Megacity pumping and preferential flow threaten groundwater quality The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Khan, Mahfuzur R. et al. “Megacity Pumping and Preferential Flow Threaten Groundwater Quality.” Nature Communications 7 (2016): 12833. As Published http://dx.doi.org/10.1038/ncomms12833 Publisher Nature Publishing Group Version Final published version Citable link http://hdl.handle.net/1721.1/107717 Terms of Use Creative Commons Attribution Detailed Terms http://creativecommons.org/licenses/by/4.0/ ARTICLE Received 16 Jan 2016 | Accepted 5 Aug 2016 | Published 27 Sep 2016 DOI: 10.1038/ncomms12833 OPEN Megacity pumping and preferential flow threaten groundwater quality Mahfuzur R. Khan1, Mohammad Koneshloo1, Peter S.K. Knappett2, Kazi M. Ahmed3, Benjamin C. Bostick4, Brian J. Mailloux5, Rajib H. Mozumder4, Anwar Zahid3,6, Charles F. Harvey7, Alexander van Geen4 & Holly A. Michael1,8 Many of the world’s megacities depend on groundwater from geologically complex aquifers that are over-exploited and threatened by contamination. Here, using the example of Dhaka, Bangladesh, we illustrate how interactions between aquifer heterogeneity and groundwater exploitation jeopardize groundwater resources regionally. Groundwater pumping in Dhaka has caused large-scale drawdown that extends into outlying areas where arsenic- contaminated shallow groundwater is pervasive and has potential to migrate downward. We evaluate the vulnerability of deep, low-arsenic groundwater with groundwater models that incorporate geostatistical simulations of aquifer heterogeneity. Simulations show that preferential flow through stratigraphy typical of fluvio-deltaic aquifers could contaminate deep (4150 m) groundwater within a decade, nearly a century faster than predicted through homogeneous models calibrated to the same data. The most critical fast flowpaths cannot be predicted by simplified models or identified by standard measurements. Such complex vulnerability beyond city limits could become a limiting factor for megacity groundwater supplies in aquifers worldwide. 1 Department of Geological Sciences, University of Delaware, Newark, Delaware 19716, USA. 2 Geology and Geophysics, Texas A&M University, College Station, Texas 77843, USA. 3 Department of Geology, University of Dhaka, Dhaka 1000, Bangladesh. 4 Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA. 5 Environmental Sciences, Barnard College, New York, New York 10027, USA. 6 Bangladesh Water Development Board, Government of Bangladesh, Green Road, Dhaka 1000, Bangladesh. 7 Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge Massachussetts, 02197, USA. 8 Department of Civil and Environmental Engineering, University of Delaware, Newark, Delaware 19716, USA. Correspondence and requests for materials should be addressed to H.M. (email: [email protected]). NATURE COMMUNICATIONS | 7:12833 | DOI: 10.1038/ncomms12833 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12833 eltas and river basins sustaining dense populations and heads are falling tens of kilometres beyond the city limits16.Over- unique ecosystems are sensitive to environmental stres- pumping is lowering water levels beyond the threshold for Dses1. Nearly half a billion people, often concentrated in handpump use16 and could induce downward migration of megacities, live in 50 deltas around the globe2. Groundwater is shallow groundwater23,24, which may transport arsenic and other often used for water supply in deltas because of uncertainty in contaminants or reactive organic carbon that can fuel reductive surface water availability and high vulnerability to contamination, dissolution of iron (oxy)hydroxides and associated release of as occurs in the Bengal Delta3. However, groundwater over- arsenic to groundwater13,27 from deeper, older sediments28.These development4–6, especially in megacities7–9, and groundwater risks threaten the sustainability of the deep groundwater resource contamination are common in deltas and river basins10–13. Under of the 10 million people living in the Dhaka metropolitan area a changing climate, uncertainty of surface water supply is likely to outside the city centre and in surrounding rural areas (Fig. 1) who increase14, and rapid population growth in urban centres15 may are not supplied with city water. exacerbate groundwater depletion and contamination problems. Predicting the extent to which a hydrologic perturbation of this One consequence of pervasive groundwater over-development scale jeopardizes the quality of groundwater resources outside of and depletion in city centres is an increase in vertical recharge in pumping centres is confounded by geologic complexity. The large- and around the city11,12,16,17 where the surface and near-surface scale heterogeneity characteristic of river basins and deltas creates waters are often contaminated with toxic metals, organics, nitrate groundwater flowpaths that can contaminate groundwater and other pollutants10,11,17–20. Although sustaining the quantity resources29,30, yet aquifers of this type host more than half of 47 and quality of city water supply is a high priority, little attention is indexed groundwater mega-depletion cases worldwide6.Typicalof given to the potentially catastrophic impacts these hydrologic such systems, the aquifer system surrounding Dhaka is a highly alterations can have on the water resources of surrounding peri- heterogeneous, kilometres-thick sequence of fluvio-deltaic deposits urban and rural communities that do not benefit from city centred in the 200,000 km2 Bengal Basin31. Numerous studies over supply21, and for which water treatment may not be feasible. the past four decades have addressed the influence of aquifer Dhaka, the capital of Bangladesh, shares many of the water heterogeneity on transport of solutes, but many focus on small- management problems common to major cities22 and is located scale, low-variance heterogeneity relevant to plumes observed on in one of the largest fluvio-deltaic basins in the world. Naturally the scale of tens of metres32–34. Indeed, the importance of local occurring arsenic in shallow groundwater (o50 m), the drinking heterogeneity in controlling transport of As and organic carbon has water source for tens of millions of people, is widespread in the been demonstrated in As-affected aquifers of Cambodia35 and basin18, as in many fluvio-deltaic aquifers of Southern Asia13. Bangladesh36–38. At the basin scale, however, we must consider Deep (4150 m), low-arsenic aquifers are increasingly relied on heterogeneity in geologic features that spans tens of kilometres and for reducing exposure of the rural population to arsenic and could many orders of magnitude in hydraulic conductivity (K)39. be for decades to come if properly managed23–25. Dhaka pumping, Predicting the vulnerability of groundwater on this large scale however, has caused groundwater levels to drop more than 60 m requires numerical modelling, but the complexity and unknown over the last half century, and levels are currently declining at a rate nature of the aquifer system often necessitates a simplified of 43 m per year in areas of the city centre7,26.Thismassive approach using spatially averaged aquifer properties40.This alteration to the subsurface hydrology has expanded the approximation may miss the small fraction of flowpaths which management problem from local to regional because hydraulic are fast and most relevant to water quality vulnerability41. ab 2.68 N m) 6 2.64 Meghna Meghan Meghna Northing (×10 GangesGanges 2.60 Coarse-grid model boundary Fine-grid model boundary 2.0 2.4 2.8 5 Easting (×10 m) Coastline % Tubewells with >50 µg l–1 As Rivers As-safe >50% Dhaka statistical >0 to Յ50% No data metropolitan area Groundwater level monitoring locations Central city Figure 1 | Map of the study area. Colours represent percentage of tubewells with 450 mglÀ 1 As (a) within the study area and (b) within the Bengal Basin. The orange polygon outlines the basin-scale (coarse-grid) model and the black rectangle outlines the locally refined (fine-grid) model. Arsenic data sources: DPHE-JICA59 for Bangladesh and Chakraborti et al.60 for West Bengal. DWASA only supplies municipal water within the central city area. 2 NATURE COMMUNICATIONS | 7:12833 | DOI: 10.1038/ncomms12833 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12833 ARTICLE ab 2.68 2.68 Head (m) 20 2.66 2.66 00 m) 6 2.64 2.64 –20 W E –40 2.62 2.62 Northing (×10 –60 2.60 2.60 2.0 2.2 2.4 2.6 2.8 2.0 2.2 2.4 2.6 2.8 Easting (×105 m) Easting (×105 m) cd Calendar year Buriganga Balu and 1980 2010 River Shitalakshya River 1990 2015 W E 2000 Steady-state 0 City 2005 Homogeneous Rivers City limit Shallow –40 Deep 2.68 Heterogeneous Shallow –80 Deep 2.66 Hydraulic head (m) m) 6 e 2.64 0.3 0.2 Homogeneous 2.62 Heterogeneous Northing (×10 0.1 2.60 0.0 Vertical gradient –0.1 2.0 2.2 2.4 2.6 2.8 2.0 2.2 2.4 2.6 2.8 5 Easting (×105 m) Easting (×10 m) Figure 2 | Effects of Dhaka pumping on the flow system. Hydraulic head (contours) and flowpaths (black arrows) within the study area simulated for homogeneous conditions at 150 m depth for (a) pre-development (before 1980) and (b) current pumping (2015) conditions. (c) Extent of 9 m water level depth (suction limit for handpump wells) at 150 m depth. The steady-state condition in c is based on current Dhaka pumping levels. (d,e) Cross-sectional views of hydraulic head at depths of 50 m (shallow) and 150 m (deep) and the vertical gradient, respectively. Cross-sections are shown along the W–E transect shown in b. Here, we explicitly represent aquifer heterogeneity in a than four times natural levels and rivers near the city changed groundwater flow model to explore the existence and predict- from net gaining to net losing (Supplementary Figs 1 and 2). Both ability of fast flowpaths, and associated vulnerability of ground- measured and modelled heads within the city centre are very low water resources to contamination around the Dhaka megacity at all depths (Supplementary Figs 3–6).