Based on the detailed studies of prevailing aquifer conditions, the scientific community has been able to delineate the principal mechanisms of the genesis and mobilization of As in many aquifers (BGS and DPHE 2001, Bhattacharya et al. 1997, 2002a, b, 2006, Smedley and Kinniburgh 2002, van Geen et al. 2003, Ahmed et al. 2004, Bundschuh et al. 2004, Hoque et al. 2004, McArthur et al. 2004, Nickson et al. 1998, Smedley et al. 2005). However, in many of the regions, the concentration of As in groundwater is extremely heterogeneous, both laterally and vertically. The safe and unsafe tubewells in many cases, installed in the same aquifer, are often found <25 m from each other. On the basis of the hydrogeological settings, the prevailing aquifer conditions and the theories of mobilization, many of the arsenic-safe tubewells should in fact have high concentrations of As. Consequently, the “patchy distribution” has often been explained in terms of “local variations in sedimentary characteristics, hydrogeological and hydrogeochemical conditions” both in the case of Bangladesh and other areas such as in the Río Dulce alluvial cone in Argentina (BGS and DPHE 2001, Bhattacharya et al. 2002a, b, McArthur et al. 2004, Smedley et al. 2002, 2005, Bundschuh et al. 2004, Bhattacharya et al. 2006) and hence no, or at the best a poor, explanation has been given for the As-safe tubewells. However, recent studies in Bangladesh and West Bengal, India ( Harvey et al. 2002, Jonsson and Lundell 2004, McArthur et al. 2004, van Geen et al. 2004, von Brömssen et al. 2007) of the aquifer and groundwater characteristics have been refined

or specified in such way that they can be used to distinguish As-safe or unsafe aquifers for the installations of tubewells.