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

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Chemical weathering in the plain and peninsular sub-basins of the Ganga: Impact on major ion chemistry and elemental fluxes
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  Chemical weathering in the plain and peninsular sub-basinsof the Ganga: Impact onmajor ion chemistry andelemental 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  87 Sr/ 86 Sr 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 Gangaand silicate and carbonate erosion of the Ganga basin. The results show high concentrations of Na and Sr in the Gomti, theYamuna and the Ganga (at Varanasi) with much of the Na in excess of Cl. The use of this ‘excess Na’ (Na* = Na riv  Cl riv ) acommon index of silicate weathering yield values of   18 tons km  2 yr  1 for silicate erosion rate (SER) in the Gomti and theYamuna basins. There are however, indications that part of this Na* can be from saline/alkaline soils abundant in theirbasins, raising questions about its use as a proxy to determine SER of the Ganga plain. Independent estimation of SER basedon dissolved Si as a proxy give an average value of   5 tons km  2 yr  1 for the peninsular and the plain drainages, several timeslower than that derived using Na*. The major source of uncertainty in this estimate is the potential removal of Si from riversby biological and chemical processes. The Si based SER and CER (carbonate erosion rate) are also much lower than that inthe Himalayan sub-basin of the Ganga. The lower relief, runoff and physical erosion in the peninsular and the plain basinsrelative 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 high-light 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 6 10% of the major ion fluxes of the Ganga at Rajma-hal during high river stages (October). The impact of both saline/alkaline soils and anthropogenic sources on the major ionabundances of the Ganga is minimum during its peak flow and therefore the SER and CO 2  consumption rates of the river isbest determined during this period.   2010 Elsevier Ltd. All rights reserved. 1. INTRODUCTION The suggestion that silicate weathering in youngorogenic belts such as the Himalaya is a key driver of climate change over million year time scales (Walkeret al., 1981; Raymo and Ruddiman, 1992; Ruddiman,1997) led to a number of studies on the chemical and isoto-pic composition of two major global river systems drainingthe Himalaya, the Ganga and the Brahmaputra (Sarinet al., 1989, 1992; Krishnaswami et al., 1992, 1999; Palmerand 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 associatedatmospheric CO 2  drawdown in their basins. Between thesetwo rivers, the Ganga is investigated in relatively more de-tail, with many of the studies focusing on the Himalayan 0016-7037/$ - see front matter    2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.gca.2010.01.008 * Corresponding author. Tel.: +91 7926314307; fax: +917926314900. E-mail address:  sunil@prl.res.in (S.K. Singh). 1 Present address: Wadia Institute of Himalayan Geology,Dehradun, Uttarakhand 248 001, India. www.elsevier.com/locate/gca  Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 74 (2010) 2340–2355  sub-basin of the Ganga drainage and a few others coveringalmost the entire stretch of the Ganga river system, from itssrcin 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 chem-ical and silicate erosion rates integrated over the entireGanga basin spread over the Himalaya, the peninsularIndia and the Ganga plain, however information on theerosion rates in the plain and peninsular sub-basins, whichaccount for   80% of the area is limited. As a result, thesignificance of chemical erosion in these sub-basins, partic-ularly the role of various lithologies including saline– alkaline soils in contributing to the fluxes of elementstransported by the Ganga to the Bay of Bengal is not wellquantified. Further, the influence of anthropogenic activi-ties in the Ganga plain, one of the most densely populatedregions of India, on the major ion chemistry of the Gangaand its impact on estimates of silicate erosion rates is alsoonly poorly constrained. Galy and France-Lanord (1999)based on material balance of elemental fluxes transportedby the Ganga at its mouth and those supplied to it by itstributaries in the Himalaya, inferred that the chemical ero-sion rate in the Ganga plain is much lower than that in theHimalaya. West et al. (2002) by comparing weatheringfluxes from small Himalayan catchments with the weather-ing fluxes from the Ganga concluded that more cations aredissolved from the Higher Himalayan silicates in the plainsthan  in-situ  in the Higher Himalaya.The aim of the present work is to determine silicateand carbonate erosion rates in the peninsular and plainsub-basins of the Ganga and compare them with erosionin its Himalayan sub-basin. Further, attempts also havebeen made to assess the impact of saline–alkaline soilsand anthropogenic activities on the water chemistry of theGanga. This work is based on the major ion compositionand Sr isotope systematics of the Gomti, the Son and theYamuna, tributaries of the Ganga which predominantlydrain its plain and peninsular sub-basins and the Ghaghra,the Gandak and the Kosi, its Himalayan tributaries, whichhave part of their drainage prior to their confluence withthe Ganga in the plain (Fig. 1). 2. GEOHYDROLOGY OF THE GANGA BASIN The geohydrology of the Ganga particularly that of itsheadwaters and tributaries in the Himalaya has been dis-cussed in detail in many earlier publications on its waterand sediment chemistry (e.g. Sarin et al., 1989; Galy andFrance-Lanord, 1999; Bickle et al., 2003; Singh et al.,2008). The emphasis in this paper is more on the geohydrol-ogy of rivers draining the peninsular basin and the Gangaplain. The characteristics of rivers studied are given inTable 1. 2.1. Hydrological setting The Gomti is a rain fed river with its entire drainage inthe Ganga plain. It srcinates from the reservoir Madho-Tanda (Fig. 1; Miankot, 28  34 0 N, 80  07 0 E) located  50 km south of the foothills of the Himalaya. It drains    K  a  r  n  a   l   i   K a  l  i     B  e  t  w a        K     e     n DELHI      Y    a    m    u    n    a  Allahabad VaranasiGhazipur PatnaRevilganj G  o  m  t  i    Son N  K  o e l   A     r    u    n     G    a   n   d    a   k         N   a   r  a  y a ni  G  h  a   g  h  r  a   Gorakhpur  R  a   p  t   i    DumarighatHazipur   C h a m b a l   S  i  n d Nainital T  I  B  E  T   N  E  P  A L  76 0 88 0 84 0 80 0 32 0 28 0 24 0 GANGA BASIN      B     A     N     G      L     A     D     E     S      H FarakkaRajmahal BR-318BR06-501 BR-327 BR-309 BR-342BR-336BR-311 BR-375 B   R-388 Y   a  m  u  n  a   G     a    n     g    a     BR-363BR06-12-1BR-346 Sample siteCity/Town RW03-2 Lucknow Madho-Tanda Bar auni BR-315 R    a   m    g   a   n    g   a    S   u  n  K   o  s  i         B    a    g    m    a     t     i  T  a   m    u       r     K    o   s   i S    e   t    i      T  r   i  s  u   l   i   B   h  e  r   i B     u    r    h     i          G     a    n    d      a    k     i          K  a   l   i G  a   n   d  a   k   i M     a   r    s     y    a   n   d     i      Narainghat     K   o   s    i Haridwar RishikeshTajew ala RW03-5 Koilawar DsrcanjBatamandiDevprayagGangotri RW03-4RW03-3BR06-14-1BR06-13-1BR06-10-1BR06-801BR06-301BR06-401BR06-104BR06-11-1BR06-901BR06-705BR06-201BR06-601  Amarkantak Fig. 1. Sampling locations of water samples from the Ganga mainstream and its tributaries. Samples were collected all along the Ganga, fromits source near Gangotri to its outflow at Rajmahal.Chemical weathering in the Ganga plain 2341  the interfluve sediments between the Ganga and the Gha-ghra, prior to joining the Ganga at Udyar Ghat near Ghaz-ipur,  30 km downstream of Varanasi (Fig. 1). Most of theannual water discharge of the Gomti occurs during thesouth west monsoon. Average annual rainfall in the Gomtibasin, during 1997–2006, is  1025 mm. whereas its runoff isonly   240 mm yr  1 . Thus, about 75% of rainfall in theGomti drainage is either returned to the atmospherethrough evaporation/evapotranspiration and/or stored asgroundwater.The Yamuna is the largest tributary of the Ganga interms of drainage area and accounts for   40% of it. Anal-ogous to the Gomti, nearly 75% of the Yamuna dischargealso occurs during June–September, the south west mon-soon period. The Chambal, the Sind, the Betwa and theKen are the major tributaries of the Yamuna, all of whichsrcinate in and flow almost entirely through peninsular In-dia (Fig. 1), as a result  63% of the Yamuna drainage is inthe peninsular basin and  34% in the Ganga plain. Rainfallin the drainage basin of the Yamuna and its runoff showsignificant spatial variation with high values in the Hima-laya and low in the Ganga plain and the peninsular region(Table 1).The Son is another tributary of the Ganga with its drain-age predominantly in peninsular India. A part of the Sonbasin (  15% based on global mapper) prior to its conflu-ence with the Ganga is in the Ganga plain. The Son srci-nates from the Amarkantak in the Bundelkhand plateauand merges with the Ganga   25 km upstream Patna(Fig. 1). The headwaters of the Son in the mountainous re-gion have a steep slope compared to its outflow in the Gan-getic plain (Table 1). The Son is also a rain fed river withmost of its flow during monsoon.In addition to these three tributaries, studies of theGanga from its source near Gangotri to its outflow nearthe Indo-Bangladesh border and its major Himalayan trib-utaries also have been carried out to determine their contri-bution of major ions and Sr to the Ganga at its out flow. Of the total drainage area of the Ganga up to Farakka,  935  10 3 km 2 (http://www.grdc.sr.unh.edu),   20% liesin the Himalaya,   33% in peninsular India and the restin the Gangetic plain. The runoff among the different re-gions of the Ganga basin varies from   1 m yr  1 in theHimalayan drainage (Donald, 1992) to   0.3 m yr  1 in theGangetic plain and peninsular India (Rao, 1975); this differ-ence helps assess the impact of runoff on chemical erosionrates. 2.2. Geological setting The Ganga catchment is spread over the Himalaya inthe north, peninsular India in the south and the Gangeticplain between them. The headwaters of the Ganga, its trib-utaries the Ghaghra, the Gandak and the Kosi have a ma- jor part of their drainage in the Himalaya. The Yamunadrains all the three sub-basins whereas the Gomti and theGanga downstream of Haridwar drain only the Gangaplain. The lithology of the peninsular basin and the Gangaplain is discussed below; for details of the Himalayan basinreference is made to Gansser (1964), Valdiya (1980), Sarinet al. (1989), Bickle et al. (2003) and Singh et al. (2008). The peninsular drainage:  The major lithologies exposedin this drainage are the Bundelkhand crystallines (granites),the Vindhyan sediments (carbonates, shales and sand-stones) and the Deccan basalts (Krishnan, 1982; Singhet al., 2008). The tributaries of the Yamuna (Chambal,Sind, Betwa and Ken) and the Son though drain peninsularIndia, they flow through different lithologies. The Yamunatributaries drain the Deccan Traps and the Vindhyanswhereas the Son lies mainly in the Vindhyan–Bundelkhandplateau. The tributaries of the Son drain the Gondwanasedimentary sequences comprising of sandstones, shalesand carbonates (Krishnan, 1982). The Ganga plain  is a major alluvial tract (Singh, 1996)formed by the accumulation of detritus from the Higherand Lesser Himalaya (Singh, 1996; Galy and France-La-nord, 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 peninsulardrainage and the Ganga plain contain evaporites formed lo-cally from river and floodwaters and ground water throughcapillary action. The evaporation of these waters precipi-tates their solutes, with silica and carbonates likely to de-posit initially. These carbonates known as  “ kankar ” accumulate as layers or irregular concretions in the soilcolumn. The residual water on subsequent evaporation Table 1Physical features and details of rivers of the Ganga system.River Location Area Rainfall b Discharge a Runoff Slope b 10 3 km 2 mm yr  1 10 12 L yr  1 mm yr  1 mm km  1 Ganga Rishikesh 21.7 1000–1600 23.9 1100 2300–45,000Yamuna Tajewala 9.6 800–1400 10.8 1130 500–30,000Ghaghra Revilganj 128 400–1400 94.4 740 300–33,000Gandak Hazipur 46.3 600–1600 52.2 1130 300–35,000Kosi Dumarighat 74.5 200–1400 62 830 110–38,000Gomti Ghazipur 30.5 800–1200 7.4 240 100–550Son Koilawar 71.3 800–1200 31.8 450 100–1500Yamuna Allahabad 366 400–1400 93 250 300–30,000Ganga Allahabad – – 59 – – Ganga Rajmahal 935 200–1600 380 410 60–45,000 a From Rao (1975) and Jain et al. (2007); the discharge at Rajmahal (taken same as at Farakka) from http://www.grdc.sr.unh.edu. b Based on http://disc2.nascom.nasa.gov/Giovanni/tovas/TRMM_V6.3B43.shtml; Sinha et al. (2005) and Bookhagen and Burbank (2006). 2342 S.K. Rai et al./Geochimica et Cosmochimica Acta 74 (2010) 2340–2355  deposits various sodium salts, chloride, sulfate and carbon-ate leading to the formation of alkaline/saline soils contain-ing up to a few percent of sodium salts (Agarwal andGupta, 1968; Agarwal et al., 1992; Bhargava and Bhatta-charjee, 1982; Chhabra, 1996; Datta et al., 2002; Palet al., 2003; Srivastava, 2001 Singh et al., 2006a,b; Singh,2005). The excessive use of river and groundwater for irri-gation in recent years has further aggravated the problem(Chhabra, 1996; Singh, 2005). The abundance and compo-sition of these soil salts varies significantly within andamong the river basins (Fig. 2) and is generally moretowards the western basin due to the conducive semi-aridclimate and basin characteristics. For example, they occurextensively in the Gomti basin, in the interfluve region be-tween the Yamuna and the Ghaghra, in the Yamuna basinbetween Delhi and Allahabad, particularly in and aroundthe 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 depositingepisodically, the recent significant deposition occurringaround   10,000 years BP coinciding with the cold arid cli-mate (Mohindra, 1995). The presence of saline soils alongwith these carbonates in the Ganga plain (Mohindra,1995; Srivastava et al., 1998) seems to indicate that thesesoils have been developing since this period. 3. SAMPLING AND ANALYSIS Surface water samples from the Gomti, the Son, theYamuna and the Ganga mainstream between Gangotriand Rajmahal (near Farakka, Fig. 1) and its Himalayantributaries were collected during three field campaigns con-ducted in May 2003 and 2004 (summer) and October 2006(post/tail-end of monsoon). The rivers are generally in theirlow stages in May and high stages in October. The dis-charge of the Ganga during October is   12% of its annualflow. After collection the samples were brought to labora-tory and two separate aliquots each of    500 mL were fil-tered using 0.2  l m nylon membrane Millipore filters. Oneof the filtered aliquot was acidified to pH   2 with dou-ble-distilled HNO 3  for cation and Sr analysis. The other fil-tered aliquot was kept unacidified for anion measurements.A separate aliquot of   250 mL of river water was collectedand stored as such for alkalinity measurements.Temperature and pH of the water samples were mea-sured at site in two of the three field campaigns followingearlier 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 acidtitration, Cl, NO 3  and SO 4  by ion chromatography, Kand Na by flame AAS, Ca and Mg by ICP-AES and ion 0-4 (Area in sq. Km)4-4040-200200-400Over 400     K  a   r   n  a    l    i   K a  l  i DELHI       Y     a     m     u     n     a AllahabadVaranasiGhazipur Patna G   o  m  t   i    Son A      r     u     n      G     a    n    d     a    k     G   h  a   g   h  r   a   S     a    r     d      a     Dum ariaghat Hazipur   C  h a m  b a  l 76 0 88 0 84 0 80 0 32 0 28 0 24 0 Rajm ahal Y    a  m  u  n  a   G      a     n      g     a      Barauni Kanpur  S   u   n  K    o  s  i     B     a     g    m    a    t     i         T   a    m     u        r      K   o    s    i     T   r    i  s   u    l    i    B    h  e   r    i      K    o    s     i RishikeshUttarkashiGangotriKoilaw ar Dsrcanj INDIASALT AFFECTED SOILS 5000 KM Saline SoilAlkali Soil 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 andPatna and in the basins of the Gomti and the Yamuna are evident.Chemical weathering in the Ganga plain 2343  chromatography and Si by spectrophotometry of its molyb-denum blue complex. The precision of these measurements,based on repeat measurements and earlier studies in thislaboratory is better than  ±5%. Sr was measured in filteredand acidified samples using graphite furnace AAS (May2003 and 2004) and isotope dilution mass-spectrometry(October 2006). The accuracy and precision for graphiteAAS is better than   5%, whereas for mass-spectrometrictechnique it is better than   0.4%.Sr isotope measurements were made following publishedprocedures (Rai and Singh, 2007; Singh et al., 2008; Rai,2008). This involved the separation of Sr from a suitablevolume of water and the measurement of its isotopic com-position using ISOPROBE-T thermal ionization mass spec-trometer (TIMS) in static multi-collection mode. Theprocedural blanks were 2–3 orders of magnitude lower thanthe total Sr analyzed in the samples, hence the river waterdata are not corrected for blank. 4. RESULTS The locations of the river water samples are presented inFig. 1. The temperature, pH, concentrations of major ions,silica, TDS, Sr and  87 Sr/ 86 Sr ratio of the Gomti, the Sonand the Yamuna are given in Table 2, the data for theGanga mainstream and its principal Himalayan tributariesare given in Table 3. In many samples from the Himalaya,pH and temperature could not be measured due to failureof the probe.Temperature of waters collected during summer (May2003 and 2004) falls in two groups, one at   17.5   C andthe other at  31   C, the cooler ones being the Ganga head-waters. The pH ranges from 7.9 to 8.6, suggesting the alka-line nature of the waters. The major ion data (Tables 2 and3) show a good balance between total cations (TZ + ) andanions (TZ  ). The NICB, normalized inorganic chargebalance, [(TZ +  TZ  )/TZ  ] for the Gomti, the Son andthe Yamuna is <3% (Table 2), well within the precision of measurements, which leads to conclude that the ions mea-sured by and large account for the charge balance. Thisinference also seems valid for the other samples (Table 3),excluding RW03-5 and BR-311 for which TZ  exceedsTZ + by 16% and 13%, respectively, an observation that isdifficult to interpret with the present data. 5. DISCUSSION5.1. General observations The major ion chemistry of the Gomti sampled duringboth May and October is nearly the same (Table 2) withhigh concentrations of Na, Mg and Ca and highest TDSamong the October samples. In the Yamuna (Allahabad)the May sample has the highest TDS, Na, Mg, Cl and Sramong the samples analyzed (Table 2). The decrease in ma- jor ions and TDS in the Yamuna during October is dilutioneffect caused by monsoon. The impact of this, however isnot discernible in the Gomti, a cause for this may be redis-solution of salts from the basin formed by evaporation dur-ing summer and/or anthropogenic inputs. The major ioncomposition of the Gomti and the Yamuna measured inthis 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 theGanga shows high concentration of Na and moderatelyhigh values of Mg, Ca and TDS at Allahabad/Varanasiduring May (Table 3). For example, Na increases from106  l M at Rishikesh to 2380  l M at Varanasi. In October,the sampling was done only downstream of Allahabad,which shows that Na is highest at Varanasi. This can resultfrom chemical weathering along the course of the Gangafrom Rishikesh to Varanasi and/or due to its supply from Table 2Major ions, TDS, Sr and  87 Sr/ 86 Sr in the Gomti, the Son and the Yamuna rivers.No. River(location)Date(dd.mm.yy)Location (N; E) Na K Mg Ca Cl NO 3  SO 4  HCO 3  SiO 2  TDS,mg L  1 NICB a Sr,nM 87 Sr/ 86 Sr l M Gomti (Ghazipur) 1 BR-375 16.05.04 25  30.35 0 ;83  08.36 0 1354 127 894 872 400 17 252 4193 – 388   2.0 3173 0.727592 BR06-11-1 19.10.06 25  30.28 0 ;83  08.45 0 1399 130 848 859 324 7 200 4286 92 390   1.5 2717 0.72714 Son (Koilawar )3 BR06-201 16.10.06 25  33.81 0 ;84  47.56 0 431 43 253 537 116 11 50 1833 161 170   0.3 944 0.72504 Yamuna (Batamandi) b 4 RW99-5 June 99 306 49 661 967 73 6 556 2508 193 285   2.4 2044 – 5 RW98-4 October 98 255 52 497 1019 60 35 333 2369 211 254 6.7 1802 0.72356 Yamuna (Allahabad) 6 BR-346 14.05.04 25  25.16 0 ;81  50.19 0 3575 136 1104 794 1493 0 335 5300 325 574 0.6 5889 0.714677 BR06-13-1 20.10.06 25  25.33 0 ;81  50.26 0 1275 85 443 673 696 50 181 2575 129 280   2.5 2717 0.71239 a NICB, normalized inorganic charge balance (see text). b At foot-hills of the Himalaya, Dalai et al. (2002).2344 S.K. Rai et al./Geochimica et Cosmochimica Acta 74 (2010) 2340–2355
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