•The spatial diversity of bacterial and archaeal community in mangrove-inhabited mudflat is studied•Gradient variation of microbial community composition is found from high tidal flat to mid/low ...tidal flat•Stratified distribution of microbial communities is detected from surface to subsurface layer at a depth of 50 cm•The metabolism functions, such as photoautotrophy, sulfur-related respiration, and nitrification are varied in the profile•DO and DTN are the main physiochemical factors affecting the distribution of functional microbes in the mudflat
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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.
Regional groundwater level forecasting is critical to water resource management, especially for arid regions which require effective management of groundwater resources to meet human and ecosystem ...needs. In this study Machine Learning and Deep Learning approaches - Support Vector Machine, Generalized Regression Neural Network, Decision Tree, Random Forest (RF), Convolutional Neural Network, Long Short Term Memory and Gated Recurrent Network- have been used to simulate the groundwater levels in the lower Tarim River basin (LTRB) which is an extreme dryland. The results showed that models developed here with easily available input data such as relative humidity, flow volume and distance to the riverbank can fully utilize spatiotemporally inconsistent groundwater monitoring data to predict the spatiotemporal variation of the groundwater system in arid regions where exist intermittent flow. The shapely additive explanations method was used to interpret the RF model and discover the effect of meteorological, hydrological and environmental variables on the regional groundwater. These explanations showed that the flow volume, the distance to the river channel and reservoir have a critical impact on groundwater changes. Within 300 m distance to the riverbank, groundwater would mainly be influenced by the flow volume and the distance to the reservoir. While far from the riverbank, these effects decreased gradually further away from the river course. The RF prediction results showed that in the next three years (2021−2023), the groundwater level on average may decline to −6.4 m, and the suitable areas for natural vegetation growth would be limited to 39% if no water conveyance in the LTRB. To guarantee the stability of ecosystems in the LTRB, it is necessary to convey the water annually. These results can support spatiotemporal predictions of groundwater levels predominantly recharged by intermittent flow, and form a scientific basis for sustainable water resources management in arid regions.
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•The machine learning (ML) model is effective in predicting groundwater levels.•Random Forest performs the best in predicting groundwater levels in this study.•The shapely additive explanations method is useful for interpreting the ML model.•No water conveyance for three years will dramatically shrink vegetation area.
Bacterial community has been significantly enrolled in the biogeochemical cycling of the coastal subsurface ecosystem. The bacterial community variations with salinity have been extensively ...investigated in the surface environment, such as lake, soil, and estuary, but not in the subsurface environment. Here we explore the responses of bacterial populations to the salinity and other environmental factors (EFs) by considering both the abundant and rare sub-community in a coastal Holocene groundwater system. Our study results indicate that the bacterial diversity was independent of the salinity in both the abundance and rare sub-community. Besides diversity, no flourishing of abundant bacteria relative abundance is observed with increasing or decreasing salinity. Yet the rare taxa exhibit a bio-growth with salinity, which has a significant correlation (p < 0.001) with sulfate concentration. The responses of the abundant sub-community taxa to nutrients, temperature, pH, and dissolved oxygen are insensitive. However, the correlation between δ18O, δD and the entire community diversity is significant, which demonstrates the bacterial community is affected by the groundwater origin. Besides, not all the species in one class or order are necessarily shaped by the same factor. To quantify the impact of EFs on the community properties, analyses in different taxonomic levels is suggested. These findings imply that the spatial organization of microbial communities is complicated and influenced by multiple factors on a regional scale. The investigated results are useful for understanding biogeochemical processes in the coastal groundwater.
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•The entire bacterial abundance and diversity are independent of the salinity in the saltwater aquifer.•Bacterial community composition is affected by the groundwater origin.•No flourishing of abundant bacteria relative abundance was observed with salinity variations.•The rare sub-community diversity has a bio-growth with salinity.
A salt-freshwater transition zone due to seawater intrusion to groundwater promotes changes in microbial diversity and community composition in a coastal aquifer. The main purpose of this study is to ...explore the effect of seawater intrusion on the groundwater quality in a salt-freshwater transition zone and identify the microbial fingerprints of seawater intrusion. The changes in microbial community diversity response to the seawater intrusion were characterized by comparing the community structures of the microbes in fresh groundwater, seawater, and salty groundwater from various monitoring wells at different depths using the high throughput 16S rDNA gene sequencing. Results show that seawater had the lowest taxon richness and evenness, and the irrigation water had the highest richness and evenness. Statistical analysis showed that DO%, ORP, and Cl− affected microbial distribution in the groundwater; while DO% was a main environmental factor influencing microbial community diversity. The analysis of microbial community structures indicates that the order Oceanospirillales and the family Alteromonadaceae could be used as indicators of seawater intrusion.
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•Determination of salt-fresh water interface location on the profile•Variations of microbial diversity in groundwater were found.•Oceanospirillales and Alteromonadaceae were indicators for seawater intrusion.•DO% affected the microbial community diversity of groundwater strongly.
A hybrid discrete‐continuum numerical model, Variable‐Density Flow and Solute Transport—Conduit Flow Process (VDFST‐CFP), is developed to simulate seawater intrusion to a coastal karst aquifer with a ...conduit network. The Darcy‐Weisbach equation is applied to simulate the nonlaminar groundwater flow in the conduit system that is conceptualized as pipes, while the Darcy equation is used for laminar groundwater flow in the continuum porous medium. Density‐dependent groundwater flow with appropriate additional density terms in the conduit is analytically derived. The flow and transport equations are coupled, and numerically solved by the finite difference method with an implicit iteration procedure. Two synthetic benchmarks are developed to compare the VDFST‐CFP model results with other numerical models, such as the variable‐density SEAWAT, constant‐density continuum MODFLOW/MT3DMS, and constant‐density discrete‐continuum CFPv2/UMT3D models. The VDFST‐CFP model compares reasonably well with the other model results in both conduit and porous medium domains, and well describes water and salt exchange between the two systems. Under turbulent flow conditions within the conduit, the Darcy‐Weisbach equation calculates the flow rate more accurately without overestimation by the Darcy equation. Sensitivity analysis indicates that conduit diameter, friction factor, matrix hydraulic conductivity, and effective medium porosity are important parameters in the VDFST‐CFP model. The pros and cons of the VDFST‐CFP model are discussed, including the model assumptions and simplifications, limitations of the discrete‐continuum modeling method, and the convergence criteria. In general, the newly developed VDFST‐CFP model provides a new numerical modeling method for simulating seawater intrusion in a coastal karst aquifer with conduits.
Key Points
The VDFST‐CFP model simulates density‐dependent groundwater flow and salt transport in the conduit and porous medium domains
Variable‐density nonlaminar conduit flow and Darcian flow in porous medium is coupled in the hybrid discrete‐continuum model
The seawater intrusion simulation results by the VDFST‐CFP model compare reasonably well with other numerical models in two benchmarks
Ongoing climate warming is largely affecting the frequency and magnitude of short‐term climatic extremes in many regions of the world, causing significant damage to the environment and society. Here, ...based on the long‐term (1980–2010) meteorological observations and reanalysis products, we investigated the spatio‐temporal evolutions of short‐term concurrent hot and dry extreme (SCHDE) events over the Pearl River basin, a typical humid and subtropical basin in southern China. We also explored the physical mechanisms of SCHDE and its relationship with a series of hydrometeorological variables and large‐scale climate indices. The results indicate an intensification of SCHDE over the study basin during the study period, especially for the period 1997–2010. SCHDE occurs mostly in the northeastern and western regions and shows a statistically significant increasing trend mainly in the midwestern and southeast regions, which is mainly associated with an increased temperature (Tair), drying trend in soil moisture (SM), and a sharp increasing trend in evapotranspiration (ET). The contribution of Tair variability to the SCHDE variability is largest (57.27%), followed by SM (28.43%) and ET (14.30%). In addition to Tair, SM deficit before the onset also plays an important role in the SCHDE occurrence. Furthermore, the SCHDE variability tends to be more sensitive to the SM deficits in the high‐altitude areas than in the low‐altitude areas. Besides, wavelet transform analysis results indicate that the cold phase of ENSO tends to trigger a higher risk of SCHDE events, while El Niño Modoki events have different influences on the variations of SCHDE during different warm/cold phases over the Pearl River basin.
SCHDE showed a significant increasing trend during 1997–2010.
Although Tair dominates SCHDE, SM deficit before the onset also plays an important role in the SCHDE occurrence.
El Niño and El Niño Modoki events had different impacts on SCHDE.
Oases environments in oases to be sensitive to anthropogenic activity because of ecological fragility. Polycyclic aromatic hydrocarbon (PAH) pollution resulting from anthropogenic activity leads to ...ecological degradation in oases. To examine the impact of anthropogenic activity on the oasis ecological environment, the present study focused on the spatial distribution and source apportionment of soil PAHs and bacterial community responses in typical oases in Xinjiang, China. The results showed that the soil PAH level were higher in the city centres of Urumqi (9–6340 μg kg−1), Aksu (8–957 μg kg−1) and Korla (8–1103 μg kg−1) and lower in the centres of Hotan city (11–268 μg kg−1) and Qira county (7–163 μg kg−1). Source apportionment suggested that gasoline emissions, diesel emissions, vehicle emissions, coal combustion, coke processing and biomass burning were the sources of soil PAHs. The integrated lifetime cancer risks of soil PAH exceeding the guideline safety values (10−6) recommended by United States Environmental Protection Agency. The ingestion and dermal exposure pathways caused the greatest health risk (contribution ≤82%). Additionally, in the soil with low PAH concentrations, the richness and evenness of the soil bacterial community were great, and the molecular ecological network (MEN) structure was complex. Among populations, Proteobacteria and Actinobacteria (relative abundance ≥17%) are the main dominant species in the bacterial communities and the keystone species in the MEN.
•Soil samples were collected from five arid oases in Xinjiang.•Soil PAH concentrations in Urumqi was the highest among the given oases.•Traffic emissions were identified as the main source of soil PAHs.•Proteobacteria and Actinobacteria were the dominant bacterial species.
In this study, a pore-scale simulation method is applied to quantitatively study the variation of solute dilution through porous media with different cementation degrees and explore the corresponding ...mechanisms. The study results indicated that the cementation degrees of the solid grains had a significant effect on the solute dilution process and that the influence was very complicated. The complexity was manifested in that the effect of rising cementation degree on the solute dilution process would be different or even completely opposite in the porous media in which the solid grains cement slightly with that in porous media with a higher cementation degree. For example, for the porous media in which the solid grains were slightly cemented (the percentage of the cemented solid grains Pc is less than 40%), the dilution effects became enhanced with the increase of cementation degree. Then, after Pc increased to about 55%, the dilution effect was obviously weakened, and the solute was in an incomplete dilution state for a long period of time. In addition, this study found that the properties of the flow fields may vary greatly in porous media with different cementation degrees and that those differences in the flow fields resulted in the distinct behavior of the solute dilution. It is interesting to note that a more heterogeneous flow field had not necessarily led to the enhancement of the dilution process.
•Clay minerals have an important influence on the mobility of PSNPs in porous media.•KL has the strongest inhibition effect on the mobility of PSNPs, followed by MT and IL.•The 2D distribution of ...DDLVO interaction energy is obtained to quantify the critical ionic strength.•k and Smax linearly decrease with the logarithm of the integrated value of the DLVO energy barrier.•Rm exponentially increases with the logarithm of the integrated value of the DLVO energy barrier.
Microplastics are widely detected in the soil-groundwater environment, which has attracted more and more attention. Clay mineral is an important component of the porous media contained in aquifers. The transport experiments of polystyrene nanoparticles (PSNPs) in quartz sand (QS) mixed with three kinds of clay minerals are conducted to investigate the effects of kaolinite (KL), montmorillonite (MT) and illite (IL) on the mobility of PSNPs in groundwater. Two-dimensional (2D) distributions of DLVO interaction energy are calculated to quantify the interactions between PSNPs and three kinds of clay minerals. The critical ionic strengths (CIS) of PSNPs-KL, PSNPs-MT and PSNPs-IL are 17.0 mM, 19.3 mM and 21.0 mM, respectively. Experimental results suggest KL has the strongest inhibition effect on the mobility of PSNPs, followed by MT and IL. Simultaneously, the change of ionic strength can alter the surface charge of PSNPs and clay minerals, thus affecting the interaction energy. Experimental and model results indicate both the deposition rate coefficient (k) and maximum deposition (Smax) linearly decrease with the logarithm of the DLVO energy barrier, while the mass recovery rate of PSNPs (Rm) exponentially increases with the logarithm of the DLVO energy barrier. Therefore, the mobility and associated kinetic parameters of PSNPs in complex porous media containing clay minerals can be predicted by 2D distributions of DLVO interaction energy. These findings could help to gain insight into understanding the environmental behavior and transport mechanism of microplastics in the multicomponent porous media, and provide a scientific basis for the accurate simulation and prediction of microplastic contamination in the groundwater system.
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The aquifer in the coastal area of the Laizhou Bay is affected by salinization processes related to intense groundwater exploitation for brine resource and for agriculture irrigation during the last ...three decades. As a result, the dynamic balances among freshwater, brine, and seawater have been disturbed and the quality of groundwater has deteriorated. To fully understand the groundwater chemical distribution and evolution in the regional aquifers, hydrogeochemical and isotopic studies have been conducted based on the water samples from 102 observation wells. Groundwater levels and salinities in four monitoring wells are as well measured to inspect the general groundwater flow and chemical patterns and seasonal variations. Chemical components such as Na
+
, K
+
, Ca
2+
, Mg
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, Sr
2+
, Cl
−
, SO
4
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, HCO
3
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, NO
3
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, F
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, and TDS during the same period are analyzed to explore geochemical evolution, water-rock interactions, sources of salt, nitrate, and fluoride pollution in fresh, brackish, saline, and brine waters. The decreased water levels without typical seasonal variation in the southeast of the study area confirm an over-exploitation of groundwater. The hydrogeochemical characteristics indicate fresh-saline-brine-saline transition pattern from inland to coast where evaporation is a vital factor to control the chemical evolution. The cation exchange processes are occurred at fresh-saline interfaces of mixtures along the hydraulic gradient. Meanwhile, isotopic data indicate that the brine in aquifers was either originated from older meteoric water with mineral dissolution and evaporation or repeatedly evaporation of retained seawater with fresher water recharge and mixing in geological time. Groundwater suitability for drinking is further evaluated according to water quality standard of China. Results reveal high risks of nitrate and fluoride contamination. The elevated nitrate concentration of 560 mg/L, which as high as 28 times of the standard content in drinking water is identified in the south region. In addition, the nitrate and ammonia data of the Wei River suggests decreasing nitrification rate in the study area from inland to estuary. High fluoride concentration, larger than 1 mg/L, is also detected in an area of about 50% of the study region. The saltwater intrusion is analyzed to be responsible for part of dissolution of minerals containing fluoride. Therefore, water treatment before drinking is needed in urgent to reduce the health expose risk.
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