Background
Arsenic (As) is highly toxic but a common element found in the atmosphere, soils, and natural waters. However, As in drinking water probably poses the greatest threat to human health – ingested As causes cancer of the skin, bladder, and lung, and has been linked to other health effects, including reproductive and developmental effects, cardiovascular disease, and skin lesions. In term of drinking water standard, WHO and US-EPA permissible limit for As is 10 ug/L..
In Bangladesh 30% of wells have elevated concentrations of As considering the Bangladesh standard 50 ug/L, which put approximately 35 millions people at risk (Fig 1, see also WHO Report). If we follow the WHO guideline, 46% exceed the limit and 57 million people are exposed. At the shallow depth (<30 m) of aquifers where hydrogeology and geochemistry are dynamic, this scenario get worse - 55% of wells exceed the safe limit. I am interested to take a closer look at this shallow part of aquifers to understand subsurface processes that control As concentrations in groundwater.
Fig 1: Groundwater arsenic distributions in Bangladesh (BGS and DPHE, 2001)
Araihazar
My study area is in Araihazar, Bangladesh where six thousands wells have been tested for As concentrations. Nearly 60% of shallow wells (<30 m) in Araihazar have elevated concentrations of As and show a wide range of variability (1-1000 ug/L) both spatially and seasonally (Fig. 2; van Geen et al., 2003).
Fig 2: Distribution of arsenic in 6000 tube wells in Araihazar (van Geen et al, 2003)
Surface Permeability and As Concentrations in Shallow Groundwaters
I compared dissolved As concentrations in 5200 shallow wells with the nature of surface soils mapped with EM31 conductivity meter (a hand held geophysical instrument). The study showed that electromagnetic (EM) conductivity reading reflects the clay percentage and the recharge capability of surface soils. I also found that low dissolved As corresponds to areas where EM conductivities of surface soils are also low and vice versa. Based on these observations we concluded that local recharge inhibits As concentrations from rising in shallow aquifers (Aziz et al., 2008).
Figure 3: Comparison of the distribution of As in shallow wells of Araihazar with a contour map of EM conductivity. Arrows point to the four well nests where recharge rates (numbers in m/y) are shown. (Aziz et al, 2008)
Trapping Mechanism of As
I tracked both physical and chemical processes that control dissolved As in shallow aquifers by installing a series of very shallow multilevel (depths range from 3 to 9m) monitoring wells at three different sites. A seasonal variation was observed in groundwater chemistry at one multilevel wells site related to a simple mixing between two water masses with different chemical compositions and depths. This mixing process was attributed to recharge processes controlled by the seasonal changes in vertical and horizontal flow directions. Data at this site also suggests a trapping mechanism of As in shallow aquifers that could explain about 50% of the spatial variability in As concentration in Bangladesh. Besides, groundwater chemistry and relative hydraulic head measurements at two other multilevel sites support the notion that long term irrigation pumping can lower the As concentration in shallow aquifers (Aziz et al., in preparation).
Groundwater Flow Modeling
I am working on a a numerical groundwater model where I have made an attempt to explain the observations and processes discussed in the second chapter. The groundwater flow model was simulated using the computer code known as "modflow" through a commercial interface GMS 6.0.
Over all, these works suggest that local hydrology plays an important role in the distribution of dissolved As in shallow aquifers.
Dissertation Committee