Vertical Hydraulic Gradient Research Articles

Sampling in Long-Screened Wells: Issues, Misconceptions, and Solutions.

The issues associated with long-screened wells (LSWs) (and open boreholes) at contaminated sites are well documented in the groundwater literature but are still not fully appreciated in practice. As established in seminal and review papers going back over three decades, the interpretation of sampling results from LSWs is challenging in the presence of vertical hydraulic gradients and borehole flow; furthermore, LSWs allow for vertical redistribution of contamination between aquifer layers. Acknowledgment of these issues has led to the development of new technologies and well designs to enable discrete-zone monitoring (DZM), yet LSWs remain common for many reasons, for example, as multipurpose wells, for geophysical logging, and (or) as legacy installations. Despite the literature on LSWs and despite the adoption of DZM at many sites, the use of LSWs persists and the challenges of interpreting sampling results from LSWs remain. In this issue paper, we provide a conceptual overview of the problems posed by LSWs and review existing literature and past work to improve the interpretation of sampling in LSWs. We draw on experience from previous studies at the Hanford Site in eastern WA, USA, and use synthetic examples to illustrate key concepts and challenges for interpretation. A recently published analytical modeling framework is used to develop illustrative synthetic examples and demonstrate a workflow for building scientific intuition to understand issues around interpreting samples from LSWs, which is critical to effective characterization and groundwater remediation at sites with LSWs.

Quantifying heterogeneities of vertical hydraulic gradients in a highly urbanized effluent river system in the Darjeeling Sub Himalayan foothills, India

Quantifying heterogeneities of vertical hydraulic gradients in a highly urbanized effluent river system in the Darjeeling Sub Himalayan foothills, India

Groundwater Abstraction has Caused Extensive Ecological Damage to the Doñana World Heritage Site, Spain

Acreman et al. (Wetlands 42:63, 2022) reviewed evidence for ecological damage to the Doñana wetlands (UNESCO World Heritage Site [WHS] and Ramsar site), Spain, associated with intensification of groundwater use, particularly for agriculture. Acreman et al. presented a multistep methodology for evidence-based risk assessment that involves identification of conservation issues, and a systematic review of scientific evidence for ecological damage and its causes. However, they involved few local scientists, used a questionable methodology in stakeholder selection and involvement, used a flawed conceptual framework, and an incomplete literature review. We propose improvements to their methodology. They overlooked or misinterpreted key evidence, and underestimated the impacts that abstraction for irrigation for red fruits (mainly strawberries), rice and other crops has had on Doñana and its biodiversity. They reported groundwater level depletion of up to 10 m in the deep aquifer, but wrongly concluded that there is no evidence for impacts on the natural marsh ecosystem, the dune ponds or the ecotone. Groundwater drawdowns are actually up to 20 m, and have inverted the formerly ascending vertical hydraulic gradient in discharge areas. Phreatic levels have been lowered from 0.5 to 2 m in some areas. Groundwater abstraction has caused multiple ecological impacts to temporary ponds and marshes in the WHS, as well as to terrestrial vegetation, and should be urgently reduced. Furthermore, Acreman et al. focused on groundwater quantity while overlooking the importance of severe impacts on quality of both surface and groundwater, intimately connected to the use of agrochemicals for irrigated crops.

Full-scale mud pumping test of ballastless trackbed under train loading

This paper presents a full-scale physical modeling investigation on the mud pumping in ballastless trackbed. The physical model was equipped with a rainfall device to simulate the in situ precipitation and various testing sensors were installed to monitor the dynamic response of the track-subgrade system. Results showed that the whipping of the track structure induced by high-speed train passages caused a separation between the track structure and the underlying roadbed, which allowed rainwater to intrude into the roadbed. The intruded rainwater infiltrated vertically and then permeated laterally from the bottom of the roadbed, eventually saturating the entire roadbed. Train moving loads partially promoted this rainwater infiltration. The excess pore water pressure (EPWP) was formed in the saturated roadbed caused by train moving loads, and it increased with train speeds and loading carriages. A noticeable upward EPWP gradient and a slight longitudinal EPWP gradient towards the expansion gap were also created in the saturated roadbed. Accordingly, both the vertical hydraulic gradient (HG) and longitudinal HG increased with train speeds and loading carriages, most of which, especially the vertical HG, exceeded the critical HG for particle migration with a diameter smaller than 7.1 mm. It caused severe upward particle migration and slight longitudinal particle migration towards the expansion gap. This spatial fine particle migration process, known as mud pumping, was driven by the HG-induced seepage force and further promoted by the EPWP amplification induced by high-speed train passages. Mud pumping resulted in a significant decrease in stiffness of the trackbed and a noticeable increase in vibration velocity of the track slab.

Impacts of a beaver dam analogue complex on streambed vertical hydraulic gradients in a 1.2‐km stream reach in Wyoming, USA

AbstractBeaver dam analogues (BDAs) have seen growing use as restoration structures across the western United States. This study investigates the patterns in streambed upwelling and downwelling along a 1.2‐km stream reach in Red Canyon Creek (RCC), Wyoming before and after the installation of 31 new BDAs and the upgrade of four existing BDAs in July 2021. Over 100 mini‐piezometers were used to measure upwelling and downwelling in low‐flow, summer periods as quantified by vertical hydraulic gradient (VHG). Both before and after BDA installation, the stream reach was dominated by downwelling patterns, suggesting that RCC was a net losing stream during this summer period, with and without BDAs. While there were spatial variations in VHG before BDA installation, this variation was not dependent on stream depth, water surface concavity, sediment characteristics, and position relative to meanders, suggesting that unobservable subsurface properties may be a control on VHG or that there are attributes that were not captured due to the 10‐m spacing of mini‐piezometers. After BDA installation, VHGs were primarily related to the magnitude of the elevation gradient across the BDA. VHGs were highest near the BDAs and diminished once moving more than a few metres from the BDAs. When VHGs were averaged over the full reach length, BDAs appeared to slightly enhance net stream loss, albeit we could not control for possible seasonal differences in water table gradient during the observtion period.

Improved Darcian streambed measurements to quantify flux and mass discharge of volatile organic compounds from a contaminated aquifer to an urban stream

Quantifying VOC transport from contaminated groundwater to streams is challenging and important for understanding off-site migration of VOCs, cross-media contamination (groundwater to surface water and eventually air), and potential impacts on downstream ecosystems and human populations. A streambed point sampling approach was used to quantify fluxes of water and 14 VOCs from groundwater to an urban stream in North Carolina, USA, during summer (June 2015) and winter (January 2016). The approach is unique in coupling measurements of vertical hydraulic conductivity, vertical hydraulic head gradient, and groundwater VOC concentration at each individual sampling point, reducing or eliminating some potential concerns with other Darcian methods for quantifying VOC inputs to streams.Most results were consistent with discharge of two main VOC plumes on opposite sides of the stream. Plume 1 from the west side was dominated by cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) at mean concentrations of 19 and 11 μg L−1, respectively. Plume 2 from the east side was dominated by benzene (mean concentration 56 μg L−1). Plume 2 was not previously known, and the improved sampling approach allowed VOC discharge from both plumes to be quantified simultaneously.For 13 of the 14 detected VOCs, the mean VOC flux from groundwater to the stream (fVOC) was higher in January 2016 than in June 2015, mainly because groundwater flux was higher in January. The only exception was cDCE, the most abundant VOC in Plume 1, which had mean fVOC values of 9.8 and 9.5 mg m−2 d−1 in June 2015 and January 2016, respectively. Benzene was the most abundant VOC in Plume 2 and had mean fVOC values of 11 and 37 mg m−2 d−1 in June 2015 and January 2016, respectively. High groundwater flux drove almost all the occurrences of high VOC flux. For a given VOC, the flow-weighted mean concentration (with each VOC concentration weighted by the upward groundwater flux at the VOC sampling point) was generally larger than the unweighted mean concentration. Thus, flow-weighting of concentrations gave a more accurate indication of the average VOC concentration in net groundwater discharge to the stream.An estimate of total VOC mass discharge from groundwater to the study reach of the stream, 3.6 kg of VOC per year, was based on the fVOC results and streambed area in the reach. The bulk of this discharge was due to benzene, cDCE, and VC, with individual mass discharges of 2.1, 0.83, and 0.40 kg yr−1, respectively. Estimates of maximum potential VOC degradation in the streambed suggest that the 3.6 kg yr−1 estimate of mass discharge was not sensitive to potential degradation of VOCs in the streambed sediments above the groundwater sampling depth.

A data-driven modeling approach for the sustainable remediation of persistent arsenic (As) groundwater contamination in a fractured rock aquifer through a groundwater recirculation well (IEG-GCW®)

Persistent arsenic (As) pollution sources from anthropogenic activities pose a serious threat to groundwater quality. This work aims to illustrate the application of an innovative remediation technology to remove As from a heavily contaminated fractured aquifer at a historically polluted industrial site. Groundwater circulation well (GCW) technology was tested to significantly increase and accelerate the mobilization and removal of As in the source area. The GCW extracts and re-injects groundwater at different depths of a vertical circulation well. By pumping out and reinjecting in different screen sections of the well, the resulting vertical hydraulic gradients create recirculation cells and affect and mobilize trapped contaminants that cannot be influenced by traditional pumping systems. The first 45-m deep IEG-GCW® system was installed in 2020, equipped with 4 screen sections at different depths and with an above-ground As removal system by oxidation and filtration on Macrolite (Enki). A geomodeling approach supports both remediation and multi-source data interpretation. The first months of operation demonstrate the hydraulic effectiveness of the IEG-GCW® system in the fractured rock aquifer and the ability to significantly enhance As removal compared to conventional pumping wells currently feeding a centralized treatment system. The recirculation flow rate amounts to about 2 m3/h. Water pumped and treated by the GCW system is reintroduced with As concentrations reduced by an average of 20%–60%. During the pilot test, the recirculating system removed 23 kg As whilst the entire central pump-and-treat (P&T) system removed 129 kg, although it treated 100 times more water volume. The P&T plant removed 259 mg As per m3 of pumped and treated groundwater while the GCW removed 4814 mg As per m3 of the treated groundwater. The results offer the opportunity for a more environmentally sustainable remediation approach by actively attacking the contamination source rather than containing the plume.

Intra‐crown variation of leaf mass per area of Fagus crenata is driven by light acclimation of leaf thickness and hydraulic acclimation of leaf density

AbstractIn trees, leaf anatomy acclimates to the vertical light and hydraulic gradients within the crown. Intra‐crown trait variation reflects phenotypic plasticity, which is considered a measure of individual‐level acclimation to environmental variation. Here, we compared plasticity of leaf anatomical traits among trees from five regional populations of Fagus crenata and assessed the contribution of these traits to leaf mass per area (LMA). Intra‐crown variation of LMA was larger than variation among trees from different sites, suggesting phenotypic plasticity is greater than genetic variation. After intra‐crown light effects were removed, LMA increased with increasing height, suggesting, in addition to light acclimation, hydraulic limitation drives intra‐crown variation. LMA was positively correlated with leaf thickness and negatively correlated with mesophyll porosity. Leaf thickness accounted for 39% of the variation in LMA, while palisade thickness accounted for 62% of the variation in leaf thickness, suggesting light was the main driver of leaf thickness. Thicker leaves also had denser mesophyll and thicker epidermal layers, both acclimations to low water availability (i.e., hydraulic limitation). Among the traits, intra‐crown plasticity was highest for mesophyll porosity, while there was no difference among trees from different sites in overall trait plasticity. Our results indicate phenotypic plasticity of leaf traits in F. crenata is mainly driven by light availability, while some traits, such as leaf density, may be driven by hydraulic limitation. In regional populations with high phenotypic plasticity, individual‐level acclimation may buffer negative effects of future climate change on growth and productivity of F. crenata.

Characterizing the Flow Regime in Brook Trout Incubation Habitats and Implications for Management in a Hydro‐Regulated River

AbstractThe operation of dams, diversions, and power generation facilities unavoidably influence the ecological function of rivers. We evaluated vertical and horizontal hydraulic gradients and subsequent water temperature changes in the hyporheic zone of a spawning area for Brook Trout Salvelinus fontinalis in a managed Lake Superior tributary in Ontario, then conducted a laboratory‐based experiment to show the impact of cold shock during incubation on the timing of larval emergence and survival. Upwelling groundwater was observed at the spawning area during the monitoring period (October 28, 2016, to January 11, 2017); hyporheic water temperatures remained above 3.7°C, whereas ambient river temperature above the substrate fell to 1.5°C until water was released from the upstream control dam. After the release event, water level increased by 0.9 m over the spawning area, and within 24 h, vertical and horizontal flow gradients were reversed in the hyporheic zone; downwelling conditions were observed for 30 h between surface and hyporheic water 1.8 m below substrate, and hyporheic water temperature decreased in unison. Hyporheic temperatures at shallow inshore sites fell below 1°C for more than 53 h. In the laboratory, applying a worst‐case scenario of dam‐induced cold shock on Brook Trout redds using historic monitoring records (i.e., <1.0°C for 40 h), we found no appreciable effect on survival and development timing of incubating Brook Trout eggs compared with a control group. Survival from fertilization to hatching was high for both treatment and control groups (>90%) and lower from fertilization to emergence (55%). Overall, our results suggest that standard winter operating procedures on a Lake Superior tributary had little impact on Brook Trout egg survival and development time to hatching and emergence; however, our findings also suggest that staged winter discharges would lessen the reversal of flow in the hyporheic zone and the magnitude of temperature changes experienced at Brook Trout redds.

Regional-scale hydrogeochemical evolution across the arsenic-enriched transboundary aquifers of the Ganges River Delta system, India and Bangladesh

Arsenic (As) dynamics within the extensively contaminated aquifers of the Ganges River delta have been widely studied over the past few decades, but the hydrogeochemical signatures across the delta aquifers remain to be characterized. Here, we characterize the varied geochemical and isotopic (δ18O, δ2H) signatures of groundwater across the delta and interpret the hydrogeochemical evolution pathways and the driving processes on a regional-scale as a function of the delta hydrostratigraphy. Our hydrostratigraphic model identifies three major aquifer sub-systems across the delta from north-west to south-east: a single continuous unconfined aquifer (Type I); a semiconfined vertically-segregated aquifer sub-system (Type II); and a nearly confined multilayered aquifer sub-system (Type III). The Type I aquifer is dominated by Ca-Mg-HCO3-rich waters, while the aquifers to the south (Type II and Type III) exhibits increasing dominance of Na-Cl hydrogeochemical facies at shallow and intermediate depths and Na-HCO3 hydrogeochemical facies in the deep aquifers. The spatial distribution of As is also found to be heavily dictated by hydrostratigraphy, wherein the Type I aquifer sub-system yields similar concentrations across depths, while the Type II and Type III aquifer sub-systems exhibit a sharp increase in As-safe aquifers with depth. Although dominant reducing conditions occur within the delta groundwater, co-occurrence of redox-sensitive solutes from varying redox stability fields indicates to the development of overlapping redox zones. Stable isotopic signatures of groundwater exhibit a progressive depletion away from the Bay of Bengal. The Type I aquifer exhibits relatively homogenous hydrogeochemical signatures, possibly suggesting deeper infiltration of recharge under higher vertical hydraulic gradients, while the Type II and Type III aquifers exhibit variability across depth, which is possibly a reflection of horizontally stratified groundwater flows, dictated by the spatial geometry of the intervening aquitard layers.

Effects of In-Channel Structure on Chinook Salmon Spawning Habitat and Embryo Production

Adult salmonids are frequently observed building redds adjacent to in-channel structure, including boulders and large woody debris. These areas are thought to be preferentially selected for a variety of reasons, including energy and/or predation refugia for spawners, and increased hyporheic exchange for incubating embryos. This research sought to quantify in-channel structure effects on local hydraulics and hyporheic flow and provide a mechanistic link between these changes and the survival, development, and growth of Chinook salmon Oncorhynchus tshawytscha embryos. Data were collected in an eight-kilometer reach, on the regulated lower Mokelumne River, in the California Central Valley. Nine paired sites, consisting of an area containing in-channel structure paired with an adjacent area lacking in-channel structure, were evaluated. Results indicated that in-channel structure disrupts surface water velocity patterns, creating pressure differences that significantly increase vertical hydraulic gradients within the subsurface. Overall, in-channel structure did not significantly increase survival, development, and growth of Chinook salmon embryos. However, at several low gradient downstream sites containing in-channel structure, embryo survival, development, and growth were significantly higher relative to paired sites lacking such features. Preliminary data indicate that adding or maintaining in-channel structure, including woody material, in suboptimal spawning reaches improves the incubation environment for salmonid embryos in regulated reaches of a lowland stream. More research examining temporal variation and a full range of incubation depths is needed to further assess these findings.

Three‐Dimensional Subsurface Flow Path Controls on Flow Permanence

AbstractIntermittent streams currently constitute >50% of the global river network, and the number of intermittent streams is expected to increase due to changes in land use and climate. Surface flows are known to expand and contract within the headwater channel network due to changes in the water table driven by climate, often changing seasonally. However, the underlying causes of disconnections and reconnections throughout the stream network remain poorly understood and may reflect subsurface flow capacity. We assess how 3D subsurface flowpaths control local surface flows at Gibson Jack Creek in the Rocky Mountains, Idaho, USA. Water table dynamics, hydraulic gradients, and hyporheic exchange were monitored along a 200‐m section of the stream throughout the seasonal recession in WY2018. Shallow lateral hillslope‐riparian‐stream connectivity was more frequent in transects spanning perennially flowing stream reaches than intermittent reaches. During low‐flow periods, larger losing vertical hydraulic gradients were observed in paired piezometers in intermittent reaches than in adjacent perennial reaches. Contrary to dominant conceptual models, longitudinal measurements of hydrologic exchange in both intermittent and perennial reaches were seasonally variable except for one perennial reach that showed consistent significant gains. Observed drying dynamics, as well as subsurface pathways, were highly variable even over short distances (30 m). Flow probability and subsurface flow capacity at upstream locations can be assessed with an outlet hydrograph and upstream flow measurements. Accurate characterization of subsurface storage, discharge, and connection is critical to understanding the drivers of drying cycles in intermittent streams and their likely responses to future change.

Vertical Hydraulic Gradient Estimation in Clay Till, Using MiHPT Advanced Direct‐Push Technology

AbstractThe vertical transport of contaminants from source areas is employed in many risk assessment models and screening tools in order to estimate the contaminant mass discharge (CMD) into underlying aquifers. The key parameters for estimating CMD are the contaminant source area and concentration, and the vertical water flux, the latter of which depends on the vertical hydraulic conductivity and the vertical hydraulic gradient in the subsurface. This study focuses on advancing the use of the combined membrane interface probe hydraulic profiling tool (MiHPT) to investigate the vertical hydraulic gradient across a clay till overlying a sandy aquifer at a contaminated site in Denmark. Only the HPT is necessary for the estimate of vertical hydraulic gradient. The hydraulic head, clay till thickness, and resulting vertical hydraulic gradients found using the MiHPT compared well with observations from nearby nested wells. The parameter with the largest discrepancy was the thickness of the clay till. The advantage of the MiHPT is its relatively quick depth discrete access to information regarding subsurface permeability, vertical hydraulic gradients and contaminant distribution across a site. In this case study, performance of additional dissipationtests during the HPT log to acquire determination of the vertical hydraulic gradient increased the cost by 3% compared to standard HPT logs.

A study of water curtain parameters of underground oil storage caverns using time series monitoring and numerical simulation

Water curtain systems (WCSs) are key components for the operation of underground oil storage caverns (UOSCs) and their optimization and design are important areas of research. Based on the time series monitoring of the first large-scale underground water-sealed storage cavern project in China, and on finite element analysis, this study explores the optimum design criteria for WCSs in water-sealed oil caverns. It shows that the optimal hole spacing of the WCS for this underground storage cavern is 10 m in order to ensure seal effectiveness. When the WCS is designed with a 10-m horizontal hole spacing and a water curtain pressure (WCP) of 80 kPa, a water curtain hole (WCH) has an influence radius of approximately 25 m. The smaller the vertical distance is between a WCH and the main cavern, the greater the water inflow into the main cavern. The vertical hydraulic gradient criterion can be satisfied when this distance is 25 m. It shows that the optimal WCP is 70 kPa, which meets sealing requirements.

Hydraulic transmissivity of geocomposite confined with soils

This research investigated the effect of fine content (F) on the transmissivity (T) of geocomposite confined with single and double surrounding soil layer under various vertical stresses (σv) and hydraulic gradients (i). A prototype hydraulic transmissivity test apparatus was developed for this investigation, based on the in-plane flow rate testing device in accordance with ASTM D4716. The relationship between the T of the geocomposite and i was non-linear for both scenarios of with and without surrounding soil. The T was reduced significantly with increasing F, due to the clogging of fine particles into the channel. Non-linear relationships between generalized transmissivity and i for single and double surrounding soil conditions at various F was developed. The developed relationships assessed satisfactorily the T of geocomposites with surrounding soil at various i, σv and F, which is significant for field drainage applications.

Using isotopes and hydrogeochemistry to characterize groundwater flow systems within intensively pumped aquifers in an arid inland basin, Northwest China

The impacts of long-term pumping on groundwater flow patterns and groundwater chemistry are unclear in the Manas River Basin, an arid inland basin in Northwest China. In this study, hydraulic heads, hydrochemistry and environmental isotopic tracers were analyzed to reveal groundwater flow patterns, recharge sources and deduce hydrogeochemical processes in this highly exploited aquifer system. Spatial trends in the isotopic ratios δ18O and δ2H indicate that groundwater originates from glacial melting and precipitation in the mountainous area. In the piedmont plains, dissolution of calcite, gypsum and dolomite yields a groundwater type dominated by Ca-HCO3-SO4 with TDS of 300 mg/L. In this area, high 3H concentrations demarcate local flow systems and imply modern precipitation recharge. Downgradient in the lowlands, irrigation return flow recharges shallow groundwater through local flow paths. Groundwater pumping has intensified vertical hydraulic gradients causing vertical mixing between aquifers. Generally, across the basin, regional flow systems contain groundwater with 14C ages ranging from modern to 33,000 years, with the oldest waters found in wells with depths greater than 200 m. From high to low elevation groundwater chemistry transitions from Ca-HCO3-SO4 to Na-Cl-SO4 type as concentrations of Cl− and overall TDS increase. The replacement of Ca2+ with Na+ as the dominant cation suggests reverse ion exchange and carbonate precipitation occur along the intermediate and regional flow paths. Anthropogenic activities (i.e. pumping, irrigation return flow, application of fertilizers) impact shallow groundwater chemistry (<60 m). This research reveals the impacts of natural and human-induced hydrogeochemical processes impacting groundwater chemistry in this basin with implications for similar semi-arid to arid, inter-montane basins around the world.

Groundwater-surface water interactions and agricultural nutrient transport in a Great Lakes clay plain system

Nutrient export from agricultural land to surface waters is a significant environmental concern within the Great Lakes Basin (GLB). A field-based watershed-scale study was completed to investigate spatial and temporal variations of phosphorus and nitrate to assess nutrient transport pathways and groundwater-surface water interactions in an agriculturally dominated clay plain system. This was conducted in the 127 km2 Upper Parkhill Watershed, near Lake Huron in southwestern Ontario, Canada. Data collection occurred from June 2018 to May 2019 via continuous sensor deployment and discrete sampling of stream water, groundwater, hyporheic zone, and tile drainage water. Samples were analyzed for various nutrient species (total, total dissolved, soluble reactive, and particulate phosphorus, and nitrate-N) to examine the hydrological dynamics of principal transport pathways of agriculturally-derived nutrients. Total phosphorus and nitrate concentrations in stream water ranged from 0.007 to 0.324 mg/L and 0.32 to 13.13 mg NO3−-N/L, respectively. Tile drainage water total phosphorous concentrations varied from 0.006 to 0.066 mg/L. Groundwater total dissolved phosphorus concentrations ranged from <0.003 to 0.085 mg/L. Transport of phosphorus through tile drainage was observed to be greater than through groundwater over the study period. No distinct relationship was observed between nutrient concentrations in the hyporheic zone and the vertical hydraulic gradient within this zone in the studied stream reach. Preliminary correlations were discerned between water quality observations and recognized land management practices. Given the elevated stream nutrient concentrations, these results are consequential for the continual improvement of strategies and programs devised to conserve water resources within the GLB.

Beaver dam analogues drive heterogeneous groundwater–surface water interactions

AbstractBeaver dam analogues (BDAs) are a cost‐effective stream restoration approach that leverages the recognized environmental benefits of natural beaver dams on channel stability and local hydrology. Although natural beaver dams are known to exert considerable influence on the hydrologic conditions of a stream system by mediating geomorphic processes, nutrient cycling, and groundwater–surface water interactions, the impacts of beaver‐derived restoration methods on groundwater–surface water exchange are poorly characterized. To address this deficit, we monitored hyporheic exchange fluxes and streambed porewater biogeochemistry across a sequence of BDAs installed along a central Wyoming stream during the summer of 2019. Streambed fluxes were quantified by heat tracing methods and vertical hydraulic gradients. Biogeochemical activity was evaluated using major ion porewater chemistry and principal component analysis. Vertical fluxes of approximately 1.0 m/day were observed around the BDAs, as was the development of spatially heterogeneous zones of nitrate production, groundwater upwelling, and anaerobic reduction. Strong contrasts in hyporheic zone processes were observed across BDAs of differing sizes. This suggests that structures may function with size‐dependent behaviour, only altering groundwater–surface water interactions after a threshold hydraulic step height is exceeded. Patterns of hyporheic exchange and biogeochemical cycling around the studied BDAs resemble those around natural beaver dams, suggesting that BDAs may provide comparable benefits to channel complexity and near‐stream function over a 1‐year period.

Observations of wave attenuation, scour, and subsurface pore pressures acrossbthree marsh restoration sill structures on a sandy bed

With rising sea levels and more frequent exposure to extreme storms, coastlines worldwide are vulnerable to increased erosion and loss of natural marsh lands. In an effort to lessen these impacts, there is a growing practice of adapting hard or “gray” coastline protection techniques to more nature-based features that promote habitat and ecosystem health. Living shoreline marsh restorations utilize natural and naturebased materials to protect marsh shores from erosion while also allowing intertidal flushing to promote the health and diversity of the marsh. Our study investigates three types of living shoreline sill designs exposed to average and storm-energy wave conditions at varying water levels. The sills were designed to mimic constructed sills in practice (rock, oyster shell, tree root wads), but more generally vary in structure porosity and material dissipation potential. Large-scale laboratory experiments were conducted in the large wave flume at the O.H. Hinsdale Wave Research Laboratory. Wave transmission and reflection are used to demonstrate wave attenuation capability of each sill structure. Scour of the sill, bedload sediment transport rates on the seaward and shoreward sides of the sill, and sediment pore-water vertical hydraulic gradients were used to demonstrate the potential for sediment transport and liquefaction. Results will contribute to understanding the effect of sill material porosity and mass on structure stability, and the effectiveness of using green living shoreline sill structures in the continued effort to establish design criteria for living shoreline implementation.

Seasonal variation of infiltration rates through pond bed in a managed aquifer recharge system in St‐André, Belgium

AbstractIn Belgium, IWVA uses managed aquifer recharge (MAR) to recharge the aquifer with treated wastewater generated from the communities to sustain the potable water supply on the Belgian coast. This MAR facility is faced with a challenge of reduced infiltration rates during the winter season when pond water temperatures near 4°C. This study involves the identification of the predominant factor influencing the rate of infiltration through the pond bed. Several factors, including pumping rates, natural recharge, tidal influences of the North Sea and pond‐water temperature, were identified as potential causes for variation of the recharge rate. Correlation statistics and linear regression analysis were used to determine the sensitivity of the infiltration rate to the aforementioned factors. Two groundwater flow models were developed in visual MODFLOW to simulate the water movement under the pond bed and to obtain the differences in flux to track the effects of variation of hydraulic conductivity during the two seasons. A 32% reduction in vertical hydraulic gradient in the top portion of the aquifer was observed in winter, causing the recharge rates to fluctuate. Results showed that water temperature caused a 30% increase in hydraulic conductivity in summer as compared with winter and has the maximum impact on infiltration rate. Cyclic variations in water viscosity, occurring because of seasonal temperature changes, influence the saturated hydraulic conductivity of the pond bed. Results from the models confirm the impact on infiltration rate by temperature‐influenced hydraulic conductivity.