Outlet Stream Research Articles

Epikarst Controls of Runoff Composition in Subterranean Stream After Rainstorm Events

ABSTRACTEpikarst plays a critical role in karst water circulation, however, its studies have often been limited to small springs or basins, ignoring its importance in larger, highly karstified subterranean streams. This study focused on the Longlingong (LLG) subterranean stream system, where the discharge and electrical conductivity (EC) of the subterranean stream outlet for 2.5 years, and found that 85% of all rainstorm events monitored during this period showed a pattern of increasing and then decreasing EC. High‐resolution hydrochemistry monitoring was conducted at the outlet during a rainstorm event when this response pattern occurred. Simultaneously, epikarst water (EW), saturated zone fissure karst water (FW), and surface water (PW) from this system were collected as three recharge sources for hydrochemical analyses based on the results of the field investigations, and end‐member mixing analyses were performed using the EMMA‐MIX model. These combined methods can accurately characterise the response of the subterranean stream system to rainstorm events, assess the contribution and response of individual recharge sources throughout the rainstorm, and reveal the mechanisms involved. The results indicate that the increase in EC is primarily driven by changes in HCO3− concentration resulting from the rapid discharge of EW through sinkholes and shafts. EW contributes 28.7% of the rain‐induced subterranean stream runoff, exceeding FW. The epikarst exhibits a rapid response to rainstorms, as evidenced by a remarkable 681% increase in EW discharge following a rainstorm event. Flood peaks in the subterranean stream are mainly composed of PW (44%) and EW (43.6%). This study highlights the key role of the epikarst as a karst groundwater reservoir. The sensitivity of the epikarst to rainstorms, particularly its role in facilitating rapid piston‐like migration of EW during initial runoff, highlights its significant influence on discharge and hydrochemistry. This research contributes to a deeper understanding of the function of epikarst within highly karstified subterranean streams.

Efficient Exergy Analysis of Chemical Processes through Process Engineering Software

In this work, authors present a simple methodology for computing physical, chemical and total exergies, and the exergy destruction and the exergetic efficiency of simple and complex processes by using the object linking and embedded (OLE) automation by connecting Aspen HYSYS to MS Excel VBA. For this purpose, a simple flowsheet was added to the main one with the same number of streams as that of the components. By introducing all these streams in a mixer and then consecutively conducted to a heater to condition the outlet stream, a separator and a mixer, the VBA application was able to compute the chemical exergy of the different streams of the main flowsheet. To demonstrate the procedure here described seven cases were considered: single streams, a CO2 mixer, a cooler, a distillation column, a reactor, a multiple recirculation process, and different configurations of separating components by distillation, which can be extended to very complex processes. The former was selected to validate the methodology here proposed whereas the rest of cases were used to demonstrate the potential of the tool here developed, which in turn could be used for discriminating among different process alternatives.

Determining kinetics of fast exothermic reactions using a flow calorimeter

Continuous flow calorimeters are considered an emerging technique due to their ability to reduce safety issues associated with highly exothermic rapid reactions. In this study, a custom-made isoperibolic flow calorimeter was constructed, which uses a pyroelectric IR Sensor for temperature profile recording. The objective of this study is to validate the reactor concept for the determination kinetic parameters. Residence time distribution (RTD) analysis was conducted to assess the mixing performance of the packed-bed reactor employed in the calorimeter. This calorimeter was connected to a custom-made conductivity flow cell (CFC), to measure the in-line conductivity of the outlet stream. An axially dispersed plug flow reactor model (AD-PFR) is required to be able to incorporate the non-ideal behaviour of the flow. The experimental apparatus design and methodology was validated by conducting the hydrolysis of acetic anhydride. The activation energy, EA and the pre-exponential factor, ln(k0) for the hydrolysis of acetic anhydride were found to be 52.05 kJ mol−1 and 12.26 (s−1), respectively, which falls within the range of reported values. The new flow calorimeter proved to be effective, and accurate.

Heat integration analysis based on recycle split vapor ethane recovery process

AbstractRecovering ethane from natural gas involves significant energy consumption. Globally, the recycle split vapor process (RSV) is widely adopted as an efficient method for ethane recovery. Nonetheless, one major challenge faced by the RSV process is the lack of adequate heat integration, despite its overall effectiveness. In this article, we investigate the heat integration of the RSV process and propose two novel ethane recovery processes: the recycle split vapor process with direct heat integration of the feed gas (RSV‐DTI) and the recycle split vapor process with split heat integration of the feed gas (RSV‐SHI). A comparative analysis is conducted among these three processes, focusing on integrated energy consumption, exergy efficiency, and economic investment. The study's findings reveal the following: (1) The RSV‐DTI process distinguishes itself with its reduced energy consumption, enhanced stability, and minimized refrigerant usage. In comparison to the RSV process, the RSV‐DTI process achieves a reduction of over 15% in total compression duty and a remarkable decrease of 68% in propane usage. (2) Electricity emerges as the predominant energy consumed in the ethane recovery process, and the RSV‐DTI process significantly improves upon this aspect. Notably, the RSV‐DTI process incurs the lowest investment cost, yielding a swift payback period of approximately 1 year for the plant. The characteristics of the RSV‐DTI process are investigated, and the effect of changes in feed gas conditions on the heat integration of the RSV‐DTI process is analyzed. The characteristics of the RSV‐DTI process show the following: (1) Different pressures of feed gas existing in the main cold box have different minimum heat integration temperatures (MHIT). When the feed gas temperature is lower than the MHIT, heat integration becomes difficult, and the heat energy can be supplied by hot liquid propane at 48°C. When the feed gas temperature is higher than the MHIT, changes in feed gas temperature have little effect on the process, only affecting the external gas temperature. (2) The heat transfer duty of the demethanizer sideline outlet stream is affected by the feed gas pressure. To enhance heat integration, it is recommended to set the heat transfer duty of the low‐temperature sideline outlet stream (DLTSS) between 40% and 90% of the reboiler duty and the heat transfer duty of the high‐temperature sideline outlet stream (DHTSS) between 40% and 75% of the reboiler duty.

The Predictability of a Heavy Rainfall Event during the Summer of 2022 Using an All-sky Radiance Assimilation Experiment

This paper presents the results of the recent development of the all-sky radiance assimilation system in the Korean Integrated Model (KIM). In the cycled analysis and forecast experiments, the increased coverage of radiance data in cloudy regions improved the quality of initial fields for mass variables, temperature and humidity. The experimental period covered the record-breaking heavy rainfall event on August 9, 2022. We examined the simulation accuracy of the western North Pacific subtropical high (WNPSH) in both clear- and all-sky experiments. In the clear-sky experiment, northward propagation of the WNPSH was restricted. A humid bias exists with clear-sky radiance assimilation over the WNPSH region. Since humid air is lighter than dry air, in this situation, the geopotential height (GPH) should be lower to achieve the same pressure, and a low-pressure bias occurs. All-sky radiance assimilation dries the moisture field, which helps elevate the GPH over the WNPSH region. The expansion of the WNPSH yielded a steeper confrontation in the air between the land and ocean around the southeastern sea of the Korean Peninsula to predict the strength of rainfall events more accurately. A more accurate simulation of the jet stream outlet was also demonstrated in an all-sky experiment. This study shows that the all-sky radiance assimilation can help to more accurately predict extreme rainfall events via proper simulations of large-scale fields.

Industrial Waste Heat Utilization in the European Union—An Engineering-Centric Review

The efficient utilization of waste heat from industrial processes can provide a significant source of energy savings for production plants, as well as be a driver of sustainable operations and the abatement of emissions. Industrial waste heat usually is contained in liquid or gaseous outlet streams. Although the possible ways to utilize waste heat are discussed in a wide variety of papers, these either provide only a general overview of utilization options and opportunities or focus on a narrow range of industrial processes. The aim of the present paper is to discuss the practical aspects of waste heat utilization in the European Union so that the reader can gain perspective on (i) the thermal classification of waste heat, (ii) liquid and gaseous waste streams and their typical temperatures for industrial use cases, (iii) the technical, economic, physical, and environmental aspects barring full utilization of the available waste heat, (iv) waste heat sources in various industries, and (v) standardized equipment and technologies applicable to industrial waste heat utilization, including their advantages, disadvantages, and weak points.

Estimation-based model predictive control of an electrically-heated steam methane reforming process

The surge in demand for hydrogen (H2) across diverse sectors, including clean energy transportation and chemical synthesis, underscores the need for a thorough investigation into H2 production dynamics and the development of effective controllers for industrial applications. This paper focuses on an electrically heated steam methane reforming (SMR) process for H2 production, offering advantages such as enhanced environmental sustainability, compactness, efficiency, and controllability compared to conventional reforming methods. Electric heating of the entire system allows for adjustments in current to control reactor temperature, thereby impacting hydrogen production rates. However, accurately modeling hydrogen production dynamics presents a formidable challenge, as complex models with high precision are computationally unsuitable for real-time control integration. Considering these factors, an accurate and efficient first-principles-based lumped-parameter model is developed to provide a dependable estimation of hydrogen production in an electrically-heated steam methane reformer. This model is validated experimentally and then utilized in a model predictive controller (MPC). To obtain the necessary state estimate information for the MPC, an extended Luenberger observer (ELO) method is employed to estimate state variables from limited, infrequent and delayed measurements of gas-phase reactor outlet stream and frequent measurements of the reactor temperature. Simulation comparisons with a proportional-integral (PI) controller reveal a much faster response in achieving the desired H2 production rate under the estimation-based MPC. Additionally, the simulations demonstrate the robustness of the controller to process variability such as a decrease in catalyst activation energy, commonly encountered in the SMR process, highlighting its effectiveness in maintaining stable operation under varying process conditions.

Molecular reconstruction of crude oil and simulation of distillation processes: An approach based on predefined molecules

Utilizing molecular management is a robust method for tackling the obstacles presented during the worldwide energy shift and for streamlining the utilization of crude oil resources. A fundamental component of this management technique is molecular reconstruction technology. In this study, a molecular model of crude oil is constructed using 151 deterministic molecules and artificially constructed molecules with average structures. To establish the molecular property matrix, the group contribution method and molecular dynamics simulations are employed to compute the molecular properties. An optimization algorithm is then employed to convert true boiling point profiles, SARA (Saturate, Aromatic, Resin, and Asphaltene) contents, and API of crude oils into detailed molecular content distributions. These distributions, alongside the actual boiling point curve data, are integrated into chemical process simulation software to model the distillation process of the crude oil. Notably, the molecular weights and mass densities of the process outlet streams derived from these two calculations are generally in good agreement.

Evaluation of hexabromocyclododecane (HBCD), polybrominated dibenzo-p-dioxins/furans (PBDD/Fs) and polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) outflows during the destruction of HBCD wastes in a hazardous waste incinerator

This paper presents an evaluation of hexabromocyclododecane (HBCD), polybrominated dibenzo-p-dioxins/furans (PBDD/Fs) and polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) outflows during the destruction of HBCD waste stockpiles in IZAYDAS Hazardous Waste Incinerator (HWI) in Kocaeli, Türkiye. HBCD wastes containing 100 % pure HBCD were in 25 kg packages with 63 % Br content were co-incinerated in a 3-day test burn with average feed rate of 26 kg/h. HBCD, PBDD/Fs and PCDD/Fs were measured in the outlet streams to quantify the amount of unintended POPs releases associated with the processing of HBCD waste and to observe the POP removal performance of air pollution control equipment (APCE) of the incinerator. Total mass outflow rate of HBCDs is calculated as 2.6 g/day, corresponding to destruction efficiency of 99.9996 %. Total toxicity of the brominated dioxins was measured as 0.00044 ng TEQ/Nm3 on average, while highly brominated congeners are dominant. PCDD/F concentrations in the outflow streams during HBCD test burns are produced similar congener distributions with those given in the previous studies, with the dominance of 7,8-chlorinated congeners. Mass flows in the outlet streams indicated that the efficiency of ESP and wet scrubbers for the removal of PCDD/Fs and HBCDs. Flue gas concentrations of PCDD/Fs, HBCDs and PBDD/Fs obtained in HBCD burn test indicated that burning HBCD wastes cause no significant emissions as operational parameters and total halogen content in the menu are kept within the incinerator limits.

Degradation of chlorinated volatile organic compounds in waste gas by catalytic plasma system

Dichloromethane (DCM), one of the main components of chlorinated volatile organic compounds (Cl-VOCs), is recognized as extremely hazardous and severely affects human health and the environment. In this study, a self-designed Dielectric Barrier Discharge (DBD) reactor combined with different catalysts to treat the exhaust DCM gas containing ppm concentration levels. A plasma system can generate high-energy electrons and free radicals (such as hydroxyl radical (OH·), and superoxide radical (O2·)) to degrade DCM in the industrial exhaust gas into most HCl and CO2. The removal efficiency can achieve 98.7% under the conditions, a flow rate of 1042 mLmin−1, the residence time of 76.72 ms, concentration of DCM of about 1650 ppm, and specific input energy of 591 JL−1. Although the plasma was highly reactive, the selectivity of the desired products was low, and there were unwanted toxic byproducts such as CHCl3 (1.7%) and CCl4 (13.8%). Thus catalysts were loaded in the plasma discharging zone to enhance removal efficiency and change product distribution. The catalysts were conducive to producing water-soluble products such as HCl. At the same time, the formation of some unwanted byproducts, CHCl3 and CCl4, were also inhibited. We found that using 11.5 kVpp plasma operating voltage combined with 200 mg Co3O4/HZSM-5 can lower the DCM concentration of the outlet stream from 1650 ppm to 53 ppm. Under this condition, the removal efficiency was about 96.7%, attaining permitted emissions.

Assessing Nitrate Leaching and Runoff Coefficients in the Dynamic Interplay of Seasonal Crop Biomass: A Study of Surface and Groundwater Nitrate Contamination in the Bilate Cropland Watershed

Effective water resource management and sustainable agricultural practices are contingent upon a thorough understanding of nitrate dynamics. This study investigates the utilization of seasonal average cropland runoff observations to improve the prediction accuracy of nitrate runoff loads to surface water originating from cropping zones. By integrating data from various sources, including seasonal rainfall/runoff observations and cropland parameters, the study explores the complex relationship between crop biomass growth, nitrogen fertilizer application, and seasonal rainfall over cropland. Monthly average observations of cropping zone rainfall/runoff in the cropping zone are analyzed to understand the response of seasonal crop biomass to nitrogen fertilizer application and seasonally calibrated rainfall/runoff in the cropland watershed. The study demonstrates the prototypical runoff observation from cultivated areas and statistically estimates seasonal runoff at downstream stream outlets. This shows a strong correlation with an R² of 0.9742 with a p-value less than 0.001 for seasonal rainfall climate parameters. Seasonal cropland runoff observations from EWX lite are presented for model calibration at downstream outlets, based on regional crop calendars, facilitating the estimation of nitrate loading to surface water and nitrate contamination from cropping zones. The findings reveal distinct cropping zones, particularly zones 5, 6, 11, and 13, which significantly contribute, each exceeding 50 mg/l of leaching load to groundwater. Moreover, more than ten zones surpass the threshold of 50mg/l in nitrate loading to surface water. These quantitative insights provide valuable spatial information regarding nitrate runoff relationships within the watershed, enabling the identification of specific zones with heightened nitrate contributions and offering actionable solutions for promoting sustainable agriculture and refining water resource management strategies.

Point Source Capture of Methane Using Ionic Liquids in Packed Bed Absorbers/Strippers: Experimental and Modelling

Fugitive methane emissions from the mining industry, particularly so-called ventilation air methane (VAM) emissions, are considered among the largest sources of greenhouse gas (GHG) emissions. VAM emissions not only contribute to the global warming but also pose a significant hazard to mining safety due to the risk of accidental fires and explosions. This research presents a novel approach that investigates the capture of CH4 in a controlled environment using 1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide [BMIM][TF2N] ionic liquid (IL), which is an environmentally friendly solvent. The experimental and modelling results confirm that CH4 absorption in [BMIM][TF2N], in a packed column, can be a promising technique for capturing CH4 from point sources, particularly the outlet streams of ventilation shafts in underground coal mines, which typically accounts for <1% v/v of the flow. This study assessed the effectiveness of CH4 removal in a packed bed column by testing various factors such as absorption temperature, liquid and gas flow rates, flow pattern, packing size, desorption temperature, and desorption pressure. According to the optimisation results, the following parameters can be used to achieve a CH4 removal efficiency of 23.8%: a gas flow rate of 0.1 L/min, a liquid flow rate of 0.5 L/min, a packing diameter of 6 mm, and absorption and desorption temperatures of 303 K and 403.15 K, respectively. Additionally, the experimental results indicated that ILs could concentrate CH4 in the simulated VAM stream by approximately 4 fold. It is important to note that the efficiency of CH4 removal was determined to be 3.5-fold higher compared to that of N2. Consequently, even though the VAM stream primarily contains N2, the IL used in the same stream shows a notably superior capacity for removing CH4 compared to N2. Furthermore, CH4 absorption with [BMIM][TF2N] is based on physical interactions, leading to reduced energy requirements for regeneration. These findings validate the method’s effectiveness in mitigating CH4 emissions within the mining sector and enabling the concentration of VAM through a secure and energy-efficient procedure.

Numerical investigation of the aerodynamic performance and loss mechanism in a low bypass ratio variable cycle engine fan

The aerodynamic performance of the variable cycle engine fan changes sharply during mode transition. Investigating the variations of flow structure and understanding the loss mechanism are helpful in providing guidance for the fan design. Three-dimensional models of single bypass and double bypass compression systems are established, and static pressure is applied at the bypass stream outlet to simulate the opening of the mode selection valve. The characteristic band of variable cycle engine fan is obtained by gradually increasing the bypass stream pressure while maintaining specific values for the core stream pressure. Results show that the overall performance of the double bypass configuration, without bypass recirculation, is almost identical to that of the conventional single bypass configuration during the throttling process. With the increase in bypass pressure, the shock wave and the trajectory of tip leakage vortex gradually move forward, thereby increasing the blockage region induced by the interaction between the shock and tip leakage vortex. In addition, the performance of fan with reverse flow is also calculated. The recirculation causes the operating point to move closer to the stability limit, reducing the isentropic efficiency. Additionally, the recirculation changes the radial distribution of axial velocity and total pressure, leading to inlet distortion in the core driven fan stage. Furthermore, the loss mechanism is clarified by modeling the splitter and conducting entropy generation analysis. The sharp expansion of bypass stream could cause severe flow separation, and reducing the curvature of casing can effectively suppress the viscous shear loss.

Accounting for costs in attainable‐region‐based reactor–separation network synthesis: Models, tradeoffs, and insights

AbstractIn reaction engineering, the attainable region defines the set of all possible states attainable in the outlet stream of any steady‐state reactor network, given specified feed and reaction kinetics. Costs in approaches that aim to determine the attainable region are often neglected, resulting in targets that require expensive processes. In this work, we propose modeling methods that allow us to account for costs in reactor–separation synthesis while accurately identifying the attainable region. Our goal is to holistically analyze cost trade‐offs within the attainable region as well as provide a method to obtain practical targets to be used as benchmarks. We generate the attainable region using the continuous stirred tank reactor (CSTR) equivalence principle and introduce separation models accounting for costs to perform the required separation tasks. Through several case studies, we show how some subregions, typically close to the attainable region boundary, are expensive to achieve.

Optimizing electrocoagulation pre-treatment efficiency during simultaneous treatment of different produced water streams in a FO-MD hybrid system

In this study, the electrocoagulation (EC) process was explored and optimized for the pre-treatment of key contaminants present in hyper-saline produced water (PW) streams before their treatment through forward osmosis-membrane distillation (FO-MD) hybrid system. Due to their hyper-saline nature and the associated high temperatures, PW streams can be treated cost-effectively and sustainably using low-pressure dual membrane filtration systems like FO-MD hybrids. Desalter effluent (DE) stream was used as the FO feed solution, and WOSEP outlet (WO) stream served simultaneously as the FO draw solution and MD feed solution. The key foulants were identified in the WO and DE streams. Silicon, calcium, and sulfur were found as the key elements contributing towards FO and MD membrane fouling in the form of CaSO4, SiO2, and CaSiO3. Applying EC for 10 min with 100 mA current (7 mA/cm2 current density) at a neutral pH of 7 demonstrated the highest removal efficiencies for silicon (97 %), calcium (3.6 %), and sulfur (12.2 %) with only 0.023 kWh/m3 electric energy requirement. FO-MD hybrid efficiency was re-assessed with pre-treated WO and DE streams. The FO flux showed more stability and increased by 32.5 % and MD by 28.9 %, with a 10 % reduction in specific reverse solute flux. EC is thus a robust and effective process for hyper-saline hazardous PW pre-treatment and can substantially improve the FO-MD hybrid system efficiency.

Parametric study and optimization of an acid gas enrichment plant with outlet streams and energy usage consideration

Acid gas enrichment (AGE) is a new method to increase the H2S conversion in conventional Clause process. In this paper effect of methyl diethanolamine (MDEA) flow rate, MDEA temperature, acid gas flow rate, and AGE tower pressure on the outlet H2S and CO2 concentration and energy consumption have been studied. A novel procedure based on enthalpy of combustion in the reaction furnace and incinerator is conducted for energy consumption calculations. By changing MDEA and inlet acid gas flow rates, a maximum H2S concentration in the enriched stream could be attainable. Increasing MDEA temperature will decrease maximum H2S concentration and enriched acid flow rate; also, CO2 concentration in off-gas will be decreased. By increasing AGE pressure, maximum H2S concentration in the enriched stream will be diminished. At higher MDEA flow rates, lower AGE pressures will bring higher H2S concentration in enriched acid gas stream. According to flame temperature simulation in the reaction furnace, energy consumption is directly related to H2S concentration in the sulfur recovery unit (SRU) feed stream. Results show that maximum H2S concentration in the enriched acid gas stream has the best conditions from energy point of view. At optimum conditions, H2S concentration in enriched acid gas is 62.45 mol%, which has been increased by 24.8%, compared with existing plant conditions, and energy consumption has been decreased by 25.05 GJ/h, and CO2 concentration in off-gas is 94.94 mol%.

Cell Designs Optimizing the Cathode Outlet Purity of CO2 Electrolyzers

Recently, advances in (photo-)electrochemical CO2 reduction have enabled record faradaic efficiencies for products containing at least two carbon atoms. However, comparatively little effort has been made to maximize the molar concentrations of the products that come out of the electrolyzer. In this presentation, I will talk about strategies and specific cell designs to optimize cathodic product concentrations, which may enable coupling the CO2 reduction cell to a downstream reactor that produces further reduced and more valuable products. Specifically, I will illustrate the concept with a system that enhances ethylene concentrations of an electrolyzer outlet stream.

Ni supported on bioapatite for WGS: Improving catalyst stability and H2 selectivity by Pt-doping and thermochemical activation of the support

Catalytic properties of Ni and NiPt catalysts supported onto naturally derived hydroxyapatite (HAp) were investigated for the Water-Gas Shift (WGS) reaction in the 200–450 °C range. HAp was obtained by thermochemical conversion of waste animal bones which yield a porous solid mainly composed by apatite. A feed composition representative of real reformer outlet stream was used (CO/H2O/CO2/H2 = 5/46/4/31 mol %) at a gas hourly space velocity of 120,000 h−1. The catalysts were thoroughly characterised by N2 physisorption, ICP-AES, H2-chemisorption, XPS, FTIR, SEM-EDX, XRD, H2-TPR, CO2-TPD, and NH3-TPD. From the light-off catalytic tests, it was found that all catalysts reached the equilibrium CO conversion in the 350–400 °C range. Pt doping into pristine Ni/HAp did not enhance neither catalytic activity nor selectivity to hydrogen. Interestingly, thermochemical activation (acid treatment) aimed to reduce the CH4 formation (@ 350 °C) by around two-fold (25 % vs. 9–14 %) with a concomitant increase in the hydrogen yield. Moreover, catalytic stability was also improved. For instance, after 30 h TOS CO conversion dropped by 50 % for the pristine Ni/HAp and only 7 % for NiPt/SHAp catalyst. Similarly, the latter showed the highest and most stable hydrogen yield throughout all the long-term test.

Impact of land use on macroinvertebrates from a multiscale perspective: enhancing structural equation models with inverse distance-weighted metrics

Effective riverine ecosystem management and restoration are predicated upon comprehending the relationship between land use patterns, the aquatic environment, and macroinvertebrates. However, the processes by which land use affects riverine ecosystems are exceedingly complex, influenced by factors across varying special scales. To explore these multi-scale relationships, we centered our study on the Taizi River Basin (TRB) in northeastern China, encompassing data on land use, macroinvertebrates, physical habitats, and water quality. We employed a synergy of correlation analysis, multiple stepwise regression, and redundant analysis techniques to discern these relationships at diverse spatial extents. This approach led to the development of 24 structural equation models (SEMs) that delineate the pathways bridging land use patterns to various biological indices. Remarkably, the interpretive capacity of SEM-Benthic Index of Biotic Integrity (SEM-BIBI), SEM-Shannon Wiener index (SEM-H’), and SEM-Average Score Per Taxon (SEM-ASPT) models was 76%, 70%, and 66% respectively. Within the TRB, the primary pathway from land use pattern to biological index consisted of: land use (predominantly rural and agricultural) feeding into physical habitat (habitat complexity, combined characteristic of speed and depth), which in turn affected benthic indicators. A secondary pathway charts a course from land use, through water quality (electrical conductivity and total dissolved solids) and culminating in benthic variations. Furthermore, our findings indicated that inverse-distance-weighted (IDW) metrics, specifically the IDW distance based on flow length to the stream outlet and the nearest stream (iFLO_m and iFLS_m, respectively) could provide more accurate interpretations of the river’s ecological health status. Given the effects of rural areas and agriculture on macroinvertebrates, we recommend establishing 100-m wide riparian buffers. The optimal buffer range should be determined by evaluating various biological needs and considerations.

Transport of nitrate-containing groundwater to coastal areas through buried tunnel valleys, Denmark

Nitrogen impact on the aquatic environment, including coastal areas, is too high in many countries worldwide, particularly in areas with intensive agriculture. Efficient mitigation initiatives demand that important pathways and the fate of nitrate in the hydrological cycle are known. In this study, we focus on groundwater nitrate contamination in two near-shore catchment areas in north-west Denmark. Groundwater in the area is mainly located in buried tunnel valleys, which are subsurface structures eroded by meltwater during Pleistocene glaciations in former glaciated areas. Groundwater samples from the aquifers inside the buried valleys reveal the presence of up to 120 mg/l nitrate down to 10 m below sea level and about 1 km down from the stream outlet towards the coast. We interpret the complex tunnel-valley infill to be responsible for the spatial heterogeneity of the groundwater geochemistry, where sandy geological windows create localised hydraulic pathways and complex redox structures. Groundwater and stream water chemistry in the study area clearly demonstrate the role of groundwater in nitrate transport within the catchment as well as the direct pathway to the coast bypassing the stream and riverine systems. Our results show that the buried tunnel valleys potentially contribute to submarine groundwater discharge and therefore could be responsible for a hitherto unaccounted input of nitrogen to the marine environment.