Chemical and strontium isotope characterization of rainwater in karst virgin forest, Southwest China

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    Received 12 April 2009Received in revised form13 October 2009Accepted 14 October 2009

    Keywords:Major ionsRainwater

    wide areas. For example, emissions of SO2 associated with coalcombustion have considerably increased in China since the late1970s, leading to signicant deposition of acid rain (Zhao et al.,1988). The estimated emissions of SO2 in China were about 22millionmetric tons in 2003 (Larssen et al., 2006). The investigations

    understand the dispersion (i.e. both local and regional scales) ofelements, whether pollutants or not, and their potential impact oneco-hydrosystems through deposition processes (Negrel et al.,2007). But chemical compositions are less sensitive to unravel thedetailed sources, which can be replenished by isotope studies(Andersson et al., 1990). Strontium isotopes (87Sr/86Sr) are expectedto provide insights into the sources of base cations in rainwaters(Herut et al., 1993; Aberg, 1995). However, integrated studies onthe chemistry and Sr isotope compositions of rainwaters are few,

    * Corresponding author. Tel.: 86 851 5891954; fax: 86 851 5891609.

    Contents lists availab

    Atmospheric E


    Atmospheric Environment 44 (2010) 174e181E-mail address: (G. Han).1. Introduction

    Acidic deposition, as a result of increasing worldwide industri-alization, has attracted more and more attention because of itsdirect adverse effects on ecosystems and indirect effects on humanhealth (Lara et al., 2001; Hu et al., 2003). Over the last few decades,China has undergone rapid and extensive industrial growthparticularly in the more densely populated. This has inevitably ledto dramatic increases in pollutant emissions to the atmosphere andconsequently the increases in deposition of these pollutants over

    of precipitation chemistry from many areas in China reveal signif-icant effects of acid rain over the last three decades (Zhao et al.,1988; Wang and Wang, 1995; Yu et al., 1998; Feng et al., 2001;Larssen et al., 2006; Han and Liu, 2006; Aas et al., 2007; Huanget al., 2008; Xu and Han, 2009). Aas et al. (2007) have noticed thatthe monitoring sites of air pollutants were mainly in urban areas,and emphasized that more integrated approach is required toaddress the acid rain problem in China (Aas et al., 2007).

    Chemical compositions of rainwaters can help distinguish themajor source types that contribute to rainwater chemistry andStrontium isotopesKarst virgin forest1352-2310/$ e see front matter 2009 Elsevier Ltd.doi:10.1016/j.atmosenv.2009.10.0194 4 3

    were measured for 52 rainwater samples collected in virgin forest in a rural region between May 2007and Dec. 2008. The rainwater pH values vary from 4.1 to 7.2 with a volume weight mean (VWM) value of5.40. 40 of 52 samples have pH value above 5.0, indicating that the regional rainwater was not acidic.Among anions and cations, sulphate concentration (40.4 meq l1, VWM) is the highest in the rainwater,followed by ammonium and calcium (30.2 and 20.8 meq l1, VWM). Rainwater quality is characterized bylow salinity and neutralized pH.The chemical compositions and 87Sr/86Sr ratios of the rainwater samples vary considerably. Using Na

    concentration as an indicator of marine origin, the proportions of sea salt and crustal elements wereestimated from elemental ratios. The 87Sr/86Sr ratios were used to characterize different sources base onthe data sets of this study and those from literatures. Such sources include weathering of limestone(87Sr/86Sr 0.7075), remote soil dust (87Sr/86Sr > 0.7135) and anthropogenic source (fertilizers:87Sr/86Sr 0.7079). The results of the present study suggest that one likely source for high ammoniumand calcium concentration is local soil. Due to a large contribution of these cations to the sulphateneutralization action, the rainwater in this region displays non-acidity, and thus has not signicantenvironmental impact. The wet precipitation in the karst virgin forest in Guizhou province is stronglyinuenced by natural sources rather than anthropogenic sources.

    2009 Elsevier Ltd. All rights reserved.Article history: Strontium isotope ratios and concentrations of Ca2, NH, Na, K, Mg2, Cl, SO2, NO and Al3, Sr2a r t i c l e i n f o a b s t r a c tChemical and strontium isotope charactforest, Southwest China

    Guilin Han a,*, Yang Tang a,b, Qixin Wu a,b, Qiu Tan c

    a The State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, CbGraduate School of Chinese Academy of Sciences, Beijing 100039, Chinac School of Geographic and Environmental Sciences, Guizhou Normal University, Guiyan

    journal homepage: www.eAll rights reserved.ization of rainwater in karst virgin

    se Academy of Sciences, Guiyang 550002, China


    le at ScienceDirect


    vier .com/locate/a tmosenv

  • limiting the application of Sr isotopes in atmospheric geochemistry(Dupre et al., 1994; Nakano and Tanaka,1997; Negrel and Roy, 1998;Chabaux et al., 2005; Han and Liu, 2006; Negrel et al., 2007; Xu andHan, 2009).

    In this study, we carried out comprehensive investigations, forthe rst time in our knowledge, of the Sr isotope compositionsintegrated with major element compositions of rainwaterscollected between May 2007 and Dec. 2008 from remote coun-tryside (the Maolan National Nature Reserved Park) in southeastGuizhou province, Southwest China. The chemical and isotopiccharacterizations of those rainwaters in karst virgin forest areexpected to provide representative background data in the Chineseacid rain control zone.

    2. Sampling site

    The Maolan area in Guizhou Province, southwest China ischaracterized by dense virgin evergreen forests growing on peakcluster karst. The study site is in the Maolan National NaturalReserved Park (between 250902000and 252005000of latitude N and1075201000 and 1080504000of longitude E; Fig. 1). This area coversapproximately 200 km2, mostly mountains of jagged carbonaterock with a forest-covering rate of more than 90%. A karst forest ofsuch magnitude is rarely seen in the world. The study site is locatedin the middle part of ChongqingeGuiyangeLiuzhou acid raincontrol zone (Hao et al., 2001). Annual rainfall at the virgin forestsite is about 1750 mm; ca 80% of which is precipitated inmonsoonal rainy season from April to September (Zhou, 1987). Themean annual air temperature at the study site is about 17 C withsummer (JuneeAugust) and winter (DecembereFebruary).

    The rock exposures in the study area are represented by sedi-

    middle and lower Carboniferous ages. The geological map of thestudy area is shown in Fig. 1.

    3. Sampling and analytical procedure

    The rainwater samples were collected manually at the begin-ning of each precipitation event by a 20-l Polypropylene bottleplaced approximately 150 cm above the building roofs. The Poly-propylene bottles were pre-cleaned by rinse using 2e3 N HCl fol-lowed by Milli-Q water (18.2 MUcm) and then dried. In order tominimize contributions from dry fallout, the samplers were openedafter the onset of rainfall as quickly as possible. 52 rainwatersamples were collected fromMay 2007 to Dec. 2008. Most of themwere collected in the rainy season from June to August.

    The pH values were measured instantaneously at the end of therain events at sampling sites with a portable pH meter. All the rain-water samples were ltered through 0.22 mm membrane lters(Millipore). A small aliquot of each sample was stored for anionmeasurements, another aliquot was acidied with ultra-puriednitric acid to pH< 2 after collection formeasurements of cations andSr isotopic compositions. Major cations (K, Na, Ca2 and Mg2)were determined by inductively coupled plasma-optical emissionspectrometry (ICP-OESVarianVistaMPX)withaprecisionbetter than5%. NH4

    concentrationwas determined by spectrophotometer usingthe Nessler method. The concentrations of Al3 and Sr2 weremeasured by ICP-MS (VG POEMS III) with a precision of 5%. Anions(Cl, SO42 and NO3) were measured by ionic chromatography (Dio-nex DX-120) with a precision of 5%. Reagent and procedural blankswere determined in parallel to the sample treatments using identicalprocedures. Each calibration curvewas evaluated byanalyses of thesequality control (QC) standards before, during and after the analyses of

    G. Han et al. / Atmospheric Environment 44 (2010) 174e181 175mentary formations of limestone, dolomite, sandstone and clay inFig. 1. Sketch map showing the lithology of Maolan National Nature Reserved Paa set of samples. The analytical precision was better than 5%.rk and sampling locations. 1. Limestone, 2. Dolomite, 3. Sandstone, 4. River.

  • Separation of Sr from other major elements for isotopic analysiswas carried out by a conventional ion-exchange technique usinga Dowex 50W-X8 200e400 mesh resin in HCl media. A total blanksfor Sr is less than 0.5 ng, negligible with regard to the total Sranalyzed in rainwaters. The isotopic compositions of Sr weredetermined by TIMS (IsoProbe T, GV Instrument) with nine Faradaycollectors in the Institute of Geochemistry, Chinese Academy ofSciences. The 87Sr/86Sr ratios were normalized to 86Sr/88Sr 0.1194.The value 87Sr/86Sr for the standard NBS987 was 0.710235 0.000018 (2s, n 50) during the measurement period of oursamples.

    4. Results and discussions

    The eld data, concentrations of major ions, aluminum andstrontium, and 87Sr/86Sr ratios are given in Table 1.

    4.1. pH

    The pH values of rain samples range between 4.1 and 7.2. Mostsamples show pH value from 5.0 to 5.6, while 12 rainwater sampleshave pH values smaller than 5.0. The mean pH value observed inMaolan was 5.4. The highest acidity was observed on August 24th,2008 with a pH of 4.1, and the lowest acidity was on July 24th, 2007with a pH of 7.2. According to previous studies, the naturallyexisting