Vertical Hydraulic Gradient Research Articles

Experimental Investigation on Suffusion Characteristics of Cohesionless Soils Along Horizontal Seepage Flow Under Controlled Vertical Stress

Suffusion is one of the main modes of internal erosion leading to selective erosion and gradual migration of fine soil particles through the voids of soil skeleton formed by coarse soil particles under seepage flow. Suffusion may induce deterioration and even failure of hydraulic geo-structures, such as embankment dams, dikes, levees, as well as landslide dams, if they are formed by gap-graded cohesionless soils. Soil mass in the hydraulic geo-structures is always subjected to overburden load and seepage through the soil mass generally has a large component along the horizontal direction. In the literature, however, little attention has been paid to the horizontal seepage-induced suffusion under controlled vertical stress. In this study, horizontal seepage-induced suffusion tests were carried out using a testing apparatus with controllable vertical stress and hydraulic gradient. The seepage flux and eroded fine particle mass, as well as local hydraulic gradients can be measured. The effects of initial dry density and initial fines content on suffusion characteristics of a cohesionless soil were investigated. Test results showed that after the initiation of suffusion, alternate decrease and increase in the coefficient of permeability occurred possibly due to alternate clogging and dredging of soil pores by the movable fine particles. Uneven distribution of local hydraulic gradients along the flow path was observed and this could be regarded as a sign of the initiation of suffusion. Both initiation and failure hydraulic gradients increased with initial dry density and initial fines content.

Characterisation of fractured carbonate aquifers using ambient borehole dilution tests

Fractured carbonate aquifers derive their transmissivity essentially from a well-developed network of solutionally-enhanced fractures and conduits that can lead to high groundwater velocities and high vulnerability to contamination of water quality. Characterisation of the variation of hydraulic properties with depth is important for delineating source protection areas, characterising contaminant fate and transport, determination of the effectiveness of aquifer remediation, and parameter estimation for models. In this work, ambient open borehole uniform and point injection dilution tests were conducted on observation boreholes in the unconfined Cretaceous Chalk aquifer of East Yorkshire, UK, and interpreted in conjunction with other data via the implementation of a new work flow. This resulted in the characterisation of flow in these boreholes and the inference of properties such as groundwater flow patterns and velocities in the surrounding aquifer formation. Our workflow allowed sections of open boreholes showing horizontal versus vertical flow to be distinguished, and the magnitude of such flows and exchanges with the aquifer to be determined. Flow within boreholes were then used to characterise: i) presence and direction of vertical hydraulic gradients; ii) nature and depth distribution of flowing features; iii) depth interval porosity and permeability estimation of the flowing features from overall borehole transmissivity and geophysical image or caliper logs; iv) groundwater velocity estimation in the surrounding aquifer. Discrete flowing features were distributed across the range of depths sampled by the observation boreholes (typically up to 45–60 mbgl), but the majority were located in the zone of water table fluctuation marked by solutionally enlarged flow features. Quantitative interpretation of both uniform injection (tracer distributed throughout the open borehole section) and point injection (slug of tracer introduced at targeted depth) yielded vertical velocities within the borehole water column in broad agreement with those measured by flow logging. Depth specific fracture kinematic porosities inferred from the ambient dilution data combined with long-interval pump test and geophysical log data ranged between 3.7 × 10−4–4.1 × 10−3 with an average of 2.1 × 10−3; these values were in excellent agreement with those from other methods applied to the same aquifer such as larger scale pumping tests. A new approach to estimation of groundwater velocities from the dilution test data using externally measured hydraulic gradients gave inferred horizontal groundwater velocities ranging between 60 and 850 m/day, in full agreement with those from previously conducted borehole-to-borehole tracer tests. These results confirm that the studied aquifer is karstic, with rapid preferential pathways which have implication for flow and transport modelling, and pollution vulnerability. Our study results indicate that ambient single-borehole dilution approaches can provide an inexpensive and reliable approach for the characterisation of fractured and karstic aquifers.

Can Borehole Heat Exchangers Trigger Cross-Contamination between Aquifers?

Borehole heat exchangers (BHEs) commonly reach depths of several tens of meters and cross different aquifers. Concerns have been raised about the possibility of boreholes to act as preferential pathways for contaminant transport among aquifers (cross-contamination). This article employs numerical modelling of contaminant transport in the subsurface to address these concerns. A common hydrogeological setup is simulated, composed of three layers: A shallow contaminated and a deep uncontaminated aquifer separated by an aquitard, all crossed by a permeable borehole. The hydraulic conductivity of the borehole and, to a lesser extent, the vertical hydraulic gradient between the aquifers are the key factors of cross-contamination. Results of the numerical simulations highlight that, despite the severe conditions hypothesized in our modelling study, the cross-contamination due to the borehole is negligible when filled with a slightly permeable material such as a geothermal grout properly mixed and injected. A good agreement was found with analytical formulas used for estimating the flow rate leaking through the borehole and for studying the propagation of leaked contaminant into the deep aquifer.

A multi-technique approach to determine temporal and spatial variability of groundwater-stream water exchange

Characterizing the spatio-temporal distribution of groundwater-surface water exchange fluxes is of paramount importance in understanding catchment behavior. A wide range of field-based techniques are available for such characterization. The objective of this study is to quantify the spatio-temporal distribution of the exchange fluxes along the Cakit stream (Nigde, Turkey) through coupling a set of geophysical techniques and in-stream measurements in a hierarchical manner. First, geological and water quality information were combined at the catchment scale to determine key areas for reach-scale focus. Second, electromagnetic induction (EMI) surveys were conducted along the reach to pinpoint potential groundwater upwelling locations. EMI anomalies guided our focus to a 665 meter-long reach of the stream. Along this selected reach, a fiber-optic distributed temperature sensing (FO-DTS) system was utilized to investigate streambed-temperature profiles at fine spatial and temporal scales. Furthermore, vertical hydraulic gradients and exchange fluxes were investigated using nested piezometers and vertical temperature profiles, respectively, at two potential upwelling locations and a potential downwelling location identified by previous surveys. The results of the study reveal heterogeneity of vertical water-flow components with seasonal variability. The EMI survey was successful in identifying a localized groundwater upwelling location. FO-DTS measurements revealed a warm temperature anomaly during cold air temperature and low streamflow conditions at the same upwelling site. Our point-based methods, namely vertical temperature profiles and vertical hydraulic gradient estimates, however, did not always provide consistent results with each other and with EMI and DTS measurements. This study, therefore, highlights the opportunities and challenges in incorporating multi-scale observations in a hierarchical manner in characterization of the groundwater-surface water exchange processes that are known to be highly heterogeneous in time and space. Overall, a combination of different methods helps to overcome the limitations of each single method and increases confidence in the obtained results.

An Automated Seepage Meter for Streams and Lakes

AbstractWe describe a new automatic seepage meter for use in soft bottom streams and lakes. The meter utilizes a thin‐walled tube that is inserted into the streambed or lakebed. A hole in the side of the tube is fitted with an electric valve. Prior to the test, the valve is open and the water level inside the tube is the same as the water level outside the tube. The test starts with closure of the valve, and the water level inside the tube changes as it moves toward the equilibrium hydraulic head that exists at the bottom of the tube. The time rate of change of the water level immediately after the valve closes is a direct measure of the seepage rate (q). The meter utilizes a precision linear actuator and a conductance circuit to sense the water level to a precision of about ±0.1 mm. The meter can also provide an estimate of vertical hydraulic conductivity (K v ) if data are collected for a characteristic time. The detection limit for q depends on the vertical hydraulic head gradient. For K v = 1 m/day, q of about 2 mm/day can be measured. Results from a laboratory sand tank show excellent agreement between measured and true q, and results from a field site are similar to values from calculations based on independent measurements of K v and vertical head gradients. The meter can provide rapid (30 min) q measurements for both gaining and losing systems and complements other methods for quantifying surface water groundwater interactions.

Improving Groundwater Model in Regional Sedimentary Basin Using Hydraulic Gradients

Groundwater flow systems are strongly influenced by heterogeneity and anisotropy of hydraulic conductivity (K). Particularly in stratified clastic sedimentary aquifers, the vertical hydraulic gradient (dh/dz) is often very high, due to the low vertical hydraulic conductivity (Kv) or high anisotropy (Kh/Kv). However, data on the vertical hydraulic gradient to calibrate the anisotropy is seldom available. We investigated the relationship between the variable Kh/Kv and the dh/dz computed by a groundwater flow model to improve the regional parameterization using an extensive data base on 3D distribution of hydraulic head. Although it is commonly assumed that the value of Kh/Kv is 10, our values typically ranged between 10 and 105, because the maximum Kh/Kv was 105 in this regional basin. The simulations used MODFLOW-2000. We found that the high Kh/Kv contributed to high dh/dz, while identifying locations of potentially good vertical connectivity between the aquifer zones, where dh/dz is very low and difficult to measure sufficiently accurately. Sensitivity analysis of dh/dz to recharge and pumping from wells showed that recharge led to change in inflow of groundwater that led to head change, which in turn led to change in dh/dz. While, pumping contributed to aquifer outflow, as change in head drawdown around the influence zone, in turn changed dh/dz. Additionally, dh/dz in a shallower aquifer was more strongly affected by recharge and pumping.

Groundwater-surface water interaction along river Kali, near Aligarh, India

River Kali, in the western gangetic plains, is posing serious threat to riparian communities by contaminating shallow riparian zone groundwater. To manage groundwater contamination, this study attempts to investigate the nature of groundwater-river water interaction, during February to April 2018, along a segment of river Kali near Aligarh in western Uttar Pradesh, India. River discharge was measured at three locations in two phases. Hydraulic heads in mini-piezometers were observed twice weekly. Forty water samples were analysed for major ions, nitrate, fluoride, TDS, pH, EC. River discharges and river bank vertical hydraulic gradients reveal that river Kali is losing water to riparian aquifers. Hydro-chemical analysis shows discordance in the pattern of ionic concentrations of river and groundwater, a distinct clustering of river and groundwater for both sampling periods. Ionic concentrations in groundwater with distance from channel indicate potentially clogged riverbed conditions that reduce the interaction of river water with riparian groundwater.

Hydropedology of depression-toe slope interaction across a soil unit boundary at the Boreal-Prairie interface

The genesis of mineral soil wetlands at the Boreal Plain – Prairie Ecozone interface is not well understood, given the transition to peatlands in this ecotone. Patterns of groundwater depth and hydraulic gradient were investigated in relation to soil development at a forested site at the Boreal Transition Ecoregion in central Alberta, Canada. Depressional and toe slope positions in each of four small catchments (up to 2 ha) oriented along a soil unit boundary were instrumented with wells and piezometers. Groundwater depth was measured manually or by sensor from 2012 to 2017. Groundwater at both slope positions fluctuated commonly 1.5 to 2 m annually and contributed to seasonal inundation in depressions. Direction of vertical hydraulic gradient reversed regularly, but was more commonly downward at toe slopes, indicating recharge dominance. An equal balance of upward and downward flow gradients in depressions was interpreted as flow-through. Lateral hydraulic gradients were commonly positive from toe to depression. Counts of subgroups for depressions were Orthic Humic Gleysol (3), Orthic Luvic Gleysol (2), Terric Mesisol (2), Orthic Gleysol (2) Terric Humisol (1) and Humic Luvic Gleysol (1). Counts of subgroups for toe slopes were Gleyed Dark Gray Chernozem (2), Gleyed Solonetzic Gray Luvisol (2), Gleyed Dark Gray Luvisol (2), Dark Gray Solodized Solonetz (1), Dark Gray Luvisol (1), Gray Solodized Solonetz (1), and Humic Luvic Gleysol (1). The infrequency of eluvial/illuvial soil features in depressions was consistent with their flow-through hydrology. The higher frequency of eluvial/illuvial soil features at toe slopes was also consistent with recharge hydrology. Soil carbonates were more abundant within the weakly saline soil unit. The seasonal hydrologic pattern in depressions included a period of rapid groundwater rise during snowmelt, a period of stability maintained by inputs of precipitation and toe slope flux, and a period of groundwater recession that was probably initiated when forest ET depleted upland soil moisture. Mineral wetlands with Gleysolic soils are found in locations of the landscape where soil saturation is too irregular to allow peatland formation. Mineral soil wetlands in this region may persist despite climate change because of the expected shift towards greater winter precipitation.

A Practical Geotechnical Analysis of in situ Stress Variations and Hydraulic Stability of Small Weirs Using SEEP/W and SIGMA/W Simulation

Geotechnical soil problems underneath foundation of hydraulic structures occurs due to engineering soil properties, geological setting and hydraulic properties of the projects. Two finite element programs of Geoslope 2012 software, SIGMA/W and SEEP/W, were used for analysis of in situ stresses, load deformation behavior, seepage quantity and vertical gradient below Teeb weir foundation, to compute factors of safety against seepage uplift. The site soil is a granular (gravel, sand and silt), weakly cemented soil cohered by gypsum and clay materials. The area has low lying topography, with slightly tectonic activities. The model results show that the upstream side stresses are reduced while the pore pressure are increased, indicating decreased stability. Soil displacement and settlement were measured and the effects of these displacements on the low cemented soil particles were discussed. The pressure distribution and vertical hydraulic gradient measured and used for analyze the weir stability. Finally, the soil potential failure zones were drawn to fix the main risks in each sides of the weir.

Heterogeneity in Hyporheic Flow, Pore Water Chemistry, and Microbial Community Composition in an Alpine Streambed

AbstractThe hyporheic zone, where surface water and groundwater mix, is an important microbial habitat where biogeochemical reactions influence water quality. We show that spatial variability in hyporheic flow in the East River near Crested Butte, CO, drives heterogeneity in streambed geochemical conditions and microbial community assemblages, but the diversity of microbial assemblages remains nearly constant throughout the reach. In July 2018, we collected approximately 100 pore water samples at 20‐cm depth and analyzed them for anions, cations, dissolved organic carbon, dissolved organic matter (DOM) quality, and basic water quality parameters. Vertical hydraulic head gradients were also measured to assess the potential for upward or downward flow, and heat tracing was used to quantify vertical flux rates at a subset of locations. We found that regions of the streambed that are more groundwater‐dominated contain less dissolved oxygen, higher concentrations of reduced metals, and more microbially processed, recalcitrant DOM, while more surface water‐dominated locations contain higher dissolved oxygen concentrations and terrestrially derived, labile DOM. 16S rRNA gene sequencing of extracted DNA revealed that microbial community composition varies with geochemical gradients related to hyporheic flow. These findings provide a better understanding of hyporheic controls on streambed biogeochemistry during the baseflow season, which is expected to lengthen with climate change in alpine watersheds due to earlier snowmelt onset and reduced snowpack.

Geo-mechanics and Hydraulic Conductivity Study of Claystone in Boyolali, Central Java, Indonesia

Claystone middle Miocene age were found in Wonosegoro sub-district, Boyolali region, Central Java, Indonesia. The purpose of the paper is to examine and discuss the geology and typical behavor of this claystone and its micro-level mechanism. The hydraulic conductivity was assessed using consilidated apparatuses; 150 mm diameter column mound using aggregated sample and 60 mm oedometer mould using slurry sample as a reference. Claystone materials used were treated under various conditions. In long term test under constand vertical stress and hydraulic gradient, the hydraulic conductvity decreases with time although the volume of void volume of the sample increased by swelling. Water contens of the individul aggregated increased by swelling, by which strength of particles decreases with and aggregate breakdown was enchanced. As a result, large void created by large particles could be redused in its size, leading the reduction of hydraulic conductuvity. The hydralic conductivities (K values) obtained from the aggregated sample varied in a broad range compared with those from slurry sample. In the test using the higher percentage of gravel-sized aggregate (up until couarse gravel-sized; retained in 26.5 mm sieve), K values changed from 10-5 to 10-7 m/s under vertical stresses from 5 up to 245 kPa. The test was repeated using smaller percentage of gravel-sized aggregate (up until fine gravel-sized, retained in 4.75 mm sieve) and the observed K values changed from 10-5 to 10-10 m/s. While the K values obtained in the specimen made from surry under same vertical stresses was 10-9 to 10-11 m/s. All of the extruded aggregate samples had higher water content than the initial ones, which suggest the alteration mechanism of soled consolidation phase to more deformable plastic phase, whice enables thesample to decrease the void size. These results conclude that using coarse gravel-sized aggregated, which is a reasonable scenario of a practically feasible aggregate size in a field, may noy produce the aimed hydraulic conductivity by the regulated standard. Therefore, breakdown of the aggrgate size and enchament of swelling are crucial factors for the application of the clay stones as a barrier material.

Impacts of Groundwater Drainage on Peatland Subsidence and Its Ecological Implications on an Atlantic Raised Bog

AbstractField data of topography, water levels, and peat hydraulic conductivity collected over a 28‐year period have revealed the impacts of marginal drainage on uncut raised bog ecohydrology and its peat properties. Drainage of the regional groundwater body has induced changes in the hydraulic properties of deep peat, with peat compression decreasing hydraulic conductivity and storativity while simultaneously introducing localized secondary porosity and effective storage. Where peat has increased in hydraulic conductivity, there is a corresponding decline in vertical hydraulic gradients and significant localized increases in recharge to the underlying substrate. Repeated topographic surveys show intense localized areas of peat consolidation (>5%) where it is underlain by highly permeable (>10 m/day) glacial till deposits. More widely, continued subsidence (4–6 mm/year) of the bog surface has been measured over 900 m from the bog margin, resulting in the progressive loss of approximately 40% of actively growing raised bog since 1991. This loss has thus been shown to be attributable to changes in the underlying groundwater head due to deep‐cut drainage, rather than near‐surface peatland drainage. However, although reinstating regional hydrostatic pressures in order to restore this ombrotrophic peatland may control the rapid drainage through preferential flow pathways, this may not eliminate the ecological impacts resulting from changed surface morphology arising from subsidence. Hence, this longitudinal study provides new insights into the role that aquifer systems and groundwater bodies play in maintaining hydrogeological processes in ombrotrophic peatland systems, while highlighting the difficulty in ecological restoration where regional groundwater dependencies are significant.

Hydrologic function of a moderate-rich fen watershed in the Athabasca Oil Sands Region of the Western Boreal Plain, northern Alberta

Peatlands are a dominant land feature in the Athabasca Oil Sands Region (AOSR) of the Western Boreal Plain (WBP), comprising >50% of the total land area, many of which are moderate-rich fens. The carbon stocks of moderate-rich fens in the WBP are susceptible to degradation through anthropogenic- and climate-related factors, yet, few studies have aimed to understand their hydrologic function. This study, located in a meltwater channel belt characterized by relatively thin outwash sand and gravel (∼6 m) underlying the peat, provides the first hydrological assessment of a moderate-rich fen in the AOSR. The lithology, hydrological function and groundwater geochemistry all point to the dominance of a local flow system supplying groundwater to the fen areas, evidenced by a thick (∼16 m) and shallow (∼7 m below ground surface) aquitard underlying the outwash, restricting hydrological connectivity between the fen and underlying regional aquifers. Vertical hydraulic gradients between the peat and underlying outwash aquifer, and horizontal hydraulic gradients between the fen and upland varied in response to both short-term and seasonal precipitation trends. Groundwater discharge to the fen was enhanced during wet periods characterized by high rainfall. Conversely, flow reversals (groundwater recharge; fen to underlying aquifer and upland), and subsequently, enhanced fen water table drawdown persisted during extended dry periods. This local groundwater flow-system influences recharge/discharge patterns at Poplar Fen, with hydraulic head in the underlying outwash aquifer highly susceptible to fluctuations in the presence and absence of precipitation-driven recharge from adjacent uplands. Moderate-rich fens similar to that studied here will likely become more susceptible to drying in the future due to a changing climate, leading to enhanced water table drawdown, peat oxidation and subsequent decomposition, vulnerability to wildfire, and seral succession to a more ombrogenous peatland system.

Interactions Between Regional Climate, Surficial Geology, and Topography: Characterizing Shallow Groundwater Systems in Subhumid, Low‐Relief Landscapes

AbstractThe Boreal Plains region of Canada is characterized by low relief, heterogeneous glacial landforms, and a subhumid climate, which result in complex and spatially variable groundwater‐surface water interactions. We test the influence of, and interactions between, short‐term climate variations, glacial deposit types, and topography on water table position and groundwater flow in a region with a subhumid long‐term climate. To do so, we evaluated water table positions, vertical hydraulic gradients, geochemistry, and stable water isotope signatures over a 19‐year period (including wet and dry climate states) along a 70‐km transect. The transect spans topographic positions on three hydrological response areas that comprise the major glacial depositional types typical of the Boreal Plains. High spatiotemporal variability of water table fluctuations and responses to climate signals illustrate the strong spatially variable controls that surficial geology, climate, and topography exert over scales of groundwater flow within and between glacial landforms across the Boreal Plains. Water tables were determined to be recharge controlled in the hummocky moraine and coarse outwash and topography controlled in the clay plain. Chemoscapes and isoscapes help delineate areas with characteristic water storage and transmission properties, which in turn control scales of groundwater flow and hydrologic responses to climate. Understanding the natural spatial and temporal variability of, and controls on, water table position, groundwater movement, and water quality under varying physical and climatic scenarios is important, as water security, ecosystem sustainability, and environmental quality become the focus of land management and reclamation efforts.

Buoyancy Force Acting on Underground Structures considering Seepage of Confined Water

The antifloating property of underground structures in areas with high underground water levels is a key design aspect. Evaluating the buoyancy forces acting on underground structures is complicated, particularly in the presence of confined water beneath the structures. Herein, the effects of the permeability coefficient of layered soil, hydraulic gradient, and embedment depth of the aquiclude on the buoyancy force acting on underground structures are investigated through three model tests: (1) calibration of the test system, (2) buoyancy force acting on a structure located in homogeneous soil considering vertical direction seepage, and (3) buoyancy force acting on a structure located in layered soil considering vertical seepage of confined water. The results show that the pore pressure along the structure and the buoyancy force acting on the underground structure considering seepage are greater than those obtained under hydrostatic conditions. The raising ratios of the pore pressure and buoyancy force are equal to the vertical hydraulic gradient when seepage occurs in homogeneous soil. In the presence of confined water, the raising ratio is significantly greater than the hydraulic gradient. In the cases studied herein, the raising ratio is approximately twice the hydraulic gradient. Simplified equations are proposed to calculate the buoyancy force acting on underground structures considering the vertical seepage of confined water. Finally, a finite element analysis is carried out to verify the conclusions obtained from the model test and the rationality of the proposed equations.

Development of a high-resolution 3D geological model for landfill leachate risk assessment

Groundwater protection and risk assessment of contaminated sites (e.g. abandoned landfills, industrial waste facilities, gasoline stations, and dry cleaners) situated in complex glacial landscapes are extremely challenging. A common method to assess the risks for leakage and contamination of groundwater and surface water is to develop hydraulic models. However, reliable models need to contain information about both the three-dimensional (3D) distribution of the deposits and their hydraulic properties. Environmental risk assessments therefore require highly detailed digital 3D geological models. In order to construct models with this degree of detail, dense data coverage with high-quality data is necessary. Many studies rely of few data sources, resulting in relatively sparse data.In this study we demonstrate how 6 different data sources can be combined to gain new insight on the geological history, which is central to the subsequent 3D geological modelling. The analyses in this study include (i) geomorphology, (ii) spear-auger mapping and near-surface electromagnetic induction data, (iii) borehole analyses, (iv) geoelectrical profiling, and (v) transient electromagnetic measurements.The study area is located on the island of Samsø, in the central part of Denmark, where a digital 3D geological model is constructed. The model consists of combined layer and voxel models and covers a small area (~1 km2) surrounding a former landfill (Pillemark). The near-surface geology is characterized by a dead-ice landscape formed by glaciers during the Weichselian ice-age. By interpreting the new data it has been possible to update and revise the geological history, which was used during the development of a highly detailed 3D geological model. This model was constructed combining layers and voxels in order to better represent the complex geology and incorporate all the details provided by the different data sources.The novel geological understanding was used to update the risk assessment of the Pillemark landfill, where a remedial pumping today is on-going. The immediate risk is related to the migration of landfill leachate downgradient through a lacustrine sandy aquifer and wetlands. Groundwater used for drinking water purposes is abstracted from an underlying sand and gravel aquifer (Tebbestrup formation), which is protected by a clay till (mid Danish till) found in the entire Pillemark area. A vertical hydraulic gradient from the upper to the lower aquifer indicates groundwater flow. However, the actual impact of landfill on the deeper aquifer has to be assessed by a detailed evaluation of the drainage system, pumping schemes and groundwater quality in the area. The 3D geological model is an important step toward the development of a groundwater flow model, required in order to establish a water balance for the hydrogeological system and estimate the vertical transport.

Effects of subsurface soil characteristics on wetland–groundwater interaction in the coastal plain of the Chesapeake Bay watershed

AbstractEcosystem services provided by depressional wetlands on the coastal plain of the Chesapeake Bay watershed (CBW) have been widely recognized and studied. However, wetland–groundwater interactions remain largely unknown in the CBW. The objective of this study was to examine the vertical interactions of depressional wetlands and groundwater with respect to different subsurface soil characteristics. This study examined two depressional wetlands with a low‐permeability and high‐permeability soil layer on the coastal plain of the CBW. The surface water level (SWL) and groundwater level (GWL) were monitored over 1 year from a well and piezometer at each site, respectively, and those data were used to examine the impacts of subsurface soil characteristics on wetland–groundwater interactions. A large difference between the SWL and GWL was observed at the wetland with a low‐permeability soil layer, although there was strong similarity between the SWL and GWL at the wetland with a high‐permeability soil layer. Our observations also identified a strong vertical hydraulic gradient between the SWL and GWL at the wetland with a high‐permeability soil layer relative to one with a low‐permeability soil layer. The hydroperiod (i.e., the total time of surface water inundation or saturation) of the wetland with a low‐permeability soil layer appeared to rely on groundwater less than the wetland with a high‐permeability soil layer. The findings showed that vertical wetland–groundwater interactions varied with subsurface soil characteristics on the coastal plain of the CBW. Therefore, subsurface soil characteristics should be carefully considered to anticipate the hydrologic behavior of wetlands in this region.

Variation in the vertical zonality of erodibility and critical shear stress of rill erosion in China's Hengduan Mountains

AbstractSoil erodibility is an essential parameter used in soil erosion prediction. This study selected the Liangshan town watershed to quantify variation in the vertical zonality of rill erodibility (kr) in China's ecologically fragile Hengduan Mountains. Soil types comprised of yellow–brown (soil A), purple (soil B), and dry‐red (soil C) in a descending order of occurrence from the summit to the valley, which roughly corresponds to the vertical climate zone (i.e. cool‐high mountain, warm‐low mountain, and dry‐hot valley sections) of the study area. With elevation, vertical soil zonality varied in both soil organic matter (SOM) content and soil particle‐size fractions. A series of rill erosion‐based scour experiments were conducted, using water discharge rates of 100, 200, 300, 400, 500, and 600 mL min‐1. Additionally, detachment rates (Dr) were measured under three hydrological conditions (the drainage, saturation, and seepage treatments). Results show that both Dr and flow shear stress (Ʈ) values increased as discharge increased. As elevation increased, the kr values decreased, while the vertical zonality of critical shear stress (Ʈc) values showed no obvious variation. The highest kr values were observed during the seepage treatment, followed by the saturation treatment then drainage treatment, indicating that variation in vertical hydraulic gradients could significantly alter kr values. This study also found that land‐use types could also alter kr and Ʈc values. Further research, however, is necessary to better quantify the effects of subsurface hydrological conditions and land‐use types on kr under different soil zonalities in China's Hengduan Mountains. © 2018 John Wiley & Sons, Ltd.

Well-screen and well-head clogging by hydrous ferric oxides

In the production of drinking water from groundwater by aeration and rapid sand filtration (RSF), iron(II) may be removed in two ways: at pH > ~8 by homogeneous oxidation and precipitation, and at ~6.0 ~7) by development of HFO flakes resulting in minimal clogging of the screen slots, and at “low” pH (pH < ~7) by massive HFO precipitates and biomass resulting in extensive clogging of the screen slots. As HFO precipitation continues, both processes result in extensive well-head clogging, i.e. fouling of submersible pumps, collector lines, etc. While well-screen clogging may be monitored conveniently, no general method is available for monitoring well-head clogging. Recognition of both chemical clogging types is important for effective well construction, operation and maintenance. During rest, HFO precipitation processes continue, further blocking pumps and water lines. If a vertical hydraulic gradient is present over the height of the well screen, HFO precipitates may accumulate on the well screen and enter the aquifer. Consequently, the behavior of the well is important not only during operation, but also during rest. These findings are illustrated by numerous observations on well fields exploited by the drinking water utilities in the Netherlands.

A Multi-Scale Approach for Improved Characterization of Surface Water—Groundwater Interactions: Integrating Thermal Remote Sensing and in-Stream Measurements

The interaction between surface water and groundwater is recognized as a key process for effective management of water resources. However, scale dependency and spatial heterogeneity of the processes at the interface are limiting factors in the characterization of this process. A hierarchical, multi–scale methodology has been devised and demonstrated to effectively and efficiently characterize the surface water-groundwater interaction along a 2-km reach in Kirmir stream, Turkey. Our methodology starts with investigation of geological information at the regional scale. At the intermediate scale, thermal infrared imagery is utilized to pinpoint groundwater seepage locations. Discharge measurements helped to understand the gains/losses. Point scale measurements including vertical hydraulic gradients, streambed vertical/horizontal temperature profiles and electrical conductivity profiles are investigated to characterize spatio-temporal variation in the vertical fluid fluxes. The results indicated that the study reach can be separated into three sections; upstream loosing section, downstream gaining section and mid-section with a seasonally variable character. Moreover, dam operation was found to control the exchange process—a rapid rise in the river stage resulted in temporary reversal of the exchange processes. Analysis of chloride concentrations indicated that the lower confined aquifer is likely the source for the seepage at a fault-controlled section of the streambed.