title: Sources of salinity and arsenic in groundwater in southwest Bangladesh.
abstract: BACKGROUND
  High salinity and arsenic (As) concentrations in groundwater are widespread problems in the tidal deltaplain of southwest Bangladesh. To identify the sources of dissolved salts and As, groundwater samples from the regional shallow Holocene aquifer were collected from tubewells during the dry (May) and wet (October) seasons in 2012-2013. Thirteen drill cores were logged and 27 radiocarbon ages measured on wood fragments to characterize subsurface stratigraphy.
  
  
  RESULTS
  Drill cuttings, exposures in pits and regional studies reveal a >5 m thick surface mud cap overlying a ~30 m thick upper unit of interbedded mud and fine sand layers, and a coarser lower unit up to 60 m thick dominated by clean sands, all with significant horizontal variation in bed continuity and thickness. This thick lower unit accreted at rates of ~2 cm/year through the early Holocene, with local subsidence or compaction rates of 1-3 mm/year. Most tubewells are screened at depths of 15-52 m in sediments deposited 8000-9000 YBP. Compositions of groundwater samples from tubewells show high spatial variability, suggesting limited mixing and low and spatially variable recharge rates and flow velocities. Groundwaters are Na-Cl type and predominantly sulfate-reducing, with specific conductivity (SpC) from 3 to 29 mS/cm, high dissolved organic carbon (DOC) 11-57 mg/L and As 2-258 ug/L, and low sulfur (S) 2-33 mg/L.
  
  
  CONCLUSIONS
  Groundwater compositions can be explained by burial of tidal channel water and subsequent reaction with dissolved organic matter, resulting in anoxia, hydrous ferric oxide (HFO) reduction, As mobilization, and sulfate (SO4) reduction and removal in the shallow aquifer. Introduction of labile organic carbon in the wet season as rice paddy fertilizer may also cause HFO reduction and As mobilization. Variable modern recharge occurred in areas where the clay cap pinches out or is breached by tidal channels, which would explain previously measured (14)C groundwater ages being less than depositional ages. Of samples collected from the shallow aquifer, Bangladesh Government guidelines are exceeded in 46 % for As and 100 % for salinity.
authors.affiliation: Department of Earth & Environmental Sciences, Vanderbilt University, PMB 351805, 2301 Vanderbilt Place, Nashville, TN 37235-1805 USA.
authors.forename: John C
authors.identifier: 0000-0003-2737-4718
authors.initials: JC
authors.lastname: Ayers
authors.affiliation: Department of Earth & Environmental Sciences, Vanderbilt University, PMB 351805, 2301 Vanderbilt Place, Nashville, TN 37235-1805 USA.
authors.forename: Steven
authors.initials: S
authors.lastname: Goodbred
authors.affiliation: Department of Earth & Environmental Sciences, Vanderbilt University, PMB 351805, 2301 Vanderbilt Place, Nashville, TN 37235-1805 USA.
authors.forename: Gregory
authors.initials: G
authors.lastname: George
authors.affiliation: Department of Earth & Environmental Sciences, Vanderbilt University, PMB 351805, 2301 Vanderbilt Place, Nashville, TN 37235-1805 USA.
authors.forename: David
authors.initials: D
authors.lastname: Fry
authors.affiliation: Department of Earth & Environmental Sciences, Vanderbilt University, PMB 351805, 2301 Vanderbilt Place, Nashville, TN 37235-1805 USA.
authors.forename: Laura
authors.initials: L
authors.lastname: Benneyworth
authors.affiliation: Department of Earth & Environmental Sciences, Vanderbilt University, PMB 351805, 2301 Vanderbilt Place, Nashville, TN 37235-1805 USA ; Department of Civil & Environmental Engineering, Vanderbilt University, PMB 351831, 2301 Vanderbilt Place, Nashville, TN 37235-1831 USA.
authors.forename: George
authors.initials: G
authors.lastname: Hornberger
authors.affiliation: Environmental Science Discipline, Khulna University, Khulna, 9208 Bangladesh.
authors.forename: Kushal
authors.initials: K
authors.lastname: Roy
authors.affiliation: Environmental Science Discipline, Khulna University, Khulna, 9208 Bangladesh.
authors.forename: Md Rezaul
authors.initials: MR
authors.lastname: Karim
authors.affiliation: Environmental Science Discipline, Khulna University, Khulna, 9208 Bangladesh.
authors.forename: Farjana
authors.initials: F
authors.lastname: Akter
keywords: Arsenic; Bangladesh; Groundwater; Salinization; Water chemistry