Presentation on theme: "Arsenic in groundwater- examples from US Superfund sites and Bangladesh Martin Stute, Barnard College & L-DEO (with help from A. Keimowitz)"— Presentation transcript:
Arsenic in groundwater- examples from US Superfund sites and Bangladesh Martin Stute, Barnard College & L-DEO (with help from A. Keimowitz)
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
BGS and DPHE (2001)
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) Children’s intellectual function
Arsenic Overview Toxic metalloid dispersed throughout earth’s 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 people’s 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 Gurian et al., ES&T
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
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
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
Two Distinct Regions
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 9 35 2 Peripheral Region10 2 16 4 5 2 2 4 2 4 1 0.005
Arsenic Mobilization– Geochemical Explanation
Transport of Reducing Power Implication: COD high in fast transport zones. –[As] and [Fe] too? –How to measure “fast transport” zones?
As Immobilization via Oxidation
Laboratory Oxidation Fe & As are both removed from reducing GW in a lab experiment of progressive oxidation Removal is decoupled
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
ORC® Pilot Experiment– Results
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
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
Source: M. Steckler, LDEO, based on GTOPO30 digital elevation model (USGS EROS Data Center). Dhaka Araihazar * *
Scale of human tragedy West Bengal, India -1 million people drink high arsenic groundwater. -200,000 cases of skin-lesions as of % 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).
Araihazar Bangladesh Arsenic Mitigation and Water Supply Program 5 million wells so far?
Arsenic in 5,966 wells
As - depth distribution in Araihazar
Wellnests in Araihazar AC E GF B H
Site A 10’s of y 1000’s of y Residence time Site A, Zheng et al., 2005 peat
What’s 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?
As and groundwater age
Recharge rate, As and local EM data Lowhigh As 500m ABFCGE lowhigh EM highlow recharge rate EM conductivity Color Scale (mS/m) cm/y coarse fine sediments
As removal Well switching Shallow wells Deep wells Pond water Rain collection New wells Surface water Existing wells Alternative sources Remediation options Safi 3-kolshi Tube well sand filter Maintenance Monitoring Bacterial growth Pond sand filter 50 ea. Bacteria 1/100 Aquaculture Boiling Rainwater harverster 1 $160/$40 ea. Storage-seasonality Dug wells Seasonality Pathogens $50 for 150 ft Installation Distribution Spatial variability Social resistance
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