Risk Of Groundwater Contamination Research Articles

Deep accumulation of soluble organic nitrogen after land-use conversion from woodlands to orchards in a subtropical hilly region

Accumulation of soluble organic nitrogen (SON) in soil poses a significant threat to groundwater quality and plays an important role in regulating the global nitrogen cycle; however, most related studies have focused only on the upper 100-cm soil layers. Surface land-use management and soil properties may affect the vertical distribution of SON; however, their influence is poorly understood in deep soil layers. Therefore, this study assessed the response of SON concentration, pattern, and storage in deep regoliths to land-use conversion from woodlands to orchards in a subtropical hilly region. Our results showed that the SON stocks of the entire soil profile (up to 19.5 m) ranged from 254.5 kg N ha−1 to 664.1 kg N ha−1. Land-use conversion not only reshaped the distribution pattern of SON, but also resulted in substantial accumulation of SON at the 0–200 cm soil profile in the orchards compared to that in the woodlands (124.1 vs 190.5 kg N ha−1). Land-use conversion also altered the SON/total dissolved nitrogen ratio throughout the regolith profile, resulting in a relatively low (<50 %) ratio in orchard soils below 200 cm. Overall, 76.8 % of SON (338.4 ± 162.0 kg N ha−1) was stored in the layers from 100 cm below the surface to the bedrock. Regolith depth (r = −0.52 and p < 0.05) was found to be significantly correlated with SON concentration, explaining 17.8 % of the variation in SON, followed by total nitrogen (14.4 %), total organic carbon/total nitrogen ratio (10.1 %), and bulk density (9.3 %). This study provides insights into the estimation of terrestrial nitrogen and guidance for mitigation of groundwater contamination risk due to deep accumulation of SON.

Hydrodynamic Behaviors and Geochemical Evolution of Groundwater for Irrigation in Yaoba Oasis, China

The Yaoba Oasis is an irrigated cropland entirely dependent on groundwater; previous investigations (1980–2015) revealed an over-abstraction of groundwater and deteriorating groundwater quality. For further exploring the hydrodynamic behaviors and geochemical processes of groundwater during the irrigation season, groundwater samples were collected and analyzed using different techniques including classical statistics, correlation analysis, Piper diagrams, and Gibbs diagrams. The results indicated that Na+, K+, SO42− and Cl− were the main ions in groundwater, which were significantly correlated with TDS. The water–rock interaction is manifested by the precipitation of calcite and dolomite and the dissolution of rock salt and gypsum as an increase in TDS related to evaporation. In addition, the increasing complexity of hydrochemical type is caused by the rapid variation of hydrodynamic regime, irrigation and evaporation, which are subjected to the constraints of salty water intrusion from the desert salty lake and infiltration of irrigation return flow. Existing wells should limit overexploitation to halt the decline in groundwater levels and cut down irrigation water to reduce the risk of groundwater contamination and restore ecological balance in desert oasis.

A new model for predicting hydraulic fracture penetration or termination at an orthogonal interface between dissimilar formations

Vertical height growth of hydraulic fractures (HFs) can unexpectedly penetrate a stratigraphic interface and propagate into neighboring layers, thereby resulting in low gas-production efficiency and high risk of groundwater contamination or fault reactivation. Understanding of hydraulic fracture behavior at the interface is of pivotal importance for the successful development of layered reservoirs. In this paper, a two-dimensional analytical model was developed to examine HF penetration and termination behavior at an orthogonal interface between two dissimilar materials. This model involves changes in the stress singularity ahead of the HF tip, which may alter at the formation interface due to material heterogeneity. Three critical stress conditions were considered to assess possible fracture behavior (i.e., crossing, slippage, and opening) at the interface. Then, this model was verified by comparing its theoretical predictions to numerical simulations and three independent experiments. Good agreement with the simulation results and experimental data was observed, which shows the validity and reliability of this model. Finally, a parametric study was conducted to investigate the effects of key formation parameters (elastic modulus, Poisson's ratio, and fracture toughness) between adjacent layers. These results indicate that the variation in the introduced parameters can limit or promote vertical HF growth by redistributing the induced normal and shear stresses at the interface. Among the three studied parameters, Poisson's ratio has the least influence on the formation interface. When the fracture toughness and elastic modulus of the bounding layer are larger than those of the pay zone layer, the influence of fracture toughness will dominate the HF behavior at the interface; otherwise, the HF behavior will more likely be influenced by elastic modulus.

Groundwater Vulnerability and Groundwater Contamination Risk in Karst Area of Southwest China

Groundwater pollution in karst areas is often the result of the interaction of hydrogeological characteristics and human activities. In this study, a chimeric evaluation system of groundwater vulnerability and pollution load was constructed, and the groundwater contamination risk was intuitively displayed using a risk map. The study area is located in Qiannan, a typical karst area in Southwest China. Groundwater vulnerability was mainly calculated using the PLEIK model. Pollution source load evaluation was evaluated using the classification method. The groundwater pollution status was evaluated by the CCME WQI method. The results show that the groundwater vulnerability in the research area was mainly at a medium level. The areas with high pollution load values were mainly distributed in industrial parks and other pollution-source-gathering areas. The results of the water quality evaluation show that the groundwater quality in the study area was generally good. However, the impact of some pollution sources on the surrounding groundwater still cannot be ignored. The results reveal the risk level of groundwater pollution in typical karst areas and provide theoretical support for regional groundwater protection.

Vulnerability methods in hard rock formation as a basis for groundwater risk assessment – from resource to source

Groundwater in a hard rock formation is most endangered at places where a potential source can discharge contaminants that can reach the saturated zone of an aquifer. In these circumstances, an essential tool for groundwater protection is the contamination risk map. This map is based on the integration of two maps: a hazards map, i.e., map of potential sources of contamination and a vulnerability map. The selection of a proper vulnerability method is an important task since the resulting vulnerability map can significantly impact the final risk of contamination map. The most appropriate method for groundwater vulnerability assessment was considered in the case study of Tara National park, in Western Serbia. The four commonly used methods were applied to assess the intrinsic vulnerability maps: DRASTIC, EPIK, PI and COP. All the applied methods resulted in different vulnerability maps in assessing the degree of vulnerability, consequently influencing the groundwater contamination risk maps. The applied research presents an example of how contamination risk should be assessed in a specific area. Comparison of the results obtained for the area of Tara National Park indicates the preference of the PI method as a well-balanced method, taking into account all the specifics of the study area. A detailed analysis of the assessed risks in the catchments of the existing sources was also conducted to indicate probable sources of contamination and confirm the degree of accuracy of the created vulnerability and risk maps. The conducted research emphasizes the necessity to adopt a clear conceptual hydrogeological model and to apply several methods simultaneously to determine the optimal one for each individual area.

Stimulated leaching of metalloids along 3D-printed fractured rock vadose zone

Fractured rock aquifers are susceptible to contamination, with metal(loid)s rapidly migrating from poorly developed overburden to the fractured rock vadose zone and thus into groundwater. Compared to typical porous aquifers, retention effects within the rock matrix are small, and rapid advection along fractures leads to a higher risk of groundwater contamination. However, the highly complex anisotropic pathways of natural fractures hinder research in this field. To construct reproducible fractures, this study used 3D printing following Computed X-ray Microtomography (μCT) scans of a fractured rock collected in a natural limestone aquifer. Stimulated metalloid release was observed in the fractured rock during column leaching, and the leachate concentrations of arsenic (As) and antimony (Sb) increased by up to 17.5 and 36.4 times, respectively, compared with the porous vadose zone. Fluctuations in fracture metalloid release patterns in dissolved and adsorbed phases were attributed to retention and filtration effects induced by soil particles within fractures. Geophysical properties of the porous overburden, especially the aggregation characteristics, greatly affected the non-equilibrium leaching behavior of As, but had a limited effect on the near-equilibrium leaching of Sb, which was explored by modifying the surficial soil layer with either montmorillonite clay or charcoal. The results of this study provide a novel method and useful information for modeling and risk assessment of fractured rock aquifers.

Nitrate transport in a fracture-skin-matrix system under non-isothermal conditions.

Subsurface leaching of agricultural runoff has been identified to pose a serious hazard to the soil-water ecosystem and human health, mostly due to the associated contamination with nitrate. Our understanding of the nature of contaminant spread in the vadose and aquifer zones has been improved from recent mechanistic models on the flow and transport of contaminants through fractured porous media. The present study aims to explore the impacts of skin formation in a fracture-matrix aquifer system onto the nitrogen species transport under non-isothermal settings using numerical modeling. A finite-difference scheme was employed to capture the nitrogen concentration profile and kinetics of transformation by solving the derived partial differential equations. The results show evidence of an additional mass transfer from fracture to skin so as to reduce the migration of nitrogen species (NO3-N and N2) at the fracture-matrix interface thereby reducing the peak concentration of N2 by nearly 1.5 times in fracture after denitrification. Although the thermal conductivity of the rock matrix has a direct impact on the temperature distribution in fracture-skin-matrix profiles, the presence of skin has a cooling effect for a high-temperature influent (45°C), which also deteriorates the propagation of organic N2 and NO3-N, within the fracture. An increase in the temperature coefficient of skin has resulted in an apparent reduction in nitrogen species migration, indicating the thermo-chemical feasibility of an intermediate skin favoring the mass transfer processes. The findings of this study can be extended toward realistic estimation of groundwater contamination risks and for the design of biological filters for in situ remediation.

Effects of solution chemistry and humic acid on transport and deposition of aged microplastics in unsaturated porous media

Microplastics (MPs) are susceptible to aging in the environment, and aged MPs are highly migratory in soil due to their smaller particle size and more negative surface charge, but the effects of soil environmental factors on the fate and transport of aged MPs are still unclear. In this study, the transport behavior of pristine/aged MPs in unsaturated sandy porous media was examined under different ionic strength (IS), cationic type (Na+, Ca2+) and humic acid (HA) conditions. The results indicated that the surface charge, surface oxygen-containing functional groups and surface morphology of MPs changed significantly after aging, and that the mobility of aged MPs was significantly enhanced than the pristine MPs under all test conditions. The retention amounts of pristine/aged MPs in unsaturated porous media increased with IS, and IS had a less inhibitory effect on the transport of aged MPs than pristine MPs. The mobility of pristine/aged MPs in Ca2+ solutions was significantly weaker than that in Na+ solutions due to enhanced straining and electrostatic adsorption. HA promoted the mobility of pristine/aged MPs in unsaturated porous media under all IS Na+ (1, 10, and 25 mM) solutions and lower IS (1 mM) Ca2+ solutions, and the ability of HA to promote the transport of aged MPs was significantly stronger than that of pristine MPs due to the higher adsorption of HA on the surface of aged MPs. However, at higher IS (10 mM) Ca2+ solution conditions, the bridging effect of Ca2+ led to the formation of HA-MPs complexes, which altered the hydrophobicity of the pristine/aged MPs surface and the pristine/aged MPs were mainly retained on the air-water interface (AWI). CFT theory and two-site kinetic retention models indicated that the retention of pristine/aged MPs in unsaturated media was dominated by monolayer adsorption, straining and clogging effects. The current research findings may provide insights into the fate and transport of aged MPs in soil and their potential risk of groundwater contamination.

Assessment of social vulnerability to groundwater pollution using K-means cluster analysis.

It is possible to assess the harm that society suffers as a consequence of groundwater contamination in aquifers. Indexing methodologies are commonly applied to assess the social vulnerability to polluted aquifers. However, they assign weighting and rating values to the different factors involved, which makes them very subjective. This research aims to assess the social vulnerability to groundwater pollution taking into account three factors: the uses of groundwater resources, the exposed population, and the socio-economic losses. In order to eliminate the subjectivity of current indexing methodologies, this work uses a K-means cluster analysis for the assessment of social vulnerability. With this method, a social vulnerability map can be produced with greater objectivity. The proposed methodology is applied to an aquifer located in central Spain, an area with significant agricultural development. Low population density and unproductive zones result in low social vulnerability in most of the area. However, high social vulnerability is observed in the southern sector due to agricultural development, which leads to higher socio-economic variables and demand for groundwater resources. Similarly, high social vulnerability is observed in the northeast, mainly influenced by the groundwater use and the exposed population. These results show that social vulnerability in most of the study area is not very significant for assessing the risk of groundwater contamination, because the damage to the social, environmental, or economic sector is low. However, in the south and northeast of the study area, pesticides and fertilizers should be used with caution, as they significantly increase the risk of groundwater contamination. The K-means clustering method proved to be an objective and reliable option for assessing social vulnerability to groundwater pollution in aquifers.

The influence of biochar pyrolysis temperatures on the efficiency of soil amendment for Imazapic sorption

Weeds are responsible for losses in sugarcane productivity, often requiring the application of herbicides to control their infestation in crops. One of the herbicides commonly used in sugarcane cultivation is Imazapic. However, this chemical can be leached into environmental compartments when applied to soil, contaminating it. Scientific research has shown that the soil amended with biochar can reduce the leaching of herbicides, avoiding environmental pollution. Therefore, the present work aimed to evaluate the adsorption efficiency of Imazapic in a dystrophic Yellow Ultisol amended with biochar cultivated with sugarcane. Biochars were produced with sugarcane bagasse by pyrolysis at temperatures of 300, 500, and 700ºC to evaluate the adsorption capacity of Imazapic by the soil amended with biochar. The experiments revealed an increase in Imazapic adsorption with soil amended with biochar and that the adsorptive potential is more significant for pyrolyzed biochars at higher temperatures. In this way, the biochars produced can reduce the risks of groundwater contamination by increasing the residence time of Imazapic in the surface layer of the soil, being subject to the action of microbial degradation and other forms of degradation.

Mechanism comparisons of transport-deposition-reentrainment between microplastics and natural mineral particles in porous media: A theoretical and experimental study

The migration and distribution of microplastic particles (MPs) in the natural environment has attracted global attention in recent years. However, little is known about the transport-deposition-reentrainment differences between MPs and natural mineral particles in porous media. In this study, polystyrene (PS) and silica (SiO2) particles, representing model MPs and natural mineral particles, respectively, were selected to study the responses of different particle types to changes in specific particle size and flow velocity. Three typical particle sizes and various flow velocities were chosen to compare and delineate the transport-deposition-reentrainment characteristics of PS and SiO2 in a packed-bed laboratory column. Collector efficiency was calculated using Tufenkji and Elimelech (TE) equation. The particle fractions released from the collector surfaces were predicted using DLVO theory and force analysis. Two types of particles were attached in the secondary minimum, which were either retained on the collector surface or reentrained to the fluid. The staged elution experiment wherein the flow velocity was increased experienced a period of flow shock, thus breaking the force balance of the particle. An increase in the flow velocity resulted in various degrees of particle elution. The breakthrough experiment at a specific flow velocity showed that the corresponding velocity alteration in staged elution experiment contributed to reentrainment to varying extents. When the effect of gravity on particle deposition was negligible, the particle size was larger, and the lower the velocity for releasing the particles. However, the opposite tendency was observed when considering the effect of gravity on particle deposition. Moreover, the deposition, mainly due to gravity, easily causes particle reentrainment as the flow velocity increases. This study further predicts and reveals the nature of transport and deposition differences between MPs and natural mineral particles, which helps to further assess the risk and potential of groundwater contamination with MPs of different sizes.

Regional Scale Assessment of Shallow Groundwater Vulnerability to Contamination from Unconventional Hydrocarbon Extraction.

Concerns over unconventional oil and gas (UOG) development persist, especially in rural communities that rely on shallow groundwater for drinking and other domestic purposes. Given the continued expansion of the industry, regional (vs local scale) models are needed to characterize groundwater contamination risks faced by the increasing proportion of the population residing in areas that accommodate UOG extraction. In this paper, we evaluate groundwater vulnerability to contamination from surface spills and shallow subsurface leakage of UOG wells within a 104,000 km2 region in the Appalachian Basin, northeastern USA. We test a computationally efficient ensemble approach for simulating groundwater flow and contaminant transport processes to quantify vulnerability with high resolution. We also examine metamodels, or machine learning models trained to emulate physically based models, and investigate their spatial transferability. We identify predictors describing proximity to UOG, hydrology, and topography that are important for metamodels to make accurate vulnerability predictions outside their training regions. Using our approach, we estimate that 21,000-30,000 individuals in our study area are dependent on domestic water wells that are vulnerable to contamination from UOG activities. Our novel modeling framework could be used to guide groundwater monitoring, provide information for public health studies, and assess environmental justice issues.

Groundwater Contamination Risk Assessment Based on Groundwater Vulnerability and Pollution Loading: A Case Study of Typical Karst Areas in China

Groundwater contamination risk assessment is an important basis to support the protection of the groundwater ecological environment. In this research, the groundwater contamination risk in typical karst areas in China was evaluated by PLEIK model (P: protective cover; L: land use; E: epikarst development; I: infiltration conditions; K: karst development) and classification and discrimination methods were used to assess groundwater vulnerability and pollution source load; the water quality index method was used to assess the status of groundwater contamination in the research area. The results show that groundwater vulnerability values in the research area range from 3.04 to 7.79, and most areas have low groundwater vulnerability. Groundwater pollution loading indexes, in the region of the pollution sources, gathered numerical up-water quality status evaluation that shows that most of the regional groundwater quality in the research area has good groundwater at present. The groundwater contamination risk assessment results show that the groundwater pollution risk is mainly at a very low level in most areas of the study area, but the groundwater pollution risk is higher in the areas where the pollution sources gather. The result reveals that the regional groundwater contamination risk level for regional groundwater ecological environment protection provides a theoretical basis for policy making.

Risk assessment methodology for Underground Coal Gasification technology

Geoenergy technologies, including the Underground Coal Gasification (UCG), are currently being considered as possible solutions for reducing emissions of CO2 and other gases to atmosphere and at the same time, provide sustainable sources of low-carbon energy. In this paper, a flexible risk assessment methodology for UCG technology is presented, based on an established methodology for radioactive waste repositories. The assessment methodology can be applied at any stage in a project, between initial planning and final site abandonment. Central to the approach is the analysis of scenarios, which represents a “source-pathway-receptor” combination and its evolution. Here, a Reference Scenario (RS) and several Alternative Scenarios (AS) are developed and analysed using a numerical model. Results of the RS suggest that contaminant concentrations at an evaluation point are far below any level that could reasonably be detected. In some AS, the calculated concentrations showed an increasing trend at the end of the assessment period, potentially approaching levels that conceivably could be detected. However, as such cases are unexpected and pessimistic, their inclusion is to illustrate worst cases that could happen, rather than to give predictions. An illustrative application demonstrates that plausibly the risks of groundwater contamination from a UCG site should be very low if the site is developed and operated appropriately. The outcomes from applying the numerical model are intended to demonstrate how the methodology and the numerical model can be readily adapted to different sites.

Groundwater vulnerability and contamination risk mapping of semi-arid Totko river basin, India using GIS-based DRASTIC model and AHP techniques

Totko river basin is a semi-arid watershed, which undergoes severe water crisis during the dry season. Presently, due to increase in population, demand of food has increased leading to a rise in growth of high yield crop variety and usage of chemical fertilizers and pesticides. So, surface water as well as groundwater is getting polluted. In this study assessment of groundwater vulnerability of Totko river basin has been done using DRASTIC and Analytic Hierarchy Process (AHP) models. For this, seven hydrogeological parameters have been considered which are; Depth to water level (D), Net Recharge (R), Aquifer media (A), Soil media (S), Topography (T), Impact of vadose zone (I) and Hydraulic Conductivity (C). Weight and rating analysis of the seven criteria and their sub-criteria have been done using generic DRASTIC algorithm and AHP comparison matrix. Ground Water Vulnerability Map (GWV) obtained from DRASTIC and AHP analysis has been divided into five vulnerable classes. Area of very high vulnerable zone is 6.53% more in AHP based vulnerability as compared to Generic DRASTIC. Similarly, these regions show a high nitrate concentration (30–50 ppm) in groundwater. GWV maps have been validated through nitrate concentration and the accuracy of the models have been assessed through Pearson's correlation coefficient and Kappa coefficient. To prevent groundwater contamination proper land use planning and watershed management are necessary, for which vulnerable zones need to be demarcated and DRASTIC is a useful model for vulnerability assessment.

Propisochlor retention and mobility in the presence of straw in Brazilian cropping systems

ABSTRACT Non-ionic herbicides, such as propisochlor, are more easily retained in organic soil fractions. The presence of straw in tropical cropping systems can affect herbicide transposition to the underlying seed banks and their environmental behavior. The aim of this study was to evaluate propisochlor soil retention in the presence of soybean and sugarcane straw (0, 5 and 10 t ha−1) through radiometric techniques. Most of the applied 14C-propisochlor was retained in the soil profile (18.2–39.4%) following an extreme rainfall simulation in the presence of straw. Decreased herbicide leaching was observed in the presence of sugarcane and soybean straw (0.7–0.5%), compared with soil without straw (1.8%). Hysteresis values ranged from 0.8 to 1.0, indicating that propisochlor sorption was reversible allowing for soil seed bank effects, as well as environmental dissipation. These findings indicate that propisochlor can overcome straw barriers, although representing low groundwater contamination risks. Because of this, reduced environmental impacts following the application of this herbicide are noted for tropical cropping systems containing straw mulch.

Groundwater contamination and human health risk assessment in the main karst areas of Romania

Long-term monitoring reveals the temporal evolution of groundwater chemistry and the potential human health risk posed by the use of contaminated waters (Giri and Singh 2015). Groundwater aquifers are an important source of drinking water in Romania. This study was conducted to appraise the groundwater chemistry and the potential non-carcinogenic risks for human health associated with the groundwater consumption through oral and dermal pathways. In order to achieve this aim, 193 samples were collected from 29 groundwater sources in the main karst areas of Romania during 2019-2021. A total of 15 parameters were analyzed (F-, Cl-, SO42-, NO3, Ca, Na, K, Mg, Fe, Mn, Cu, Zn, Ni, Cr, and As) and compared with the World Health Organization standards for drinking water (WHO 2017). The heavy metals pollution index showed that no groundwater samples had detectable levels of metal contamination, while the heavy metal evaluation index revealed that 4 out of 29 groundwater sources were classified as contaminated: 3 sources with a medium level of pollution and 1 source with a high level of pollution, indicating a potential risk for human health. The human health risk for oral exposure indicated a potential non-carcinogenic risk only in the karst area from South Dobrogea. The non-carcinogenic risk posed by nitrates is higher than that posed by metals in the aquifers from South Dobrogea. Therefore, control and remedial actions should be taken to ensure appropriate water quality for the locals using the contaminated water sources.

Does the timing of short-term biowaste compost application affect crop growth and potential nitrate leaching? The case studies of processing tomato and cauliflower under field conditions

The feasibility of municipal solid waste compost (MSWC) as a substitute for mineral nitrogen (N) fertiliser was tested for a spring-summer (i.e., processing tomato) and an autumn-winter (i.e., cauliflower) vegetable crop grown in Mediterranean open field conditions. Two different doses (10 and 20 t dm C ha–1) and two distribution timings for each dose (i.e., early application at about nine months before processing tomato transplanting and five months before cauliflower transplanting: C10_early and C20_early; late application at about one month before processing tomato and cauliflower transplanting: C10_late and C20_late) were compared in a two-year field experiment. An unfertilised control and a 100% mineral N fertilisation (MIN, 200 kg N ha–1 for processing tomato and 150 kg N ha–1 for cauliflower) were added to the experiment. The application of MSWC significantly reduced the aboveground DM accumulation compared to the MIN in both crops, and it was inadequate to ensure a high yield for spring-summer and autumn-winter vegetables. However, the timing of compost application seems to play an essential role in reducing the reduction of crop growth due to compost application. In both tomato and cauliflower, when the MSWC was applied a few months earlier than the transplanting (i.e., in the previous summer in tomato and the previous spring in cauliflower), the DM and yield reduction was less apparent than in soil where compost was applied immediately before transplanting. Despite the lowest N-uptake associated with the MSWC application, the N-NO3 concentration in the soil solution was reduced by MSWC. In addition to the amendment effect, compost use may positively impact lowering N leaching risks in the groundwater. Combining the use of MSWC applied early before the crop season with mineral N fertiliser, it is possible to gain high yield, increase soil organic carbon and reduce groundwater contamination risk both in spring-summer and autumn-winter vegetable crops. Highlights- Biowaste compost decreased the aboveground biomass accumulation and yield in processing tomato and cauliflower. - Biowaste compost alone did not meet the N requirement in processing tomato and cauliflower. - Biowaste compost distribution in the summer before the processing tomato growing season alleviated its depressive effect in reducing DM and yield. - Biowaste compost distribution in the spring before the cauliflower growing season alleviated its depressive effect in reducing DM and yield. - Biowaste compost decreased the N-NO3 concentration in soil solution compared to mineral fertilisation with a positive effect in reducing N leaching risks in the groundwater.

Discrete Bubble Flow in Granular Porous Media via Multiphase Computational Fluid Dynamic Simulation

The coal seam gas industry has raised public concerns about the potential risk of groundwater contamination, where gases leaked from coal seams are thought to pollute groundwater. However, the basic principles and controlling parameters for gas seepage from deep ground formations to the ground surface have not been fully understood. As a possible mechanism for gas transport in the subsurface environment, discrete bubble flow was previously investigated using laboratory experiments by Ma et al. (Water Resour. Res, 2015, 51 (6), 4359–4373). This study developed a multiphase computational fluid dynamic (CFD) model to simulate discrete bubbly flow in a two-dimensional granular porous media at the pore scale. Following the experimental setup from Ma et al. (Water Resour. Res, 2015, 51 (6), 4359–4373), a “point source” with preset bubble fluxes was specified in a simulating domain representing the flume size in the earlier experiments. There were around 7,000 granular particles within this domain to model the porous media. This numerical model was validated by conserving the gas mass in the simulating domain. The simulation results provide more physical insights into complex bubble transport behaviour in porous media through specific plume parameters. The breakthrough time of the bubble plume and the cross-sectional averaged velocity of ambient pore water flow were manifested to be proportional to the gas release rates in the logarithmic scales. Also, the bubble plume width was also observed to be proportional to the gas release rates. Moreover, the gas distribution on the top boundary could be observed. The outcomes were further tested against the scaling solutions proposed by Ma et al. (Water Resour. Res, 2015, 51 (6), 4359–4373) with disagreements. The limitations of this multiphase computational fluid dynamic model were finally discussed.

Modeling the Water and Nitrogen Management Practices in Paddy Fields with HYDRUS-1D

Rice production involves abundant water and fertilizer inputs and is prone to nitrogen (N) loss via surface runoff and leaching, resulting in agricultural diffuse pollution. Based on a two-season paddy field experiment in Jiangsu Province, China, field water and N dynamics and their balances were determined with the well-calibrated HYDRUS-1D model. Then, scenarios of different controlled drainage and N fertilizer applications were simulated using the HYDRUS-1D model to analyze the features and factors of N loss from paddy fields. Evapotranspiration and deep percolation were the two dominant losses of total water input over the two seasons, with an average loss of 50.9% and 38.8%, respectively. Additionally, gaseous loss of N from the whole soil column accounted for more than half of total N input on average, i.e., ammonia volatilization (17.5% on average for two seasons) and denitrification (39.7%), while the N uptake by rice accounted for 37.1% on average. The ratio of N loss via surface runoff to total N input exceeded 20% when the N fertilizer rate reached 300 kg ha−1. More and longer rainwater storage in rice fields under controlled drainage reduced surface runoff losses but increased the risk of groundwater contamination by N leaching. Therefore, compared with raising the maximum ponding rainwater depth for controlled drainage, optimizing N fertilizer inputs may be more beneficial for controlling agricultural diffuse pollution by reducing N loss via surface runoff and leaching. The HYDRUS-1D model provides an approach for the quantitative decision-making process of sustainable agricultural water and N management.