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"Subsurface flow"
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Surface and subsurface runoff generation processes in a poorly gauged tropical coastal catchment : a study from Nicaragua : dissertation
Hydrological research in humid tropics is particularly challenging because of highly variable hydrological conditions and high socio-economic stresses caused by rapid population increase, as is the case of Nicaragua. The objective of this research is to understand the surface and subsurface runoff generation processes in a poorly gauged coastal catchment in Nicaragua under variable humid tropical conditions. Specifically, it focuses on identifying geomorphological and hydro-climatic controls on catchment response at different spatio-temporal scales and studies the link between hydrological processes and ecosystem conditions.
Book
Understanding hydraulic fracturing: a multi-scale problem
Despite the impact that hydraulic fracturing has had on the energy sector, the physical mechanisms that control its efficiency and environmental impacts remain poorly understood in part because the length scales involved range from nanometres to kilometres. We characterize flow and transport in shale formations across and between these scales using integrated computational, theoretical and experimental efforts/methods. At the field scale, we use discrete fracture network modelling to simulate production of a hydraulically fractured well from a fracture network that is based on the site characterization of a shale gas reservoir. At the core scale, we use triaxial fracture experiments and a finite-discrete element model to study dynamic fracture/crack propagation in low permeability shale. We use lattice Boltzmann pore-scale simulations and microfluidic experiments in both synthetic and shale rock micromodels to study pore-scale flow and transport phenomena, including multi-phase flow and fluids mixing. A mechanistic description and integration of these multiple scales is required for accurate predictions of production and the eventual optimization of hydrocarbon extraction from unconventional reservoirs. Finally, we discuss the potential of CO2 as an alternative working fluid, both in fracturing and re-stimulating activities, beyond its environmental advantages.
This article is part of the themed issue ‘Energy and the subsurface’.
Journal Article
Groundwater lowering in construction : a practical guide to dewatering
Many engineering construction projects entail excavations into water bearing substrates. The authors explain the drainage techniques required to lower groundwater sufficiently to allow projects to be undertaken with confidence.
Book
Generative Subsurface Flow Modeling With Pretrained Diffusion Model and Training‐Free Knowledge Alignment
We introduce a versatile generative learning framework that integrates probabilistic diffusion models, observational data, and domain knowledge for stochastic modeling of flow in porous media. The framework begins by pretraining an unconditional diffusion model to approximate the joint distribution of subsurface parameters and state variables, effectively capturing prior information of dynamical systems. By leveraging Bayesian conditional sampling, the model flexibly incorporates specific constraints and adapts to multiple modeling tasks without retraining or fine‐tuning. Furthermore, we devise a training‐free knowledge alignment strategy that embeds domain‐specific knowledge into the sampling process to generate spatiotemporal fields more compatible with physical principles. Extensive evaluations on diverse subsurface flow problems demonstrate that a single pretrained diffusion model, equipped with optimized generative paths, delivers superior performance in unconditional generation, forward prediction, uncertainty quantification, and inverse modeling with sparse and noisy data. These findings underscore the potential of knowledge‐aligned generative learning to advance subsurface flow modeling research.
Journal Article
A comparative experimental and multiphysics computational fluid dynamics study of coupled surface-subsurface flow in bed forms
The use of multiphysics computational fluid dynamics (CFD) approaches to simulate surface–subsurface flow processes is evaluated by comparison with flume experiments on current‐exposed permeable bed forms. The unique experimental data include measurements of the time‐averaged surface water flow velocities, the pressure distribution at the sediment–water interface, and pore water flow paths. The modeling approach first simulates the time‐averaged turbulent flow in the channel with CFD and then uses the predicted pressure distribution at the sediment–water interface to drive a flow and transport model for the sediment. The CFD‐modeled velocity and pressure distribution and transient particle tracks within the sediment agree reasonably well with observations. Differences that exist between observations and simulations mainly concern the eddies in the wake zone downstream of the ripple crests that are slightly shorter than those predicted by the model. This deviation propagates from the surface to the subsurface domain, appearing in the pressure distribution along the bed and, consequently, the subsurface flow patterns. The good representation of general patterns and rates makes multiphysics CFD modeling a powerful and sufficiently accurate tool that can replace measurements for many studies of surface–subsurface processes involving current‐exposed immobile bed forms. The approach can be used for predicting transport processes where they cannot easily be observed, such as in large rivers and coastal systems where boundary conditions such as mean currents and bed forms can be mapped. Key Points Validation of surface‐subsurface flow model by direct comparison to experiments Unique experimental data includes surface and subsurface flow and pressures Simulations match experiments with slight differences in eddy representation
Journal Article
Biochar-modified constructed wetlands using Eclipta alba as a plant for sustainable rural wastewater treatment
Constructed wetlands (CWs) provide a low-cost, effective solution for domestic wastewater treatment in developing nations compared to costly traditional wastewater systems. Biochar which is an organic material created by pyrolysis offers straightforward, affordable methods for treating wastewater and lowering carbon footprint by acting as a substrate in CWs. Batch mode biochar-amended subsurface flow (SSF) CWs planted with
Eclipta alba
(L) with a hydraulic retention time (HRT) of 3 days were used for the treatment of rural domestic wastewater in the present investigation. Two control CWs, without plants (C1) and with plants (C2), and five different amendments of biochar 5% (B5), 10% (B10), 15% (B15), 20% (B20) and 25% (B25) in ratio with soil were set up to check the treatment efficiency of CWs. Removal efficiency (RE%) of the CWs for parameters namely chemical oxygen demand (COD), biochemical oxygen demand (BOD), phosphate (PO
4
2−
), sulphate (SO
4
2−
), nitrate (NO
3
−
) and total Kjeldhal nitrogen (TKN) was determined using standard methods. Removal efficiency of 93%, 91%, 74% and 77% was observed for BOD, COD, nitrate and sulphate, respectively, in the B25 amendment at HRT 72 h. The highest removal of TKN (67%) was also observed in the B25 amendment at HRT of 72 h. No stable trend for the removal of phosphates was found during the study, and maximum removal was observed at HRT 48 h; afterward, phosphate was slightly inclined with the increasing HRT. The findings of one-way ANOVA using Tukey’s test show significant variations (
p
< 0.05) in the removal efficiencies of pollutants after 72 h between two controls (C1 and C2) and various biochar amendments in CWs, indicating a significant role of the wetland plants and concentration of the biochar as substrate. Biochar shows a positive impact on the removal of organic pollutants and nitrates. Hence, biochar-amended CWs can be a sustainable way of treating rural domestic wastewater.
Journal Article
Combined role of ground cover management in altering orchard surface‒subsurface erosion and associated carbon–nitrogen-phosphorus loss
The combined role of ground cover management in controlling soil erosion and nutrient loss from new orchards is still less understood. In this study, four ground cover management practices, orchard with grass cover (OG), orchard with interplant cover (OI), orchard with straw cover (OS), and orchard with bare ground (OB), were designed to identify their impacts on soil erosion and associated carbon–nitrogen-phosphorus loss in new orchards by rainfall simulation tests with rainfall intensities of 60, 90, and 120 mm h
−1
and 90 min rainfall duration. The results showed that OS had the lowest surface flow coefficient (6.6%) and highest subsurface flow coefficient (32.5%). The highest soil loss rate occurred in the OB plot (65.4 g m
−2
min
−1
), and the lowest soil loss rate occurred in the OS plot (0.5 g m
−2
min
−1
). OS plot showed better effectiveness in improving soil erosion. However, the increased infiltration capacity was facilitated in terms of causing non-point source pollution. The C-N-P ratios of surface flow in different cover measures (OB, OI, OG, and OS) were 1.4:1.2:0.9:1, 1.8:1.7:1.2:1, and 2.3:1.9:1.2:1, respectively. The ratios of sediment in different cover measures were 7.3:9:2.3:1, 2:1.5:1.2:1, and 1.2:1:0.8:0.7, respectively. Cover management plots play an active role in reducing nutrient loss in surface flow and sediment, but the increased infiltration in covered management plots is associated with the risk of groundwater contamination in subsurface flow. The C-N-P ratios of subsurface flow in OB and covered managed plots (OI, OG, and OS) were 1:3.3:1.6:2.7, 1:1.5:2.2:2.4 and 1:1.2:1.5:1.3, respectively. Therefore, when managing the phenomenon of soil erosion through ground cover measures, attention should also be focused on the function of cover measures in regulating non-point source pollution underground, such as subsurface flow. This research recommends a combination of cover management measures to further mitigate erosion and the risk of groundwater contamination.
Journal Article
How Well Bucket Lysimeters Correspond with Whole-catchment Runoff and its Chemistry: A Case Study of Artificial Experimental Catchments at a Post-mining Site
This study evaluated efficiency of bucket lysimeters for measuring water fluxes and ion transport in four hydrologically isolated experimental catchments representing reclaimed (levelled and planted by alder) and unreclaimed (wave like topography, unvegetated) post-mining sites near Sokolov, Czech Republic. Weekly measurements of leachate from lysimeters and surface/subsurface runoff from experimental catchments, in which lysimeters were installed, were collected from 2021 to 2024. Ion concentrations (Ca
, Na
, Li
, NH
, K
) were quantified using ion-selective electrodes. Upscaled estimates showed higher accuracy at the unreclaimed site (R
= 0.81 for total runoff, R
= 0.88 for evapotranspiration) than at the reclaimed site (R
= 0.72 and R
= 0.77). Lysimeter leachate explained surface runoff variance at unreclaimed (R
= 0.75) and reclaimed (R
= 0.47) sites, but was not predictive for subsurface flow. Among ions, Li
showed the highest predictive capacity (R
= 0.44 - 0.56), while NH
showed consistent patterns across sites. K
, Na
, and Ca
showed variable transport influenced by soil and vegetation development. Lysimeters captured surface water fluxes and evapotranspiration but did not represent subsurface flow or solute transport well. Better lysimeter performance at the unreclaimed site suggests that vegetation development reduces hydrological predictability during ecosystem recovery.
Journal Article
Rhizosphere Microorganisms in Subsurface Flow Garden Constructed Wetland and their Influence on Nitrogen Removal Efficiency
Subsurface flow garden constructed wetland (SFGCW) is a type of constructed wetland with garden characteristics. This study explores the efficiency of SFGCW in the removal of ammonia nitrogen (NH4+-N) and total nitrogen (TN) from domestic sewage and examines the structure of the rhizosphere microbial community and prevalence of nitrifying and denitrifying bacteria. An L4(23) orthogonal experiment was conducted using different factors such as substrates, plants, and hydraulic retention times (HRT). The results of range and variance analyses revealed that HRT and substrate considerably influenced nitrogen removal by SFGCWs, with plant factors playing a notable role. The use of fluidized bed slag as the substrate enhanced nitrogen removal, particularly when HRT was set at 3 or 6 days. At the phylum level, Proteobacteria predominated the rhizosphere microbial abundance, comprising 42.58%–55.38% of the microbial population, followed by Chloroflexi (7.19%–17.16%). It exhibited higher counts in winter than in autumn. Anaerolineaceae, which belongs to Chloroflexi, was predominant in each wetland group. Seasonal variations significantly impacted the abundance of ammonia-oxidizing and nitrite-oxidizing bacteria among nitrifying bacteria in the rhizosphere microbial community, with higher levels observed in autumn than in winter, and played a crucial role in NH4+-N transformation. Additionally, a correlation was observed between the nitrogen removal effectiveness and abundance of rhizosphere microbial nitrifying bacteria, providing technical insights for further optimization of the structure and operational parameters of SFGCW.
Journal Article
Effects of different substrates on nitrogen and phosphorus removal in horizontal subsurface flow constructed wetlands
This study aimed to explore the nitrogen and phosphorus removal performance of the horizontal submerged constructed wetland (HSCW) with Ti-bearing blast furnace slag (T). Another two HSCWs, with the converter steelmaking slag (G) and the stone (S) as wetland substrates, respectively, were simultaneously running as control. The results showed that the nitrogen and phosphorus removal capacities of the T-HSCW were generally better than those of another two HSCWs. When the hydraulic retention time (HRT) was 6 days, the effluent concentrations of ammonia nitrogen (NH
4
+
-N) and total nitrogen (TN) were 6.66 mg L
−1
and 14.02 mg L
−1
, respectively, and the removal rates of NH
4
+
-N and TN reached 77.54% and 71.07%, respectively. The T-HSCW had better removal efficiency of phosphorus. The effluent concentration of total phosphorus (TP) was lower than 0.3 mg L
−1
, and the maximum removal rate could reach 98%. Through the characterization of the three substrates before and after experiments, it was found that the removal of nitrogen and phosphorus by T and G mainly relied on chemical adsorption, while S mainly relied on physical adsorption. Ti could also promote the absorption of nitrogen by plants and increase the nitrogen removal capacity of T-HSCWs.
Journal Article