Riparian lowlands act as interfaces between the streams and upland areas. This study identified and quantified local flow paths in four transects of a 26 ha Danish riparian lowland in a glacial till landscape. The riparian lowland was fed by drain water from the upland agricultural drainage catchments. Precipitation, stream stage, and drainage discharge into the riparian lowland were measured continuously, while groundwater hydraulic heads were measured in piezometer pipes twice per month. A water balance model was developed to quantify water fluxes leaving the riparian lowland area via evapotranspiration, leakage to a deeper aquifer, and via groundwater flow, drain flow, and overland flow to the adjacent stream. Overland flow originating from the tile drains was the main flow path in all four transects, and also fluxes to the stream via groundwater or lowland tile drains were significant in some subareas. The presence of a secondary tile drainage network within parts of the riparian lowland reduced overland flow and increased interaction with the riparian lowland soils. Area‐normalized fluxes varied greatly between transects, largely reflecting variations in hydraulic loading rate (ratio of water input rate to the area of the receiving riparian lowland). This, combined with the significance of groundwater flow and riparian lowland tile drain flow in some of the investigated transects, revealed a heterogeneous distribution of flow paths within the small headwater lowland. The rate of overland flow was highly correlated to the hydraulic loading rate, which in turn was dominated by the drainage discharge rate at the hillslope boundary.

Study region: Eastern shore of Ringkobing Fjord, a coastal lagoon at the west coast of Denmark Study focus: A dual tracer approach based on salinity and delta O-18 is used to assess seasonal dynamics at the saltwater-freshwater interface of a coastal lagoon. At the site, salinity is prone to vary on a sub-seasonal or daily frequency due to riverine freshwater inputs to the lagoon. In contrast, delta O-18 compositions of end-members only vary seasonally. New hydrological insights: The dual tracer approach shows to be valuable in coastal settings where end-member concentrations vary substantially over the seasons and hence, an unambiguous endmember definition does not exist. Calculated mixing fractions using only salinity, deviated from the dual tracer approach on average by 18%, but were as high as 97%. Although, these differences decrease to 6% on average when using only delta O-18, our study strongly suggests their simultaneous application. Moreover, we found that seawater intrusion occurs during the summer when salinity in the lagoon is high and fresh submarine groundwater discharge (SGD) is low. This process reverses during the winter (wet season) when SGD increases by a factor of 2-3, due to the recession of the saltwater wedge from land. Our findings show that in absence of waves and tides, density-driven dynamics, and particularly the terrestrial freshwater fluxes, create a major impact on saltwater wedge dynamics.

This study presents field and modelling data of how dense seawater plumes move into a shallow coastal freshwater aquifer, following a storm flooding. The locations of the seawater plumes appear to be controlled by topographical depressions where seawater ponds were observed after the flooding. The rapid development and sinking of the seawater plumes in the aquifer was monitored by pore water electrical conductivity measurements in a 120 m transect of about 100 piezometers and by drive-point continuous vertical resistivity logs (el-logs). Large parts of the plumes reached the bottom of the aquifer 10 mbs in less than 30 days, but the plume migration caused by the unstable density distribution was found to be highly variable. The variable density code SUTRA was used for numerical modelling the flow of seawater down into the aquifer. The model captures the overall trends of the plume development and migration velocity observed in the field. However, even after introducing measured medium scale heterogeneity in the model, it could not adequately describe the complex details in the seawater distribution. This variability is believed to be caused by the geological heterogeneity and much of the variability appears to be on a small scale (~ cm), finer than the sampling scale of this study. The vastly different chemical composition of the seawater compared to the fresh groundwater triggered several geochemical reactions in the affected part of the aquifer. The high seawater sulfate content shifted the dominating redox-process from methanogenesis to sulfate reduction. The results of this field and modelling study highlight the complexity of how an aquifer, subject to seawater flooding, is contaminated by seawater. In turn, the results give insight into the vulnerability of coastal freshwater aquifers to catastrophic flooding events and to future sea-level rise. The study also indicates how other groundwater contaminants in dense fluids released at the surface may spread in the aquifer, and, points to the possible use of density driven flow as a novel method for introducing reactants for the remediation of contaminated sites.

A mass and age transport numerical model of Motril-Salobreña coastal aquifer has been developed considering a new age dating survey with environmental tracers (3H–3He, 85Kr, and 39Ar). The method applied for the simulation of age transport was the direct age, which enables the direct simulation of the age defined as a transport specie. The model includes also variable density since saltwater encroaches from the sea boundary. The results of the model reproduce the saline wedge and the age distribution explaining the hydrodynamic processes in the discharge zone of the aquifer. The model describes the aquifer hydrogeological processes complementing the uncertainties of groundwater age dating, as well as the integration of other hydrogeological information from previous studies. The calibration based on the age dating yields a better fit of the advective-dispersive parameters than the common approaches using groundwater levels or concentrations and could be applied to other aquifers. This information will allow a better management of the water resources against the changes due to anthropogenic activity and climate change.

This study aims to investigate the depth distribution of the Nitrate Reduction Potential (NRP) on a natural and a re-established wetland. The obtained NRP provides a valuable data of the driving factors affecting denitrification, the Dissimilatory Nitrate Reduction to Ammonium (DNRA) process and the performance of a re-established wetland. Intact soil cores were collected and divided in slices for the determination of Organic Matter (OM) through Loss of Ignition (LOI) as well as Dissolved Organic Carbon (DOC) and NRP spiking nitrate in batch tests. The Nitrate Reduction (NR) was fitted as a pseudo-first order rate constant (k) from where NRPs were obtained. NR took place in a narrow superficial zone showing a dropping natural logarithmic trend along depth. The main driving factor of denitrification, besides depth, was OM. Although, DOC and LOI could not express by themselves and absolute correlation with NRP, high amounts of DOC ensured enough quantity and quality of labile OM for NR. Besides, high concentration of LOI but a scarce abundance of DOC failed to drive NR. DNRA was only important in superficial samples with high contents of OM. Lastly, the high NRP of the re-established wetland confirms that wetlands can be restored satisfactorily.

Groundwater discharge into a seepage lake was investigated by combining flux measurements, hydrochemical tracers, geological information, and a telescopic modeling approach using first two-dimensional (2-D) regional then 2-D local flow and flow path models. Discharge measurements and hydrochemical tracers supplement each other. Discharge measurements yield flux estimates but rarely provide information about the origin and flow path of the water. Hydrochemical tracers may reveal the origin and flow path of the water but rarely provide any information about the flux. While aquifer interacting with the lake remained under seemingly steady state conditions across seasons, a high spatial and temporal heterogeneity in the discharge to the lake was observed. The results showed that part of the groundwater flowing from the west passes beneath the lake and discharges at the eastern shore, where groundwater springs and high discharge zones (HDZs) are observed at the lake bottom and at seepage faces adjacent to the lake. In the 2-D cross section, surface runoff from the seepage faces delivers 64% of the total groundwater inputs to the lake, and a 2 m wide offshore HDZ delivers 13%. Presence of HDZs may control nutrient fluxes to the lake.

Streambed hydraulic conductivity is one of the main factors controlling variability in surface water‐groundwater interactions, but only few studies aim at quantifying its spatial and temporal variability in different stream morphologies. Streambed horizontal hydraulic conductivities (Kh) were therefore determined from in‐stream slug tests, vertical hydraulic conductivities (Kv) were calculated with in‐stream permeameter tests and hydraulic heads were measured to obtain vertical head gradients at eight transects, each comprising five test locations, in a groundwater‐dominated stream. Seasonal small‐scale measurements were taken in December 2011 and August 2012, both in a straight stream channel with homogeneous elevation and downstream of a channel meander with heterogeneous elevation. All streambed attributes showed large spatial variability. Kh values were the highest at the depositional inner bend of the stream, whereas high Kv values were observed at the erosional outer bend and near the middle of the channel. Calculated Kv values were related to the thickness of the organic streambed sediment layer and also showed higher temporal variability than Kh because of sedimentation and scouring processes affecting the upper layers of the streambed. Test locations at the channel bend showed a more heterogeneous distribution of streambed properties than test locations in the straight channel, whereas within the channel bend, higher spatial variability in streambed attributes was observed across the stream than along the stream channel. Copyright © 2014 John Wiley & Sons, Ltd.

Electrical resistivity tomography (ERT) can be used to constrain seawater intrusion models because of its high sensitivity to total dissolved solid contents (TDS) in groundwater and its relatively high lateral coverage. However, the spatial variability of resolution in electrical imaging may prevent the correct recovery of the desired hydrochemical properties such as salt mass fraction. This paper presents a sequential approach to evaluate the feasibility of identifying hydraulic conductivity and dispersivity in density‐dependent flow and transport models from surface ERT‐derived mass fraction. In the course of this study, geophysical inversion was performed by using a smoothness constraint Tikhonov approach, whereas the hydrological inversion was performed using a gradient‐based Levenberg‐Marquardt algorithm. Two synthetic benchmarks were tested. They represent a pumping experiment in a homogeneous and heterogeneous coastal aquifer, respectively. These simulations demonstrated that only the lower salt mass fraction of the seawater‐freshwater transition zone can be recovered for different times. This ability has here been quantified in terms of cumulative sensitivity and our study has further demonstrated that the mismatch between the targeted and the recovered salt mass fraction occurs from a certain threshold. We were additionally able to explore the capability of sensitivity‐filtered ERT images using ground surface data only to recover (in both synthetic cases) the hydraulic conductivity while the dispersivity is more difficult to estimate. We attribute the latter mainly to the lack of ERT‐derived data at depth (where resolution is poorer) as well as to the smoothing effect of the ERT inversion.

The effects of the hydraulic characteristics of a groundwater-lake interface on discharge and nitrate loading to a lake were investigated. The interface is defined as the zone separating the adjacent aquifer (10's of m) and the lake bed (10's of cm) itself. The study combines field data using several tracers (water, oxygen isotopes, and nitrate) and numerical modeling. The hydraulic head distribution, a nitrate plume and seepage rates were observed over a two-year period along a similar to 100 m long transect reaching from an agricultural field into the lake. The groundwater-lake interface system was simulated with a 2D steady state flow and nitrate transport model (FEFLOW). The observations showed that discharge to the lake was doubled-peaked, with a peak discharge near the shore line followed by an almost (classical) exponential decrease, and a second peak further off-shore. The nitrate plume also extended 60-80 m off-shore. By calibrating the model to measured discharge and the outline of the nitrate plume it was demonstrated that; (1) the ratio of horizontal to vertical hydraulic conductivity (anisotropy) was very important and on the order of 50 and (2) the lake bed acted as a hydraulic barrier by having a much lower hydraulic conductivity than that of the relatively homogeneous aquifer. We suggest that the barrier is formed by an extensive plant cover that can trap finer materials and produce a surface colmation layer. The simulation results show that when a barrier is present the total groundwater discharge to the lake can be up to a factor of two lower and that approximately 50% of the nitrate bypasses the barrier. This proportion of the nitrate loading will therefore also bypass the plant cover and discharge directly to the lake off-shore potentially leading to algal blooms under N-limited conditions in the lake water column. (C) 2014 Elsevier B.V. All rights reserved.

Distributed temperature sensing (DTS) was used to map spatial and temporal changes in temperature on a 25 m by 6 m lakebed area in the winter (February), spring (May), and summer (August) of 2012. A constant and high discharge of groundwater with the average temperature of around 8 degrees C to the lake will result in either lower (summer) or higher (winter) daily temperatures and reduce temperature variability at the sediment-water interface (SWI). DTS data were used as a proxy for groundwater discharge using three metrics; daily minimum temperature, diel amplitude, and daily standard deviation of temperature. During the seasons, the daily minimum temperatures at the SWI indicate a discharge zone 4-6 m offshore. From winter to summer, the extent of this zone changes and the SWI temperatures also show a shift of discharge locations toward the shore. Fluxes estimated on the basis of vertical temperature profiles from the top 50 cm of the lakebed and seepage meters in August compare well with the locations of the high-discharge zones detected by the DTS in the same period, giving confidence in the ability of the method to map both the areas and spatial variability of groundwater discharge to lakes. Compared to February, the DTS was able to detect new relatively cold temperature zones at the SWI in May and August indicating that groundwater discharge to the lake changes over time and that DTS can be used to monitor temporal variability in areas of discharge.

This intermediate scale laboratory experimental study was designed to improve the conceptual understanding of aquifer flushing time associated with diffuse saltwater contamination of coastal aquifers due to a tsunami-like event. The motivation comes from field observations made after the tsunami in December, 2004 in South Asia. The focus is on the role and effects of heterogeneity on flushing effectiveness. A scheme that combines experimentation in a 4.8m long laboratory tank and numerical modeling was used. To demonstrate the effects of geologic heterogeneity, plume migration and flushing times were analyzed in both homogeneous and layered media and under different boundary conditions (ambient flow, saltwater infiltration rate, freshwater recharge). Saltwater and freshwater infiltrations imitate the results of the groundwater salinization from the tsunami and freshening from the monsoon rainfall. The saltwater plume behavior was monitored both through visual observations (digital photography) of the dyed salt water and using measurements taken from several electrical conductivity sensors installed through the tank walls. The variable-density, three dimensional code HST3D was used to simulate the tank experiments and understand the fate and movement of the saltwater plume under field conditions. The results from the tank experiments and modeling demonstrated that macro-scale heterogeneity significantly influenced the migration patterns and flushing times of diffuse saltwater contamination. Ambient flow had a direct influence on total flush-out time, and heterogeneity impacted flush-out times for the top part of the tank and total flush-out times. The presence of a continuous low-permeability layer caused a 40% increase in complete flush-out time due to the slower flow of salt water in the low-permeability layer. When a relatively small opening was introduced in the low-permeability layer, salt water migrated quickly into a higher-permeable layer below causing a reduction in flush-out time. Freshwater recharge caused an early dilution of salt water in the top part of the tank in the case of a layered media, but also pushed the saltwater plume into the low-permeability layer which led to increased total flush-out times.

On December 26,2004,the Indian Ocean tsunami waves hit Sri Lanka resulting in the devastation of the coastal aquifers from saltwater contamination.In an attempt to speed up the cleaning process of the contaminated water in wells,large-scale and intensive campaigns went into cleaning and rehabilitating wells through pumping out saline water（physical cleansing）. However,it was unclear whether these attempts improved the well-water quality,especially in terms of salinity due to density driven flow and solute transport phenomenon.Changes in water quality of a sand aquifer on the east coast of Sri Lanka owing to the December 26,2004 tsunami and subsequent

We have investigated the potential of 2D electrical imaging for the characterization of seawater intrusion using field data from a site in Almeria, SE Spain. Numerical simulations have been run for several scenarios, with a hydrogeological model reflecting the local site conditions. The simulations showed that only the lower salt concentrations of the seawater-freshwater transition zone could be recovered, due to the loss of resolution with depth. We quantified this capability in terms of the cumulative sensitivity associated with the measurement setup and showed that the mismatch between the targeted and imaged parameter values occurs from a certain sensitivity threshold. Similarly, heterogeneity may only be determined accurately if located in an adequately sensitive area. At the field site, we identified seawater intrusion at the scale of a few kilometres down to a hundred metres. Borehole logs show a remarkable correlation with the image obtained from surface data but indicate that the electrically derived mass fraction of pure seawater could not be recovered due to the discrepancy between the in-situ and laboratory-derived petrophysical relationships. Surface-to-hole inversion results suggest that the laterally varying resolution pattern associated with such a setup dominates the image characteristics compared to the laterally more homogeneous resolution pattern of surface only inversion results and hence, surface-to-hole images are not easily interpretable in terms of larger-scale features. Our results indicate that electrical imaging can be used to constrain seawater intrusion models if image appraisal tools are appropriately used to quantify the spatial variation of sensitivity and resolution. The most crucial limitation is probably the apparent non-stationarity of the petrophysical relationship during the imaging process.

Changes in water quality of a sand aquifer on the east coast of Sri Lanka due to the 26 December 2004 tsunami and subsequent remediation attempt by pumping were investigated. Two transects, disturbed (where pumping of groundwater took place) and undisturbed (where no pumping occurred), were monitored. In the undisturbed area, the average electrical conductivity (EC) in the wells affected by the tsunami showed a decrease from 3000 to 1200 μS/cm after the first full rainy season following the disaster; however, in the disturbed area the average EC stabilized around 1500 μS/cm. The observations were further analyzed by calculating hydrogeochemical mixing ratios, which showed that the seawater fraction was higher in the disturbed site than in the undisturbed site. The disturbance caused by physical cleaning by extensive pumping and subsequent disposal of the pumped water adjacent to the wells likely led to retention of salinity in the aquifer.

A seawater intrusion experiment was carried out in a shallow coastal unconfined aquifer at Skansehage, Denmark. The aquifer consists of 6–10m of postglacial (Holocene) eolian and marine sands and gravels resting on Pleistocene glacial sequence of tills and glacio-fluviatile sediments on a basement of Palaeocene (Danian) limestone. Stable isotopes (18O and 2H), strontium isotopes (87Sr/86Sr) and the elements Sr and Cl were measured to delineate the mixing of seawater and groundwater in the shallow aquifer and to determine the contribution from different water sources along the flow paths. The stable isotopes reflect recharge conditions and mixing between groundwater and seawater. The 87Sr/86Sr isotope ratios are controlled by aquifer matrix chemistry. The 87Sr/86Sr ratio distribution and Sr content reveal a ternary mixing system in the shallow aquifer, encompassing brackish seawater, fresh shallow groundwater and deep groundwater from the underlying limestone aquifer as end-members. A mixing analysis is performed using the Cl contents and the 87Sr/86Sr ratios. The mixing analysis indicates a zone with leakage of deep groundwater from the limestone aquifer into the lower part of the shallow sandy aquifer through the Pleistocene sedimentary sequence. The upper part of the sandy aquifer is thoroughly dominated by mixing processes between infiltrating fresh groundwater and brackish seawater from Isefjord. Prior to the intrusion experiment there appeared to be limited mixing between the upper and lower part of the water body in the shallow sandy aquifer. The leakage of deep groundwater increased during the intrusion experiment as a result of the large abstraction of groundwater. Following the ceasing of the groundwater pumping, the interface went from being vertical to being predominantly horizontal due to density flow. As a consequence the deep groundwater from the limestone aquifer was pushed by the seawater into the upper part of the aquifer to mix with the upper fresh groundwater.

Using environmental tracers in groundwater dating partly relies on the assumption that groundwater age distribution can be described analytically. To investigate the applicability of age dating in complex multiaquifer systems, a methodology for simulating well specific groundwater age distribution was developed. Using a groundwater model and particle tracking we modeled age distributions at screen locations. By enveloping modeled age distributions and estimated recharge concentrations, environmental tracer breakthroughs were simulated for specific screens. Simulated age distributions are of irregular shapes and sizes without being similar to the assumed age distributions used in the analytical approach. The shape of age distribution to some extent depends on sampling size and on whether the system is modeled in a transient or in a steady state, but shape and size were largely driven by the heterogeneity of the model and by topographical variations as well. Analytically derived groundwater ages are dependent on sampling time. This time dependence relates to the nonlinearity of recharge concentrations and the shape and size of age distribution that has no coherence with the simplified assumptions of traditional approaches. Accordingly, constraining flow models by "age observations" may lead to misrepresentations that are biased depending on sampling time. If environmental tracers are used directly in terms of concentrations instead of ages, spatial as well as temporal variations become useful in constraining models.

Vertical profiles of the chlorofluorocarbons CFC‐11, CFC‐12, and CFC‐113 penetrating aerobic and anaerobic parts of a shallow sandy aquifer show that the CFC gases are degraded in the <1 m thick transition zone from aerobic to anaerobic groundwater in a pyritic sand aquifer at Rabis Creek, Denmark. Two‐dimensional solute transport simulations with either zero‐order or first‐order degradation in the anaerobic zone corroborate this interpretation. The transport model was previously calibrated against detailed tritium profiles in the same wells. First‐order degradation is found to best match the observed CFC profiles yielding an approximate half‐life of a few months for CFC‐11. Degradation is not as clearly recognized for CFC‐12 and CFC‐113, but it may occur with rates corresponding to a half‐life of a few years or more. Data indicate a geochemical control of the CFC concentration gradient at the redox front and that denitrification and denitrifiers are not of major importance for the observed CFC degradation. The responsible mechanism behind the observed degradation is not known but we suggest that reductive dehalogenation by surface‐bound Fe(II) on pyrite possibly enhanced by the presence of Fe(III)‐bearing weathering products (green rust) may be a plausible mechanism. The observed data and the performed simulations confirm the potential application of the CFC gases as age‐dating tools in the aerobic part of the investigated aquifer, but also that CFC data must be analyzed carefully before it is used as a dating tool in reducing aquifers because degradation may have occurred. The use of multiple or alternative tracers should be considered in anaerobic environments.