the effect of surface lithology on arsenic and other heavy metals in surface water and groundwater...

The Effect of Surface Lithology on Arsenic and other Heavy Metals in Surface Water and Groundwater in Mustang Valley, Nepal Himalaya

Steven H. Emerman1, Janae R. Nelson1, J. Kade Carlson1 Tracy Kemp Anderson1, Anusha Sharma2, Basanta Raj Adhikari2

1Department of Earth Science, Utah Valley University2Department of Geology, Tri-Chandra Campus, Tribhuvan University

Dominant Paradigm for Arsenic Contamination of Groundwater in South Asia

• In strongly reducing conditions of thick sedimentary package, micro-organisms break down iron oxyhydroxides for energy (reductive-dissolution model).

• This model assigns no responsibility to any human activity.

Competing Sulfide-Oxidation Model for Arsenic Contamination of Groundwater

• Oxidation of sulfide minerals results in release of co-precipitated arsenic.

• Sulfide oxidation could result from overpumping of aquifers.

Comparison of Fluvial As between Higher Himalayan and other Zones

0

2

4

6

8

10

12

14

16

As

(mcg

/L)

Khumbu Glacial (2005)

Khumbu Non-Glacial(2005)

Khumbu Glacial (2006)

Khumbu Non-Glacial(2006)

Kathmandu (2003)

Other Zones (2003-05)

Global Average

Higher Himalayan

a

a abb

c

c

Fluvial Recharge Model

• All of the existing models are thermodynamically plausible, but the kinetics are too slow.

• Groundwater is recharged by losing streams with elevated arsenic due to accelerated erosion caused by monsoon climate, tectonic uplift, deforestation and rangeland degradation.

Why does Accelerated Erosion cause Elevated Arsenic in Surface Water?

• Accelerated erosion results in higher concentration of dissolved solids in overland flow.

• In rapid overland flow, the large As oxyanion has too much kinetic energy to attach to sorption sites and not enough residence time at a sorption site.

Objectives• Document the occurrence of elevated arsenic

in the three major tectonic valleys of Nepal Himalaya (Kathmandu Valley, Pokhara Valley and Mustang Valley) with particular attention to sources of drinking water

• Test the fluvial recharge, reductive-dissolution and sulfide-oxidation models in these tectonic valleys

Comparison of Geometric Mean Concentrations for Groundwater and Surface Water

0.00001

0.0001

0.001

0.01

0.1

1

As Fe Cu Ni Co Zn

Geo

met

ric

Mea

n C

on

cen

trat

ion

(m

g/L

)Groundwater Surface Water Background

*

******

***

Methods: Mustang Valley• Water samples were collected from 24

streams, four canals or pipes fed by streams, five public faucets fed by streams, 10 springs, and 14 public faucets fed by springs.

• There are no wells in Mustang Valley.• pH, electrical conductivity and temperature

measured on-site• As, transition elements (Fe, Cu, Ni, Co, Mn,

Zn, Cr) and SO4 measured with Hach DR-2700 Portable Spectrophotometer

Nepal Interim Arsenic Standard (As = 0.05 mg/L) was exceeded in:

31% of surface water sites

63% of groundwater sites

all nine public faucets in Lo-Manthang

All other public faucets in Mustang Valley did not exceed Nepal Interim Arsenic Standard.

Table 3: Statistically Significant Differences between Geochemistry of Regions I and II

Region I (high As) Region II (low As)

Total Subsurface Surface Total Subsurface Surface

As (mg/L)a 0.071 0.087 0.056 0.001*** 0.001*** 0.001***

Fe (mg/L) 0.06 0.08 0.04 0.03 0.01** 0.04

Cu (mg/L) 0.07 0.14 0.03 0.03* 0.02* 0.03

Zn (mg/L) 0.14 0.17 0.12 0.08* 0.07 0.08

Co (mg/L) 0.011 0.013 0.009 0.007 0.002** 0.012

pH 8.62 8.58 8.67 8.42* 8.17* 8.50*

aValues are geometric means for elemental concentrations and arithmetic mean for pH. *,*,*** indicates differences between values for Region I and II are statistically significant at the 95%, 99%, and 99.9% confidence intervals.

Tentative Conclusions• Arsenic contamination of groundwater in south Asia

is not confined to the Ganges-Brahmaputra floodplain.

• The dominant reductive-dissolution model is not supported by the geochemical data in the Himalaya.

• Arsenic contamination of groundwater in the Himalaya results from accelerated erosion.

• Arsenic contamination of groundwater in the Himalaya cannot be addressed without addressing the problems of deforestation and rangeland degradation.

Acknowledgements

• Center for Engaged Learning (UVU)• Presidential Faculty Scholarly Activities Award

(UVU)• Department of Earth Science (UVU)• Institute for Professional Engagement (UVU)• Student Scholarly and Creative Opportunities

(SCOP) Award (UVU)• Student Scholarly Activities Committee (SAC)

Award (College of Science and Health, UVU)

Thanks for listening! Any questions?