Characteristics and ecological risk assessment of polycyclic aromatic hydrocarbons in soil seepage water in karst terrains, southwest China.

Concentrations of polycyclic aromatic hydrocarbons (PAHs) in soils and soil seepage waters were analyzed along with dissolved organic carbon (DOC) to investigate the ecological risks and factors controlling the subsurface transport of PAHs in karst terrain in southwest China. The concentrations of dissolved PAHs in soil seepage water increased with depth and the mean concentrations at a depth of 80 cm were relatively high (exceeding 1147 ng/L). PAH composition in soil seepage water was dominated by low molecular weight (LMW) PAHs, whereas those associated with soil matrix were dominated by high molecular weight (HMW) PAHs. The results revealed that HMW PAHs in soil seepage water were closer to the equilibrium of dissolution than LMW and medium molecular weight (MMW) PAHs. However, due to the carrier functions of dissolved organic matter (DOM) for HMW PAHs, all PAHs can be continuously dissolved in the soil seepage water from soil matrix as the water moved vertical downward through the profiles. During this process, dynamic sorption and desorption processes were occurring between constituents in the soil seepage water and the soil matrix. This study demonstrates soil seepage water has posed a high risk to the groundwater, and effective protection is urgently needed.

Carbonic anhydrase: a key regulatory and detoxifying enzyme for Karst plants.

Karstification is a rapid process during which calcidic stones/limestones undergo dissolution with the consequence of a desertification of karst regions. A slow-down of those dissolution processes of Ca-carbonate can be approached by a reforestation program using karst-resistant plants that can resist alkaline pH and higher bicarbonate (HCO3⁻) concentrations in the soil. Carbonic anhydrases (CA) are enzymes that mediate a rapid and reversible interconversion of CO2 and HCO3⁻. In the present study, the steady-state expression of a CA gene, encoding for the plant carbonic anhydrase from the parsley Petroselinum crispum, is monitored. The studies were primarily been performed during germination of the seeds up to the 12/14-day-old embryos. The CA cDNA was cloned. Quantitative polymerase chain reaction (qPCR) analysis revealed that the gene expression level of the P. crispum CA is strongly and significantly affected at more alkaline pH in the growth medium (pH 8.3). This abolishing effect is counteracted both by addition of HCO3⁻ and by addition of polyphosphate (polyP) to the culture medium. In response to polyP, the increased pH in the vacuoles of the growing plants is normalized. The effect of polyP let us to propose that this polymer acts as a buffer system that facilitates the adjustment of the pH in the cytoplasm. In addition, it is proposed that polyP has the potential to act, especially in the karst, as a fertilizer that allows the karstic plants to cope with the adverse pH and HCO3⁻ condition in the soil.

The concentration and distribution of organochlorine pesticides in the air from the karst cave, South China.

Organochlorine pesticides (OCPs) in the air of Dayan Cave in Guilin were analyzed, to investigate the source and contamination levels. Generally, air outside of the cave had much higher concentrations than inside air, and both outside and inside air showed much lower contamination levels than those observed in other regions in China and abroad. The ratios of α-/γ-HCHs and the percentages of ß-(or δ-)HCH implied a residue of historical local technical HCH contamination. The DDT concentrations, o,p'-DDT/p,p'-DDT, and p,p'-DDE/p,p'-DDT suggested fresh DDT-containing products acted as the primary source of DDT for the air in Dayan Cave. An intermediate level concentration of α-endosulfan (156.00 pg/m(3)) was observed in the outside air, implying local source was dominant. Based on the similar sources of HCH and DDT and the relationships between outside and inside air, it appeared that outside air pollutants affected on inside HCH and DDT's distribution. For other OCPs, outside air might be major contributor to inside air contamination under conditions of high concentrations in outside air compared with low levels in inside air for semi-closed karstic caves.

Nitrate sources and nitrogen dynamics in a karst aquifer with mixed nitrogen inputs (Southwest China): Revealed by multiple stable isotopic and hydro-chemical proxies.

The nitrate (NO3-) contamination of karst aquifers as an important drinking water reservoir is increasing globally. Understanding the behavior of nitrogen (N) in karst aquifers is imperative for effective groundwater quality management. This study combined multiple stable isotopes (δ2H-H2O, δ18O-H2O, δ13C-DIC, δ15N-NO3, and δ18O-NO3), including hydro-chemical data, with a tracer test and a Bayesian isotope mixing (SIAR) model to elucidate the NO3- sources and N cycling within the Babu karst aquifer in Guizhou Province, Southwest China. Nitrate isotopes and SIAR model revealed that manure and sewage, nitrogen fertilizer, and soil organic nitrogen were the three dominant NO3- sources in winter, contributing to 37%, 32%, and 31% to spring NO3-, and 38%, 31%, and 31% to surface water NO3-, respectively. The δ18O-NO3 values of sampled waters ranging from 0.3‰ to 13.7‰ (mean of 7.7 ± 3.0‰; N = 63) and the significant negative correlations between δ15N-NO3 and δ13C-DIC in the spring waters (P < 0.01) revealed that nitrification was the primary N transformation process in the Babu watershed. Whereas, denitrification might still occur locally, confirmed by the enriched values of δ15N-NO3 (14.3 ± 7.6‰; N = 6) and high denitrification extent (46.6 ± 22.2%; N = 6) in the springs from residential areas, and by elevated δ13C-DIC (-11.2 ± 0.6‰; N = 26) and δ15N-NO3 values (18.9 ± 5.2‰; N = 26) in the boreholes. During the base flow period, point-inputs of the AMD-impacted stream and sewage waters, and short transit time (<5 days) were conducive to nitrification processes in the karst conduit, resulting in elevated NO3- concentration and NO3-/Cl- ratio at the watershed outlet. Approximately 50% of NO3- flux at the outlet was derived from nitrification, indicating that a significant extent of nitrification occurred in the NH4+-contaminated karst conduit, which may be a new NO3- source to receiving rivers and lakes. This study provided an integrated method for exploring the N dynamics in contaminated karst aquifers. Moreover, the study highlighted that the point N sources control required particular attention for groundwater protection and restoration.

Arsenic in a speleothem from Central China: stadial-interstadial variations and implications.

In a pilot study, arsenic in a stalagmite (SJ3) collected from Central China was measured, and its association with past climate and environment was explored. Most of the SJ3 arsenic concentrations ranged from 120 to 320 ppb with the highest concentrations associated with relatively warm and humid climatic phases and lowest concentrations with cold and dry phases. The SJ3 arsenic record was very similar to the manganese record of SJ3. Variations of arsenic in SJ3 might be controlled by metal oxides of iron, manganese, and aluminum in karst groundwater at the study site, which in turn were closely related with changes in past climate and environment. A considerable proportion of arsenic was in excess over manganese in SJ3, which might be related with incorporation of arsenic into the calcite lattice during the formation of SJ3. It was speculated that more arsenic was released due to stronger weathering of the surface soils and sequestrated by metal oxides in karst groundwater under warm-humid climatic phases than under cold-dry phases. This suggested that climate shift might alter arsenic balance in sedimentary areas and aquifer systems and potentially exert significant influence on global arsenic contamination.

Organochlorine pesticides in the soil of a karst cave in Guilin, China.

Organochlorine pesticides (OCPs) including HCH, DDT, chlordane, endosulfan, and endosulfan sulfate were analyzed in surface soil of the Dayan Cave to investigate their source and concentration levels in September 2006. Generally, the data showed that outside soil of the cave had much higher concentrations than inside soil for the most detected OCPs in the cave and both inside soil and outside soil showed much lower concentration levels (basically, the levels of OCPs were less than 0.7 ng/g) than those observed in other regions within and outside China other than TC (ranging from 3.22 to 5.00 ng/g) and CC (ranging from 3.89 to 5.08 ng/g) in the soil outside. The ratios of α-/γ-HCH ranged from 0.88 to 1.20 in the soil of the cave, together with the averaged percentages of ß-HCH and δ-HCH among the total HCH isomers (accounting for 39.0% and 14.2%, respectively), implied that a historical residue of local technical HCH contamination was likely to be dominant in the soil of the cave. Based on (1) the accordance of TC/CC ratios (ranging from 0.83 to 0.98) between the values observed in the outside soil and the potentially available chlordane products in the markets, and (2) the high concentrations of TC and CC observed in the outside soil, it appeared that the illegal usage of technical chlordane was done in Guilin. The low concentrations of TC (0.02 to 0.56 ng/g) and CC (0.10 to 1.71 ng/g) in the inside soil, together with the significant distinctions of TC/CC ratios between the inside soil (ranging from 0.03 to 0.33) and the outside soil, implied that the chlordane in the inside soil of the cave was a historical residue of local technical chlordane contamination. The similar ratios of DDT isomers (o, p (')-DDT/p, p (')-DDT and p, p (')-DDE+DDD/DDT) between the outside soil and the inside soil of the cave suggested that they may have the similar DDT source. The ratios of p, p (')-DDE+DDD/DDT (ranging from 3 to 8) indicated that DDT was relatively aged. The values of o, p (')-DDT/p, p (')-DDT ratios (ranging from 3 to 7.5 with a mean value of 5.45) were found to be much higher than that of technical DDT, and very similar to that of dicofol products, implied that the primary source of DDT in the soil of the cave was most probably from dicofol-type DDT products. The low concentration levels of endosulfan and the higher levels of metabolite endosulfan sulfate indicated that the residue from historical usage of technical endosulfan was likely to be dominant in the soil of the cave.

Impacts of acid mine drainage on karst aquifers: Evidence from hydrogeochemistry, stable sulfur and oxygen isotopes.

The pollution of karst aquifers by acid mine drainage (AMD) waters is increasing. Major and minor ions (Ca2+, Mg2+, HCO3-, SO42-, F-, and Fe), stable sulfur and oxygen isotopes of dissolved sulfates (δ34SSO4 and δ18OSO4) and oxygen isotope of water (δ18OH2O), were analyzed in rainwater, surface water, groundwater, and AMD water sampled from the Babu subterranean stream watershed, in Southwest China. The principal aim of this study was to explore the impact of AMD waters on the evolution of karst aquifers. Based on hydrogeochemistry and stable isotopes (δ18OH2O, δ18OSO4 and δ34SSO4): (1) the chemistry of AMD waters was primarily controlled by pyrite oxidation, karst conduit water by AMD waters and mixing with calcite and dolomite dissolution, and spring water by atmospheric precipitation and carbonate dissolution; (2) contamination of the karst conduit water was mainly attributed to the input of AMD waters, resulting in a shift of δ34SSO4 towards more negative values (from 3.4‰ to -13.2‰); (3) the quality of karst conduit water changed from suitable to unsuitable for irrigation and drinking, particularly due to the increase in total Fe, SO42-, and F- concentrations, reflecting the cumulative effect of AMD waters derived from tailings dumps; this influence was enhanced during rainstorm/drought and anthropogenic activities; and (4) the flow of contaminated groundwater through the conduit promoted the dissolution of carbonates, especially during the dry season due to the greater proportion of AMD in the groundwater. This released more CO2 to the atmosphere. We believe that analysis of stable isotopes (δ18OH2O, δ18OSO4, and δ34SSO4), combined with hydrogeochemistry, is effective for exploring the impact of AMD on karst aquifers. Therefore, reasonable treatment methods should be taken to reduce the negative impacts of tailings dumps on karst aquifers.

Varying thermal structure controls the dynamics of CO2 emissions from a subtropical reservoir, south China.

Thermal stratification and mixing are important to the physicochemical composition of reservoirs and lakes and impact their water quality and biogeochemical cycles. However, it remains unclear how thermal stratification and mixing process control the exchange of CO2 between surface water and the Earth's atmosphere. To address this issue, we examine the temporal characteristics of some physicochemical parameters, partial pressure of CO2 (pCO2), the δ13CDIC, and CO2 emission from a typical karst groundwater-fed reservoir (Dalongdong reservoir). During the 23 month study (2016-2018) thermal stratification limited CO2 emission, in part from photosynthetic uptake of CO2, from early April to late October, while mixing processes stimulated CO2 emission of CO2 generated from organic matter remineralization in bottom water from October to April. The Dalongdong reservoir is an atmospheric source of CO2 for most of the study period; however, during periods of stratification, approximately 0.37 ± 0.44 Gg CO2 (1 Gg = 109g) dissolved into the water from the atmosphere, while approximately 6.24 ± 3.73 Gg CO2 was lost to the atmosphere during periods lacking stratification. Limited emissions during stratified period may thus represent a negative feedback to CO2 contributions to global warming, which has increased lengths of stratified periods. These study results are important to optimize sampling monitoring strategies to reduce errors of regional CO2 emission estimation.

Source and flux of anthropogenically enhanced dissolved inorganic carbon: A comparative study of urban and forest karst catchments in Southwest China.

Anthropogenic disturbance has become an accelerator of dissolved inorganic carbon flux (FDIC), but the magnitude by which FDIC is enhanced by various anthropogenic activities, especially urbanization, remains largely unknown. In this study, rainfall, discharge, pH, and specific conductivity were continuously monitored during a one-year period in two adjacent karst catchments affected by anthropogenic activities in Southwest China. Groundwater, rainwater, and untreated sewage were collected monthly, and their hydrogeochemical characteristics and isotopic compositions (δ13CDIC, δ34SSO4, δ18OSO4, δ15NNO3 and δ18ONO3) were measured. The results showed that the urban catchment exported 70% more FDIC than its forest counterpart, with considerable differences in the concentration and isotopic composition of DIC, SO42- and NO3- between the two subterranean streams. Based on the contribution by various SO42- or NO3- sources estimated with a Bayesian isotope mixing model, carbonate weathering by H2CO3, H2SO4 and HNO3 contributed 67.5%, 26.0% and 6.5% of FDIC in the urban catchment and 93.3%, 3.4% and 3.3% in the forest catchment, respectively. Anthropogenic activities enhanced the FDIC by 24.0% in the urban catchment and 5.8% in the forest catchment. Anthropogenically enhanced DIC flux (FDIC-A) primarily arose from carbonate dissolution due to acid precipitation (especially S deposition), which accounted for approximately 64% of the FDIC-A in the two catchments. Sewage (including manure) and fertilizer applications also enhanced carbonate dissolution through nitrification processes. Furthermore, FDIC-A was much higher (>83%) during the wet and warm season (May to October) in both catchments. The results of this study suggest that anthropogenic impacts on carbonate dissolution should be considered in global carbon cycle assessments, especially in urban areas.

Integrated understanding of the Critical Zone processes in a subtropical karst watershed (Qingmuguan, Southwestern China): Hydrochemical and isotopic constraints.

Improving the management and protection of karst groundwater resources and addressing karst-related environmental and ecological problems still face challenges raised from the limited knowledge on the entire karstic Critical Zone (K-CZ), including soil, epikarst, the vadose and saturated zones. Particularly, there is still a lack of integrated understanding of K-CZ properties and major CZ processes across space and time. In this study, we measured and analyzed the hydrochemical and multiple stable isotopic compositions of soil water, surface- and groundwaters from various compartments of the K-CZ in a typical subtropical karst watershed - Qingmuguan (QKW), Southwestern China, in order to explore the source and spatiotemporal variations of water and solutes (C, N, S) within the K-CZ; thereby elucidating the hydrological and biogeochemical processes and their affecting factors. The results show that (i) the K-CZ of QKW is characterized by high heterogeneity and permeability, with fast and strong hydrologic variations in response to rainfall variability; (ii) water-CO2­carbonate interactions (i.e. carbonate weathering) are remarkably active in different zones and are significantly modulated by hydrologic dynamics and seasonal change in biological activities; (iii) efficient migration of nitrate and sulfate occurs across the surface toward the saturated aquifer zone, which is affected by the source availability, elemental transformation and flow transport processes; (iv) human activities have clearly influenced groundwater quality and the natural K-CZ processes, for example, exogenic acids of anthropogenic origin (e.g. acid precipitation and nitrogenous fertilizers from crop lands) have been proven to be involved in the carbonate weathering, with a contribution of ~20%-30%. Our study highlights the strong coupling of hydrological and various biogeochemical processes and the interactive connection among various layers of K-CZ. Thus, systematical monitoring along the CZ profile and a process-based dynamic approach to elucidating climatic and anthropogenic forcing are necessary to better understand the K-CZ properties and functions.

Concentration, distribution and sources of polyaromatic hydrocarbons in soils from the Karst tiankengs, South China.

Polycyclic aromatic hydrocarbons (PAHs) were investigated in the surface soils from Dashiwei and Datuo Karst tiankengs, Guangxi Province, South China. Concentrations of total PAHs ranged from 16.93 ng g(-1) to 190 ng g(-1) with a mean of 58.29 ng g(-1). In Dashiwei more PAHs were concentrated than in Datuo (mean 74.42 ng g(-1) > 42.15 ng g(-1)). Dashiwei and Datuo accounted for the majority of all PAHs concentrated in the lower location through increased rates (58% and 37%) from the upper to lower geographical levels, respectively. Dashiwei presented the stronger trapping ability for heavier molecular weights on the tiankeng floor with a concentration increase rate of 167% when compared to that of the lighter molecular weights (which had a concentration increase of rate 6%), which was in contrast to the Datuo with a concentration increase rate of 37% for lighter molecular weights and 31% for heavier molecular weights. In Dashiwei, it was found that the ratio of heavier PAH compound molecular weight resulted in its higher precipitation recording on the tiankeng floor. The differentia between two tiankengs, therefore, are related to their environmental characteristics and the PAHs physiochemical properties. The analytical result from the PAHs indicators suggest that a petrogenic source was dominant in the area studied. Therefore, the karst tiankeng has the potential to act as a geophysical well for PAHs deposition, especially for the heavier molecular weighted PAH.

High spatial and seasonal heterogeneity of pCO2 and CO2 emissions in a karst groundwater-stream continuum, southern China.

Accurate quantification of the emission of CO2 from streams and rivers is one of the primary challenges in determining the global carbon budget because our knowledge of the spatial and seasonal heterogeneity on these CO2 emissions is limited. In karst areas, the groundwater-stream continuum is likely ubiquitous because the carbon-rich groundwater discharges into some of the streams through springs or subterranean streams, which results in more complex spatial and seasonal variations in the CO2 emissions. To address this issue, the spatial and seasonal characteristics of partial pressure of CO2 (pCO2), the δ13CDIC, and the CO2 emission flux of the Guancun surface stream (GSS) karst groundwater-stream continuum in southern China were investigated from the stream head (groundwater outlet) to the downstream mouth during the 2014-2017 period. Our results reveal that the pCO2 and CO2 emissions exhibit high spatial and seasonal heterogeneities over ~ 1300 m in the GSS. Spatially, the pCO2 and CO2 emissions decrease sharply from the stream head (mean 8818.4 µatm for pCO2 and mean 423.4 mg m-2 h-1 for CO2 emission) to the site farthest downstream (mean 2752.7 µatm for pCO2 and 257.0 mg m-2 h-1 for CO2 emission). Except for the dates when extreme rainfall occurred, the pCO2 and CO2 emission values were higher in the rainy season than in the dry season. This suggests that in a groundwater-stream continuum, CO2 emission occurs very soon after the water is transferred from the karst groundwater to the surface water. We estimate that the total amount of CO2 released to the atmosphere from the GSS is 21.75 t CO2/year, which is only 1.71-5.62% of the dissolved inorganic carbon loss flux in the GSS during the study period. It is important to note that the measured CO2 emission and pCO2 levels decrease farther downstream, so carbon loss is underestimated when it is calculated using downstream sampling points. Therefore, accurate assessments of the CO2 emission flux need to take into consideration the high spatio-temporal heterogeneity in order to reduce the bias of the entire CO2 emission flux.

Contaminant sources and processes affecting spring water quality in a typical karst basin (Hongjiadu Basin, SW China): insights provided by hydrochemical and isotopic data.

Springs are an important source of drinking water supply in mountainous karst areas of SW China. However, the quality of many spring waters has deteriorated greatly in recent years, which leads to a significant problem of drinking water scarcity. In this study, hydrochemistry and stable sulfur and oxygen isotopic compositions of SO42- (δ34S and δ18OSO4) of 38 representative samples of waters (incl. spring water, surface water, rainwater, and sewage) from the Hongjiadu Basin, Guizhou province, SW China, were investigated in order to identify the sources of contaminates in spring waters and trace the processes affecting the karst groundwater quality. Approximately 28% of the total investigated springs has been suffered from serious contamination and the concentrations of NO3-, SO42-, and total iron (TFe) in many spring waters have exceeded the standards for drinking water. The springs that have NO3- concentrations of > 30 mg/L are concentrated in residential and agricultural areas, suggesting that NO3- in spring water are mainly derived from chemical fertilizers, manure, and sewage. δ34S and δ18OSO4 data indicate that SO42- in spring water mainly originates from sulfide oxidation, acid rain, and sewage. Furthermore, the high δ34S and δ18OSO4 values of SO42- in some spring waters may be related to the occurrence of bacterial sulfate reduction. Some springs that are discharged from abandoned coal mines have SO42- concentrations of > 250 mg/L, demonstrating that mining activities have accelerated the deterioration of spring water quality. Also, springs with TFe concentrations of > 0.3 mg/L are discharged from coal-bearing strata, revealing that iron in spring waters is mainly derived from the oxidation of pyrite. Our results show that the karst spring waters are highly vulnerable to anthropogenic contaminations and human activities, such as agricultural fertilizing and sewage and waste disposal as well as mining activities, which exert a great impact on the quality of groundwater in karst areas.

Diel-scale variation of dissolved inorganic carbon during a rainfall event in a small karst stream in southern China.

Metabolic processes of the submerged aquatic community (photosynthesis and respiration) play important roles in regulating diel cycles of dissolved inorganic carbon (DIC) and sequestering carbon in a karst stream. However, little is known of whether diel DIC cycling occurs during rainfall in a karst groundwater-fed stream, even though this question is critical for the accurate estimation of what may be a major terrestrial carbon sink. Here, we measured diel variations of water chemical composition in a small karst groundwater-fed stream in southwest China during a rainfall event to assess the influences of rainfall and rising discharge on DIC diel cycling and the potential carbon sink produced by in-stream metabolism. Our results show that water chemical composition at the source spring (CK site) is relatively stable due to chemostatic behavior during rising discharge after a rainfall period. This site lacked submerged aquatic vegetation and, thus, had no diel variations in water chemistry. However, diel cycles of all hydrochemical parameters occurred at a site 1.3 km downstream (LY site). Diel variations in pH, DO, and δ13CDIC were inversely related to diel changes in SpC, DIC, Ca2+, and pCO2. These results indicated that diel cycling of DIC due to in-stream metabolism of submerged aquatic community was still occurring during elevated discharge from rainfall. We estimate the carbon sink through the in-stream metabolism of the submerged aquatic community to be 5.6 kg C/day during the studied rainfall event. These results imply that submerged aquatic communities in a karst stream can significantly stabilize carbon originating from the carbonate rock weathering processes in karst areas.

Vertical migration from surface soils to groundwater and source appointment of polycyclic aromatic hydrocarbons in epikarst spring systems, southwest China.

Understanding the transfer process of polycyclic aromatic hydrocarbons (PAHs) in the karst terrain is of great importance to their ecological risk assessments, however, the impact of the vertical transfer of the soil PAHs on the underground water is largely unknown in the karst system. Here, the vertical distribution and the seasonal variation of 16 PAHs in the soils and the water of 4 epikarst spring catchments in Southwest China were investigated. The total concentration of the PAHs ranged within 61-3285 ng g-1 in the soils, and 341-4969 ng L-1 in the spring water. The vertical distribution of the PAHs in soils varied with ring numbers and altitude of the catchment. PAHs concentrations were linearly related with the total organic carbon (TOC) at different depths in the catchments 563-783 m above the sea level (A.S.L.). However, no correlation with TOC was observed in the catchment of a high altitude (2090 m A.S.L.), because the large water flux led to the fast migration of the 2-3 rings PAHs in soils. The PAHs in soils and springs were mainly derived from the combustion of grass/wood/coal, closely related with the primary fossil fuels used in this area. This study demonstrate that the groundwater was heavily polluted by PAHs in the karst terrains of Southwest China, due to the vertical transfer of PAHs from the surface soils, and effective protection was urgently needed.

Transport of polycyclic aromatic hydrocarbons in a highly vulnerable karst underground river system of southwest China.

The concentration and fluxes of polycyclic aromatic hydrocarbons (PAHs) were investigated in a karst underground river system in southwest China. Groundwater, particles, and sediments from underground river, topsoil, and surface water were monitored, allowing establishment of a conceptual model of PAH transport at the watershed scale. The results showed that PAHs could be transported from the surface to the subsurface through two migration pathways, which were slow-flowing water in the karst fissure and fast-flowing water in conduits. During rainfall events, increasing PAH levels (concentrations and fluxes) at the underground river exit indicated that hydrodynamic force could facilitate PAH transport. The PAHs in water were dominated by dissolved PAHs, accounting for 58.7% of total, especially in the freely dissolved phase, in which SPM-associated PAHs accounted for 41.3% of the total PAHs. Low molecular weight PAHs dominated transport and were mainly transported in dissolved form, whereas high molecular weight PAHs were dominated by SPM-associated transport during the rainfall events. A significantly positive correlation was observed between two-ring and three-ring freely dissolved PAHs and dissolved organic carbon (p < 0.01), respectively. Moreover, PAHs with four to five rings were relatively more abundant in the dissolved organic matter (DOM) associated phase than in the freely dissolved phase, suggesting a major role of DOM in their transport during rainfall events. The trend of PAH fluxes suggested that particle-facilitated transport was another dominant cause of PAH mobilization.

Enhancement of Ca2+ release from limestone by microbial extracellular carbonic anhydrase.

Three experimental systems were set up to investigate whether carbonic anhydrase (CA) from microorganisms actually plays the driving role in limestone dissolution. In one, redistilled water served as negative control. In a second, microbial origin CA enzyme solution supplemented with CA special inhibitor acetazolamide (AZ) served as positive control. A third contained a crude enzyme solution of microbial CA. The results showed that the amount of released Ca2+ from limestone in a CA non-inhibited system increased by 2.4 times compared to a CA inhibited system, and increased by 11.7 times compared to the redistilled water control. These experiments demonstrated that microbial origin CA significantly enhanced Ca2+ release from limestone (P < 0.01), and therefore, proved the significant driving effect of microbial CA on limestone dissolution. The results also suggested that microbially derived CA might exert an important influence on biokarst process.

In-stream metabolism and atmospheric carbon sequestration in a groundwater-fed karst stream.

Atmospheric carbon sequestered in karst systems through dissolution of carbonate minerals is considered to have no net effect on long-term regional and global carbon budgets because precipitation of dissolved carbonate minerals emits CO2 back to the atmosphere. Even though recent studies have implied that rapid kinetics of carbonate dissolution coupled with the aquatic photosynthetic uptake of dissolve inorganic carbon (DIC) could facilitate a stable atmospheric C sink in karst rivers and streams, little is known about the magnitudes and long-term stability of this C sink. To assess in-stream biogeochemical processes and their role on stream C cycling, we measured diel cycles of water characteristics and chemical composition (temperature, pH, DO, SpC, DIC, Ca2+, δ13CDIC) in a groundwater-fed karst stream in southwest China. Our results show no diel variations at the groundwater discharge point (CK site) due to the absence of a sub-aquatic community (SAC). However, all hydrochemical parameters show significant diel cycle 1.3km downstream (LY site). Diel variations in pH, DO, and δ13CDIC were inversely related to diel changes in SpC, DIC, Ca2+ and pCO2. This result indicates that in-stream metabolism (photosynthesis and respiration) of SAC controls diel variations in stream water chemistry. Significant diel cycles of net ecosystem production (NEP) influences in-stream diel fluctuation of pH, DO, SIc, DIC, pCO2, Ca2+ and δ13CDIC, with gross primary production (GPP) dominating in day and ecosystem respiration (ER) dominating at the night. Absence of in-stream metabolism at CK enhances CO2 degassing from stream to the atmosphere, which is estimated to be 3-5 times higher than at LY. We estimate the carbon sink through in-stream metabolism of SAC to be 73tCkm-2a-1, which is around half the rate of the oceanic biological pump. These results imply in-stream photosynthesis sequesters DIC originating from karst weathering and controls CO2 evasion.

[Spatial Variations of CO2 Degassing Across Water-air Interface and Its Impact Factors in Summer in Guijiang River, China].

River is characterized by obvious spatial heterogeneity in catchment, which is exacerbated by special environment features of calcium-rich, alkaline and DIC-rich(dissolved inorganic carbon) in karst river. Thus, it also leads to significant spatial variation in the CO2 degassing across water-air interface. Main ions, physicochemical parameters, δ13CDIC value and two common approaches(floating chamber(FC) and thin boundary layer models(TBL) were used to analyze the CO2 degassing characteristics in Guijiang River, a karst river, China. The results were as follows:1 Hydrochemistry in Guijiang River basin showed a significant spatial change. All of HCO3-, Ca2+, specific conductivity, total dissolved solids(TDS), SIc and pCO2 showed similar distribution characteristics in the following order:tributaries in the middle reaches > middle reaches > Downstream > Upstream of Guijiang River. 2 During the monitoring period, CO2 degassing occurred in all the sampling sites and it was the CO2 source for the atmosphere. The mean CO2 evasion was 237 mg·(m2·h)-1 in Guijiang River, which located in the range of average CO2 evasion of global river. However, significant spatial variations also occurred along Guijiang River. The CO2 degassing flux in tributaries of the middle reaches and middle reaches of the mainstream were obviously larger than those in downstream and upstream of the mainstream. 3 CO2 degassing was mainly affected by carbonate equilibrium system in tributaries in the middle reaches and middle reaches in the mainstream of the Guijiang River basin, which resulted in obviously larger CO2 degassing than those in downstream and upstream of mainstream. However, the CO2 degassing flux in tributaries of the middle reaches was also simultaneously affected by biological photosynthesis, and the minimum CO2 degassing flux[6.38 mg·(m2·h)-1] appeared in tributaries of the middle reaches. In addition, the CO2 degassing flux in mainstream upstream was mainly affected by atmospheric environmental factors, while it was synergetically influenced by many factors in mainstream downstream.

River sequesters atmospheric carbon and limits the CO2 degassing in karst area, southwest China.

CO2 fluxes across water-air interfaces of river systems play important roles in regulating the regional and global carbon cycle. However, great uncertainty remains as to the contribution of these inland water bodies to the global carbon budget. Part of the uncertainty stems from limited understanding of the CO2 fluxes at diurnal and seasonal frequencies caused by aquatic metabolism. Here, we measured surface water characteristics (temperature, pH, and DO, DIC, Ca2+ concentrations) and CO2 fluxes across the air-water interface at two transects of Guijiang River, southwest China to assess the seasonal and diurnal dynamics of fluvial carbon cycling and its potential role in regional and global carbon budgets. The two transects had differing bedrock; DM transect is underlain by carbonate and detrital rock and PY is underlain by pure carbonate. Our results show that the river water both degasses CO2 to and absorbs CO2 from the atmosphere in both summer and winter, but the degassing and absorption varied between the two transects. Further, CO2 fluxes evolve through diurnal cycles. At DM, the river evaded CO2 from early morning through noon and absorbed CO2 from afternoon through early morning. At PY in summer, the CO2 evasion decreased during the daytime and increased at night while in winter at night, CO2 uptake increased in the morning and decreased in the afternoon but remained relatively stable at night. Although the river is a net source of carbon to the atmosphere (~15mMm-2day-1), the evasion rate is the smallest of all reported world's inland water bodies reflecting sequestration of atmospheric carbon through the carbonate dissolution and high primary productivity. These results emphasize the need of seasonal and diurnal monitoring of CO2 fluxes across water-air interface, particularly in highly productive rivers, to reduce uncertainty in current estimates of global riverine CO2 emission.