New insight into the response and transport of nitrate in karst groundwater to rainfall events.

Although numerous studies focused on nitrate source, transformation and transport of river water in karst area have been reported, it's still unclear in understanding nitrate main source and transformation in karst groundwater system and how nitrate transport from soil to water during rainfall events in karst critical zone. In order to explore the response and transport of nitrate in karst groundwater to rainfall events, different depths of well water before, during and after rainfall event were sampled, and hillslope runoff, surface runoff of different land-use types during rainfall event were sampled synchronously at a typical karst agricultural catchment in Southwest China. Results showed that fluctuations of EC, pH and DO in deep borehole well (W1) and artesian well (W2) were small, on the contrary, variations of EC and DO in shallow well (W3) were large during sampling period. The nitrate concentrations and isotopic values indicated that nitrate in karst groundwater mainly originated from chemical fertilizer (CF), and influenced by denitrification process. High intensity of denitrification was observed in deep groundwater (87%) and artesian well water (almost 100%). Extremely high dual nitrate isotope values up to 46.8 ± 1.5‰ and 24.7 ± 0.5‰ were found in the deep artesian well. The small variation of water chemistry (EC, DO and pH), nitrate concentration and dual nitrate isotope values in deep wells during sampling period suggested that newly supplied nitrogen in deep groundwater during rainfall events also comes from deep groundwater. Low nitrogen concentrations in hillslope subsurface flow and surface runoff suggests that nitrogen transport process leading to increase of water nitrogen content mainly occur in depression. Nitrogen in depression soil is mainly transported to groundwater through fissures, fractures and conduits, rather than through vertical migration processes in the soil during rainfall events.

Determining rainwater chemistry to reveal alkaline rain trend in Southwest China: Evidence from a frequent-rainy karst area with extensive agricultural production.

Rainwater chemistry plays an important role in the earth-surficial ecosystem, but studies on rainwater chemical composition of karst agro-ecosystem are rare. To explore the rainwater alkalization and the provenance of components responsible for neutralization, two-years chemical monitoring of rainwater was carried out in a karst agricultural catchment in Southwest China. The main findings suggest that SO42-, NO3-, Ca2+, and NH4+ are the principal ions. All the ionic contents show distinctly seasonal variation (highest in winter) in response to variations in seasonal precipitation because the rain-scour process can efficiently remove atmospheric materials. Source identification indicates that Cl- and Na+ are mainly derived from marine input whereas SO42- and NO3- are controlled by anthropogenic emission, in particular, fixed emission sources. The source of NH4+ is attributed to intense agricultural production, while Ca2+ and Mg2+ are mainly derived from calcite dissolution. The rainwater alkalization caused by the seasonal acid neutralization (via basic components, Ca2+ and NH4+) is beneficial to crop growth but also reflect agricultural overfertilization. Sulfur controlled the total wet acid deposition (68%-94%) and could be a potential agent of weathering.

[Effect of Low Molecular Weight Organic Acids on the Chemical Speciation and Activity of Mercury in the Soils of the Water-Level-Fluctuating Zone of the Three Gorges Reservoir].

To investigate the effect of low molecular weight organic acids ( LMWOA) on the ability of migration and the species of mercury in the soil of the Water-Level-Fluctuating Zone of the Three Gorges Reservoir, citric acid, tartaric acid and oxalic acid were dded into the soil to conduct simulation experiments. The results showed that the percentage of exchangeable mercury increased with the increase of the concentration of citric acid, but the value declined slightly as the concentration of tartaric acid and oxalic acid increased. While all three acids elevated the bioavailability of mercury, which increased with the increase of the concentration of acids. Vhen the concentration of citric acid reached 15 mmol x L(-1), the activation effect was the best. But for oxalic acid and citric acid, 10 mmol x L(-1) was the optimal concentration. In general, the effect of three organic acids on the activation of mercury in the soil followed the trend of citric acid > tartaric acid > oxalic acid. In the soil supplemented with 15 mmol x L(-1) citric acid, the change of mercury pecies was more and more striking with the prolonged incubation, and the conversion did not stop until 14 d, at that time the stomach cid dissolved mercury increased obviously, which was mainly converted from elemental mercury.

[Experimental Research of Hg (II) Removal from Aqueous Solutions of HgCl2 with Nano-TiO2].

Mercury removal from aqueous solutions of HgCl2 was studied by indoor simulation experiments, and the effects of three different diameter of particles of Nano-TiO2 ( Nano-Titanium Dioxide) at different dosage, pH, adsorption time and the initial concentration of Hg2+ on the mercury adsorption from simulated wastewater were investigated. The single factor experiments showed that the optimal conditions were: 7.5 g x L(-1) of 5 nm TiO2 or 2.0 g x L(-1) of 100 nm TiO2, pH 8.0, initial concentration of Hg2+ 15 x mg x L(-1) adsorption time 5 min, and under these conditions the adsorption rates reached 99.5% and 99.3%, relatively. When the content of 25 nm TiO2 was 10 g x L(-1), and the other conditions were pH 8.0, initial concentration of Hg2+ 15 mg x L(-1), adsorption time 60 min, the adsorption rate was 62.8%. The Hg(II) removal effects of the TiO2 particles with different diameters followed the order of 100 nm TiO2 > 5 nm TiO2 > 25 nm TiO2. Component adsorption results showed that the 5 nm TiO2 component adsorption effect was superior to its single adsorption effect, while there was little difference between 100 nm TiO2 component adsorption effect and its single adsorption effect. The results of orthogonal experiments indicated that the influencing factors of the adsorption rate followed the order of pH > the initial concentration of Hg2+ > time > dosage. The optimal experiment scheme was: pH 8.0, a dosage of 100 nm Nano-TiO2 of 2.0 g x L(-1) an initial Hg2+ concentration of 25 mg x L(-1) and adsorption time of 10 min. Under the experimental conditions, the maximum adsorption rate reached 99.9%, at the same time, the equilibrium concentration of Hg(II) was 0.033 mg x L(-1) < 0.05 mg x L(-1), below the current enterprise rules of water pollutants in mercury emissions limits. In addition, the maximum adsorptive capacity was 26.95 mg x g(-1). The adsorption isotherm was in line with the Langmuir isotherm equation, indicating that the Hg(II) uptake by 100 nm Nano-TiO2 was typical monolayer adsorption.

[Concentrations and deposition fluxes of different mercury species in precipitation in Jinyun Mountain, Chongqing].

Concentrations of mercury (Hg) species in precipitation were measured during the period from April 2013 to March 2014 in Jinyun Mountain, Chongqing, and corresponding deposition fluxes were also estimated. The result showed that the ranges of concentration of total mercury (THg) , dissolved mercury (DHg), particulate mercury (PHg), reactive mercury (RHg), total methyl mercury (MeHg), dissolved methyl mercury (DMeHg), particulate methyl mercury (PMeHg) were 7.47-120.11, 2.51-43.03, 2.28-77.99, 0.14-15.14, 2.58 x 10(-2)-101.62 x 10(-2), 0.30 x 10(-2)-72.29 x 10(-2), and 1.45 x 10(-2)-63.55 x 10(-2) ng x L(-1), respectively. And their estimated annual deposition fluxes were 42.71, 23.51, 19.20, 5.87, 0.61, 0.34 and 0.27 µg x (m2 x a)(-1), respectively. The proportion of MeHg in THg ranged from 0.07% to 3.79% with a mean value of 1.34%, and both PHg and PMeHg in precipitation accounted for approximately 50% of THg (ranged from 10.49% to 89.30%) and TMeHg (ranged from 4.31% to 98.86%). Obvious seasonal variations of Hg concentrations and deposition fluxes were observed, with the highest VWM levels of THg, DHg and PHg occurring in winter and the lowest value occurring in summer. And the RHg concentrations in precipitation in winter and spring were significantly higher than those found in summer and autumn. The variations of deposition fluxes of THg, DHg, MeHg, DMeHg had a similar seasonal trend with the rainfall, decreasing from spring to summer and to autumn and then to winter. The maximum deposition fluxes of RHg also appeared in spring and the minimum value occurred in winter. The wet deposition of Hg in Jinyun Mountain was influenced by rain amount, rainfall frequency and other meteorological conditions. Hg levels in the precipitation were also affected by human activities.

[Effects of Citric Acid on Activation and Methylation of Mercury in the Soils of Water-Level-Fluctuating Zone of the Three Gorges.Reservoir].

To investigate effects of the main component of vegetation root exudates-citric acid on activation and methylation of mercury in the soil of water-level-fluctuating zone (WLFZ) of the Three Gorges Reservoir area, simulation experiments were conducted by extracting and cultivating soil with different concentrations of citric acid. The results showed that after adding citric acid, the total mercury content in leaching solution before reaching peak were higher than that of the control, and increased with the increase of citric acid concentrations. The maximum amount of mercury complexes increased initially and then reached plateaus with the percentage against the total mercury in soil of 1.03%, 1.67%, 1.99%, 2.47%, 2.68%, 2.73% and 2.73% for different citric acid concentrations (0, 1, 2, 4, 5, 6 and 8 mmol · L⁻¹). In addition, concentrations of methylmercury ( MeHg) in soil remained stable in the first 3 hours, and then increased accompanying with the increasing rate rising with the concentration of citric acid ( besides the control group) . This result indicated that citric acid probably could promote the transformation process from inorganic mercury to MeHg in soil. which increased with the concentration of citric acid.

[Concentrations and deposition fluxes of heavy metals in precipitation in core urban areas, Chongqing].

Concentrations and deposition fluxes of heavy metals in the precipitation of core urban areas of Chongqing were investigated for one year from December 2011 to November 2012. Precipitation samples were collected with an automated precipitation sampler from three sampling sites. Concentrations of 13 heavy metals were analyzed using ICP-MS. Results showed that the concentrations and annual deposition fluxes of most elements in precipitation were higher than those in other domestic cities and regions overseas. For instance, the concentrations of Cd, Pb and As at the three sampling sites were up to 0.55 microg x L(-1), 37.94 microg x L(-1) and 5.65 microg x L(-) respectively, and annual deposition fluxes of Cd, Pb and As reached 0.44 mg x (m2 x a)(-1), 30.25 mg x (m2 x a)(-1), and 4.50 mg x (m2 x a)(-1) respectively. In addition, there were no obviously spatial differences for concentrations and deposition fluxes of heavy metals in urban Chongqing, but significantly seasonal variations were found. Maximum concentrations occurred in autumn and winter, while the highest deposition fluxes appeared in spring and summer. Moreover, the enrichment factor (EF) was employed to estimate the pollution level of heavy metals. Results showed that EFs of Cu, Pb, Zn, Cd, Ag, As and Se were over 100, suggesting that these metals in atmosphere were seriously influenced by human activities, especially for Cd and Se, the EFs of which were 1 740 and 4 133 respectively.