Chemical weathering in the plain and peninsular sub-basins of the Ganga: Impact on major ion chemistry and elemental fluxes

Available online at www.sciencedirect.com

Geochimica et Cosmochimica Acta 74 (2010) 2340–2355 www.elsevier.com/locate/gca

Chemical weathering in the plain and peninsular sub-basins of the Ganga: Impact on major ion chemistry and elemental fluxes Santosh K. Rai 1, Sunil K. Singh *, S. Krishnaswami Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India Received 27 April 2009; accepted in revised form 4 January 2010; available online 13 January 2010

Abstract Concentrations of major ions, Sr and 87Sr/86Sr have been measured in the Gomti, the Son and the Yamuna, tributaries of the Ganga draining its peninsular and plain sub-basins to determine their contribution to the water chemistry of the Ganga and silicate and carbonate erosion of the Ganga basin. The results show high concentrations of Na and Sr in the Gomti, the Yamuna and the Ganga (at Varanasi) with much of the Na in excess of Cl. The use of this ‘excess Na’ (Na* = Nariv  Clriv) a common index of silicate weathering yield values of 18 tons km2 yr1 for silicate erosion rate (SER) in the Gomti and the Yamuna basins. There are however, indications that part of this Na* can be from saline/alkaline soils abundant in their basins, raising questions about its use as a proxy to determine SER of the Ganga plain. Independent estimation of SER based on dissolved Si as a proxy give an average value of 5 tons km2 yr1 for the peninsular and the plain drainages, several times lower than that derived using Na*. The major source of uncertainty in this estimate is the potential removal of Si from rivers by biological and chemical processes. The Si based SER and CER (carbonate erosion rate) are also much lower than that in the Himalayan sub-basin of the Ganga. The lower relief, runoff and physical erosion in the peninsular and the plain basins relative to the Himalayan sub-basin and calcite precipitation in them all could be contributing to their lower erosion rates. Budget calculations show that the Yamuna, the Son and Gomti together account for 75% Na, 41% Mg and 53% Sr and 87 Sr of their supply to the Ganga from its major tributaries, with the Yamuna dominating the contribution. The results highlight the important role of the plain and peninsular sub-basins in determining the solute and Sr isotope budgets of the Ganga. The study also shows that the anthropogenic contribution accounts for 610% of the major ion fluxes of the Ganga at Rajmahal during high river stages (October). The impact of both saline/alkaline soils and anthropogenic sources on the major ion abundances of the Ganga is minimum during its peak flow and therefore the SER and CO2 consumption rates of the river is best determined during this period. Ó 2010 Elsevier Ltd. All rights reserved.

1. INTRODUCTION The suggestion that silicate weathering in young orogenic belts such as the Himalaya is a key driver of climate change over million year time scales (Walker et al., 1981; Raymo and Ruddiman, 1992; Ruddiman,

* Corresponding author. Tel.: +91 7926314307; fax: +91 7926314900. E-mail address: [email protected] (S.K. Singh). 1 Present address: Wadia Institute of Himalayan Geology, Dehradun, Uttarakhand 248 001, India.

0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.01.008

1997) led to a number of studies on the chemical and isotopic composition of two major global river systems draining the Himalaya, the Ganga and the Brahmaputra (Sarin et al., 1989, 1992; Krishnaswami et al., 1992, 1999; Palmer and Edmond, 1992; Edmond and Huh, 1997; Harris et al., 1998; Galy and France-Lanord, 1999; Dalai et al., 2002; Singh and France-Lanord, 2002; Bickle et al., 2003, 2005; Singh et al., 2006; Tipper et al., 2006; Hren et al., 2007; Rai and Singh, 2007). These studies provided estimates of contemporary silicate weathering rates and associated atmospheric CO2 drawdown in their basins. Between these two rivers, the Ganga is investigated in relatively more detail, with many of the studies focusing on the Himalayan

Chemical weathering in the Ganga plain

sub-basins of the Ganga and compare them with erosion in its Himalayan sub-basin. Further, attempts also have been made to assess the impact of saline–alkaline soils and anthropogenic activities on the water chemistry of the Ganga. This work is based on the major ion composition and Sr isotope systematics of the Gomti, the Son and the Yamuna, tributaries of the Ganga which predominantly drain its plain and peninsular sub-basins and the Ghaghra, the Gandak and the Kosi, its Himalayan tributaries, which have part of their drainage prior to their confluence with the Ganga in the plain (Fig. 1).

sub-basin of the Ganga drainage and a few others covering almost the entire stretch of the Ganga river system, from its origin in the Himalaya to its outflow to the Bay of Bengal (e.g. Sarin et al., 1989; Galy and France-Lanord, 1999; Krishnaswami et al., 1999). The latter studies yielded chemical and silicate erosion rates integrated over the entire Ganga basin spread over the Himalaya, the peninsular India and the Ganga plain, however information on the erosion rates in the plain and peninsular sub-basins, which account for 80% of the area is limited. As a result, the significance of chemical erosion in these sub-basins, particularly the role of various lithologies including saline– alkaline soils in contributing to the fluxes of elements transported by the Ganga to the Bay of Bengal is not well quantified. Further, the influence of anthropogenic activities in the Ganga plain, one of the most densely populated regions of India, on the major ion chemistry of the Ganga and its impact on estimates of silicate erosion rates is also only poorly constrained. Galy and France-Lanord (1999) based on material balance of elemental fluxes transported by the Ganga at its mouth and those supplied to it by its tributaries in the Himalaya, inferred that the chemical erosion rate in the Ganga plain is much lower than that in the Himalaya. West et al. (2002) by comparing weathering fluxes from small Himalayan catchments with the weathering fluxes from the Ganga concluded that more cations are dissolved from the Higher Himalayan silicates in the plains than in-situ in the Higher Himalaya. The aim of the present work is to determine silicate and carbonate erosion rates in the peninsular and plain

0

2. GEOHYDROLOGY OF THE GANGA BASIN The geohydrology of the Ganga particularly that of its headwaters and tributaries in the Himalaya has been discussed in detail in many earlier publications on its water and sediment chemistry (e.g. Sarin et al., 1989; Galy and France-Lanord, 1999; Bickle et al., 2003; Singh et al., 2008). The emphasis in this paper is more on the geohydrology of rivers draining the peninsular basin and the Ganga plain. The characteristics of rivers studied are given in Table 1. 2.1. Hydrological setting The Gomti is a rain fed river with its entire drainage in the Ganga plain. It originates from the reservoir MadhoTanda (Fig. 1; Miankot, 28°340 N, 80°070 E) located 50 km south of the foothills of the Himalaya. It drains

0

76

0

80

0

32

2341

0

84

88

T I

GANGA BASIN

B E

RW03-3

uli

t Se

Tr i s

Ar un

ki

L

and a

A

ndi

0

28

a rsy

ga

P

Madho-Tanda

Ma

E

nda ki

N

gan

DELHI

Ga

Nainital

City/Town

i

iG Burh

m Ra

Yamuna

l Ka

Ka li

Haridwar

Sample site

T

RW03-4 RW03-2

Ka r na li

Rishikesh Tajewala

ri

Bataman di

Gangotri Devprayag

Bh e

RW03-5

BR-342 BR-375 BR06-901 BR06-12-1 BR06-11-1 BR06-801

Allahabad

BR06-10-1

BR06-201

Patna

Varanasi

Ghazipur

Son 0

24

BR-309 BR 06-301

Koilawar

BR-388 BR06-14-1

N

u r Ko si

m

BR06-705 B R06-601 Dumarighat BR-311 BR-327 Hazipur B R06-501

Doriganj

BR -346 BR06-13-1

Ta

Ko

el

B R-315 BR06-401

Baraun i

BR-318 BR06-104

Rajmahal

Farakka

BA N GLA D E SH

Bagmati

G h BR-336 ag hr a

Revilganj

Ko si

Kosi

Su n

BR -363

Lucknow

Ken

B

a etw

Naraing hat

Gorakhpur

mti

k

d

a ni ay

a nd

n Si

ti

Ga

Ya mu na

ga

mb

n Ga

a Ch

al

Go

Nar

i

Ra p

Amarkantak

Fig. 1. Sampling locations of water samples from the Ganga mainstream and its tributaries. Samples were collected all along the Ganga, from its source near Gangotri to its outflow at Rajmahal.

2342

S.K. Rai et al. / Geochimica et Cosmochimica Acta 74 (2010) 2340–2355

Table 1 Physical features and details of rivers of the Ganga system. River

Location

Area 103 km2

Rainfallb mm yr1

Dischargea 1012 L yr1

Runoff mm yr1

Slopeb mm km1

Ganga Yamuna Ghaghra Gandak Kosi Gomti Son Yamuna Ganga Ganga

Rishikesh Tajewala Revilganj Hazipur Dumarighat Ghazipur Koilawar Allahabad Allahabad Rajmahal

21.7 9.6 128 46.3 74.5 30.5 71.3 366 – 935

1000–1600 800–1400 400–1400 600–1600 200–1400 800–1200 800–1200 400–1400 – 200–1600

23.9 10.8 94.4 52.2 62 7.4 31.8 93 59 380

1100 1130 740 1130 830 240 450 250 – 410

2300–45,000 500–30,000 300–33,000 300–35,000 110–38,000 100–550 100–1500 300–30,000 – 60–45,000

a b

From Rao (1975) and Jain et al. (2007); the discharge at Rajmahal (taken same as at Farakka) from http://www.grdc.sr.unh.edu. Based on http://disc2.nascom.nasa.gov/Giovanni/tovas/TRMM_V6.3B43.shtml; Sinha et al. (2005) and Bookhagen and Burbank (2006).

the interfluve sediments between the Ganga and the Ghaghra, prior to joining the Ganga at Udyar Ghat near Ghazipur, 30 km downstream of Varanasi (Fig. 1). Most of the annual water discharge of the Gomti occurs during the south west monsoon. Average annual rainfall in the Gomti basin, during 1997–2006, is 1025 mm. whereas its runoff is only 240 mm yr1. Thus, about 75% of rainfall in the Gomti drainage is either returned to the atmosphere through evaporation/evapotranspiration and/or stored as groundwater. The Yamuna is the largest tributary of the Ganga in terms of drainage area and accounts for 40% of it. Analogous to the Gomti, nearly 75% of the Yamuna discharge also occurs during June–September, the south west monsoon period. The Chambal, the Sind, the Betwa and the Ken are the major tributaries of the Yamuna, all of which originate in and flow almost entirely through peninsular India (Fig. 1), as a result 63% of the Yamuna drainage is in the peninsular basin and 34% in the Ganga plain. Rainfall in the drainage basin of the Yamuna and its runoff show significant spatial variation with high values in the Himalaya and low in the Ganga plain and the peninsular region (Table 1). The Son is another tributary of the Ganga with its drainage predominantly in peninsular India. A part of the Son basin (15% based on global mapper) prior to its confluence with the Ganga is in the Ganga plain. The Son originates from the Amarkantak in the Bundelkhand plateau and merges with the Ganga 25 km upstream Patna (Fig. 1). The headwaters of the Son in the mountainous region have a steep slope compared to its outflow in the Gangetic plain (Table 1). The Son is also a rain fed river with most of its flow during monsoon. In addition to these three tributaries, studies of the Ganga from its source near Gangotri to its outflow near the Indo-Bangladesh border and its major Himalayan tributaries also have been carried out to determine their contribution of major ions and Sr to the Ganga at its out flow. Of the total drainage area of the Ganga up to Farakka, 935  103 km2 (http://www.grdc.sr.unh.edu), 20% lies in the Himalaya, 33% in peninsular India and the rest in the Gangetic plain. The runoff among the different regions of the Ganga basin varies from 1 m yr1 in the

Himalayan drainage (Donald, 1992) to 0.3 m yr1 in the Gangetic plain and peninsular India (Rao, 1975); this difference helps assess the impact of runoff on chemical erosion rates. 2.2. Geological setting The Ganga catchment is spread over the Himalaya in the north, peninsular India in the south and the Gangetic plain between them. The headwaters of the Ganga, its tributaries the Ghaghra, the Gandak and the Kosi have a major part of their drainage in the Himalaya. The Yamuna drains all the three sub-basins whereas the Gomti and the Ganga downstream of Haridwar drain only the Ganga plain. The lithology of the peninsular basin and the Ganga plain is discussed below; for details of the Himalayan basin reference is made to Gansser (1964), Valdiya (1980), Sarin et al. (1989), Bickle et al. (2003) and Singh et al. (2008). The peninsular drainage: The major lithologies exposed in this drainage are the Bundelkhand crystallines (granites), the Vindhyan sediments (carbonates, shales and sandstones) and the Deccan basalts (Krishnan, 1982; Singh et al., 2008). The tributaries of the Yamuna (Chambal, Sind, Betwa and Ken) and the Son though drain peninsular India, they flow through different lithologies. The Yamuna tributaries drain the Deccan Traps and the Vindhyans whereas the Son lies mainly in the Vindhyan–Bundelkhand plateau. The tributaries of the Son drain the Gondwana sedimentary sequences comprising of sandstones, shales and carbonates (Krishnan, 1982). The Ganga plain is a major alluvial tract (Singh, 1996) formed by the accumulation of detritus from the Higher and Lesser Himalaya (Singh, 1996; Galy and France-Lanord, 2001; Singh et al., 2008). It is composed of beds of clay, sand and gravel (Sinha et al., 2005). In addition, the alluvial sediments of both the peninsular drainage and the Ganga plain contain evaporites formed locally from river and floodwaters and ground water through capillary action. The evaporation of these waters precipitates their solutes, with silica and carbonates likely to deposit initially. These carbonates known as “kankar” accumulate as layers or irregular concretions in the soil column. The residual water on subsequent evaporation

Chemical weathering in the Ganga plain

deposits various sodium salts, chloride, sulfate and carbonate leading to the formation of alkaline/saline soils containing up to a few percent of sodium salts (Agarwal and Gupta, 1968; Agarwal et al., 1992; Bhargava and Bhattacharjee, 1982; Chhabra, 1996; Datta et al., 2002; Pal et al., 2003; Srivastava, 2001 Singh et al., 2006a,b; Singh, 2005). The excessive use of river and groundwater for irrigation in recent years has further aggravated the problem (Chhabra, 1996; Singh, 2005). The abundance and composition of these soil salts varies significantly within and among the river basins (Fig. 2) and is generally more towards the western basin due to the conducive semi-arid climate and basin characteristics. For example, they occur extensively in the Gomti basin, in the interfluve region between the Yamuna and the Ghaghra, in the Yamuna basin between Delhi and Allahabad, particularly in and around the confluence of the Chambal and the Yamuna (Fig. 2). Radiocarbon ages of carbonates and kankars of the region (Rajagopalan, 1992) show that they have been depositing episodically, the recent significant deposition occurring around 10,000 years BP coinciding with the cold arid climate (Mohindra, 1995). The presence of saline soils along with these carbonates in the Ganga plain (Mohindra, 1995; Srivastava et al., 1998) seems to indicate that these soils have been developing since this period.

2343

3. SAMPLING AND ANALYSIS Surface water samples from the Gomti, the Son, the Yamuna and the Ganga mainstream between Gangotri and Rajmahal (near Farakka, Fig. 1) and its Himalayan tributaries were collected during three field campaigns conducted in May 2003 and 2004 (summer) and October 2006 (post/tail-end of monsoon). The rivers are generally in their low stages in May and high stages in October. The discharge of the Ganga during October is 12% of its annual flow. After collection the samples were brought to laboratory and two separate aliquots each of 500 mL were filtered using 0.2 lm nylon membrane Millipore filters. One of the filtered aliquot was acidified to pH 2 with double-distilled HNO3 for cation and Sr analysis. The other filtered aliquot was kept unacidified for anion measurements. A separate aliquot of 250 mL of river water was collected and stored as such for alkalinity measurements. Temperature and pH of the water samples were measured at site in two of the three field campaigns following earlier procedures (Dalai et al., 2002). The details of chemical analyses are given in Dalai et al. (2002) and Das et al. (2005). Briefly, alkalinity was measured by acid titration, Cl, NO3 and SO4 by ion chromatography, K and Na by flame AAS, Ca and Mg by ICP-AES and ion

INDIA SALT AFFECTED SOILS

0-4 (Area in sq. Km)

0

4-40

500 KM

40-200 200-400

320

Over 400

G a n g o tr i Alkali Soil

U tta rk a s h i

Saline Soil

R is h ik e s h

si

28

0

m

u r Ko

Tr is A l la h a b a d G h a z ip u r

ti ma

Ta

Kos i

ag hr a

Ko si

g Ba

Gh Ka np ur

k

a

a nd

Ya mu n

Su n mti

Ga

a ng Ga

C

al mb ha

Go

Arun

a rd Sa

D E LH I

uli

Bh e

ri

na li

i

Ka r

Yamuna

l Ka

D u m a r ia g h a t

H a z ip u r P a tn a B a r a u n i D o rig a n j

V a ra n a s i

R a jm a h a l

K o i la w a r So n

24 76

0

80

0

84

0

0

880

Fig. 2. Map of saline/alkaline soils distribution. The map of India modified from the publication of Central Soil Salinity Research Institute (CSSRI, 2007), Karnal, is shown as inset. The detailed map redrawn from Agarwal and Gupta (1968) is for the state of Uttar Pradesh which forms a significant part of the Ganga drainage in the plain. The high abundances of saline/alkaline soils in the stretch between Kanpur and Patna and in the basins of the Gomti and the Yamuna are evident.

2344

S.K. Rai et al. / Geochimica et Cosmochimica Acta 74 (2010) 2340–2355

chromatography and Si by spectrophotometry of its molybdenum blue complex. The precision of these measurements, based on repeat measurements and earlier studies in this laboratory is better than ±5%. Sr was measured in filtered and acidified samples using graphite furnace AAS (May 2003 and 2004) and isotope dilution mass-spectrometry (October 2006). The accuracy and precision for graphite AAS is better than 5%, whereas for mass-spectrometric technique it is better than 0.4%. Sr isotope measurements were made following published procedures (Rai and Singh, 2007; Singh et al., 2008; Rai, 2008). This involved the separation of Sr from a suitable volume of water and the measurement of its isotopic composition using ISOPROBE-T thermal ionization mass spectrometer (TIMS) in static multi-collection mode. The procedural blanks were 2–3 orders of magnitude lower than the total Sr analyzed in the samples, hence the river water data are not corrected for blank. 4. RESULTS The locations of the river water samples are presented in Fig. 1. The temperature, pH, concentrations of major ions, silica, TDS, Sr and 87Sr/86Sr ratio of the Gomti, the Son and the Yamuna are given in Table 2, the data for the Ganga mainstream and its principal Himalayan tributaries are given in Table 3. In many samples from the Himalaya, pH and temperature could not be measured due to failure of the probe. Temperature of waters collected during summer (May 2003 and 2004) falls in two groups, one at 17.5 °C and the other at 31 °C, the cooler ones being the Ganga headwaters. The pH ranges from 7.9 to 8.6, suggesting the alkaline nature of the waters. The major ion data (Tables 2 and 3) show a good balance between total cations (TZ+) and anions (TZ). The NICB, normalized inorganic charge Table 2 Major ions, TDS, Sr and No. River (location)

5.1. General observations The major ion chemistry of the Gomti sampled during both May and October is nearly the same (Table 2) with high concentrations of Na, Mg and Ca and highest TDS among the October samples. In the Yamuna (Allahabad) the May sample has the highest TDS, Na, Mg, Cl and Sr among the samples analyzed (Table 2). The decrease in major ions and TDS in the Yamuna during October is dilution effect caused by monsoon. The impact of this, however is not discernible in the Gomti, a cause for this may be redissolution of salts from the basin formed by evaporation during summer and/or anthropogenic inputs. The major ion composition of the Gomti and the Yamuna measured in this study is similar to that reported by Sarin et al. (1989) and Gupta and Subramanian (1994). Spatial variability in major ions along the course of the Ganga shows high concentration of Na and moderately high values of Mg, Ca and TDS at Allahabad/Varanasi during May (Table 3). For example, Na increases from 106 lM at Rishikesh to 2380 lM at Varanasi. In October, the sampling was done only downstream of Allahabad, which shows that Na is highest at Varanasi. This can result from chemical weathering along the course of the Ganga from Rishikesh to Varanasi and/or due to its supply from

Sr/86Sr in the Gomti, the Son and the Yamuna rivers.

Date (dd.mm.yy)

1 2

BR06-11-1

3

Son (Koilawar) BR06-201 16.10.06

4 5

Yamuna (Batamandi)b RW99-5 June 99 RW98-4 October 98

6

Yamuna (Allahabad) BR-346 14.05.04

7

BR06-13-1 a

5. DISCUSSION

87

Gomti (Ghazipur) BR-375 16.05.04

b

balance, [(TZ+  TZ)/TZ] for the Gomti, the Son and the Yamuna is <3% (Table 2), well within the precision of measurements, which leads to conclude that the ions measured by and large account for the charge balance. This inference also seems valid for the other samples (Table 3), excluding RW03-5 and BR-311 for which TZ exceeds TZ+ by 16% and 13%, respectively, an observation that is difficult to interpret with the present data.

19.10.06

20.10.06

Location (N; E) Na

K

Mg Ca

lM

Cl

NICBa Sr, nM

87

388

2.0

3173

0.72759

NO3 SO4 HCO3 SiO2 TDS, mg L1

Sr/86Sr

25°30.350 ; 83°08.360 25°30.280 ; 83°08.450

1354 127 894 872 400 17

252 4193

1399 130 848 859 324 7

200 4286

92 390

1.5

2717

0.72714

25°33.810 ; 84°47.560

431 43 253 537 116 11

50 1833

161 170

0.3

944

0.72504

73 6 60 35

556 2508 333 2369

193 285 211 254

2.4 6.7

2044 1802

– 0.72356

3575 136 1104 794 1493 0

335 5300

325 574

0.6

5889

0.71467

1275 85 443 673 696 50

181 2575

129 280

2.5

2717

0.71239

306 49 661 967 255 52 497 1019 25°25.160 ; 81°50.190 25°25.330 ; 81°50.260

NICB, normalized inorganic charge balance (see text). At foot-hills of the Himalaya, Dalai et al. (2002).

–

Table 3 Sample details, dissolved major ions, TDS and Sr in the Ganga and its Himalayan tributaries. Sample

River (location)

8 9 10 11

Ganga headwaters RW03-5 Bhagirathi (Gangotri) RW03-3 Bhagirathi (DevPrayag) RW03-4 Alaknanda (DevPrayag) RW03-2 Ganga (Rishikesh)

02.05.03 01.05.03 01.05.03 01.05.03

30°59.640 ; 30°08.730 ; 30°08.730 ; 30°30.870 ;

12 13 14

Ganga in plain (2004) BR-388 Ganga (Varanasi) BR-309 Ganga (Patna) BR-318 Ganga (Rajmahal)

17.05.04 07.05.04 09.05.04

15 17 16 18

Tributaries joining in plain (2004) BR-342 Ghaghra (Revilganj) BR-363 Rapti (Gorakhpur) BR-311 Gandak (Hazipur) BR-327 Kosi (Dumarighat)

19 20 21 22 23 24 25

Ganga in plain (2006) BR06-12-1 Ganga BR06-14-1 Ganga BR06-10-1 Ganga BR06-801 Ganga BR06-301 Ganga BR06-401 Ganga BR06-104 Ganga

26 27 28 29

Tributaries joining in plain (2006) BR06-901 Ghaghra (Revilganj) BR06-705 Gandak (Hazipur) BR06-501 Kosi (Dumarighat) BR06-601 Bagmati (Dumarighat)

(Allahabad) (Varanasi) (Ghazipur) (Doriganj) (Patna) (Barauni) (Rajmahal)

Date Location N; E (dd.mm.yy)

pH

Temp. Na (°C) lM

– 8.6 8.1 8.1–8.6

– 17.5 17.5 17.9

25°17.840 ; 84°00.400 25°37.400 ; 85°09.050 25°03.660 ; 87°50.420

8.3 8.1 8.1

12.05.04 15.05.04 07.05.04 10.05.04

25°49.170 ; 26°44.200 ; 25°41.290 ; 25°32.400 ;

84°35.090 82°20.710 85°11.300 86°43.270

8.2 8.3 8 7.9

20.10.06 21.10.06 19.10.06 19.10.06 16.10.06 17.10.06 15.10.06

25°30.470 ; 25°17.830 ; 25°32.050 ; 25°43.670 ; 25°37.440 ; 25°22.480 ; 25°03.410 ;

19.10.06 18.10.06 18.10.06 18.10.06

25°48.780 ; 25°41.330 ; 25°32.700 ; 25°32.570 ;

78°56.480 78°35.870 78°35.870 78°20.810

90 130 91 106

K

Mg

Ca

43 86 245 42 137 347 40 165 403 41 164 419

Cl

Sr, NO SO4 HCO3 SiO2 TDS, mg L1 nM

468 16 571 7.5 822 9.2 731 3.8

0.09 0.23 0.05

1.6 1.6 1.8

0.62 0.44

266 331 216 157

1952 4 2157 2.4 3675 13.1 742 4.5

1.13 0.54 0.83 0.20

17 61 24 49 50 61 75

261 247 229 228 217 213 192

1655 2078 2033 1530 1561 1447 1122

0.2 0.3 2 2.7 0.1 0.3 1.5

102 91 114 94

230 184 118 141

1373 1064 467 588

1.5 3.3 0.2 1.5

12 33 20 29

0 0 0 0

353 184 150 175

188 811 1066 973

127 121 95 107

69 98 110 108

32.4 29 30.9

2380 179 922 814 962 1354 116 722 903 463 800 156 510 784 258

0 0 0

257 166 172

4631 3986 2865

321 275 197

477 381 280

3937 3070 2157

30.4 31.8 30.4 31.4

446 102 569 762 72 565 166 703 950 115 217 83 356 714 111 283 81 187 544 69

0 0 0 0

163 136 253 125

2932 3737 2182 1578

172 197 146 174

81°51.760 83°00.600 83°11.910 84°49.460 85°09.210 86°00.090 87°50.230

829 164 528 685 300 24 966 102 415 658 447 19 876 94 367 632 420 15 422 79 369 800 128 9 495 75 376 725 164 9 407 78 364 754 131 8 334 75 273 709 94 4

238 165 156 132 124 137 96

2606 2424 2276 2513 2343 2294 2111

84°35.810 85°11.490 86°43.140 86°43.240

256 138 173 224

131 187 67 51

2555 1911 1252 1551

74 82 69 70

384 308 166 155

867 712 421 542

43 11 39 11 15 7 48 8

NICB CSI

Chemical weathering in the Ganga plain

No.

#, number; CSI, calcite saturation index.

2345

2346

S.K. Rai et al. / Geochimica et Cosmochimica Acta 74 (2010) 2340–2355

the Yamuna at Allahabad, a few tens of kilometers upstream of Varanasi (Fig. 1). Considering that the Yamuna has higher Na than the Ganga at Varanasi during both May and October and that Yamuna’s annual water discharge is 1.6 times that of the Ganga at their confluence at Allahabad (Table 1), it is likely that the Yamuna is the dominant contributor to the high Na of the Ganga at Varanasi. Fig. 3 is Mg/Na (Sr/Na) vs Cl/Na ratio plot for the May and October samples of the Ganga mainstream in the plain. Also included in the plot are data for the Ghaghra at Revilganj and the Yamuna at Allahabad. It is seen from the plot that during May the data show a near linear trend attributable to two component mixing, between the Himalayan end member (for example, Ghaghra) and the Yamuna. The October samples show larger scatter though there seem to be an overall mixing trend. The Ganga sample from Varanasi lies closest to Yamuna. The trend demonstrates the important role of the Yamuna in determining the composition of the Ganga downstream of their confluence. In comparison to both the Yamuna and the Gomti, the abundances of Ca, Mg and Na in the Son are generally lower (Table 2). This can result from a number of factors that include higher rainfall and runoff in its drainage, lesser exposure of carbonates and alkaline–saline soils and the less weatherability of the Archean granites and sand stones of the Gondwana and the Vindhyan in the Son basin.

1.6 May

Gh

October

Mg/Na

1.2

0.8

0.4 Y

Y

0.0 0.0

0.2

0.4

0.6

Cl/Na 0.8 May October

0.6

Sr/Na

Gh

0.4

0.2

Y Y

0.0 0.0

0.2

0.4

0.6

Cl/Na Fig. 3. Mg/Na vs Cl/Na property plots of the Ganga mainstream in the plain. The plot also shows the data for the Yamuna (Y) and the Ghaghra (Gh). The trend in the data seems to be a result of two end member mixing.

5.2. Sources of major ions Dissolved major ions in rivers are derived primarily from three sources, atmospheric deposition, chemical weathering of various lithologies of the basin and anthropogenic input (Holland, 1978; Stallard and Edmond, 1983; Stallard, 1995; Meybeck, 2005). The role of these sources in contributing to the major ion chemistry of the Ganga is discussed in the following sections. 5.2.1. Atmospheric contribution to water chemistry The atmospheric contribution to the Ganga water chemistry is evaluated separately for the Ganga headwaters in the Himalaya and for the Ganga and its tributaries in the plain. The calculation assumes that all chloride in river water samples with lowest Cl is of atmospheric origin. This approach for estimating the atmospheric contribution has the advantage that it does not require corrections for evapotranspiration if the samples are collected at about the same time as its effect is already factored in determining the chloride abundance of rivers. For the headwaters, the average of three river samples with the lowest chloride (a glacier melt stream, the Birahi Ganga and the Bhagirathi at Gangotri, of these only the last sample is listed in Table 3) yield (10 ± 2) lM for atmospheric contribution. This value is within the range of Cl in rains (3–7 lM; Galy and France-Lanord, 1999; Dalai et al., 2002) and snow/glaciers (15 lM, Dalai et al., 2002) from the Himalaya. Based on this, the rain water contribution to chloride in the three remaining Ganga headwater samples (Table 3) is calculated to range from 30% to 50%. For the October samples of the Ganga and its tributaries in the plain, contribution from rain is estimated to be (32 ± 15) lM, (the average of BR06-901, -705 and -501) nearly identical to the average Cl (31 ± 0.6 lM) in rains of Agra, Gopalpura and Mallikadevi from the Ganga plain (Rengarajan et al., 2009). This yields values in the range of 5% (BR-06-13-1, Yamuna) to 67% (BR06-601, Bagmati) for atmospheric contribution to rivers in the plain for October sampling. For the May samples, inclusion of evapotranspiration in the calculation (50%, Galy and France-Lanord, 1999) suggests atmospheric deposition to supply between 4% (BR-346, Yamuna) and 93% (BR-327, Kosi). The atmospheric contributions for Na, Mg and Ca are calculated from (element/Cl) ratios in precipitation. These ratios for the Ganga headwater and the plain regions are 0.6 and 0.8 for Na/Cl, 0.3 and 0.8 for Mg/Cl and 0.7 and 1.3 for Ca/Cl, respectively (Dalai et al., 2002; Rengarajan et al., 2009). It is estimated based on these ratios that the atmospheric contribution for Na average 6% for the head waters and 610% and 620% for Mg and Ca for most samples of the plain during October and May, respectively. Chloride in excess of atmospheric input in rivers has to be derived from hot springs, evaporites and anthropogenic sources. The contributions from these sources to the chloride budget of the Ganga mainstream, its Himalayan and peninsular tributaries range between 7% and 96%. Among these samples, those collected in summer from the Ganga mainstream in the plain and the Gomti and the Yamuna have some of the highest Cl contribution from non-atmospheric sources.

Chemical weathering in the Ganga plain

5.2.2. Anthropogenic input The Ganga plain and the basins of some of the Ganga tributaries (the Yamuna, its tributaries, and the Gomti) are densely populated and account for 40% of India’s population (Jain et al., 2007). These regions generate large quantities of domestic, agricultural and industrial wastes some of which are disposed in the Ganga river and/or its tributaries. Estimates of waste water discharge (GAP, 2004; Jain et al., 2007; Singh and Singh, 2007; Kar et al., 2008) range from 1 to 6.4 billion liters per day 0.1% to 0.6% of the annual water flux of the Ganga at Rajmahal. The major ion composition of these effluents is not well constrained, some of the available data are given in Electronic Annex EA-1. The anthropogenic contribution of Na, Mg, Ca, Cl and alkalinity to their annual fluxes at Rajmahal estimated from the composition of the Ganga in October, the average composition of waste waters (Table EA1) and an effluent discharge of 5 billion liters per day (National River Conservation estimate, GAP, 2004), yield <2% for Na, Ca and HCO3 and 10–20% for Mg and Cl (Table 4). The contribution from anthropogenic sources is subject to spatial and temporal variability as the river discharge varies with season and the pollution sources are localized. Thus during summer when the river discharge is low, the pollution contribution to the riverine flux could be more pronounced. For example, anthropogenic contributions to the Ganga in May calculated from its measured composition (Table 3) and water flux (5  1012 L) at Rajmahal is 40% for Mg and Cl and <10% for Na, Ca and HCO3, significantly higher than the annual average values. Similar calculations for October give 8% and 14% for Mg and Cl and <1% for Na, Ca and HCO3. These calculations thus show that during high water stages of the Ganga the impact of anthropogenic inputs on the dissolved major ion fluxes is minor whereas during summer it can be important, particularly in localized regions where these effluents are discharged. An independent assessment of anthropogenic contribution to the Ganga is also made by comparing its composition measured in this study (2004–2006) with that

2347

reported nearly five decades ago for samples from Farakka (35 km downstream of Rajmahal; Deb and Chadha, 1964). The results (Electronic Annex EA-2) show that for the same month samples the concentrations of Na, Mg, Ca and HCO3 are same within errors in both the sets, indicating that during this period any enhancement in the anthropogenic contribution to their abundances is not clearly discernible. 5.2.3. Silicate weathering and solution of saline/alkaline soils There is significant excess of Na over Cl in the Gomti, the Yamuna and the Ganga main stream samples. Potential sources for the excess Na are silicate weathering and dissolution of sodium salts from alkaline/saline soils. Anthropogenic input as discussed earlier does not seem to be a significant source for Na to the Ganga at Rajmahal on an annual basis. In the Gomti basin and the Ganga plain, silicates are primarily from the Higher and Lesser Himalaya (Singh et al., 2008) whereas Deccan basalts are an important source of silicate cations to the Yamuna and its tributaries (Rengarajan et al., 2009). The importance of saline and alkaline soils as a source of excess Na can be inferred from the high concentrations of Na salts they release to solution during their extraction with water (Table 5). The high Na*(= Nariv  Clriv) in the Gomti, the Yamuna and the Ganga downstream of their confluence even during their high stages, if attributed to silicate weathering would suggest intense silicate erosion of their basins. Earlier attempts to determine silicate erosion rates (SER) in the Ganga plain (Galy and France-Lanord, 1999; West et al., 2002) based on mass balance of silicate cation fluxes yielded contrasting results. Galy and France-Lanord (1999) observed that silicate and carbonate erosion in the Ganga plain were much lower than that in the Himalaya, a result they attributed to drier climate in the plain. In contrast, West et al. (2002) noted that silicates of the High Himalaya depositing in the Ganga plain undergo far more intense weathering in the plain than in their source regions. Fig. 4 is the Ca/Na–Mg/Na ratio plot of the river water data (Tables 2 and 3) along with the values for the four end

Table 4 Estimates of anthropogenic input to the Ganga at Rajmahal. Element

Concentration (lM) in a

Flux (109 moles)

% Anthropogenic

Ganga

Waste water

Anthropogenicb

Totalc

(at Rajmahal)

2.8 4.7 12 7 12

127 269 104 36 802

2 2 12 20 0.2

Annual

Na Ca Mg Cl HCO3

334 709 273 94 2111

1500 2600 6500 3800 6700

May 2004

Na Ca Mg Cl HCO3

800 784 510 258 2865

1500 2600 6500 3800 6700

0.23 0.40 1.0 0.58 1.0

May 2004 flux based on May concentration and river water discharge of 5  1012 L. a At Rajmahal. b Based on average concentrations (Table EA1) and waste water flux of 5  109 L day1. c Annual flux based on October concentrations and water discharge of 380  1012 L yr1.

4 4 2.6 1.3 14.3

6 10 38 44 7

2348

S.K. Rai et al. / Geochimica et Cosmochimica Acta 74 (2010) 2340–2355

Table 5 Chemical composition of saturation extracts of saline/alkaline soils. Code

Location 0

00

0

00

Na

K

Mg –

Ca 5.2

HCO3

CO3

75

422

252

27

132

234

Cl

SO4

252

UP III

27°13 30 N, 78°58 10 E Mainpuri, UP

1037

0.6

UP VIII

26°230 0700 N, 80°000 1600 E Kanpur, UP

412

0.8

3.4

–

41

Pedon 6

Ganga Yamuna Interfluve Jalesar, UP

42

1.0

22.4

14.7

48

Pedon 25

Ganga Yamuna Interfluve Jalesar, UP

1001

3.5

6.4

4.2

290

441

85

Pedon 29

Ganga Yamuna Interfluve Awagarh, UP

69

4.5

1.1

3.6

24

22

28

Pedon 24

Ganga Yamuna Interfluve Jalesar, UP

520

2.0

2.6

6.7

89

27

203

9.3

23.3

– 198 4.4 211

Concentrations in meq L1. Data source: Bhargava et al. (1981) and Pal et al. (2003).

3 Dolomite Pc-Carbonates

27

4

2

5

Mg/Na (molar)

Basalt

10 16 26

11

15

Biotite weathering

17 Saline/alkalineSoil

9

1

12 6

24

23

25

22

1 19 2 13

7

Ca precipitation

8 28 29

14

3

18

20 21 Na Addition HimalayanSilicates

0 0

2

4

6

Ca/Na (molar) Fig. 4. Ca/Na–Mg/Na ratio plot of water composition given in Tables 2 and 3. The plot also includes data for saline/alkaline soil, Pc-carbonates and the two silicate end members. The data show that many of the October samples (filled squares) lie along a line connecting Himalayan silicates and Pc-carbonates. The slope of this line is 0.4. The May samples of the rivers (connected by arrows with the October samples) plot along a line with a steeper slope of 0.7 bringing out the importance of calcite precipitation. The Gomti, the Yamuna and some of the Ganga main stream samples near their confluence plot farther to the left (see text). The numbers in the figure corresponds to samples in Tables 2 and 3.

members that contribute major ions to the rivers. It is seen from Fig. 4 that most of the river water data fall around a line connecting the Himalayan silicates and Pc-carbonates bringing out the importance of silicate–carbonate mixing in determining their Ca/Na, Mg/Na ratios. Exception to this general trend are the Gomti, the Yamuna and the Ganga mainstream (Varanasi) data; these samples plot more towards the left indicating the importance of saline/ alkaline soils in their major ion budget.

The SER of the Gomti basin calculated using Na* as an index, its October composition (Table 2) and the forward model (Krishnaswami et al., 1999) is 17.8 ± 6.1 tons km2 yr1, similar to the value for the Himalayan sub-basin of the Ganga (Rishikesh, 13.8 ± 4.3 tons km2 yr1). Such a high silicate erosion rate for the Gomti basin, though is consistent with the transport limiting conditions in the plain and the observations of West et al. (2002) is difficult to reconcile with its Si and Sr-isotope data. Further, the wide spread occurrence of

Chemical weathering in the Ganga plain

alkaline/saline soils in its basin with easily soluble Na salts indicates that they can be a source of Na* to the river, questioning its use as a proxy of silicate weathering. Among the rivers analyzed, the Gomti has the lowest SiO2/Na* ratio, 0.1, at least an order of magnitude lower than the value of 1.4 ± 0.2 in the Ganga headwaters (Table 3), 1.0 ± 0.2 in the Yamuna at Batamandi, and 1.4 ± 0.3 in the Karnali, Rapti and the Narayani (Galy and FranceLanord, 1999). The Yamuna (at Allahabad) and the Ganga downstream of Allahabad also have low (SiO2/Na*) ratios, 60.4. Considering that the Ganga headwaters and the Gomti predominantly weather Higher Himalayan silicates, the significantly lower (SiO2/Na*) in the Gomti has to result from either preferential release of Na over SiO2 during redissolution of alkaline–saline soils in addition to silicate weathering or the removal of Si from solution by biological/geochemical processes (Meybeck, 2005). The various salts formed by the evaporation of river and ground water undergo dissolution during their wetting cycle; there is however no field data on the dissolution kinetics and associated release ratios of SiO2/Na* from these deposits. Laboratory studies on the re-solution characteristics of soil salts (Drever and Smith, 1978) show that the dissolution rate decrease as Na  Cl > SO4 > carbonates > SiO2. Extension of these results to the soil salts of the Ganga plain would suggest that during their dissolution by monsoon rains, Na would be released preferentially over Si, a fractionation that can contribute to the low SiO2/Na* in rivers. Another process that can also result in low SiO2/Na* is the release of Na from clays by ion exchange. This process has been suggested as a potential source of Na for the formation of alkaline soils (Singh, 2005). The ultimate source of Na* involved in these reactions though is silicate weathering, its use as a proxy to determine SER is in doubt as its relation to contemporary silicate weathering is unclear. In case of alkaline–saline soils, the SER based on Na* would be valid if the supply of Na by rivers for their formation and its removal by their dissolution is in steady-state. It is uncertain if this requirement is met especially considering the changes in land use and the increasing use of ground water for irrigation which also serves as an additional source of solutes. Another proxy which is often used as an index of silicate erosion and related studies in river basins, including the G–B system is dissolved Si (e.g. Edmond and Huh, 1997; Galy and France-Lanord, 1999; Bickle et al., 2005; Tipper et al., 2006; Rengarajan et al., 2009). These applications require conservative behavior of Si in rivers. Such a behavior is expected in this study as the high river stage and turbid waters during October would considerably restrict diatom and other plant growth, consistent with the report of minimum phytoplankton density in the Ganga during monsoon (Sreenivasaprasad, 1991). Further the SiO2 concentration in the Gomti, 92 lM, is undersaturated w.r.t. both quartz and amorphous silica. Thus, the low (SiO2/Na*) in the Gomti is unlikely to be due to removal of Si through biological and precipitation processes. As a result, in this study SER has been estimated using dissolved Si as a proxy assuming that it behaves conservatively. The validity of this assumption needs further study considering the role of clay mineral for-

2349

mation in these river basins and its impact on dissolved Si concentration and SiO2/Na* ratio of rivers. In the basins of the Ganga headwaters and its Himalayan tributaries, illite is the dominant clay (60–80%) with kaolinite and chlorite making up the balance whereas in the Ganga plain, the Gomti and the Yamuna smectite is the significant/major component of clays with 5–10% each of kaolinite and chlorite (Sarin et al., 1989). It has been suggested (Singh et al., 2005) that the Ganga plain is a potential site for the formation of smectite type clays through reactions involving degraded aluminosilicates and dissolved silica. Similarly in soils of the Ganga plain, there is substantial weathering of biotite and formation of smectite and smectite–kaolinite with increasing age due to pedogenesis (Mohindra et al., 1992; Srivastava et al., 1998). If these processes contribute to the low dissolved Si and SiO2/Na* ratio in rivers, the use of dissolved Si as a proxy would underestimate SER. The silicate derived Na (Nasil) in the Gomti is 66 ± 9 lM for October (Table 6) only 6% of Na* suggesting that most of Na* in the Gomti is of saline/alkaline soil origin. The low silicate component in the Gomti is also consistent with its 87 Sr/86Sr data, 0.72714 (Table 2) far less than the 87Sr/86Sr of the silicate component of its sediments (0.79276; Singh et al., 2008; Rai, 2008). This can be interpreted in terms of mixing of more radiogenic Sr from silicates with the less radiogenic Sr from carbonates/alkaline–saline soils. Using a 87Sr/86Sr of 0.715 for the later (Singh et al., 1998; Rengarajan et al., 2009) the silicate Sr contribution to dissolved Sr of the Gomti is estimated to be <15%. The above arguments based on Nasil and Sr, thus indicate that in the Gomti silicate contribution to their dissolved concentrations (and therefore other major cations) is only minor. This inference also draws support from the chemical composition of the Ganga plain sediments and their CIA values which suggest that they are not subjected to significant chemical weathering (Singh et al., 2005; Tripathi et al., 2007). In contrast to the Gomti the major ion concentrations of the Yamuna at Allahabad is a composite of contributions from the Himalaya, peninsular and plain sub-basins. The fluxes of major ions from the peninsular and plain sub-basins, calculated as the difference between their fluxes at Allahabad (calculated from data in Table 2) and their Himalayan fluxes (based on data for Batamandi, Table 2) are 5.2, 3.6, 5.2 and 0.97  1010 moles yr1 for Na*, Mg, Ca and Si. The SiO2/Na* fluxes yield a molar ratio of 0.19, significantly lower than that observed for weathering of Himalayan silicates and the Deccan basalts (6 ± 1.5, Rengarajan et al., 2009) the major silicate sources of the Yamuna. The low SiO2/Na* in the peninsular and plain sub-basins of the Yamuna therefore suggests that analogous to the Gomti, it also receives Na* from alkaline and saline soils, consistent with their abundant presence in its basin (Fig. 2). The Nasil in the Yamuna is estimated to be 92 ± 13 lM (Table 6) about 18% of Na*. This is likely to be an overestimate as it does not consider contribution from the Deccan basalts which releases (SiO2/Na) with a higher ratio than the Himalayan silicates. Thus, similar to the Gomti, the Yamuna data also show that estimates of Nasil based on dissolved Si is much less than Na*.

2350

River

Drainage

Date

Location

Area

Na*

Nasil

SER

(103 km2)

(lM)

(lM)

(a)

(b)

(a)

CER (b)

Ref.

Ganga

H

May 2004

Rishikesh

21.7

77

77

13.6

5.2

53

20

This work

Yamuna Gandak Trishuli Ghaghra

H H H H

October 1998 (Narayani)

9.6 31.8 4.6 12.3

195

195

(Bheri)

Batamandi Narayanghat Betrawati Sampujighat Average

28 18.2 13 13 17.1 ± 6.0

11 7 5 5 6.6 ± 2.3

115 135 55 156 109 ± 38

43 52 21 60 42 ± 15

Dalai et al. (2002) Galy and France-Lanord (1999) Galy and France-Lanord (1999) Galy and France-Lanord (1999)

Gomti Son Yamuna

P Pe + P Pe + P

October 2006 October 2006 October 2006

Ghazipur Koilawar (Pe + plain) Average

30.5 71.3 356

1075 315

66 316

2.6 10.7 3.9 4.9 ± 1.7

1.0 4.1 1.5 1.9 ± 0.7

36.7 26.9 18.4 20.9 ± 7.3

13.6 10.0 6.8 7.8 ± 2.7

This work This work This work

Ghaghra Gandak Kosi

H+P H+P H+P

October 2006 October 2006 October 2006

Revilganj Hazipur Dumarighat Average

128 46.3 74.5

213 99 158

73 65 81

8.7 11.9 11.0 10.0 ± 3.5

3.3 4.6 4.2 3.8 ± 1.3

82.7 102.6 40.2 73 ± 26

30.6 38.0 14.9 27 ± 10

This work This work This work

H, Himalayan; P, plain; Pe, peninsular. a See Electronic Annex EA-3 for calculations. Uncertainties in the SER and CER estimates are 35% propagated based on interannual variations in water fluxes and errors associated with cation release ratios. (a) tons km2 yr1; (b) mm kyr1. The averages given are area weighted.

S.K. Rai et al. / Geochimica et Cosmochimica Acta 74 (2010) 2340–2355

Table 6 Silicate and carbonate cation erosion rates in the Ganga sub-basins.a

Chemical weathering in the Ganga plain

The Son drainage also lies in the peninsular basin and the Ganga plain, the sample from Koilawar (Fig. 1) receives contribution dominantly from the peninsular basin. Data on the distribution of saline/alkaline soils in the sub-basin upstream Koilawar is sparse, however compared to the Gomti and the Yamuna, the Son basin is likely to be less impacted by these soils as it receives higher rain fall and is less endoeric in nature. Therefore, in this case Nasil can be expected to be nearly the same as Na*. If, this approximation is in error due to supply of Na* from other sources, the calculated SER would be upper limits. The silicate erosion rates (Electronic Annexure EA-3-1) in the peninsular and plain basins of the Gomti, the Yamuna and the Son are 2.6, 3.9 and 10.7 tons km2 yr1 (corresponding to 1, 1.5 and 4 mm kyr1, respectively, Table 6). The SERs of the Gomti and Yamuna basins are similar to the value of 1 mm kyr1 reported for silicate chemical erosion of the Ganga flood plain (Galy and France-Lanord, 1999) but are 5 and 3 times lower than that for the Himalayan sub-basins of the Ganga (Rishikesh) and the Gandak (Galy and France-Lanord, 1999, Table 6). The lower SER in the peninsular and plain sub-basins relative to the Himalayan sub-basins can be a result of higher relief and runoff coupled with higher physical erosion in the Himalaya. Further, the semi-arid climate of the peninsular/plain basins and their endoeric nature may also contribute to their lower SER. The SER of the peninsular–plain basin though is lower than that of the Himalayan basin the fluxes of silicate derived cations transported by rivers draining them are similar. This is because the drainage area of the peninsular and plain sub-basins of the Ganga is 4 times the area of its Himalayan sub-basin. This observation underscores the importance of the rivers draining the peninsular and plain sub-basins in determining the flux of silicate derived elements transported by the Ganga to the Bay of Bengal. Efforts to determine the SER in the peninsular and plain sub-basins of the Ganga based on the difference in the elemental fluxes between their output at Rajmahal and the sum of their inputs from the Himalayan tributaries were

2351

unsuccessful as the input fluxes of Na, Mg, Sr and Cl to the Ganga from its tributaries are in excess of their output at Rajmahal whereas it is a few percent lower for Ca and HCO3 (Table 7). The opposite trend in Ca and HCO3 relative to other elements is likely due to their removal in the plain via calcite precipitation, as discussed in the next section. The excess, which ranges from 18% (for Mg) to 120% (for Cl), can be due either to differences in the water discharge between the year of sampling and the long term mean used in calculations or retention of a part of the solutes in the plains or a combination of them. The water discharges of the Ganga and the Yamuna show large year to year variation, by factors of 2 and 8, respectively, with average spread of ±20% for the Ganga during 1950–1960 and 1965–1973 (http://www.grdc.sr.unh.edu), and ±50% for the Yamuna (Allahabad) for 1991–2001 (Electronic Annex EA-4). The average variation is roughly similar to the observed imbalance between input and output fluxes (except for Cl). The retention of river solutes in the plain in the form of saline/alkaline soils can also contribute to the pronounced discrepancies in the budgets of Cl and Na. 5.2.4. Carbonate weathering In majority of the October samples (Ca + Mg) account for P75% of the cation charge (after chloride correction). The presence of carbonates in the drainage basins of the Himalayan rivers and to a lesser degree the Ganga plain and considering that they weather more easily than Ca, Mg silicates hints at the possibility that they can be the dominant source of Ca and Mg to these rivers. This is also borne out from Fig. 4 which shows that headwaters of the Ganga, its Himalayan tributaries and some of its mainstream samples have Ca/Na and Mg/Na significantly higher than that in the Himalayan silicates, bringing out the important role of carbonates in contributing to their Ca and Mg. This is also supported by the Sr-isotope data. The dissolved 87Sr/86Sr of the Ganga and its tributaries in the plain (0.71239–0.75732) is much lower than that of silicate Sr in their bed sediments (0.74620–0.80369; Singh et al., 2008; Rai, 2008). This suggests that dissolved 87 Sr/86Sr is a mixture of Sr from radiogenic silicates and less

Table 7 Annual fluxes of cations and anions from the Ganga and its tributaries. River

Location

Ganga Yamuna Gomti Son Yamuna Ghaghra Gandak Kosi

Rishikesh Batamandi Ghazipur Koilawar Allahabad Revilganj Hazipur Dumarighat

Flux (109 moles, yr1)a Na*

R [Fluxes of the rivers (1, 3–8)] Ganga Yamuna a

Rajmahal Pe + plain

Sr

Cl

HCO3

SO4

SiO2

1.8 2.1 8.0 10.0 53.8 20.1 5.2 9.8

Na 2.5 2.8 10.4 13.7 118.6 24.2 7.2 10.7

Mg 3.9 5.4 6.3 8.0 41.2 36.2 16.1 10.3

Ca 10.0 11.0 6.4 17.1 62.6 81.8 37.2 26.1

0.017 0.019 0.020 0.030 0.253 0.130 0.056 0.029

0.7 0.6 2.4 3.7 64.7 4.1 2.0 0.9

23 26 32 58 239 241 100 78

4.2 3.6 1.5 1.6 16.8 12.4 9.8 4.2

2.6 2.3 0.7 5.1 12.0 9.6 4.8 7.1

108.7

187.3

122.1

241.1

0.53

78.5

771

50.4

41.8

91.2 51.7

126.9 115.8

103.7 35.8

269.4 51.6

0.426 0.23

35.7 64.1

802 214

36.5 13.2

28.5 9.7

Based on October 2006 data [except for Rishikesh (May 2004) and Yamuna at Batamandi (October 1998), Tables 2 and 3].

2352

S.K. Rai et al. / Geochimica et Cosmochimica Acta 74 (2010) 2340–2355

radiogenic carbonates/saline–alkaline soils, with significant contribution from the later component. The Ca/Na of many summer samples are lower than that in the October (monsoon) samples (Fig. 4), most likely a result of preferential removal of Ca via calcite precipitation. This is consistent with their calcite saturation index (CSI) which shows that all the summer samples, except the Kosi and the Alaknanda, are supersaturated in calcite and therefore dissolved Ca is susceptible for removal via calcite precipitation from them. (CSI calculation could be made only for the summer samples, as pH data were available only for them). Support for this hypothesis comes from the observation of decreasing trend in Ca/Mg with increasing CSI (Fig. 5) and the presence of kankar carbonates in these river sediments. Thus, these different observations seem to suggest that Ca behaves non-conservatively in these rivers and that part of it is precipitated as carbonate, supporting some of the earlier studies on the Himalayan rivers (Sarin et al., 1989; Dalai et al., 2002; Jacobson et al., 2002). Alternatively, the steeper trend in Fig. 4 for some of the summer samples can be a result of mixing of saline–alkaline soils with a relatively high Mg/Na source such as weathering of dolomite rich carbonates or Deccan basalts (for the Yamuna) or in-situ weathering of biotite. The high concentration of Sr in the Gomti and Yamuna waters would argue against dolomite weathering as a major source of Ca and Mg to these waters. Deccan basalts can be a source of high Mg/Na in the Yamuna as they have Mg/Na molar ratio of 2.0 ± 0.5 (Das et al., 2005) much higher than that in Himalayan silicates. Biotite weathering could be another source contributing Mg to these rivers (Kapoor et al., 1981). Similarly anthropogenic inputs, which is relatively more pronounced during summer especially in the Gomti, the Yamuna and the Ganga in plain (Jain et al., 2007) can also contribute to the difference in the trends between October and May samples. The carbonate erosion rates (CER; Electronic Annex EA-3-2) calculated for the peninsular and plain catchments of the Gomti, the Yamuna and the Son are 36.7, 18.4 and 26.9 tons km2 yr1 (corresponding to 14, 7 and 10 mm kyr1; Table 6). These values yield a weighted average carbonate cation erosion rate of 20.9 tons km2 yr1 (8 mm

Ca/Mg (molar)

4

3

2

1

0 -0.25

0

0.25

0.5

0.75

1

Calcite Saturation Index Fig. 5. Variation of Ca/Mg with calcite saturation index. The decrease in Ca/Mg with increasing CSI can be interpreted in terms of calcite precipitation which preferentially removes Ca over Mg.

kyr1), a factor 4 higher than SER. The ratio of silicate to carbonate erosion rates in the Himalayan and the (peninsular + plain) sub-basins overlap within errors suggesting that on an average the relative intensities of erosion of silicates and carbonates in all these basins are similar. The lower CER in the plain relative to those in the Himalayan sub-basins of the Ganga (Table 6) is most likely a result of lower relief and runoff in the peninsular and plain subbasins. Another factor that can contribute to the low CER in the plain is the precipitation of calcite; the significance of this process however remains to be evaluated. 5.3. Impact of alkaline/saline soils on estimates of SERs and elemental fluxes The large excess of Na over Cl in the peninsular and plain sub-basins, 77% and 45% of Na in the October samples of the Gomti and the Yamuna (Table 2) is dominated by contribution from alkaline/saline soils questioning the use of Na* to determine SER of these river basins. This ‘Na source problem’ can be more widespread and can compromise the estimation of SERs of river basins from other semi-arid regions with characteristics similar to the Ganga plain. Further, the precipitation and accumulation of carbonates, silica and other salts during evaporation can coat the surfaces of minerals in the basin influencing their chemical weathering (Drever and Smith, 1978). Table 7 lists the fluxes of various cations and anions supplied to the Ganga from its tributaries which shows that 89% Cl, 75% Na, 31% Ca, 41% Mg and 53% Sr supplied to the Ganga from its tributaries is derived from the peninsular and plain sub-basins. This compares with their contribution of 49% to the drainage area of the Ganga and 32% towards its annual water discharge. These estimates highlight the major role of rivers draining the peninsular basin and the Ganga plain in contributing Na, Cl and Sr to the Ganga disproportionately higher than their contribution to water discharge. 5.4. Sr and

87

Sr/86Sr budget

Sr concentrations in the rivers range widely (Tables 2 and 3), with the Yamuna summer sample having the highest value. Analogous to Na, the Sr concentration of the Ganga mainstream also decreases downstream of Varanasi due to dilution by the Himalayan tributaries, this is evident from the strong positive correlation between Sr and Na in the Ganga mainstream. Among the tributaries, the Yamuna has the highest Sr flux (2.2  104 tons yr1, Table 7). It contributes about 47% of Sr and 87Sr to the cumulative flux from the tributaries of the Ganga despite an important component of its peninsular drainage being Deccan Traps, which is quite unradiogenic in Sr (Das et al., 2006). The Sr flux of the Ganga at Rajmahal based on the October sample (Table 3) is 3.7  104 tons yr1 with 87 Sr/86Sr 0.72707, similar to earlier reported values (Krishnaswami et al., 1992). The collective flux of Sr from the tributaries is 4.7  104 tons yr1 (Table 7); 25% more compared to that measured in the Ganga at Rajmahal a

Chemical weathering in the Ganga plain

result similar to Na and Mg (Table 7). Interannual variations in water discharge as mentioned earlier could be a cause for this discrepancy. 6. CONCLUSIONS Silicate and carbonate erosion rates in the plain and peninsular sub-basins of the Ganga have been determined from the major ion chemistry and Sr isotope systematics of the Gomti, the Yamuna and the Son, tributaries of the Ganga. The SER determined using Si as an index average 5 tons km2 yr1 and CER 20 tons km2 yr1. Dissolved Si was preferred over the more commonly used Na* as an index of silicate weathering as the input of non-chloride sodium to rivers in the Ganga plain and peninsular basins from saline/alkaline soils raise questions about its application. There are also concerns on the use of Si as a proxy arising from its potential non-conservative behavior in rivers, if such processes result in its removal the estimated SER would be lower limits. The Si based SER and CER in general are 3–5 times lower than those in the Himalayan sub-basin of the Ganga, attributable to the lower runoff in the peninsular and plain sub-basins and the endoeric nature of their drainages. In addition, precipitation of calcite in the Ganga plain can also contribute to the lower carbonate erosion. The fluxes of silicate derived major cations and Sr from the plain and peninsular sub-basins, however, are similar to those from the Himalayan sub-basin as their aerial coverage is 4 times that of the Himalayan sub-basin. This study also demonstrates the importance of rivers draining the Ganga plain and peninsular basin as a major source of Na, Cl and Sr to the Ganga, they supply disproportionately higher amounts of these elements relative to their contribution to water flux. Further, the role of anthropogenic inputs estimated based on the composition of effluent discharge seems to be minor during high river stages. The evaporation and precipitation of various dissolved constituents of the Ganga in the plain can, in addition to modifying the fluxes of major ions transported by the Ganga, also influence the chemical weathering rates of minerals in the basin and the transport of minor elements that can be sequestered to solid phases. ACKNOWLEDGMENTS We thank Mr. J.P. Bhavsar for help during the field trips. S.K. thanks the Indian National Science Academy, New Delhi for Senior Scientistship and Physical Research Laboratory for logistical support. The comments by E. Tipper and three anonymous reviewers and E. Oelkers, Associate editor have helped to improve the manuscript considerably.

APPENDIX A. SUPPLEMENTARY DATA Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.gca.2010. 01.008.

2353 REFERENCES

Agarwal R. R. and Gupta R. N. (1968) Saline–Alkali Soils in India. Indian Council of Agricultural Research, New Delhi, pp. 221. Agarwal A. K., Rizvi M. H., Singh I. B. and Chandra S. (1992) Carbonate deposits in Ganga plain. In Gangetic Plain: Terra Incognita (ed. I. B. Singh). Lucknow University, pp. 35–43. Bhargava G. P. and Bhattacharjee J. C. (1982) Morphology, genesis and classification of salt affected soils. In Review of Soil Research in India, Part II. Indian Society of Soil Science, New Delhi, pp. 508–528. Bhargava G. P., Paul D. K., Kapoor B. S. and Goswami S. C. (1981) Characteristics and genesis of some sodic soils in the Indo-Gangetic alluvial plain of Haryana and Uttar Pradesh. J. Indian Soc. Soil Sci. 29, 60–71. Bickle M. J., Bunbury J., Chapman H. J., Harris N. B. W., Fairchild I. J. and Ahmad T. (2003) Fluxes of Sr into the headwaters of the Ganges. Geochim. Cosmochim. Acta 67, 2567–2584. Bickle M. J., Chapman H. J., Bunbury J., Harris N. B. W., Fairchild I. J., Ahmad T. and Pomie`s C. (2005) Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges. Geochim. Cosmochim. Acta 69, 2221–2240. Bookhagen B. and Burbank D. W. (2006) Topography, relief, and TRMM-derived rainfall variations along the Himalaya. Geophys. Res. Lett. 33. doi:10.1029/2006GL026037. Chhabra R. (1996) Soil Salinity and Water Quality. A.A. Balkemia, Rotterdam, pp. 284. CSSRI (2007) CSSRI – Perspective Plan: Vision 2025. Central Soil Salinity Research Institute, Karnal, India. Dalai T. K., Krishnaswami S. and Sarin M. M. (2002) Major ion chemistry in the headwaters of the Yamuna river system: chemical weathering, its temperature dependence and CO2 consumption in the Himalaya. Geochim. Cosmochim. Acta 66, 3397–3416. Das A., Krishnaswami S. and Kumar A. (2006) Sr and 87Sr/86Sr in rivers draining the Deccan Traps (India): implications to weathering, Sr fluxes and marine 87Sr/86Sr record around K/ T. Geochem. Geophys. Geosyst. 7, Q06014. doi:10.1029/ 2005GC001081. Das A., Krishnaswami S., Sarin M. M. and Pande K. (2005) Chemical weathering in the Krishna basin and the western ghats of the Deccan Traps: rates of basalt weathering and their controls. Geochim. Cosmochim. Acta 69, 2067–2084. Datta S., Thibault Y., Fyfe W. S., Powell M. A., Har B. R., Martin R. R. and Tripathy S. (2002) Occurrence of trona in alkaline soils of the Indo-Gangetic plains of Uttar Pradesh (U.P.), India. Episodes 25, 236–239. Deb B. C. and Chadha S. P. (1964) A Study of the Quality of Indian River Waters, Technical Memorandum IRD 1. Central Water and Power Research Station, Poona, India, pp. 51. Donald A. (1992) Hydrological Aspects of the Himalayan Region, International Centre for Integrated Mountain Development, Kathmandu. (ICIMOD) Occasional Paper No. 18, pp. 68. Drever J. I. and Smith C. L. (1978) Cyclic wetting and drying of the soil zone as an influence on the chemistry of groundwater in arid terrains. Am. J. Sci. 278, 1448–1454. Edmond J. M. and Huh Y. (1997) Chemical weathering yields in hot and cold climates. In Tectonic Uplift and Climate Change (ed. W. F. Ruddiman). Plenum Press, New York, pp. 329–351. Galy A. and France-Lanord C. (1999) Weathering processes in the Ganges–Brahmaputra basin and the riverine alkalinity budget. Chem. Geol. 159, 31–60.

2354

S.K. Rai et al. / Geochimica et Cosmochimica Acta 74 (2010) 2340–2355

Galy A. and France-Lanord C. (2001) Higher erosion rates in the Himalaya: geochemical constraints on riverine fluxes. Geology 29, 23–26. GAP, Ganga Action Plan Report (2004) Central Water Commission, India. Gansser A. (1964) Geology of the Himalayas. Wiley–Interscience, London, pp. 284. Gupta L. P. and Subramanian V. (1994) Environmental geochemistry of the river Gomti: a tributary of the river Ganges. Environ. Geol. 24, 235–243. Harris N., Bickle M. J., Chapman H., Fairchild I. and Bunbury J. (1998) The significance of Himalayan rivers for silicate weathering rates: evidence from the Bhote Kosi tributary. Chem. Geol. 144, 205–220. Holland H. D. (1978) The Chemistry of the Atmosphere and Oceans. Wiley–Interscience, New York. Hren M. T., Chamberlain C. P., Hilley G. E. and Bookhagen B. (2007) Major ion chemistry of the Yarlung Tsangpo–Brahmaputra river: chemical weathering, erosion, and CO2 consumption in the southern Tibetan plateau and eastern syntaxis of the Himalaya. Geochim. Cosmochim. Acta. 71, 2907–2935. Jacobson A. D., Blum J. D. and Walter L. M. (2002) Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: insights from the carbonate geochemistry of stream waters. Geochim. Cosmochim. Acta 66, 3417–3429. Jain S. K., Agarwal P. K. and Singh V. P. (2007) Hydrology and Water Resources of India. Springer, The Netherlands, pp. 333– 418. Kapoor B. S., Singh H. B., Goswami S. C., Arbol I. P., Bhargava G. P. and Pal D. K. (1981) Weathering of micaceous minerals in some salt affected soils. J. Indian Soc. Soil Sci. 29, 486–492. Kar D., Sur P., Mandal S. K., Saha T. and Kole R. K. (2008) Assessment of heavy metal pollution in surface water. Int. J. Environ. Sci. Technol. 5, 119–124. Krishnan M. S. (1982) Geology of India and Burma, 6th ed. CBS Publishers & Distributors, New Delhi, pp. 536. Krishnaswami S., Singh S. K. and Dalai T. (1999) Silicate weathering in the Himalaya: role in contributing to major ions and radiogenic Sr to the Bay of Bengal. In Ocean Science, Trends and Future Directions (ed. B. L. K. Somayajulu). Indian National Science Academy and Academia International, New Delhi, pp. 23–51. Krishnaswami S., Trivedi J. R., Sarin M. M., Ramesh R. and Sharma K. K. (1992) Strontium isotope and rubidium in the Ganga–Brahmaputra river system: weathering in the Himalaya, fluxes to Bay of Bengal and contribution to the evolution of oceanic 87Sr/86Sr. Earth Planet. Sci. Lett. 109, 243–253. Meybeck M. (2005) Global occurrence of major elements in rivers. In Surface and Groundwater, Weathering, and Soils, vol. 5 (ed. J. I. Drever). Treatise on Geochemistry Elsevier, pp. 207–223. Mohindra R. (1995) Holocene soil chronoassociation in part of the Middle Gangetic plain: morphological and micromorphological characteristics. Terra Nova 7, 305–314. Mohindra R., Prakash B. and Prasad J. (1992) Historical geomorphology and pedology of the Gandak Megafan, Middle Gangetic plains, India. Earth Surf. Processes Landforms 17, 643–672. Pal D. K., Srivastava P., Durge S. L. and Bhattacharyya T. (2003) Role of microtopography in the formation of sodic soils in the semi-arid part of the Indo-Gangetic plains, India. Catena 51, 3– 31. Palmer M. R. and Edmond J. M. (1992) Controls over the strontium isotope composition of river water. Geochim. Cosmochim. Acta 56, 2099–2111. Rajagopalan G. (1992) Radiocarbon ages of carbonate materials from Gangetic Alluvium. In Gangetic Plain: Terra Incognita

(ed. I. B. Singh). Geology Department, Lucknow University, pp. 45–48. Rai S. K. (2008) Geochemical and isotopic studies of ancient and modern sediments. Ph.D. Thesis, MS University of Baroda, Vadodara, India, pp. 195. Rai S. K. and Singh S. K. (2007) Temporal variation in Sr and 87 Sr/86Sr of the Brahmaputra: implications for annual fluxes and tracking flash floods through chemical and isotope composition. Geochem. Geophys. Geosyst. 8, Q08008. doi:10.1029/ 2007GC001610. Rao K. L. (1975) India’s Water Wealth. Orient Longman Ltd., New Delhi, pp. 255. Raymo M. E. and Ruddiman W. F. (1992) Tectonic forcing of late Cenozoic climate. Nature, 117–122. Rengarajan R., Singh S. K., Sarin M. M. and Krishnaswami S. (2009) Strontium isotopes and major ion chemistry in the Chambal River system, India: implications to silicate erosion rates of the Ganga. Chem. Geol. 260, 87–101. Ruddiman W. F. (1997) Tectonic Uplift and Climate Change. Plenum Publishing Corporation, New York, pp. 535. Sarin M. M., Krishnaswami S., Dilli K., Somayajulu B. L. K. and Moore W. S. (1989) Major ion chemistry of the Ganga– Brahmaputra river system weathering processes and fluxes to the Bay of Bengal. Geochim. Cosmochim. Acta 53, 997–1009. Sarin M. M., Krishnaswami S., Trivedi J. R. and Sharma K. K. (1992) Major ion chemistry of the Ganga source waters Weathering in the high altitude Himalaya. Proc. Indian Acad. Sci. (Earth Planet. Sci.) 101, 89–98. Singh I. B. (1996) Geological evolution of Ganga plain – an overview. J. Paleontol. Soc. India 41, 99–137. Singh M., Sharma M. and Tobschall H. J. (2005) Weathering of the Ganga alluvial plain, northern India: implications from fluvial geochemistry of the Gomti river. Appl. Geochem. 20, 1–21. Singh M. and Singh A. K. (2007) Bibliography of environmental studies in natural characteristics and anthropogenic influences on the Ganga river. Environ. Monit. Assess. 129, 421–432. Singh S. K. and France-Lanord C. (2002) Tracing the distribution of erosion in the Brahmaputra watershed from isotopic compositions of river sediments. Earth Planet. Sci. Lett. 202, 645–662. Singh S. K., Kumar A. and France-Lanord C. (2006) Sr and 87 Sr/86Sr in waters and sediments of the Brahmaputra river system: silicate weathering, CO2 consumption and Sr flux. Chem. Geol. 234, 308–320. Singh S. K., Rai S. K. and Krishnaswami S. (2008) Sr and Nd isotopes in river sediments from the Ganga basin: sediment provenance and hotspots of physical erosion. J. Geophys. Res. 113, F03006. doi:10.1029/2007JF000909. Singh S. K., Trivedi J. R., Pande K., Ramesh R. and Krishnaswami S. (1998) Chemical and Sr, O, C, isotopic compositions of carbonates from the Lesser Himalaya implications to the Sr isotope composition of the source waters of the Ganga, Ghaghara and the Indus rivers. Geochim. Cosmochim. Acta 62, 743–755. Singh T. N. (2005) Irrigation and Soil Salinity in the Indian Subcontinent Past and Present. Lehigh University Press, Bethlehem, pp. 404. Sinha D. K., Saxena S. and Saxena R. (2006a) Seasonal variation in the aquatic environment of Ramganga river at Moradabad: a quantitative study. Indian J. Environ. Prot. 26, 488–496. Sinha R., Jain V., Babu G. P. and Ghosh S. (2005) Geomorphic characterization and diversity of the fluvial systems of the Gangetic plains. Geomorphology 70, 207–225. Sinha R., Tandon S. K., Sanyal P., Gibling M. R., Stuben D., Berner Z. and Ghazanfari P. (2006b) Calcretes from a Late Quaternary interfluve in the Ganga plains, India: carbonate

Chemical weathering in the Ganga plain types and isotopic systems in a monsoonal setting. Palaeogeogr. Palaeoclimatol. Palaeoecol. 242, 214–239. Sreenivasaprasad S. (1991) Biological profile of the Ganga phytoplankton. In The Ganga: A Scientific Study (ed. C. R. Krishnamurty et al.). Northern Book Centre, New Delhi, pp. 53–66. Srivastava P. (2001) Paleoclimatic implications of pedogenic carbonates in Holocene soils of the Gangetic plains, India. Palaeogeogr. Palaeoclimatol. Palaeoecol. 172, 207–222. Srivastava P., Prakash B. and Pal D. K. (1998) Clay minerals in soils as evidence of Holocene climatic change, Central Indo-Gangetic plains, North-Central India. Quat. Res. 50, 230–239. Stallard R. F. (1995) Tectonic, environmental and human aspects of weathering and erosion: a global review using a steady-state perspective. Annu. Rev. Earth Planet. Sci. 23, 11–39. Stallard R. F. and Edmond J. M. (1983) Geochemistry of the Amazon 2. The influence of geology and weathering environment on the dissolved load. J. Geophys. Res. 88, 9671–9688.

2355

Tipper E. T., Bickle M. J., Galy A., West A. J., Pomies C. and Chapman H. J. (2006) The short term climatic sensitivity of carbonate and silicate weathering fluxes: insight from seasonal variations in river chemistry. Geochim. Cosmochim. Acta 70, 2737–2754. Tripathi J. K., Ghazanfari P., Rajamani V. and Tandon S. K. (2007) Geochemistry of sediments of the Ganges alluvial plains: evidence of large-scale sediment recycling. Quat. Int. 159, 119– 130. Valdiya K. S. (1980) Geology of Kumaun Lesser Himalaya. Wadia Inst. Himalayan Geol., Dehradun, India, pp. 291. Walker J. C. G., Hays P. B. and Kasting J. F. (1981) A negative feedback mechanism for the long-term stabilization of the Earth’s temperature. J. Geophys. Res. 86, 9776–9782. West A. J., Bickle M. J., Collins R. and Brasington J. (2002) Smallcatchment perspective on Himalayan weathering fluxes. Geology 30, 355–358. Associate editor: Eric Oelkers