer wells indicate reductive dissolution of iron hydroxides during organic matter degradation as major process, triggering As release to groundwater. Nevertheless, results from the sand lens point out, that dissolution of As and Fe may be de-coupled.
On-site P-spike experiments, reaching maximum added concentrations of 2.3 mg PO4-3/L in groundwaters, showed that no Arsenic could be released by competitive sorption with the connatural Phosphate anion from sediments in 25 m depth. Also positive significant correlations between As and P indicate a common underlying release mechanism.
Modelling with Visual Modflow emphasized that evapo(transpiration)ration mainly controls water table decline after monsoon season. Thereby, irrigation activities significantly enhance, whereas artificially elevated village and street basements distinctly inhibit direct evapo(transpi)ration from groundwater. High potential hydraulical permeabilities of 17 m/d in the shallow aquifer were calculated, indicating that recent intense water withdrawals have the potential to significantly alter and accelerate water exchange processes in a natural low-flow aquifer system. Accelerated intrusion of waters during dry season led to distinctly decreasing mineralisation of 115 ft-waters, accompanied by an increase in As-concentrations.
The distinct depth-depending co-increase of As and DOC within groundwaters, related to peatd eposits below 27 m depth, indicates, that an accelerated shift of young organic carbon to deeper strata may not play a significant role for As-mobilization at Titas. But, it may indicate that As and organic Carbon were co-deposited.
A seasonal, slight increase of As was detected during the first half of the year for four of the monitored wells. This may be an indicator for seasonal variation trends. Nevertheless, reliable conclusions only can be achieved by continual long-term monitorings over several years.
Slightly acidic pH, extremely high Fe, NH4+, elevated DOC, As and P concentrations in the deep aquifer groundwaters appear to be linked to reductive dissolution processes in the huge peat deposit within the overlying aquitard layer. Relatively low As-concentrations compared to dissolved Iron in the deep groundwaters indicate, that As after its release, partially may be re-adsorbed on pH-dependent surface-sites of kaolinite detected within the aquitard, or on solid reddish-brown iron hydroxides within the deep aquifer.
Altogether, everything points on predominantly natural causes behind Arsenic mobilization. At Titas irrigation eventually may worsen the situation by enhanced shift of As-polluted waters to more shallow strata.