Groundwater flow in the Ganges Delta.

Basu et al. (1) reported that 2 3 10 m/year of groundwater flows directly into the Bay of Bengal, an outflow equivalent to 19% of the discharge from the Ganges-Brahmaputra river system. They showed that this estimate of flow could have important consequences for the interpretation of marine strontium isotope records, because strontium concentrations are higher in Ganges delta groundwater than in Ganges-Brahmaputra river water. The flow could also have implications for the origin and fate of other groundwater constituents in the Ganges delta that could be flushed by such rapid regional flow, such as the dangerously high concentrations of arsenic contaminating millions of drinking water wells (2). Here, I show that the large estimate of regional groundwater flow by Basu et al. is implausible given the extremely flat topography of the Ganges delta, and that the young helium-tritium ratios that they find in groundwater may reflect irrigation pumping rather than basin-scale flow. The physical plausibility of the Basu et al. flow estimate may be considered by applying Darcy’s law, without application of hydrologic models that require specification of boundary fluxes. One hundred kilometers inland from the coast of the Ganges delta, elevations reach only 5 to 10 m above sea level, and the water table reaches only 2 to 4 m above sea level (3). Assuming upper bounds for aquifer thickness and hydraulic conductivity [pp. 2.1–2.22 in (4)], Darcy’s law indicates that maintaining the discharge estimated by Basu et al. requires a hydraulic gradient of ;0.06 perpendicular to the coast (5). Such a gradient implies dramatic artesian conditions, with a potentiometric surface ;300 m above sea level 5 kilometers inland; such conditions do not exist in this area. Even if the assumed aquifer thickness is further increased to 2 km—an implausible depth for rapid, topographically driven flow in lowrelief terrain—the implied artesian conditions remain incorrect. Furthermore, many aquifers in coastal areas suffer from high salinity [pp. 2.1–2.22 in (4)] that would be flushed out if groundwater flowed so quickly to the coast. Basu et al. estimated groundwater flow to the ocean by first calculating an average groundwater recharge rate of 0.6 m/year from helium-tritium dating of wellwater extracted below 30 m, and then assuming that this recharge flows directly to the Bay of Bengal. As an alternative hypothesis, irrigation pumping and local flow cells, rather than regional flow to the ocean, may draw young groundwater to depth. Although the highest groundwater extraction rates are in the northwest of Bangladesh, groundwater extraction is also widespread on the delta. The gross groundwater extraction rate for the delta region can be estimated at approximately 25 cm/year (6), which implies an annual downward groundwater velocity of approximately 1.25 m/year assuming an aquifer porosity of 0.2. This downward velocity is somewhat less than the 2 to 3 additional meters that Kinniburgh and Smedley have estimated for drawdown by irrigation pumping (7) for a hypothetical cross section. Some references describe irrigation pumping at the specific locations sampled by Basu et al. (8). Many wells are screened below 30 m, and pumping drives complex flow patterns that extend below the screened depth. In some areas, local topography may also drive groundwater flow below 30 m, but in these areas most of the discharge is also local (9). Basu et al. considered groundwater discharge directly to the Bay of Bengal. However, such local flow cells could conceivably provide some additional flux of strontium into rivers downstream of the locations where strontium concentrations and river flows were measured. During the onset and recession of monsoon floodwaters, hydraulic head values change dramatically. However, flooding drives very little groundwater flow because inundation erases lateral gradients and downward flow by compression of water is negligible. Hydraulic gradients that emerge after the floodwaters recede are bounded by the mild topographic gradient. Basu et al. supported their estimate of groundwater flow to the ocean by citing the large coastal groundwater discharge estimated by Moore et al. (10) from offshore measurements of barium and radium. In that study, however, it was recognized that the estimated fluxes may represent saltwater circulation as well as freshwater discharge. The large observed barium and radium fluxes during the dry season, when water table elevations and fresh groundwater discharge along the coast are low, may be explained by desorption of barium and radium by saline water that moves up rivers and circulates through sandy sediments with the tidal cycle. Here, it is clear that Basu et al. have considered only fresh groundwater discharge, because their strontium measurements and age dates were determined from inland freshwater wells. In summary, the groundwater fluxes used by Basu et al. to calculate subsurface strontium fluxes to the Bay of Bengal are not physically realistic. However, their heliumtritium isotopic dates provide valuable information for characterizing groundwater flow in the Ganges Delta. The young ages of deep groundwater may indicate active circulation by pumping and irrigation, rather than subsurface flowpaths that continue for up to 100 kilometers to the coast. Circulation by pumping and irrigation, rather than large-scale flow to the ocean, implies less flushing of groundwater solutes and, hence, smaller fluxes of strontium to the ocean.