Arsenic and other Heavy Metals in the Rivers of Nepal Steven H. Emerman, Tara N. Bhattarai, Danda P....

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Arsenic and other Heavy Metals in the Rivers of Nepal

Steven H. Emerman, Tara N. Bhattarai, Danda P. Adhikari, Sunendra R. Joshi, Siddhi L. Lakhe, Aimee J. Luhrs, Kangada R. Prasai, Kristine L. Robson

Problem

• As of January 2004, 18,635 wells in the Terai Region (Indo-Gangetic Plain) of Nepal had been tested for arsenic.

• 23.7% of wells exceed the WHO Standard of 10 mcg/L

• 7.4% of wells exceed the Nepal Interim Standard of 50 mcg/L

Objectives

• Measure fluvial As in the rivers that drain into the Terai in order to determine the source of As

• Look for correlations between fluvial As and the elements most commonly associated with As (Fe, Cu, Co, Ni, Zn) in order to better determine the source of As mineralization

Methods

• 303 samples were collected from 120 sites over three years

• pH was measured on site

• Fe, Cu, Co, Ni and Zn were measured the same day using Hach Portable Colorimeters

• Arsenic was measured the same day using the Hach Arsenic Test Kit

Questions to Ask about Fluvial Chemistry

• Is there a different fluvial chemistry between rivers fed by glaciers and rivers fed by springs and snowmelt?

• Is there a relation between fluvial chemistry and the geological zone dissected by the river?

• Which of the heavy metals in rivers of the heavily polluted Kathmandu Valley are naturally occurring?

• Is there an annual variation in fluvial chemistry?

Comparison of Fluvial Fe between Higher Himalayan and other Zones

Khumbu Glacial (2005)

Khumbu Non-Glacial(2005)

Kathmandu (2003)

Other Zones (2003-05)

Global Average

Higher Himalayan

Comparison of Fluvial Cu between Higher Himalayan and Other Zones

Kathmandu (2003)

Global Average

Higher Himalayan

Comparison of Fluvial Co between Higher Himalayan and other Zones

Kathmandu (2003)

Global Average

Higher Himalayan

Comparison of Fluvial Zn between Higher Himalayan and other Zones

Kathmandu (2003)

Global Average

Higher Himalayan

Comparison of Fluvial As between Higher Himalayan and other Zones

Kathmandu (2003)

Global Average

Higher Himalayan

Comparison of Fluvial pH between Higher Himalayan and other Zones

Kathmandu (2003)

Higher Himalayan

Search for Correlations

• Fluvial As does not correlate with Fe, Cu, Co, Ni or Zn or any combinations of those elements.

• There are multiple sources of As that may be associated with mineralization of Fe, Cu, Co or Ni.

• Arsenic is not associated with Pb-Zn mineralization.

Glacial Meltwater and Fluvial Chemistry• Rivers in the Khumbu Region that are fed

by glaciers have considerably more Fe and Co than rivers fed by springs or snowmelt.

• Glaciers in the Khumbu Region are probably incorporating rock from unmapped deposits of Fe-Co in the Tibetan-Tethyan Zone.

Pollution of Rivers in Kathmandu by Heavy Metals• Elevated levels of Cu, Co, Ni and As in

rivers in Kathmandu are naturally occurring.

• Elevated levels of Fe in rivers in Kathmandu probably result from the many rusted pipes that discharge directly into rivers.

Geological Zones and Fluvial Chemistry• For rivers dissecting rocks of the Higher

Himalayan Zone, <1% of samples had As concentrations ≥ 10 mcg/L (WHO standard).

• For rivers dissecting rocks and sediments of all other zones, 53% of samples had As concentrations ≥ 10 mcg/L.

Typical As Concentrations(measurements not made in Nepal)

Geological Zone Dominant Rock/Sediment As concentration (mg/kg)

Higher Himalayan Zone Schist/gneiss 1.1 (<0.1-18.5)

Lesser Himalayan Zone Phyllite/slate 18 (0.5-143)

Limestone/dolomite 2.6 (0.1-20.1)

Siwalik Group Sandstone 4.1 (0.6-120)

Terai Zone, Recent Deposits

Various sediments 3 (0.6-50)

Models for As Contamination of Groundwater in South Asia

• Over-pumping of aquifers causes oxidation of sulfide minerals and release of co-precipitated As into groundwater (Badal et al. 1996, Mallick and Rajgopal 1996)

• Excessive use of phosphate fertilizers results in displacement of As from sediment adsorption sites by phosphate (Acharyya et al. 1999, 2000)

• Strongly reducing conditions cause release of As from adsorption sites on Fe oxyhydroxides after dissolution of Fe oxyhydroxides (Nickson et al. 2000, McArthur at al. 2001, Bose and Sharma 2002, Harvey et al. 2002) or after reduction of adsorbed As from As+5 (arsenate) to As+3 (arsenite) (Bose and Sharma 2002)

• Arsenic is displaced from adsorption sites by carbonate after sediments deposited in surface waters with low carbonate concentrations are later exposed to groundwater with high carbonate concentration (Appelo et al. 2002)

• Widespread deforestation causes loss of the microbial population that maintains soil As in an immobile, organic form (Emerman 2004)

What all these Models have in Common is that there is no Role for Fluvial As

• Is it a coincidence that there is high fluvial As in rivers draining into a region with high groundwater As?

• Hypothesis: All of the existing models are thermodynamically plausible, but the kinetics may be too slow. In that case, the groundwater chemistry may simply be a rough reflection of the surface water

chemistry. • The hypothesis predicts that groundwater in the Terai

region should have elevated Co, Ni and Cu.

Acknowledgements

• Funding from Fulbright Alumni Initiative Award• Prof. Bishal Nath Upreti, Dean of Science,

Tribhuvan University• Dr. Peter Moran, Executive Director, United

States Educational Foundation / Nepal• Mr. Michael Gill, Former Executive Director,

United States Educational Foundation / Nepal

Arsenic and other Heavy Metals in the Rivers of Nepal Steven H. Emerman, Tara N. Bhattarai, Danda P....

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