Slide 1

Arsenic in groundwater- examples from US Superfund sites and Bangladesh Martin Stute, Barnard College & L-DEO (with help from A. Keimowitz) Slide 2 Outline As standards, health effects Basic geochemistry As mobilization at a former landfill site in ME As in Bangladesh Causes for As mobilization Remediation options Slide 3 BGS and DPHE (2001) Slide 4 Health effects of (chronic) arsenic exposure Previous studies in Taiwan, Argentina, and Chile Cardiovascular disease Skin lesions (few years of exposure) Cancers of the skin, lung, liver, and bladder (several decades of exposure) Childrens intellectual function Slide 5 http://superfund.ciesin.columbia.edu/home.html Slide 6 Arsenic Overview Toxic metalloid dispersed throughout earths crust average crustal concentration 2mg/kg = 2ppm Used in insecticides, herbicides, semiconductors Can be released into groundwater WHO and US EPA standards: 10 ppb ( g/L) Bangladesh standard: 50 ppb In South Asia, ~100 million peoples drinking water exceeds 10 ppb In the US, cost estimates of implementing this standard are $100-$600 million / year At US Superfund sites, arsenic is the 2 nd most common contaminant (after lead); present at 718 sites. Nordstrom, Science 2002. Gurian et al., ES&T 2001. http://www.epa.gov/superfund/sites/query/basic.htm. Slide 7 Arsenic Overview Arsenic is stable in multiple oxidation states: -3, 0, +3, and +5 This influences As mobility in the subsurface both natural As and anthropogenic pollution Slide 8 Landfill History 1930 Dump operations begin including local industrial wastes 1972 Landfilling begins and serves adjacent towns 1979 Buried drums found at landfill 1982 Landfilling ceases 1987 Landfill cover system installed 1995 SVE and P&T remediation commences N. Nikolaidis, UCon Slide 9 Slide 10 As Source: Not the Landfill Even distribution of sediment As no hotspots Typical crustal abundance Dissolved As does not correlate with Cl a leachate tracer in swamps Implies As controlled by a different transport or mobilization process Slide 11 Two Distinct Regions Slide 12 Reduction Induced by Leachate Sed As mg kg -1 Dissolved Species mean, mg L -1 Cl - DOCSulfideFe 2+ IronCODWater As 7 1 Central Region29 6 26 14 53 49 24 15 37 9 35 23 0.3 0.1 8 2 Peripheral Region10 2 16 4 5 2 2 4 2 4 1 0 0.01 0.005 Slide 13 Arsenic Mobilization Geochemical Explanation Slide 14 Transport of Reducing Power Implication: COD high in fast transport zones. [As] and [Fe] too? How to measure fast transport zones? Slide 15 As Immobilization via Oxidation Slide 16 Laboratory Oxidation Fe & As are both removed from reducing GW in a lab experiment of progressive oxidation Removal is decoupled Slide 17 Purposeful in situ Oxidation ORC Oxygen Release Compound Contains magnesium peroxide, phosphate and minor components Designed to release O 2 over ~6 months 1400 kg ORC injected through aquifer thickness Slide 18 ORC Pilot Experiment Results Slide 19 ORC Pilot Experiment 1400 kg ORC released ~330 kg O 2 But what is the subsurface oxygen demand? BOD BOD of water satisfied 100 times Only 20% of BOD of sediments met COD COD of water satisfied 18 times Only 0.1% of COD of sediments met Slide 20 Conclusions Winthrop, ME Leachate induces reducing conditions These reducing conditions permit As mobilization Hydrogeology has controlled where leachate influence is strongest; this in turn effects where As is mobilized As is removed from solution by oxidation In situ oxidation is hampered by redox demand of sediments Slide 21 Source: M. Steckler, LDEO, based on GTOPO30 digital elevation model (USGS EROS Data Center). Dhaka Araihazar * * Slide 22 Scale of human tragedy West Bengal, India -1 million people drink high arsenic groundwater. -200,000 cases of skin-lesions as of 1996. -62% of 20,000 sampled tube wells exceed 50 g/L. Bangladesh: -25/51 million people drink groundwater with arsenic. above Bangladesh (50 g/L) /WHO (10 g/L) standard. -21% of 18,000 people examined with skin lesions. -35% of 22,000 sampled tube wells exceed 50 g/L. Number of deaths of children under 5 has declined from 250/1000 live births in 1974 to ~100/1000 in 1994 cause of decline disputed. Sources:Das et al. (1996), Saha (1998), Dhar et al. (1997), Mandal et al. (1998), BGS/Mott McDonald (1999) D. Chakraborti et al. (2000), Kabir et al. (1999). Slide 23 Araihazar Bangladesh Arsenic Mitigation and Water Supply Program 5 million wells so far? http://www.bamwsp.org/ Slide 24 Slide 25 Slide 26 Arsenic in 5,966 wells Slide 27 As - depth distribution in Araihazar Slide 28 Wellnests Slide 29 Wellnests in Araihazar AC E GF B H Slide 30 Slide 31 Site A 10s of y 1000s of y Residence time Site A, Zheng et al., 2005 peat Slide 32 Whats needed for elevated As concentrations? Iron oxyhydroxides with adsorbed As Perhaps other phases? reducing conditions (no O 2, low ORP) natural organic matter peat? anthropogenic organic matter? Slide 33 As and groundwater age Slide 34 Recharge rate, As and local EM data Lowhigh As 500m ABFCGE lowhigh EM highlow recharge rate EM conductivity Color Scale (mS/m) 62 63 9 13 6 cm/y coarse fine sediments Slide 35 As removal Well switching Shallow wells Deep wells Pond water Rain collection New wells Surface water Existing wells Alternative sources Remediation options Safi filter @$18 3-kolshi filter @$5 Tube well sand filter Maintenance Monitoring Bacterial growth Pond sand filter 50 families @$16 ea. Bacteria 1/100 Aquaculture Boiling Rainwater harverster 1 family @ $160/$40 ea. Storage-seasonality Dug wells Seasonality Pathogens $50 for 150 ft Installation Distribution Spatial variability Social resistance Slide 36 Conclusions As concentrations are highly variable in Bangladesh As is of natural, but we do not know yet if there are anthropogenic factors influencing the As distribution Hydrology (groundwater age) is an important factor Deeper wells appear a feasible remediation option, although we need to keep irrigation in check