Bill X. Hu

Characterizing groundwater flow in a translational rock landslide of southwestern China

Characterizing the groundwater flow pattern in a landslide would help to establish a monitor-warning system to predict the movement of a highly concealed and extremely hazard translational landslide. The complex fracture network in this type of landslide affects the rainfall infiltration process and the groundwater flow. In this paper, multi-tracer tests, injection tests and electrical resistivity tomography were used to investigate the hydrogeological characteristics and groundwater dynamics of the Kualiangzi translational rock landslide, which is located in the northeast of the Sichuan Basin, China. The study results indicate that there are two kinds of groundwater flow mode, in the landslide, the concentrated mode and the dispersed mode. Tracer and injection test results indicate that the groundwater flow in the landslide is mainly controlled by a vertical preferential flow pathway (concentrated mode), of which the direction is approximately perpendicular to the sliding direction of the landslide. The main runoff direction in the middle of this landslide is southwest according to electrical resistivity tomography. The hydraulic conductivity in the preferential direction is more than 3750 times larger than that perpendicular to the direction. The groundwater flow along the weathered sandstone and mudstone media in the vicinity of the slip surface is very slow (dispersed mode). The results reveal the existence of a preferential flow pathway. The hydrogeological conceptual model is considered as an unbounded domain with an anisotropic medium in the whole area. This hydrogeological model clearly describes the hydrological conditions, and can help establish a monitor-warning system in a translational landslide.

The change of representative elementary volume of DNAPL influenced by surface active agents during long-term remediation period in heterogeneous porous media

Representative elementary volume (REV) is important to characterize dense nonaqueous phase liquids (DNAPLs) during surfactant-Enhanced aquifer remediation (SEAR) period. To investigate the REVs of DNAPL in remediation, a perchloroethylene (PCE) SEAR experiment is conducted in a two dimensional (2D) heterogeneous translucent porous media. Light transmission techniques are used to quantify PCE saturation (Soil) and PCE-water interfacial area (AOW). Afterward, corresponding REVs are estimated using a criterion of relative gradient error (εgi) to reveal the change of REVs of DNAPL over the entire remediation period. Results from this work suggest the presence of surface active agents strongly affect the REVs of DNAPL. At the beginning of the SEAR experiment, the frequency of minimum Soil-REV size closely follows a Gaussian distribution in 0.0mm-11.0mm. Simultaneously, the frequency of minimum AOW-REV size is close to a Gaussian distribution in 2.0mm-9.0mm and appears a peak value in 13.0mm-14.0mm. As SEAR experiment proceeds, both the shapes of frequency and cumulative frequency of REV sizes are changed. At the end of SEAR experiment, the frequency of minimum Soil-REV and minimum AOW-REV size tend to Gaussian distributions in 0.0mm-6.0mm and 0.0mm-9.0mm, respectively, which suggest both minimum Soil-REV size and minimum AOW-REV size show decreasing tendency. Continuous quantification of the REVs of DNAPL is realized in this study to reveal the change of REVs influenced by surface active agent. The finding has important significance on improving our understanding of the characteristics of DNAPL in SEAR process, simulating DNAPL remediation and designing appropriate remediation scheme with high-resolution.

Hydrogeochemical Characteristics and Geothermometry Applications of Thermal Waters in Coastal Xinzhou and Shenzao Geothermal Fields, Guangdong, China

Two separate groups of geothermal waters have been identified in the coastal region of Guangdong, China. One is Xinzhou thermal water of regional groundwater flow system in a granite batholith and the other is thermal water derived from shallow coastal aquifers in Shenzao geothermal field, characterized by high salinity. The hydrochemical characteristics of the thermal waters were examined and characterized as Na-Cl and Ca-Na-Cl types, which are very similar to that of seawater. The hydrochemical evolution is revealed by analyzing the correlations of components versus Cl and their relative changes for different water samples, reflecting different extents of water-rock interactions and clear mixing trends with seawaters. Nevertheless, isotopic data indicate that thermal waters are all of the meteoric origins. Isotopic data also allowed determination of different recharge elevations and presentation of different mixing proportions of seawater with thermal waters. The reservoir temperatures were estimated by chemical geothermometries and validated by fluid-mineral equilibrium calculations. The most reliable estimates of reservoir temperature lie in the range of 148–162°C for Xinzhou and the range of 135–144°C for Shenzao thermal waters, based on the retrograde and prograde solubilities of anhydrite and chalcedony. Finally, a schematic cross-sectional fault-hydrology conceptual model was proposed.

Projected changes of temperature extremes over nine major basins in China based on the CMIP5 multimodel ensembles

Based on the outputs of 27 global climate models (GCMs) from the Coupled Model Inter-comparison Project Phase 5 (CMIP5), projected changes of extreme temperature events have been analyzed over nine river basins in China by the end of the twenty-first century relative to the reference period 1961–1990. The temporal and spatial changes and their projection uncertainty are studied using the extreme temperature indices defined by the Expert Team of Climate Change Detection and Indices (ETCCDI) under two Representative Concentration Pathways (RCPs). The model simulations predict a general increasing (decreasing) trend in warm (cold) extremes over China in the twenty-first century, with more pronounced warming trend under a higher emission scenario. The projected changes in cold and warm extremes exhibit a large difference in spatial patterns. The high-latitude and high-elevation regions of China (e.g., Continental and Southwest basins) are projected to respond more strongly to changes in cold extremes, while eastern and southern China (e.g., Yangtze River and Pearl River basins) tend to be more sensitive to the increases in warm extremes. In general, projected changes in cold indices based on minimum temperature tend to be more pronounced than in warm indices based on maximum temperature. Uncertainty analysis shows a large difference among the 27 GCMs under 2 RCP scenarios, and the uncertainty ranges tend to be larger under a higher emission scenario. Overall, the uncertainties in the emission scenarios are smaller than those from the climate models in the near future period. However, for the long-term climate projections (e.g., by the end of the twenty-first century), the projected difference under various emission scenarios tends to be larger than those by different climate models and hence can be the dominant contributor to the projection uncertainty of temperature indices.

Composition and functional diversity of microbial community across a mangrove-inhabited mudflat as revealed by 16S rDNA gene sequences

The gradient distribution of microbial communities has been detected in profiles along many natural environments. In a mangrove seedlings inhabited mudflat, the microbes drive a variety of biogeochemical processes and are associated with a dramatically changed environment across the tidal zones of mudflat. A better understanding of microbial composition, diversity and associated functional profiles in relation to physicochemical influences could provide more insights into the ecological functions of microbes in a coastal mangrove ecosystem. In this study, the variation of microbial community along successive tidal flats inhabited by mangrove seedlings were characterized based on the 16S rDNA gene sequences, and then the factors that shape the bacterial and archaeal communities were determined. Results showed that the tidal cycles strongly influence the distribution of bacterial and archaeal communities. Dissimilarity and gradient distribution of microbial communities were found among high tidal flat, mid-low tidal flat and seawater. Discrepancies were also as well observed from the surface to subsurface layers specifically in the high tidal flat. For example, Alphaproteobacteria displayed an increasing trend from low tidal to high tidal flat and vice versa for Deltaproteobacteria; Cyanobacteria and Thaumarchaeota were more dominant in the surface layer than the subsurface. In addition, by classifying the microorganisms into metabolic functional groups, we were able to identify the biogeochemical pathway that was dominant in each zone. The (oxygenic) photoautotrophy and nitrate reduction were enhanced in the mangrove inhabited mid tidal flat. It revealed the ability of xenobiotic metabolism microbes to degrade, transform, or accumulate environmental hydrocarbon pollutants in seawater, increasing sulfur-related respiration from high tidal to low tidal flat. An opposite distribution was found for major nitrogen cycling processes. The shift of both composition and function of microbial communities were significantly related to light, oxygen availability and total dissolved nitrogen instead of sediment types or salinity.

Numerical simulation of seawater intrusion to coastal aquifers and brine water/freshwater interaction in south coast of Laizhou Bay, China

We associate to each symplectic 4-orbifold X a canonical smooth symplectic resolution π : X̃ → X, which can be done equivariantly if X comes with a symplectic G-action by a finite group. Moreover, we show that the resolutions of the symplectic 4-orbifolds X/G and X̃/G are in the same symplectic birational equivalence class; in fact, the resolution of X̃/G can be reduced to that of X/G by successively blowing down symplectic (−1)-spheres. To any finite symplectic G-action on a 4-manifold M , we associate a pair (MG, D), where π : MG → M/G is the canonical resolution of the quotient orbifold and D is the pre-image of the singular set of M/G under π. We propose to study the group action on M by analyzing the smooth or symplectic topology of MG as well as the embedding of D in MG. In this paper, an investigation on the symplectic Kodaira dimension κs of MG is initiated. In particular, we conjecture that κ(MG) ≤ κs(M). The inequality is verified for several classes of symplectic G-actions, including any actions on a rational surface or a symplectic 4-manifold with κs = 0.Seawater intrusion and brine water/freshwater interaction have significantly affected agriculture, industry and public water supply at Laizhou Bay, Shandong Province, China. In this study, a two-dimensional SEAWAT model is developed to simulate the seawater intrusion to coastal aquifers and brine water/fresh water interaction in the south of Laizhou Bay. This model is applied to predict the seawater intrusion and brine water/freshwater interface development in the coming years. The model profile is perpendicular to the coastal line with two interfaces, freshwater-saline water interface near the shore and inland brine water-saline water-seawater interface. The hydrogeological parameters in the SEAWAT-2000 model are calibrated by the head and salinity measurements. The precipitation infiltration coefficient, boundary conditions and thicknesses of aquifers are studied in a sensitivity analysis. The predicted results indicate that equivalent freshwater head in shallow freshwater-saline water area will decline 2.0 m by the end of the forecasting period, caused by groundwater over-pumping for farmland irrigation. The groundwater head in the brine-saline water area will also decrease about 1.8 m by the end of forecasting period, caused by excessive brine mining. Salinity finally decreases below 105 g/L in the brine area, but increases in other areas and contaminates fresh groundwater resources.

Evaluating the interactions between surface water and groundwater in the arid mid-eastern Yanqi Basin, northwestern China

ABSTRACT The environment of Bosten Lake in the Mid-Eastern Yanqi Basin (MEYB), an arid inland area in northwest China, has deteriorated greatly due to increasing groundwater exploitation and changes in the interactions between groundwater and surface water. This study intended to simulate the spatio-temporal variability of groundwater and surface water across the entire MEYB over the period 2000–2013. The applicable groundwater flow model and mass balance calculation method for river water were constructed to evaluate the change in groundwater recharged by and discharged to different segments of the Kaidu River. Simulation results show that the entire river seepage in the MEYB increased from 1.05 to 6.17 × 108 m3/year between 2000 and 2013. The increasing river seepage, induced by increasing groundwater exploitation, plays the most important role in the water level decline in the downstream reaches of the Kaidu River and in Bosten Lake. This implies that the current utilization of groundwater resources in the MEYB is unsustainable.

Assessment of alternative adsorption models and global sensitivity analysis to characterize hexavalent chromium loss from soil to surface runoff

We investigate our ability to assess transfer of hexavalent chromium, Cr(VI), from the soil to surface runoff by considering the effect of coupling diverse adsorption models with a two‐layer solute transfer model. Our analyses are grounded on a set of two experiments associated with soils characterized by diverse particle size distributions. Our study is motivated by the observation that Cr(VI) is receiving much attention for the assessment of environmental risks due to its high solubility, mobility, and toxicological significance. Adsorption of Cr(VI) is considered to be at equilibrium in the mixing layer under our experimental conditions. Four adsorption models, that is, the Langmuir, Freundlich, Temkin, and linear models, constitute our set of alternative (competing) mathematical formulations. Experimental results reveal that the soil samples characterized by the finest grain sizes are associated with the highest release of Cr(VI) to runoff. We compare the relative abilities of the four models to interpret experimental results through maximum likelihood model calibration and four model identification criteria (i.e., the Akaike information criteria [AIC and AICC] and the Bayesian and Kashyap information criteria). Our study results enable us to rank the tested models on the basis of a set of posterior weights assigned to each of them. A classical variance‐based global sensitivity analysis is then performed to assess the relative importance of the uncertain parameters associated with each of the models considered, within subregions of the parameter space. In this context, the modelling strategy resulting from coupling the Langmuir isotherm with a two‐layer solute transfer model is then evaluated as the most skilful for the overall interpretation of both sets of experiments. Our results document that (a) the depth of the mixing layer is the most influential factor for all models tested, with the exception of the Freundlich isotherm, and (b) the total sensitivity of the adsorption parameters varies in time, with a trend to increase as time progresses for all of the models. These results suggest that adsorption has a significant effect on the uncertainty associated with the release of Cr(VI) from the soil to the surface runoff component.

Evaluating equilibrium and non-equilibrium transport of ammonium in a loam soil column

School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing, China MOE Key Laboratory of Groundwater Circulation and Evolution, China University of Geosciences (Beijing), Beijing, China 3 Institute of Groundwater and Earth Science, Jinan University, Guangzhou, China Department of Bioresource Engineering, McGill University, Montreal, QC, Canada Correspondence Bill X. Hu, School of Water Resources and Environment, China University of Geosciences (Beijing), No. 29, Xueyuan Road, Haidian District, Beijing 100083, China. Email: bill.x.hu@gmail.com Funding information National Natural Science Foundation of China, Grant/Award Numbers: 91125024, 41172225 and 41772267

Characterization, modeling, and remediation of karst in a changing environment

This introductory editorial paper provides a review and prospective outlook of the achievements and challenges in karst research under a changing environment. A brief discussion of the past and future karst research has been focused on: (1) data and new technologies; (2) modeling of karst flow and reactive transport; (3) responses of karst hydrosystems to climate variability and changes across scales.