THE SENSITIVITY OF HEADWATER STREAMS IN THE HINDU KUSHHIMALAYAS TO ACIDIFICATION

ALAN JENKINSCentre for Ecology and Hydrology, Wallingford, Oxon, U.K.

(∗ author for correspondence, e-mail: JINX@ceh.ac.uk, fax: +44 1491 692340)

(Accepted 12 February 2001)

Abstract. The mountain ecosystems of the Hindu Kush Himalayas are generally considered to bepristine despite little available data to confirm it. Is there any evidence of anthropogenic disturbanceand how sensitive are these systems likely to be to changes in atmospheric deposition and climatechange? A total of 797 headwater streams across the HKH were sampled between 1992 and 1996.The chemistry database provides a reference against which future changes can be measured. Thechemistry is controlled by bedrock geology and waters are well buffered with respect to acidity. Nev-ertheless, some 30% of streams sampled are ‘sensitive’ to acidification and despite their remoteness,evidence of atmospheric N deposition was detected.

Keywords: atmospheric deposition, geology, Himalayas, stream chemistry

1. Introduction

The mountain ecosystems of the Hindu-Kush Himalayas (HKH) are remote fromsources of atmospheric pollution or marine influence and their remoteness haslargely protected them from direct anthropogenic disturbance. Their diverse geo-logy and altitudinal and climatic ranges also provide markedly different biogeo-chemical regimes. This environment is generally considered, therefore, to be pristi-ne although is potentially sensitive to change from anthropogenic activity (Eck-holm, 1975). In particular, there has been speculation about the effects of humanactivity, in the face of a rapid regional population growth, for over two decadesin terms of major land use change especially deforestation (Collins and Jenkins,1996). More recently, concern has arisen over the possible impacts of air pollutionand climate change. This latter concern is, however, largely supposition since littlehard evidence exists on the chemical characteristics of the region with which tocharacterise the current chemical status and against which any future changes canbe assessed. The biodiversity that these ecosystems support, however, is globallysignificant in view of pronounced endemism, physico-chemical heterogeneity andlocation at the borders of the Oriental and Palearctic biological realms (Ormerodand Juttner, 1996). This importance extends into Himalayan rivers which holdthe world’s most species-rich associations of some groups (Dudgeon, 1992; Or-merod et al., 1995). Himalayan streams, therefore, provide a unique opportunity

Water, Air, and Soil Pollution: Focus 2: 181–189, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

182 A. JENKINS

Figure 1. The location of the 10 regional surveys undertaken in India and Nepal. The Likhu Kholaidentifies the location of the deposition collector referred to in the text.

to determine the controls on chemical status and to determine the sensitivity toatmospheric pollution.

This article presents chemical ‘baseline’ data describing the status of headwa-ter streams of the HKH, surveys of 10 regions in Nepal and NE India conductedbetween 1992 and 1996 and totaling 797 samples.

2. Methods and Study Site

The regions were chosen to represent an E-W transect across the HKH (Figure 1).The surveys generally followed major river valley systems draining through theMiddle Hills and High Himalaya. The 10 surveys were; Roopkund (1995, numberof samples n = 70), Pindar (1995, n = 81), Simikot (1994, n = 61), Dunai (1994,n = 82), Annapurna (1992, n = 79), Manaslu (1996, n = 80), Langtang (1992,1994, 1995, n = 55, 25, 34, respectively), Everest (1992, n = 64), Makalu (1994,n = 86) and Kangchenjunga (1996, n = 80). Samples were collected from first and

HIMALAYAN STREAM WATER CHEMISTRY 183

second order streams and were conducted during low flow conditions in either pre-or post-monsoon seasons. The catchments they drained were characterised by awide range of natural vegetation types which were closely related to altitude. Fromapproximately 1000–1700 m pine forests predominate; between 1700 and 3000 mmixed forests of oak, rhododendron and fir predominate; in the sub-alpine zonebetween 3000 and 3800 m sparse cover of fir and birch predominates; and, in thealpine zone above 3800 m, above the tree line and below the limit of permanentsnow and ice cover, juniper and rhododendron shrubs are mixed with grassland.The altitudes of the stream sampling points ranged from 350 to 4900 m. Noneof the streams were significantly affected by agricultural use nor were they feddirectly by glacial meltwater.

Bedrock geology across the HKH is essentially comprised of west to east ori-ented structures in four main lithological groups; (i) limestone, sandstone andshales of Late Cambrian to Cretaceous age; (ii) gneisses, quartzites and marblesof the Higher Himalayan Crystalline group; (iii) dolomites, marbles and phyllitesof the Kuncha and Nuwakot groups; and, (iv) phyllites, schists and gneisses of theKuncha group. In general, the highly calcareous rocks (limestone and dolomite)dominate in NE India and W Nepal and silicious metamorphic and igneous rocks(gneisses, quartzites, phyllites and schists) dominate in E Nepal (Figure 2).

At each stream, water was collected in a 100 mL syringe and filtered imme-diately through a sterile 0.45 µm cellulose-ester membrane into two 60 mL highdensity polyethylene bottles. One bottle was acidified immediately to 1% usingconcentrated AristaR nitric acid. Samples were stored in the dark and transpor-ted to the U.K. and analysed 4–6 weeks after collection. Electrical conductivity,temperature and pH were measured directly in the stream using portable meters.

In the laboratory, the acidified sample was analysed for Ca, Mg, Na, K, Si,Sr, Ba, Fe, Mn, SO4 and Al using inductively coupled plasma optical emissionspectrometry (ICP-OES). The un-acidified sample was analysed for NO3, F, Cland PO4 using colorimetric procedures. To give an indication of weak acid anionconcentration, acid neutralizing capacity (ANC) was calculated (Hemond, 1990).Given the circum-neutral pH and low flow conditions at the time of sample collec-tion the contribution from dissolved organic acids to the ANC of these waters willbe minimal and it is assumed that the major weak acid anion is bicarbonate. Criticalloads have been calculated for N and S using the Steady State Water Chemistrymethod (Henriksen et al., 1992).

3. Results and Discussion

It is assumed that surveys carried out in different years are comparable since themain variability in stream chemistry results from the strong relationship with rain-fall and runoff and these are almost uniquely seasonal with the greatest proportionof rainfall falling during the monsoon period (Collins and Jenkins, 1996). To assess

184 A. JENKINS

Fig

ure

2.A

sim

plifi

edge

olog

ical

map

ofN

epal

.Thr

eeof

the

stud

yre

gion

sar

ein

dica

ted

tode

mon

stra

teth

esa

mpl

ing

regi

me

whi

chfo

llow

edm

ajor

rive

rva

lley

s.

HIMALAYAN STREAM WATER CHEMISTRY 185

TABLE I

Mean chemistry characteristics of the Langtang region surveyed in 1992 (L1), 1994(L2) and 1995 (L3)

L1 (n = 55) L2 (n = 25) L3 (n = 34)

Mean SD Mean SD Mean SD

Cl 0.31 0.46 0.17 0.17 0.36 0.36

NO3-N 0.09 0.13 0.00 0.06 0.19 0.17

SO4 7.20 6.00 6.08 4.21 5.83 3.98

Na 2.25 1.21 1.69 0.79 1.74 0.77

K 1.52 1.18 1.20 1.25 1.62 1.68

Mg 1.57 2.11 1.85 3.43 2.55 3.89

Ca 7.47 6.17 6.55 6.17 8.50 8.53

Si 4.27 1.90 3.55 1.43 3.75 1.35

Sr 22.00 17.00 16.00 8.00 23.00 18.00

Ba 5.00 6.00 5.00 7.00 7.00 10.00

Fe 15.00 19.00 29.00 41.00 24.00 39.00

F 65.00 68.00 64.00 54.00 74.00 54.00

pH 7.35 0.45 7.95 0.45 7.74 0.59

Cond 90.00 58.00 58.00 53.00 69.00 64.00

Number of streams sampled = n. Standard deviation = SD.Units are mg L−1 for major anions and cations, µg L−1 for metals and µS cm−1

for Electrical Conductivity (Cond). Significant differences in true means (ANOVA) areidentified for Na in 1992 (significance = 95%), pH in 1992 (significance = 99.9%) andNO3-N in 1995 (significance = 99%)

year-to-year variability, streams in one region, the Langtang, were sampled in threeseparate years. Statistical analysis of the stream chemistry data (Table I) indicatesthat generally the major ion chemistry is highly consistent from year to year. Theonly significant differences are in NO3 in 1995, in pH in 1992 and in Na in 1992but these may well result from differences in the total number of samples taken ineach survey.

In the dry periods before and after the monsoon, the major source of ionsin streamwater is from chemical weathering. The supply of protons required forweathering reactions in Alpine environments depends mainly on the dissolution ofCO2 and oxidation of pyrite (Reynolds et al., 1998). These processes also accountfor the dominance of HCO3 in over 90% of the streams sampled (Figure 3) andfor relatively high SO4 concentrations in the absence of S deposition from theatmosphere. Chloride provides less than 20% of anion charge in all but 0.1% ofstreams and reflects the absence of marine influence on rainfall inputs. Calciumand Mg dominate the cation charge and the relative importance of each reflects theregional bedrock geology.

186 A. JENKINS

Figure 3. Ionic relationships for all samples collected. Different symbols reflect streams on differ-ent geology; circles represent calcareous limestone; squares represent dolomitised limestone andtriangles represent igneous or highly metamorphosed facies.

Stream base cations closely follow the west to east geological differences (Fig-ure 4) reflecting the different weathering processes with carbonate dissolution inthe west and silicate weathering in the east. The differences in supply of ions fromweathering processes (ie weathering rate) is well demonstrated by the strong W–Evariation in electrical conductivity (Figure 4) with very dilute waters occurring inthe eastern regions as a result of lower weathering rates. The regional distributionof SO4 (Figure 4) shows very high concentrations in the centre of the study regionwhich coincides with the limestone, sandstone, shale geology and indicates thepossibility of local occurrence of gypsum. Nitrate concentrations are uniformlylow across all regions, but is nevertheless found in a majority of samples. Thesource of this N is presumably atmospheric deposition and this supposes that thebiological systems are incapable of retaining all of the deposited N, despite thedeposition being an assumed small flux. Calculated ANC (Figure 4) reflects the

HIMALAYAN STREAM WATER CHEMISTRY 187

Figure 4. Distributions of major ion chemistry and critical loads (S+N) for each region from Westto East across the Himalayas (critical loads for Roopkund and Pindar could not be calculated). Thebox delineates the 25 and 75 percentiles and the whiskers the 5 and 95 percentiles. Units are mg L−1

except for ANC (µeq L−1) and critical load (keq ha−1).

188 A. JENKINS

regional pattern in base cation concentration and highlights the potential sensitivityto S and N deposition in the eastern regions dominated by gneissic, quartzitic andphyllitic bedrock. The well-buffered nature of the streams is reflected in pH whichis rarely observed below 6.5, even in the more acid sensitive areas (Figure 4).

In general, critical loads are very high, especially compared with those for acid‘sensitive’ regions in NW Europe (Figure 4), and the eastern regions have lowercritical loads. Low critical loads indicate lower weathering rate and so in generalfollow patterns of ANC and conductivity. In total only ca. 10% of all streams havea critical load of less than 2 keq ha−1. This figure, however, is significant given thatdeposition fluxes of S and N have been estimated for a valley close to Kathmanduin the Middle Hills as being each of the order of 1 keq ha−1. In the more remoteregions to the E of Kathmandu, the deposition flux is likely to be lower but givenexpected increased S and N deposition to this whole region in the future (Galloway,1989) a potential for ecosystem change exists. It is in these more dilute and lesswell buffered streams that potential ecological impacts from acidic deposition aremost likely if emissions of S and N in the wider region of SE Asia rise in line withpredictions.

4. Conclusion

The data indicate that streams in the HKH region are generally well buffered withrespect to acidity and are not affected by atmospheric pollution. From the observedchemistry, evidence of atmospheric anthropogenic S deposition is not apparentsince contributions from weathering are high. On the other hand, NO3 is measuredconsistently in all regions although at a low level and this indicates that even theseremote areas are currently receiving N deposition. Continued deposition of S andN to the more acid sensitive areas in the east of the HKH may lead to chemicalchanges towards higher acidity in the future.

Acknowledgements

This work was funded in part by the USEPA, the Overseas Development Agency(now the Department for International Development) and the U.K. Department ofthe Environment, Transport and the Regions under the Darwin Initiative. It hasnot been subjected to peer and/or policy review by the USEPA and so does notnecessarily reflect the views of the USEPA.

References

Collins, R. P. and Jenkins, A.: 1996, ‘The impact of agricultural land use on stream chemistry in theMiddle Hills of the Himalayas, Nepal’, J. Hydrol. 185, 71–86.

HIMALAYAN STREAM WATER CHEMISTRY 189

Dudgeon, D.: 1992, ‘Endangered ecosystems: A review of the conservation status of tropical Asianrivers’, Hydrobiologia 248, 167–191.

Eckholm, E.: 1975, ‘The deterioration of mountain environments’, Science 189, 764–770.Galloway, J. N.: 1988, ‘Atmospheric acidification: Projections for the future’, Ambio 18, 161–166.Hemond, H. F.: 1990, ‘Acid neutralising capacity, alkalinity and acid-base status of natural waters

containing organic acids’, Environ. Sci. Technol. 24, 1486–1489.Henriksen, A., Kämäri, J., Posch, M. and Wilander, A.: 1992, ‘Critical loads of acidity: Nordic

surface waters’, Ambio 21, 356–363.Ormerod, S. J. and Juttner, I.: 1996, ‘Catchment Sustainability and River Biodiversity in Asia: A

Case Study from Nepal’, in D. M. Harper and A. Brown (eds), Sustainable Management andTropical Catchments, John Wiley, London.

Ormerod, S. J., Baral, H. S., Brewin, P. A., Buckton, S. T., Juttner, I., Rothfritz, R. and Surren, A.M.: 1996, ‘River Habitat Surveys and Biodiversity in the Nepal Himalaya’, in P. J. Boon and D.L. Howell (eds), Freshwater Quality: Defining the Indefinable, HMSO, Edinburgh

Reynolds, B., Jenkins, A., Chapman, P. J. and Wilkinson, J.: 1998, ‘Stream Hydrochemistry of theKhumbus Annapurna and Langtang Regions of Nepal’, in R. Bando and G. Tartini (eds), Topof the World Environmental Research: Mount Everest Himalayan Ecosystem, Ecovision WorldMonograph series, Backhuys Publishers, Leiden, The Netherlands, pp. 123–141.