River-groundwater Interaction Research Articles

Intercomparison of hydrological model structures and calibration approaches in climate scenario impact projections

Summary The objective of this paper is to investigate the effects of hydrological model structure and calibration on climate change impact results in hydrology. The uncertainty in the hydrological impact results is assessed by the relative change in runoff volumes and peak and low flow extremes from historical and future climate conditions. The effect of the hydrological model structure is examined through the use of five hydrological models with different spatial resolutions and process descriptions. These were applied to a medium sized catchment in Belgium. The models vary from the lumped conceptual NAM, PDM and VHM models over the intermediate detailed and distributed WetSpa model to the fully distributed MIKE SHE model. The latter model accounts for the 3D groundwater processes and interacts bi-directionally with a full hydrodynamic MIKE 11 river model. After careful and manual calibration of these models, accounting for the accuracy of the peak and low flow extremes and runoff subflows, and the changes in these extremes for changing rainfall conditions, the five models respond in a similar way to the climate scenarios over Belgium. Future projections on peak flows are highly uncertain with expected increases as well as decreases depending on the climate scenario. The projections on future low flows are more uniform; low flows decrease (up to 60%) for all models and for all climate scenarios. However, the uncertainties in the impact projections are high, mainly in the dry season. With respect to the model structural uncertainty, the PDM model simulates significantly higher runoff peak flows under future wet scenarios, which is explained by its specific model structure. For the low flow extremes, the MIKE SHE model projects significantly lower low flows in dry scenario conditions in comparison to the other models, probably due to its large difference in process descriptions for the groundwater component, the groundwater–river interactions. The effect of the model calibration was tested by comparing the manual calibration approach with automatic calibrations of the VHM model based on different objective functions. The calibration approach did not significantly alter the model results for peak flow, but the low flow projections were again highly influenced. Model choice as well as calibration strategy hence have a critical impact on low flows, more than on peak flows. These results highlight the high uncertainty in low flow modelling, especially in a climate change context.

The thermal consequences of river-level variations in an urban groundwater body highly affected by groundwater heat pumps

The extensive implementation of ground source heat pumps in urban aquifers is an important issue related to groundwater quality and the future economic feasibility of existent geothermal installations. Although many cities are in the immediate vicinity of large rivers, little is known about the thermal river–groundwater interaction at a kilometric-scale. The aim of this work is to evaluate the thermal impact of river water recharges induced by flood events into an urban alluvial aquifer anthropogenically influenced by geothermal exploitations. The present thermal state of an urban aquifer at a regional scale, including 27 groundwater heat pump installations, has been evaluated. The thermal impacts of these installations in the aquifer together with the thermal impacts from “cold” winter floods have also been spatially and temporally evaluated to ensure better geothermal management of the aquifer. The results showed a variable direct thermal impact from 0 to 6°C depending on the groundwater–surface water interaction along the river trajectory. The thermal plumes far away from the riverbed also present minor indirect thermal impacts due to hydraulic gradient variations.

Controls of groundwater floodwave propagation in a gravelly floodplain

Interactions between surface water and groundwater can occur over a wide range of spatial and temporal scales within a high hydraulic conductivity gravelly floodplain. In this research, dynamics of river–groundwater interactions in the floodplain of the Matane River (eastern Canada) are described on a flood event basis. Eleven piezometers equipped with pressure sensors were installed to monitor river stage and groundwater levels at a 15-min interval during the summer and fall of 2011. Results suggest that the alluvial aquifer of the Matane Valley is hydraulically connected and primarily controlled by river stage fluctuations, flood duration and magnitude. The largest flood event recorded affected local groundwater flow orientation by generating an inversion of the hydraulic gradient for 16 h. Piezometric data show the propagation of a well-defined groundwater floodwave for every flood recorded as well as for discharges below bankfull (<0.5 Qbf). A wave propagated through the entire floodplain (250 m) for each measured flood while its amplitude and velocity were highly dependent on hydroclimatic conditions. The groundwater floodwave, which is interpreted as a dynamic wave, propagated through the floodplain at 2–3 orders of magnitude faster than groundwater flux velocities. It was found that groundwater exfiltration can occur in areas distant from the channel even at stream discharges that are well below bankfull. This study supports the idea that a river flood has a much larger effect in time and space than what is occurring within the channel.

Assessing the effect of different river water level interpolation schemes on modeled groundwater residence times

Summary Obtaining a quantitative understanding of river–groundwater interactions is of high practical relevance, for instance within the context of riverbank filtration and river restoration. Modeling interactions between river and groundwater requires knowledge of the river’s spatiotemporal water level distribution. The dynamic nature of riverbed morphology in restored river reaches might result in complex river water level distributions, including disconnected river branches, nonlinear longitudinal water level profiles and morphologically induced lateral water level gradients. Recently, two new methods were proposed to accurately and efficiently capture 2D water level distributions of dynamic rivers. In this study, we assessed the predictive capability of these methods with respect to simulated groundwater residence times. Both methods were used to generate surface water level distributions of a 1.2 km long partly restored river reach of the Thur River in northeastern Switzerland. We then assigned these water level distributions as boundary conditions to a 3D steady-state groundwater flow and transport model. When applying either of the new methods, the calibration-constrained groundwater flow field accurately predicted the spatial distribution of groundwater residence times; deviations were within a range of 30% when compared to residence times obtained using a reference method. We further tested the sensitivity of the simulated groundwater residence times to a simplified river water level distribution. The negligence of lateral river water level gradients of 20–30 cm on a length of 200 m caused errors of 40–80% in the calibration-constrained groundwater residence time distribution compared to results that included lateral water level gradients. The additional assumption of a linear water level distribution in longitudinal river direction led to deviations from the complete river water level distribution of up to 50 cm, which caused wide-spread errors in simulated groundwater residence times of 200–500%. For an accurate simulation of groundwater residence times, it is therefore imperative that the longitudinal water level distribution is correctly captured and described. Based on the confirmed predictive capability of the new methods to estimate 2D river water level distributions, we can recommend their application to future studies that model dynamic river–groundwater systems.

Influence of calcite on uranium(VI) reactive transport in the groundwater–river mixing zone

Calcite is an important, relatively soluble mineral phase that can affect uranium reactive transport in subsurface sediments. This study was conducted to investigate the distribution of calcite and its influence on uranium adsorption and reactive transport in the groundwater-river mixing zone of the Hanford 300A site, Washington State. Simulations using a two-dimensional (2D) reactive transport model under field-relevant hydrological and hydrogeochemical conditions revealed the development of a calcite reaction front through the mixing zone as a result of dynamic groundwater-river interactions. The calcite concentration distribution, in turn, affected the concentrations of aqueous carbonate and calcium, and pH through dissolution, as river waters intruded and receded from the site at different velocities in response to stage changes. The composition variations in groundwater subsequently influenced uranium mobility and discharge rates into the river in a complex fashion. The results implied that calcite distribution and concentration are important variables that need to be quantified for accurate reactive transport predictions of uranium, especially in dynamic groundwater-river mixing zones.

Groundwater–river interaction and management in the context of inter-basin transfers

This study is focused on the management of coupled groundwater–river interaction (GRI) in the context of inter-basin transfers. The semi-coupled models, HEC-RAS and MODFLOW, were utilized in this study to replicate the River Tees experiment that had been conducted in 1976. Then the ‘surrogate’ models were applied to two “what if” scenarios in order to understand and manage GRI behavior and the nature of the reservoir control system. The results on the River Tees showed that the total temporary bankside storage volume was up to 17.5 % of the total reservoir release, given about 22 h of travel time. In addition, zones 4 and 5 of the river model with significant alluvium deposits were the best storage sites with values of 4.5 and 6.7 % of the total reservoir release, respectively, estimated as temporary losses. The final direct runoff received downstream at broken scar was about 95 % of the Cow Green total reservoir release. Furthermore, the closure rate proved to be vital to compute the effective return flow in respect of this study area. Finally, with careful reservoir and groundwater management, the River Tees proved to be an ideal natural aqueduct for inter-basin transfers when used as part of a major regional water resources scheme.

Fe(II)- and sulfide-facilitated reduction of 99Tc(VII)O4− in microbially reduced hyporheic zone sediments

Redox-reactive, biogeochemical phases generated by reductive microbial activity in hyporheic zone sediments from a dynamic groundwater–river interaction zone were evaluated for their ability to reduce soluble pertechnetate [99Tc(VII)O4−] to less soluble Tc(IV). The sediments were bioreduced by indigenous microorganisms that were stimulated by organic substrate addition in synthetic groundwater with or without sulfate. In most treatments, 20μmolL−1 initial aqueous Tc(VII) was reduced to near or below detection (3.82×10−9molL−1) over periods of days to months in suspensions of variable solids concentrations. Native sediments containing significant lithogenic Fe(II) in various phases were, in contrast, unreactive with Tc(VII). The reduction rates in the bioreduced sediments increased with increases in sediment mass, in proportion to weak acid-extractable Fe(II) and sediment-associated sulfide (AVS). The rate of Tc(VII) reduction was first order with respect to both aqueous Tc(VII) concentration and sediment mass, but correlations between specific reductant concentrations and reaction rate were not found. X-ray microprobe measurements revealed a strong correlation between Tc hot spots and Fe-containing mineral particles in the sediment. However, only a portion of Fe-containing particles were Tc-hosts. The Tc-hot spots displayed a chemical signature (by EDXRF) similar to pyroxene. The application of autoradiography and electron microprobe allowed further isolation of Tc-containing particles that were invariably found to be ca 100μm aggregates of primary mineral material embedded within a fine-grained phyllosilicate matrix. EXAFS spectroscopy revealed that the Tc(IV) within these were a combination of a Tc(IV)O2-like phase and Tc(IV)–Fe surface clusters, with a significant fraction of a TcSx-like phase in sediments incubated with SO42−. AVS was implicated as a more selective reductant at low solids concentration even though its concentration was below that required for stoichiometric reduction of Tc(VII). These results demonstrate that composite mineral aggregates may be redox reaction centers in coarse-textured hyporheic zone sediments regardless of the dominant anoxic biogeochemical processes.

Unraveling the heat island effect observed in urban groundwater bodies – Definition of a potential natural state

Summary A superposition of several thermal processes leads to an elevation of groundwater temperatures of up to 9 °C above the natural state in the city of Basel, Switzerland. The urban thermal groundwater regime is influenced by: (1) urbanization and annual heating periods; (2) thermal groundwater use; (3) seasonal trends; (4) river-groundwater interaction; and (5) climate change and consequences thereof. The combination of short- and long-term data analysis, including conventional and high-resolution multilevel groundwater temperature monitoring, as well as 3D numerical groundwater flow and heat-transport modeling allowed quantifying the thermal influences on the investigated urban groundwater body. Results facilitate to describe the “present state” of the urban thermal groundwater regime and to derive a “potential natural state” of the investigated groundwater body. The study originated from a request of the executive council to provide a basis for cost estimates of infrastructure adaptation measures necessary to mitigate the impact of climate change. It is shown that the principal trigger for the observed thermal development is not climate change but that local and regional anthropogenic factors are dominating. Although in urban areas, groundwater is increasingly used for cooling purposes; the geothermal potential, resulting from elevated groundwater temperatures, is generally not exploited. The presented approach provides a basis for the setup of combined and thermally balanced heating and cooling systems.

Thermal management of an unconsolidated shallow urban groundwater body

Abstract. This study presents the development of tools for the sustainable thermal management of a shallow unconsolidated urban groundwater body in the city of Basel (Switzerland). The concept of the investigations is based on (1) a characterization of the present thermal state of the urban groundwater body, and (2) the evaluation of potential mitigation measures for the future thermal management of specific regions within the groundwater body. The investigations focus on thermal processes down-gradient of thermal groundwater use, effects of heated buildings in the subsurface as well as the thermal influence of river–groundwater interaction. Investigation methods include (1) short- and long-term data analysis, (2) high-resolution multilevel groundwater temperature monitoring, as well as (3) 3-D numerical groundwater flow and heat transport modeling and scenario development. The combination of these methods allows for the quantifying of the thermal influences on the investigated urban groundwater body, including the influences of thermal groundwater use and heated subsurface constructions. Subsequently, first implications for management strategies are discussed, including minimizing further groundwater temperature increase, targeting "potential natural" groundwater temperatures for specific aquifer regions and exploiting the thermal potential.

In situ and laboratory treatment tests for lowering of excess manganese and iron in drinking water sourced from river–groundwater interaction

Manganese and iron are essential nutrients at low doses. However, long-term exposure in high doses may be harmful. Human activities and natural sources are responsible for manganese and iron contamination in water. This study aimed to investigate the source of high manganese and iron contents in shallow groundwater, where the groundwater is used for drinking purpose, and to decrease the excess manganese and iron from shallow groundwater. Based on the on-site analytical results taken from the wells in the study area, iron contents in water samples varied between 30 and 200 μg/L, which were under the allowable limits of Turkish Drinking Water Standards (TDWS) (Water intended for human consumption, http://rega.basbakanlik.gov.tr/eskiler/2005/02/20050217-3.htm , 2005). However, manganese levels varied from 30 to 248 μg/L, which some of them are higher than the allowable limits of TDWS (Water intended for human consumption, http://rega.basbakanlik.gov.tr/eskiler/2005/02/20050217-3.htm , 2005) (50 μg/L), and EPA ( http://water.epa.gov/drink/contaminants/index.cfm , 2006) (50 μg/L). The source of excess manganese is originated mainly from geogenic source besides Porsuk River interaction in shallow aquifer in the specific section of the study area. To decrease high manganese content from the well water and reservoir water, laboratory and in situ treatment tests were applied. Among these tests, chlorination, associated with filtration (by fine sand, active carbon and zeolite) and the use of different filtration procedures by cation exchange resin were determined as the most effective methods, which was not previously applied on-site as a combinative approaches to reduce the excess manganese in water.

Analysis of hydrogeological parameters and numerical modeling groundwater in a karst watershed, southwest China

A numerical groundwater model was developed on the basis of a conceptualized, equivalent, continuous medium for a karst basin located in Guizhou Province, southwest China. In this model, simulation of underground drainage and groundwater–river interactions were executed using the Drain and River packages of MODFLOW, respectively. Hydraulic conductivities were first determined using flow recession analysis methods proposed by Brutsaert and Lopez (Water Resour Res 34(2):233–240, 1998) and Mendoza et al. (J Hydrol 279:57–69, 2003). These parameters, together with recharge from precipitation, were further calibrated against observed water levels of subterranean rivers during 1988 and 1989 at three observation stations. The established model can be used to simulate temporal and spatial variations of groundwater levels and subterranean river discharges. The simulated results demonstrate that the total aquifer recharge comes mostly from precipitation and river leakage. Precipitation recharge and river leakage are about 70 and 13 % of the total aquifer recharge, respectively. Meanwhile, most discharge of the study watershed is from subterranean rivers. It accounts for 62 % of the total discharge and river drains only 8 % of the total discharge.

Persistence of uranium groundwater plumes: Contrasting mechanisms at two DOE sites in the groundwater–river interaction zone

We examine subsurface uranium (U) plumes at two U.S. Department of Energy sites that are located near large river systems and are influenced by groundwater-river hydrologic interaction. Following surface excavation of contaminated materials, both sites were projected to naturally flush remnant uranium contamination to levels below regulatory limits (e.g., 30 μg/L or 0.126 μmol/L; U.S. EPA drinking water standard), with 10 years projected for the Hanford 300 Area (Columbia River) and 12 years for the Rifle site (Colorado River). The rate of observed uranium decrease was much lower than expected at both sites. While uncertainty remains, a comparison of current understanding suggests that the two sites have common, but also different mechanisms controlling plume persistence. At the Hanford 300 A, the persistent source is adsorbed U(VI) in the vadose zone that is released to the aquifer during spring water table excursions. The release of U(VI) from the vadose zone and its transport within the oxic, coarse-textured aquifer sediments is dominated by kinetically-limited surface complexation. Modeling implies that annual plume discharge volumes to the Columbia River are small (<one pore volume). At the Rifle site, slow oxidation of naturally reduced, contaminant U(IV) in the saturated zone and a continuous influx of U(VI) from natural, up-gradient sources influence plume persistence. Rate-limited mass transfer and surface complexation also control U(VI) migration velocity in the sub-oxic Rifle groundwater. Flux of U(VI) from the vadose zone at the Rifle site may be locally important, but it is not the dominant process that sustains the plume. A wide range in microbiologic functional diversity exists at both sites. Strains of Geobacter and other metal reducing bacteria are present at low natural abundance that are capable of enzymatic U(VI) reduction in localized zones of accumulated detrital organic carbon or after organic carbon amendment. Major differences between the sites include the geochemical nature of residual, contaminant U; the rates of current kinetic processes (both biotic and abiotic) influencing U(VI) solid-liquid distribution; the presence of detrital organic matter and the resulting spatial heterogeneity in microbially-driven redox properties; and the magnitude of groundwater hydrologic dynamics controlled by river-stage fluctuations, geologic structures, and aquifer hydraulic properties. The comparative analysis of these sites provides important guidance to the characterization, understanding, modeling, and remediation of groundwater contaminant plumes influenced by surface water interaction that are common world-wide.

Zeitliche und räumliche Skalen der Fluss-Grundwasser-Interaktion: Ein multidimensionaler hydrogeologischer Untersuchungsansatz

River-groundwater interactions show strong scale-dependencies and are often strongly transient. In this regard, small-scale flow conditions in the hyporheic zone at the interface between surface- and groundwater can be important for process-understanding. This especially includes questions concerning flow conditions in salmonid redds of gravel-bed rivers. The Swiss subalpine river Enziwigger was chosen as an example for a small channelized river with artificial steps within the riverbed. Several methods were developed, tested and combined that capture the four dimensions (three spatial and one temporal) of the interactions between surface water, the hyporheic zone and groundwater, for individual river segments. The setup of a monitoring network as well as the realization of field-measurements provided data for groundwater flow models. Continuous time series of hydraulic data, temperature and electrical conductivity within the river and the riverbed, as well as within the riverine groundwater, allowed identifying zones with significant exchange of surface water and groundwater. Additionally, the data helped describe the transient character of groundwater flow-paths under various hydrological boundary conditions. Results of the field-measurements in combination with transient groundwater flow modeling and scenario analyses illustrate the relevance of dynamically changing infiltration and exfiltration patterns within the riverbed.

Climate change impact on river flows and catchment hydrology: a comparison of two spatially distributed models

AbstractHydrological models have been widely used to assess changes in stream discharges by climate change; however, concern might arise on the accurateness of the model predictions under the changed conditions particularly during low flow periods. In this study, two spatially distributed hydrological models MIKE SHE and WetSpa, each representing a different model complexity in terms of process description, data needs, parameter space, degree of calibration, were compared in their estimation of the climate change impact on the flow regimes in a medium‐sized catchment in Belgium. The fully integrated, physically based MIKE SHE model, comprising a three‐dimensional groundwater flow and river model, was applied to better understand the groundwater flow and groundwater–river interactions under changed climate conditions. Both models were able to capture the flow dynamics very well with high efficiencies and simulated the flow extremes very accurately. The groundwater heads in MIKE SHE and their seasonal variation had a high model performance. The two models simulated similar changes to future flows because of climatic changes. Peak flows were expected to increase or decrease depending, taking the large uncertainty in future climate trends into account. The model structural uncertainties on these high flow predictions were rather limited. Low flows were expected to reduce because of drier conditions by future climate change, indicating elevated low flow risks for Belgium. However, the projected low flow changes differed significantly over the models and even exceeded the uncertainty by the expected climate trends. Smaller impact was predicted by considering the groundwater physics and river interaction in the MIKE SHE model. These changes to the surface water regimes were verified by the changes in groundwater heads. Future groundwater conditions also point towards drier conditions with a small decrease in heads for future summer and autumn periods. Projected variations in winter heads depend on the climate scenario. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Establishing a geochemical heterogeneity model for a contaminated vadose zone — Aquifer system

A large set of sediment samples from a 1600 m² experimental plot within a 2.2 km² vadose zone and groundwater uranium (VI) plume was subject to physical, chemical, and mineralogic characterization. The plot is being used for field experimentation on U(VI) recharge and transport processes within a persistent groundwater plume that exists in the groundwater-river interaction zone of the Columbia River at the U.S. DOE Hanford site. The samples were obtained during the installation of 35 tightly spaced (10 m separation) groundwater monitoring wells. The characterization measurements for each sample included total contaminant concentrations (U and Cu primarily), bicarbonate extractable U(VI), sequential ²³⁸U(VI) contaminant desorption Kd, ²³³U(VI) adsorption K(d), grain size distribution, surface area, extractable poorly crystalline Fe(III) oxides, and mineralogy. The characterization objective was to inform a conceptual model of coupled processes controlling the anomalous longevity of the plume, and to quantify the spatial heterogeneity of the contaminant inventory and the primary properties effecting reactive transport. Correlations were drawn between chemical, physical, and reaction properties, and Gaussian simulation was used to compute multiple 3-D realizations of extractable U(VI), the ²³³U(VI) adsorption K(d), and the distribution of the reactive <2 mm fraction. Adsorbed contaminant U(VI) was highest in the vadose zone and the zone of seasonal water table fluctuation lying at its base. Adsorbed U(VI) was measureable, but low, in the groundwater plume region where very high hydraulic conductivities existed. The distribution of adsorbed U(VI) displayed no apparent correlation with sediment physical or chemical properties. Desorption [²³⁸U(IV)] and adsorption [²³³U(VI)] K(d) values showed appreciable differences due to mass transfer controlled surface complexation and the effects of long subsurface residence times. The ²³³U(VI) adsorption K(d), a combined measure of surface complexation strength and site concentration, was relatively uniform throughout the domain, displaying correlation with fines distribution and surface area. The characterization results revealed U(VI) supplied to the groundwater plume through spatially heterogeneous recharge from residual contamination in the zone of seasonal water table fluctuation, and transport of U(VI) controlled by weak, kinetically-controlled surface complexation in the coarse-textured saturated zone. Geostatistical relationships for the adsorbed contaminant U distribution in the characterization domain allow an extrapolation to inventory at the plume scale, a critical unknown for remedial action.

Principal component analysis of time series for identifying indicator variables for riverine groundwater extraction management

Summary Although alluvial aquifers connected to rivers can be a rich source of drinking water, they are susceptible to contamination by infiltrating river water. The processes governing river–groundwater interaction are variable in time and space. Natural filtration mechanisms are often not sufficient during high discharge events in the river. To capture the dynamics of river–groundwater interaction, indicator parameters that can serve as proxies for river water infiltration need to be derived. Principal component analysis of continuously measured time series was used to identify indicator wells and derive indicator parameters for a study area in NW Switzerland. The results showed different sources of variation in the parameters, including river stage fluctuations. The multivariate approach highlighted differences between observation wells based on the response of the measured parameters to effects of damping and delay of the input signals. Three observation wells were shown to be representative of river–groundwater interaction dynamics in the study area. Of the three parameters analysed, groundwater head and electrical conductivity are recommended as a combined proxy for river water infiltration in the study area. In contrast, temperature proved not to be a reliable indicator.

Nitrogen balance and fate in a heavily impacted watershed (Oglio River, Northern Italy): in quest of the missing sources and sinks

Abstract. We present data from a comprehensive investigation carried out from 2007 to 2010, focussing on nitrogen pollution in the Oglio River basin (3800 km2, Po Plain, Northern Italy). Nitrogen mass balances, computed for the whole basin with 2000 and 2008 data, suggest a large N surplus in this area, over 40 000 t N yr−1, and increasing between 2000 and 2008. Calculations indicate a very large impact of animal husbandry and agricultural activities in this watershed, with livestock manure and synthetic fertilizers contributing 85% of total N inputs (about 100 000 t N yr−1) and largely exceeding crop uptake and other N losses (about 60 000 t N yr−1). Nitrogen from domestic and industrial origin is estimated as about 5800 and 7200 t N yr−1, respectively, although these loads are overestimated, as denitrification in treatment plants is not considered; nonetheless, they represent a minor term of the N budget. Annual export of nitrogen from the basin, calculated from flow data and water chemistry at the mouth of the Oglio River, is estimated at 13 000 t N yr−1, and represents a relatively small fraction of N inputs and surplus (∼12% and 34%, respectively). After considering N sinks in crop uptake, soil denitrification and volatilization, a large excess remains unaccounted (∼26 000 t N yr−1) in unknown temporary or permanent N sinks. Nitrogen removal via denitrification was evaluated in the Oglio riverbed with stable isotope techniques (δ15N and δ18O in nitrate). The downstream final segment of the river displays an enriched nitrate stable isotope composition but calculations suggest a N removal corresponding to at most 20% of the unaccounted for N amount. Denitrification was also evaluated in riverine wetlands with the isotope pairing technique. Areal rates are elevated but overall N removal is low (about 1% of the missing N amount), due to small wetland surfaces and limited lateral connectivity. The secondary drainage channel network has a much higher potential for nitrogen removal via denitrification, due to its great linear development, estimated in over 12 500 km, and its capillary distribution in the watershed. In particular, we estimated a maximum N loss up to 8500 t N yr−1, which represents up to 33% of the unaccounted for N amount in the basin. Overall, denitrification in surface aquatic habitats within this basin can be responsible for the permanent removal of about 12 000 t N yr−1; but the fate of some 14 000 t remains unknown. Available data on nitrate concentration in wells suggest that in the central part of the watershed groundwater accumulates nitrogen. Simultaneously, we provide evidences that part of the stored nitrate can be substantially recycled via springs and can pollute surface waters via river-groundwater interactions. This probably explains the ten fold increase of nitrate concentration in a reach of the Oglio River where no point pollutions sources are present.

Faecal Indicator Bacteria: Groundwater Dynamics and Transport Following Precipitation and River Water Infiltration

Faecal contamination of drinking water extracted from alluvial aquifers can lead to severe problems. River water infiltration can be a hazard for extraction wells located nearby, especially during high discharge events. The high dimensionality of river–groundwater interaction and the many factors affecting bacterial survival and transport in groundwater make a simple assessment of actual water quality difficult. The identification of proxy indicators for river water infiltration and bacterial contamination is an important step in managing groundwater resources and hazard assessment. The time resolution of microbial monitoring studies is often too low to establish this relationship. A proxy-based approach in such highly dynamic systems requires in-depth knowledge of the relationship between the variable of interest, e.g. river water infiltration, and its proxy indicator. In this study, continuously recorded physico-chemical parameters (temperature, electrical conductivity, turbidity, spectral absorption coefficient, particle density) were compared to the counts for faecal indicator bacteria, Escherichia coli and Enterococcus sp. obtained from intermittent sampling. Sampling for faecal indicator bacteria was conducted on two temporal scales: (a) routine bi-weekly monitoring over a month and (b) intense (bi-hourly) event-based sampling over 3 days triggered by a high discharge event. Both sampling set-ups showed that the highest bacterial concentrations occurred in the river. E. coli and Enterococcus sp. concentrations decreased with time and length of flow path in the aquifer. The event-based sampling was able to demonstrate differences in bacterial removal between clusters of observation wells linked to aquifer composition. Although no individual proxy indicator for bacterial contamination could be established, it was shown that a combined approach based on time-series of physico-chemical parameters could be used to assess river water infiltration as a hazard for drinking water quality management.

Improving the representation of river–groundwater interactions in land surface modeling at the regional scale: Observational evidence and parameterization applied in the Community Land Model

Summary Groundwater is an important component of the hydrological cycle, included in many land surface models to provide a lower boundary condition for soil moisture, which in turn plays a key role in the land–vegetation–atmosphere interactions and the ecosystem dynamics. In regional-scale climate applications land surface models (LSMs) are commonly coupled to atmospheric models to close the surface energy, mass and carbon balance. LSMs in these applications are used to resolve the momentum, heat, water and carbon vertical fluxes, accounting for the effect of vegetation, soil type and other surface parameters, while lack of adequate resolution prevents using them to resolve horizontal sub-grid processes. Specifically, LSMs resolve the large-scale runoff production associated with infiltration excess and sub-grid groundwater convergence, but they neglect the effect from loosing streams to groundwater. Through the analysis of observed data of soil moisture obtained from the Oklahoma Mesoscale Network stations and land surface temperature derived from MODIS we provide evidence that the regional scale soil moisture and surface temperature patterns are affected by the rivers. This is demonstrated on the basis of simulations from a land surface model (i.e., Community Land Model – CLM, version 3.5). We show that the model cannot reproduce the features of the observed soil moisture and temperature spatial patterns that are related to the underlying mechanism of reinfiltration of river water to groundwater. Therefore, we implement a simple parameterization of this process in CLM showing the ability to reproduce the soil moisture and surface temperature spatial variabilities that relate to the river distribution at regional scale. The CLM with this new parameterization is used to evaluate impacts of the improved representation of river–groundwater interactions on the simulated water cycle parameters and the surface energy budget at the regional scale.

Towards improved instrumentation for assessing river-groundwater interactions in a restored river corridor

Abstract. River restoration projects have been launched over the last two decades to improve the ecological status and water quality of regulated rivers. As most restored rivers are not monitored at all, it is difficult to predict consequences of restoration projects or analyze why restorations fail or are successful. It is thus necessary to implement efficient field assessment strategies, for example by employing sensor networks that continuously measure physical parameters at high spatial and temporal resolution. This paper focuses on the design and implementation of an instrumentation strategy for monitoring changes in bank filtration, hydrological connectivity, groundwater travel time and quality due to river restoration. We specifically designed and instrumented a network of monitoring wells at the Thur River (NE Switzerland), which is partly restored and has been mainly channelized for more than 100 years. Our results show that bank filtration – especially in a restored section with alternating riverbed morphology – is variable in time and space. Consequently, our monitoring network has been adapted in response to that variability. Although not available at our test site, we consider long-term measurements – ideally initiated before and continued after restoration – as a fundamental step towards predicting consequences of river restoration for groundwater quality. As a result, process-based models could be adapted and evaluated using these types of high-resolution data sets.