Geochemical and isotopic characteristics of shallow groundwater within the Lake Qinghai catchment, NE Tibetan Plateau

Major ions, isotopic ratios of strontium (Sr-87/Sr-86), hydrogen (delta D) and oxygen (delta O-18) of groundwater samples were analyzed to decipher spatial variation, controlling factors, solute sources, and the recharge source of shallow groundwaters within the Lake Qinghai catchment. Shallow groundwaters in this area are slightly alkaline, with 97% being fresh water of good quality, though there are high concentrations of nitrate and sulfide in Buha and lakeside groundwaters. Most of the shallow groundwaters are of the Ca2+-HCO3- type, whereas part of groundwaters surrounding the lake (LS) belongs to the Na+-Cl- type as lake water (QHL). Groundwater geochemistry is controlled by regional lithological association, ion exchange, and mineral precipitation. The dissolved Sr concentrations and Sr-87/Sr-86 ratios vary from 1.0 to 15.6 mu mol/L, and from 0.709859 to 0.715779, respectively. The first quantitative calculation in groundwater using a forward model shows that 40% of dissolved Sr is from carbonate weathering, 33% from evaporite dissolution, 17% from silicate weathering, and the remainder from atmospheric input for the whole catchment. Carbonate weathering dominates groundwater geochemistry in Shaliu (SL), Hargai (HG) and Buha (BR) samples, while evaporite dissolution dominates LS and Daotang (DT) samples. delta D and delta O-18 data show that rain water is the major recharge source of both river water and shallow groundwater within the Lake Qinghai catchment. Qinghai Lake water is characterized by Buha-type water, but its Sr geochemistry is different from the shallow groundwater due to carbonate precipitation. Although shallow groundwater contributes similar to 5% of the dissolved Sr to the QHL, groundwater must be taken into account when the chemistry and budget of lake water are characterized.</p

Solute geochemistry and its sources of the groundwaters in the Qinghai Lake catchment, NW China

Major ion compositions were analyzed in 65 groundwater samples to decipher water quality, solute geochemistry and sources of groundwater within the Qinghai Lake catchment, NE Tibetan Plateau. Ground-waters were slightly alkaline with pH varying from 7.2 to 8.7. The total dissolved solids (TDS) varied over two orders of magnitude from fresh (88%) to brackish (12%) with a mean value of 672 mg/L, higher than river waters within the Qinghai Lake catchment and river waters draining the Himalayas and the southeastern Tibetan Plateau. Most of the samples, approximately 80%, were the Ca2+-Mg2+-HCO3- type and suitable for drinking and irrigation. Some of the Lakeside, the Buha and the Qinghai Lake water were the Na+-Cl- type and not suitable for drinking and irrigation. Water quality of Hargai and Shaliu samples was better than the others. Rock weathering, ion exchange and precipitation are the major geochemical processes responsible for the solutes in the groundwater within the Qinghai Lake catchment. Anthropogenic input to the groundwater is minor. The forward models on the input of groundwaters from various sources showed that the contributions were 40.1% for carbonate weathering, 29.0% for evaporite dissolution, 16.8% for atmospheric input and 14.1% for silicate weathering of the total dissolved cations for the whole catchment. Evaporite dissolution was dominating in the Lakeside, the Buha and the Daotang samples, contributing 15.3-45.9%, 20.4-61.1% and 21.3-63.9% of the total dissolved cations. respectively. Carbonate weathering was dominated in the Shaliu and the Hargai samples, contributing 23.9-71.7% and 31.8-95.4% of the total dissolved cations, respectively. The result also demonstrated that carbonate weathering had higher contribution to the groundwaters than silicate weathering.</p

Major ion geochemistry of shallow groundwater in the Qinghai Lake catchment, NE Qinghai-Tibet Plateau

Conventional hydrochemical techniques and statistical analyses were applied to better understand the solute geochemistry and the hydrochemical process of shallow groundwater in the Qinghai Lake catchment. Shallow groundwater in the Qinghai Lake catchment is slightly alkaline, and is characterized by a high ion concentrations and low water temperature. The total dissolved solids (TDS) in most of the samples are &lt; 1,000 mg/L, i.e. fresh water and depend mainly on the concentration of SO4 (2-), Cl- and Na+. Groundwater table is influenced directly by the residents&#39; groundwater consumption. Most of the groundwaters in the Qinghai Lake catchment belong to the Ca2+(Na+) -HCO3 (-) type, while the Qinghai Lake, part of the Buha (BHR) and the Lake Side (LS) samples belong to the Na+-Cl- type. The groundwater is oversaturated with respect to aragonite, calcite and dolomite, but not to magnesite and gypsum. Solutes are mainly derived from strong evaporite dissolution in Daotang, BHR and LS samples and from strong carbonate weathering in Hargai and Shaliu samples. Carbonate weathering is stronger than evaporite dissolution with weak silicate weathering in the Qinghai Lake catchment. Carbonate weathering, ion exchange reaction and precipitation are the major hydrogeochemical processes responsible for the solutes in the groundwater in the Qinghai Lake catchment. Most of the shallow groundwaters are suitable for drinking. More attention should be paid to the potential pollution of nitrate, chloride and sulfide in shallow groundwater in the future.</p

Geochemistry and solute sources of surface waters of the Tarim River Basin in the extreme arid region, NW Tibetan Plateau

Major ion concentrations of river, lake and snow waters were measured to better understand the water quality, hydrochemical processes and solute sources of surface waters within the Tarim River Basin in the extreme arid region. Surface waters are slightly alkaline and are characterized by high total dissolved solids (TDS). TDS values varies over two orders of magnitude from fresh (76%) to brackish (24%) with a mean value of 1000 mg/L, higher than the global river average and river waters draining the Himalayas and the southeastern Tibetan Plateau. Most of the samples were Ca(2+)-(Mg(2+))-HCO(3)(-) type and suited for drinking and irrigation. Water quality of Aksu River (AK), Hotan River (HT) and Northern Rivers (NR) is better than the others. Rock weathering, ion exchange and precipitation are the major hydrogeochemical processes responsible for the solutes in rivers waters. Anthropogenic input to the water chemistry is minor and human activities accelerate increase of river TDS. The quantitative solute sources are first calculated using a forward model in this area. The results show that evaporite dissolution, carbonate weathering, atmospheric input, and silicate weathering contributed 58.3%, 25.7%, 8.7%, and 8.2% of the total dissolved cations for the whole basin. Evaporite dissolution dominated in Lake Waters (LW), HT, Yarkant River (YK), Tarim River (TR), and Southern Rivers (SR), contributing 73.5%, 53.4%, 56.7%, 77%, and 74.2% of the total dissolved cations, respectively. Carbonate weathering dominated in AK and NR, contributing 48% and 44.4% of the total dissolved cations, respectively. The TDS flux of HT, TR, AK, YK was 66.0, 118.6, 134.9, and 170.4 t/km(2)/yr, respectively, higher than most of the rivers in the world. Knowledge of our research can promote effective management of water resources in this desert environment and add new data to global river database.</p

Hydrogeochemical processes between surface and groundwaters on thenortheastern Chinese Loess Plateau: Implications for water chemistryand environmental evolutions in semi-arid regions

A large area of the continents is covered by loess that is subject to fast erosion, but detailed research is lacking about processes influencing the hydrogeochemistry in loess-covered regions. This study presents the first &delta;18O and &delta;D, and major ion contents of various waters (rain, rivers, lake, springs and wells) from Daihai Lake catchment on the Chinese Loess Plateau (CLP). In combination with historical hydrological and meteorological data during the past 60 years, we investigate factors affecting water chemistry and lake evolution on the CLP, and thereby provide insight into hydrogeochemical processes under semi-arid climatic conditions. On the northeast CLP, river and groundwaters show elevated TDS (450 and 461 mg/L, respectively), about ~4 times higher than the global river mean value. Their water chemistry is dominantly influenced by carbonate weathering. Lake waters show even higher TDS at 5758 mg/L, ~50 times of the global mean, which is attributed to strong evaporation and associated with precipitation of calcite and dolomite. The order of carbonate (calcite and dolomite) saturation indexes follows lake water N river water N groundwater. Downstream rivers to the west of Daihai Lake are characterized by elevated SO4 2&minus;, indicating high lake levels in the past. Comparison of &delta;18O, &delta;D, TDS and ion contents of river and groundwaters implies that shallow groundwaters are derived from surface runoff via fast infiltration, a hydrological process different from limited infiltration of groundwater on the Tibetan Plateau. Water quality assessment indicates that all river and 79% of well waters belong to moderately hard to hard-fresh waters, suitable for drinking and irrigation of plants with moderate salt tolerance. In contrast, all lake and spring waters and 21% of well waters belong to very bad water quality, and hence are not suitable for irrigation or drinking. Owing to intensified human activities under drying and warming climate, the lake water level of Daihai declined by 4.83 m from 1955 to 2003, and started to accelerate since 1980. As a result, TDS and Cl&minus; increased by 1.8 times and Mg/Ca rose by 156 times from 1953 to 2010. Our data show that environment degradation poses a great threat to human occupation on the CLP. Considering the uniformity of loess, hydrogeochemical processes between surface and groundwaters on the loess regions may represent a widespread status of the CLP.</p

Increasing dust fluxes on the northeastern Tibetan Plateau linked withthe Little Ice Age and recent human activity since the 1950s

Arid and semi-arid areas in inner Asia contribute lots of mineral dust in the northern hemisphere, but dust flux evolution in the past is poorly constrained. Based on particle sizes and elemental compositions of a sediment core from Lake Qinghai on the northeastern Tibetan Plateau, dust fluxes during &sim;1518&ndash;2011 A.D. were reconstructed based on 18&ndash;100&nbsp;&mu;m fractions of the lake sediment. The dust fluxes during the past &sim;500&nbsp;years ranged between 100 and 300&nbsp;g/m2/yr, averaging 202&nbsp;g/m2/yr, experiencing four stages: Stage 1 (&sim;1518&ndash;1590s), the flux was averaged 165&nbsp;g/m2/yr, much lower than that in the Stage 2 (1590s&ndash;1730s, 254&nbsp;g/m2/yr); similarly, an average flux of 169&nbsp;g/m2/yr in the Stage 3 (1730s&ndash;1950s) was followed by an increased flux of 259&nbsp;g/m2/yr in the Stage 4 (1950s&ndash;2011). During the first three stages the fluxes were dominated by natural dust activities in arid inner Asia, having a positive relation with wind intensity but a poor correlation with effective moisture (or precipitation) and temperature. The high dust flux in Stage 2 was due to relatively strong wind during the maximum Little Ice Age, whereas the remarkably high flux in 1950s&ndash;2011 was resulted from recent increasing human activities in northwestern China. The dust record not only documents past dust fluxes on the northeastern Tibetan Plateau but also reflects evolutions and mechanisms of dust activity/emission in inner Asia during the past &sim;500&nbsp;years.</p

Accurate and Precise Determination of Boron Isotopic Ratios at Low Concentration by Positive Thermal Ionization Mass Spectrometry Using Static Multicollection of Cs2BO2+ Ions

A static double-collector system for accurate, precise, and rapid boron isotope analysis has been established by employing a newly fixed Faraday H3 and H4 cup enabling simultaneously collected Cs2BO2+ ion beams (m/z = 308 and 309) on a Finnigan-MAT Triton thermal ionization mass spectrometer of boron (Triton B). The experimental result indicated that Cs2BO2+ ion beams (m/z = 308 and 309) were simultaneously collected using a fixed Faraday H3 and H4 cup without using the &quot;Zoom Quad&quot; function and reduced accelerating voltage. Furthermore, the method enabled the measurement of samples containing as little as 20 ng of boron. An analysis of the National Institute of Standards and Technology standard reference material (NIST SRM) 951 standard showed external reproducibility (2RSD) of +/- 0.013 parts per thousand, +/- 0.013 parts per thousand, and +/- 0.019 parts per thousand for 100, 50, and 20 ng of boron, respectively. The present method of static multicollection of Cs2BO2+ ions is applicable to a wide field of boron isotopic research that requires high precision and accuracy to analyze samples with low boron concentrations, including pore fluids, foraminifera, rivers, rainwater, and other natural samples.</p

The significant role of inorganic matters in preservation and stability of soil organic carbon in the Baoji and Luochuan loess/paleosol profiles, Central China

The preservation and stability mechanisms of soil organic carbon (SOC) are the important factors to evaluate the capacity of soil carbon pool and the potential of sustainable utilization. To understand the preservation time and mechanisms of SOC under burial conditions, in the present study, the distributions of total organic carbon (TOC) and stable organic carbon (StOC), and their correlations with the contents of clays and clay minerals and different forms of iron oxides were investigated in the Baoji and Luochuan loess-paleosol profiles. Four facts were observed as the followings. (1) The labile SOC almost was decomposed and the mostly stable SOC was preserved in the loess and paleosol after 375 kyr since their formation. StOC could be preserved at least 762 kyr in both loess and paleosol under burial condition. (2) The TOC was positively correlated with clay contents, with correlation coefficients of 0.72 (Baoji) and 0.63 (Luochuan). (3) The TOC, StOC, mineral-protected organic carbon (MOC), and recalcitrant organic carbon (ROC) were positively correlated with kaolinite, with correlation coefficients of 0.93, 0.72, 0.52, 0.81 (Baoji) and 0.78, 0.58, 0.50, 0.49 (Luochuan), respectively, both with neither illite nor vermiculite. (4) The TOC was highly correlated with complex iron (Fe-p) with correlation coefficients of 0.90 (Baoji) and 0.82 (Luochuan), so with amorphous oxides of iron (Fe-o) as well. Among them, Fe-o mainly affected by sorption and Fe-p by complexation on SOC preservation, whereas kaolinite had both chemical and physical effects. The values of coefficients further highlight that the contributions of inorganic matters to the fixation of organic carbon were ranked to an order of kaolinite &gt; Fe-p &gt; Fe-o.</p

The dominance of loess weathering on water and sediment chemistry within the Daihai Lake catchment, northeastern Chinese Loess Plateau

This study investigated modern loess weathering and its control on the chemistry of surface water and sediment within the Daihai Lake catchment. The mineral types and the abundances of major and trace elements in loess, sediments and bedrocks were determined to ascertain the provenance of river sediment. The major cation compositions and Sr isotopic ratios of surface and subsurface waters were measured to distinguish the contributions of dissolved loads from various parent materials. The data show that mineralogical characteristics and elemental abundances of the river sediments are almost identical with those of the loess, but are different from the bedrocks, indicating that river sediments are predominantly derived from loess. River waters feeding Daihai Lake show a similar range in Sr-87/Sr-86 ratios as those of HOAc-soluble carbonate minerals in loess from the Chinese Loess Plateau. The slightly lower Sr-87/Sr-86 of river waters in the southern catchment relative to other rivers reflect potential weathering of large areas of outcropping basalt. These results imply that (1) surface processes are dominated by weathering of loess which only accounts for 18% of the total catchment area, and (2) loess weathering but not basalt controls the river Sr isotopic signature, although the latter covers a larger catchment area. For groundwater, Sr-87/Sr-86 ratios indicate that subsurface processes might be controlled by interactions with ambient lithology and hydrological flowpaths. Comparing the rivers draining the Chinese Loess Plateau with global rivers, both Mg/Ca and Sr-87/Sr-86 in the Daihai surprisingly agree well with those in the upper and downstream Huanghe (Yellow River), as well as HOAc-soluble loess, but differ significantly from other global rivers. This result reinforces the argument that loess weathering plays the most important role in controlling the sediment and water chemistry in the loess-covered areas, whereas the influence of bedrock weathering is minor. This study on modern processes might provide baselines to decipher down core records for paleoclimate reconstructions, especially for lake/river sediments in (semi-)arid areas.</p

Controls on seasonal variations of silicate weathering and CO2 consumption in, two river catchments on the NE Tibetan Plateau

Water samples from the Buha and Shaliu Rivers, located on the semi-arid northeastern Tibetan Plateau, were collected weekly over a one year period. The major ionic compositions of water samples were measured and the daily contents of suspended particulate material (SPM) were monitored in both rivers in order to investigate the influence of lithology, climate and physical erosion on seasonal silicate weathering. In the Shaliu River, weathering of trace amounts of calcite contributes more than 50% of the ca(2+) and HCO3- to the river water. Through high-resolution variations of Ca2+ concentrations and elemental ratios, the signal of carbonate precipitation is captured at the end of monsoon in this river. The measured physical erosion rate is only 8.7-16.0 mm/kyr in this semi-arid region, which is 2-3 orders of magnitude lower than that in the Himalaya and nearby regions. In contrast with several orders of magnitude in seasonal variations of silicate weathering rates in both catchments, the distinct lithology between the catchments only leads to a 15 times difference of annual net CO2 consumption. The correlation analysis shows that seasonal silicate weathering is strongly dependent on water discharge in the semi-arid area. The most important observation is that, unrecognized by the previous studies, both physical erosion rate and air temperature exhibit two distinct trends with silicate weathering rates (and net CO2 consumption) during the years. The two trends might suggest that temperature plays a more important role on the CO2 consumption rate before the mid-monsoon under a condition of low water discharge than that after the monsoon with a high water discharge. During the period before the mid-monsoon, the relationship between temperature and silicate weathering rate exhibits higher activation energy than after the mid-monsoon, suggesting a greater dissolution of uneasily weatherable minerals from groundwater, frozen soil, and/or dust input. The relationship between erosion and weathering during the period before the mid-monsoon indicates a faster increase of silicate weathering rate, because freezing erosion produces large amount of high surface area minerals.</p