major, minor, and trace element chemistry of surface...

Biogeochemistry of Seasonally Snow-Covered Catchments (Proceedings of a Boulder Symposium July 1995). IAHS Publ. no. 228, 1995. 405

Major, minor, and trace element chemistry of surface waters in the Everest region of Nepal

B. REYNOLDS Institute of Terrestrial Ecology, Bangor Research Unit, UWB, Deiniol Road, Bangor, Gwynedd LL57 2UP, UK

P. J. CHAPMAN The Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB9 2QJ, UK

M. C. FRENCH Institute of Terrestrial Ecology, Monks Wood Experimental Station, Abbots Ripton, Huntingdon PE17 2LS, UK

A. JENKINS Institute of Hydrology, Wallingford, Oxfordshire 0X10 8BB, UK

H. S. WHEATER Department of Civil Engineering, Imperial College, London SW7 2BU, UK

Abstract The chemistry of water samples collected during the premonsoon dry season of 1992 from streams, lakes, glacial meltwaters, and springs in the vicinity of Mount Everest in eastern Nepal is reported. Surface waters had a low conductivity (mean 38 ptS cm4), were dominated by Ca2+, S04

2"-S and HC03" with Mg2+, Na+, K+, N03-N, Si, and Al present at lower concentrations. Lithium, Cr, Rb, Sr, Cs, Ba, Pb, and U were in sufficient abundance to be determined quantitatively, but concentrations were generally low compared to global "average" values. Solute concentrations were spatially variable reflecting the complexity of the geology and the presence of moraines containing a variety of rock fragments. Whilst alumino-silicate weathering was the main source of dissolved cations and Si, the relative abundance of divalent and monovalent base cations implies that carbonates exert an important control on surface water chemistry. Within the Dudh Khosi valley, streams draining catchments dominated by forest and agriculture contained more Ca2+, S04

2"-S, K+, and Mg2+ than high-altitude headwaters, probably reflecting the effects of terracing and cultivation.

INTRODUCTION

In order to understand and predict the effect of changing environmental conditions upon the natural processes that control surface water composition, there is a requirement to study pristine catchments, unaffected by atmospheric pollution and man's activities (Brans et al., 1991). Such work is now more urgent given that concentrations of N03" and S04

2" have been observed in snow from remote, high-altitude Himalayan glaciers

406 B. Reynolds et al.

which are comparable to those measured in "acidified" areas of northwest Europe (Mayewski etal., 1986; Nijampurkar etal, 1993). Furthermore, the emission, long-range transport and deposition of pollutants is likely to increase with future industrialization of developing parts of Asia (Galloway, 1988).

In the remote parts of the Nepal Himalaya, attention on the aquatic environment has focused mainly on lakes (e.g. Loffler, 1969; Aizaki etal., 1987). Relatively little work has been undertaken on moving waters (Jenkins et al., in press), although maj or lowland rivers in the Kathmandu valley have been studied (Upadhyaya & Roy, 1982; Tiwari & Ali, 1987). This paper presents data for major, minor, and trace elements in a range of high-altitude surface waters within the Khumbu region of the Himalaya, Nepal.

STUDY AREA

The sampling area was located within the Khumbu Valley in the Everest region of the Himalaya in eastern Nepal (Fig. 1). The work was based around the Ev-K2-CNR Pyramid laboratory, which was established in 1990 close to the Lobuche glacier at an altitude of 5050 m. The area is dominated by rock, glaciers, and permanent snowfields with the climatic snow line currently at about 5700 m (Bâumler & Zech, 1994). Vegetation is sparse and consists of alpine turf and shrubs. There are seasonal settlements at Lobuche and Gorak Shep, supported by subsistence farming and tourism.

Fig. 1 Location map of the Khumbu glacier in eastern Nepal.

Chemistry of surface waters in the Everest region of Nepal 407

The sampling region lies in a geologically complex transition zone between the High Himalaya and Tibet. The main lithological units in the region consist of: (1) Black Gneiss (a series of biotite-sillimanite paragneiss), (2) Everest metapelites (comprising pelitic and calcareous metasediments), and (3) the Island Peak Complex (a biotite paragneiss associated with augen gneiss and quartzites) (Bortolami et al., 1976). Late Tertiary granite outcrops are also present in the mountains to the east of the Khumbu Glacier.

The climate of Nepal is dominated by the monsoon which occurs from June to September and falls as snow in the study area. Snowfall may also occur in the winter between December and February.

METHODS

Water samples were collected from first-order streams, springs, lakes, and open meltwater streams flowing within and from glaciers and permanent snowfields. Sampling took place between 10 May and 4 June 1992, near the end of the dry season and prior to the onset of the monsoon or any noticeable thaw of ice sheets.

At each sampling site stream water temperature and conductivity were recorded using portable hand-held meters and two samples were filtered through sterilized 0.45-/xm pore size membrane filters using a 50-ml syringe and filter holder into 60-ml polypropylene bottles. One of the 60-ml water samples was used for the determination of major and minor anions (CI", N03-N, S04

2-S) and cations (Na+, K+, Mg2+, Ca2+, NH4

+-N) by ion chromatography (IC) using a Dionex system equipped with an lonPac AS9 column set up in the Pyramid laboratory (Tartari et al., 1993). This system typically gave detection limits of 4, 1, and 4 /xg l"1 respectively for CI", N03"-N, and S04

2"-S (Tartari et al., 1993). The other 60-ml sample was stored and returned to the UK where Si was determined on an unacidified aliquot by the Si-molybdenum blue method using a continuous flow autoanalyzer. The remainder of the sample was acidified with concentrated AristaR nitric acid and scanned by ICPMS for the presence of a wide range of trace metals of which Li, Cr, Rb, Sr, Cs, Ba, Pb, and U were in sufficient abundance to be determined quantitatively. Total filtrable aluminium (Al(tot)) was also determined on the acidified sample by graphite furnace atomic absorption spectrophotometry.

Unfortunately, the determination of pH and alkalinity (by Gran titration) had to be abandoned due to problems of instrument instability and rapid degassing of C02 from the samples leading to a rapid upwards drift in pH. However in order to give an indication of weak acid anion concentrations, charge balance alkalinity (CBALK; Hemond, 1990) has been calculated as:

(Na+ + K+ + Ca2+ + Mg2+) - (S042" + N03" + CI") in Meq l"1

Given the circumneutral pH (determined on a few samples) and low flow conditions when the samples were collected, the contribution from dissolved organic acids to the CBALK of these waters will be minimal. It is assumed, therefore, that the major weak acid anion contributing to the CBALK is bicarbonate.


RESULTS

The chemical data are summarized by water type in Tables 1 and 2 as arithmetic means for major ions, conductivity, and CBALK and means of log transformed data for Al(tot) and the trace metals. Where trace metal concentrations were below the limit of detection, a value midway between zero and the detection limit was inserted to avoid the problem of log transforming zero values.

The waters are dilute with an overall mean conductivity of 38 piS cm"1 (corrected to 25°C; Cond25), and are dominated by Ca2+ and S04

2~-S (Fig. 2). The high CBALK values indicate that HC03 was the other major anion. Concentrations of N03"-N and CI" were low (ca. 0.1 mg l"1) and ammonium was undetected in all samples. For the whole data set, conductivity was a good predictor of the sum of base cations + acid anions (SBCAA) and the sum of (Ca2+ +Mg2+) such that:

SBCAA (meq l"1) = 0.04 + 0.011Cond25 (r2 = 0.811; N = 27)

and

(Ca2+ + Mg2+ meq l"1) = 0.025 + 0.007Cond25 (r2 = 0.748; N = 27)

Trace metal concentrations were generally close to or below "global" average values, with Pb, Cs, and Cr being detected (i.e. >0.01 pg l"1) in less than 50% of the samples. Uranium was present in all samples, with unusually high concentrations (20 and 41 ^g i"1) observed in two glacial meltwater streams. The uranium content of fresh waters is very variable, but is probably in the range of 1-4 ^g l"1 (Wedepohl, 1978) in

Table 1 Mean and range of solute concentrations (/ig l"1), CBALK (jtteq l"1) and conductivity (/tS cm"1) in stream and lake waters in the Everest region of Nepal.

Na+

K+

Ca2+

Mg2 +

N03--N cr so4

2--s Si Al(tot) Sr Ba Li Cr Rb Cs Pb U CBALK Cond25

Mean

880 630

5380 350 120 120

2100 1870

43.9 8.9 0.8 1.3 0.02 1.5 0.02 0.01 1.30

222 39

Streams (14 samples)

M in

340 230

2440 110 40 70

740 1110

5.4 4.2

<0.1 0.2

<0.01 0.5

<0.01 <0.01

0.01 86 19

Max

1610 1020 8980 520 290 180

3710 2850 807

17.9 5.8 9.4 1.60 4.2 1.18 0.55

14.09 347

57

Mean

470 540

4360 330 90 90

1840 950 40.8 7.0 0.7 0.5 0.03 1.7 0.02 0.02 0.48

158 29

Lakes (4 samples)

Min

300 350

3370 210 30 70

1140 380

5.4 5.0 0.3 0.2

<0.01 1.0

<0.01 <0.01

0.12 129 20

Max

740 860

5750 460 170 110

2930 1610 296

9.9 2.3 1.2 0.73 2.8 0.14 0.08 2.19

185 43


Table 2 Mean and range of solute concentrations (>g l"1), CBALK (fteq l"1) and conductivity (ftS cm"1) in springs and glacial meltwater streams in the Everest region of Nepal.

Na+ K+

Ca2+

Mg2+

NO3--N ci-so4

2--s Si Al(tot) Sr Ba Li Cr Rb Cs Pb U CBALK Cond25

Sprin;

Mean

1040 650

5700 560 160 90

2360 2880

15.2 9.4 1.7 0.9 0.06 1.1 0.01 0.03 0.86

232 42

gs (3 samples)

M in

710 510

5390 490 150 70

1520 1970

1.0 7.6

<0.1 0.3

<0.01 0.4

<0.01 <0.01

0.60 179 40

Max

1610 740

5870 670 170 110

2870 4010

404 10.7 9.1 2.5 0.43 2.5 0.04 0.71 1.77

322 43

Glacial meltwater (6 :

Mean

610 690

5350 340 100 80

1610 1100 446

11.8 1.9 3.4 0.16 3.2 0.12 0.05 3.98

253 42

Min

210 470 520 180 30 70

160 540

2.0 5.4 0.4 0.8

<0.01 1.0

<0.01 <0.01

0.27 156 24

samples)

Max

1560 980

8500 430 210 140

2740 2650 915 26.1 4.7 8.9 1.55 6.1 1.34 0.89

41.2 405 58

the absence of mineral deposits. Whilst the source of the U in these samples is unknown, the uranyl ion is strongly complexed by the carbonate ion, which may enhance U mobility in these waters.

Although the data vary widely, lake waters generally had the lowest average solute concentrations of the four water types (Tables 1 and 2). Concentrations of Na2+, Ca2+, Mg2+, and Si were larger in spring waters compared to other waters, but only for Mg2+

and Si was this relationship statistically significant (p < 0.05 and p < 0.01, respectively, one-way ANOVA & F test). Trace metal concentrations were greatest in glacial meltwaters, but this observation was statistically significant only for Rb (p < 0.01, one-way ANOVA & F test). Many glacial meltwater samples contained a high particulate load some of which passed through 0.45 fim filters. Subsequent filtration of these samples through 0.22 jtim filters reduced Al(tot) concentrations by over 80% indicating a strong association between Al(tot) and colloidal particles which may also have influenced the observed trace metal concentrations. Unfortunately, the samples filtered through 0.22-/xm filters were not analyzed for trace metals.

The study was undertaken during a dry period with low atmospheric inputs and low stream flows so that the major source of cations and Si to surface waters was from chemical weathering. In the absence of significant atmospheric deposition of acid pollutants, the supply of protons required for weathering reactions in Alpine glacial environments depends mainly on the dissolution of C02 and oxidation of pyrite, processes which account for the dominance of HC03" and S04

2" amongst the anions in solution (Tranter et al, 1993). The granites and gneisses of the area typically contain less than 1 % Ca2 + and Mg2 + and between 3 and 3.5 % of Na+ and K+ (Bortolami et al., 1983). On an equivalent basis, base cation concentrations in the water samples do not


Base Cations Concentration (ueq/L)

100 200

Streams

Lakes

Springs

Glacial streams

§^$S5St£ïSS

WSSSIÊ

ffiT™*,™"a^

^ Ca ++ £H Mg ++ • Na + ^ K +

Strong Acid Anions Concentration (ueq/L) 50 100 150

Glacial streams p

3S04-- «BN03- • C l -

Fig. 2 Mean concentrations of base cations and strong acid anions in surface waters of the Everest region of Nepal.

reflect this, with Ca2+ > > Mg2+ = Na+ > K+ (Fig. 2). This suggests that Na+ and K+ occur in minerals which are relatively resistant to weathering compared to the mineral sources of Ca2+ and Mg2+. The main sources of dissolved Na+ and K+ from within the granites and gneisses will be albite and orthoclase feldspars together with biotite and muscovite micas. The latter may also be a source of Li. In addition to alumino-silicate minerals, carbonates identified in a number of geological formations within the region, will provide a readily weatherable source of Ca2+ and Mg2+. Carbonates occur in (1) the Everest metapelites located on the higher parts of the main peaks (Everest, Lhotse and Nuptse) at the head of the Khumbu glacier (Bortolami et al., 1976) (2) the so-called "Yellow Band" of Lhotse which contains marbles, and (3) rocks containing calcite produced by contact metamorphism of the Everest metapelites by the granites. These metamorphosed rocks are found in the southern part of the Everest region, and fragments are particularly common in the moraines (Bortolami et al., 1983). Potassium may also be less geochemically mobile than the other base cations, since it is both a major plant nutrient and it is readily incorporated into the inter-layer structure of clay minerals such as illite, which is abundant in soils of the region (Bàumler & Zech, 1994). Despite the apparent influence of carbonates on water chemistry, average Sr/Ca ratios for streams, lakes and springs (ca. 1.6 X 10"3) are more characteristic of drainage waters from igneous and metamorphic rocks than for carbonate terrain, where ratios are typically greater than 5 X 10 3 (Wedepohl, 1978). Glacial meltwaters had a higher mean


Sr/Ca ratio of 2.2 X 10"3, possibly reflecting the glacial transport of carbonate bearing rocks from the high peaks in the north of the area.

The geological complexity of the region and the effects of glaciation both contribute to the high degree of variability in the data. Although this has obscured statistically significant differences between water types, the results from this survey were generally consistent with those from other work in the area. The lake data in Table 3 were collected in the postmonsoon period, and differences in chemistry may reflect seasonal factors such as increased biological activity in warmer waters and dilution by surface monsoon runoff. However, this is a very tentative hypothesis given the large variability in the data sets.

The data from this survey can also be compared with information gathered during a more extensive survey of the Dudh Khosi catchment (see Jenkins et al., in press) of which the Everest region comprises the northern most part. Samples for the Dudh Khosi survey were collected from first and second order tributary streams within an altitude range of 1400 to 3500 m during February to March 1992 between the winter and summer monsoon periods. The tributary catchments have been classified according to the dominant landscape type namely alpine rock/ice, scrub/grazing, mixed forest, and terraced agricultural land and summarized stream chemistry data for each landscape type are presented in Table 4. The streams draining catchments dominated by rock and scrub in Table 4 contained less Ca2+ and S04

2~-S but more Na+ compared to the high-altitude streams in the Everest region (Table 1), presumably reflecting regional differences in geology. Within the Dudh Khosi survey, larger mean concentrations of Ca2+, S04

2"-S,

Table 3 Mean and range of solute concentrations (ng l"1) and conductivity (ixS cm"1) in nine lakes in the Everest region of Nepal (Data from Gosso et al., in press).

Mean Min Max

Na+ 370 K+ 510 Ca2+ 3650 Mg2+ 230 N03"-N 70 CI" 350 S04

2--S 990 Cond-,, 21

Table 4 Mean solute concentrations (/xg 1"') in streams draining different landscape types within the Dudh Khosi catchment of Nepal.

20 270

2000 60 10 70

180 11

920 860

6310 440 100

1420 2350

34

Na+

K+

Ca2+

Mg2+

NO,--N so4

2-s Si No. of samples

Alpine rock

1700 500

1750 450 150

1450 1800

2

Scrub/grazing

1840 500

1470 340

80 1100 2190

7

Forest

1690 770

3930 1020

90 2600 1970

37

Agriculture

1860 1100 3970 1420 100

2320 2110

13


K+, and Mg+ were observed in streams flowing from catchments dominated by forest and agriculture. These results are consistent with the anticipated effects on water quality of cultivation and mineral fertilizer additions associated with terraced agriculture in the region, but without detailed geological, soil, and landuse data at the appropriate catchment scale, such inferences are highly speculative. However, the chemistry data from both surveys demonstrate that the streams are well buffered against any potential increase in acidic deposition, and that levels of nitrate leaching are currently low.

Acknowledgments The authors would like to thank the staff of Project Ev-K2-CNR in Kathmandu and Milan for their assistance with the organization of the visit to the Pyramid. We would particularly like to thank G. P. Verza, M. Rossi, and the Sherpas at the Pyramid for their support throughout our visit. The authors are also grateful to S. Cavalli of Dionex (Italy) for providing valuable technical assistance with the Dionex ion chromatograph. The work was part funded by the CEC under Grant no. CI1 *CT900855.

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