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This module looks at the feasibility of using deeper wells as a source of low As water. The data sets are described in detail by van Geen et al. (van Geen et al., 2003; van Geen et al., 2002). Students are being introduced to background information about the Arsenic problem in Bangladesh in lecture format. This includes health aspects and the history of the issue. They also have been using the sand tank groundwater model distributed by the University of Wisconsin Stevens Point (http://www.co.portage.wi.us/groundwater/teach/purchase.htm) to develop an intuitive understanding of groundwater flow and transport and are familiar with basic hydrogeological concepts. They inject a dye into the shallow aquifer of the model and study how pumping effects the migration of the Arsenic plume (Fig 1). Students get an Excel spreadsheet that contains the longitude, latitude, and depth of 6000 wells and a satellite image that shows the area of investigation. They use Arc GIS software to plot data on the satellite image (Fig. 2), or alternatively plot the data as a function of longitude and latitude as a bubble plot in Excel. They find that the distribution of As in many regions is very heterogeneous. They then select sub-regions and look at the depth distribution and find that often there is a gap in the depth population of wells which turns out to be due to a clay layer varying in thickness that separates the shallow aquifer from the deep aquifer. The depth distribution (Fig. 3) of As also shows a characteristic pattern with most of the elevated As concentrated in the top 30 meters. Students then discuss remediation options, in particular the possibility of switching to neighboring wells and using deeper groundwater as an alternative source of drinking water. They find that in many regions there are safe wells within a few hundred m of the high As well. However, it is not clear how long these wells will remain low in dissolved As and there are social barriers as well to use the neighbors well. They then determine a depth below which As concentrations are low in their region and elevate the risk of using deeper groundwater for drinking water and irrigation. They find that personal use is resulting in only ~1cm year-1 of water use, while irrigation (~1 m year-1) would considerably lower the water table and potentially could contaminate the deeper aquifer as well. The conclusion is that if deeper groundwater is utilized its use should be limited to personal use.
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