Christian Schmidt

Helmholtz Centre for Environmental Research | UFZ · Department of Hydrogeology

Spatiotemporal distribution patterns of microplastic (MP) particles in lakes hinge on both the physical conditions in the lake and particle properties. Using numerical simulations, we systematically investigated the influence of lake depth and bathymetry, wind and temperature conditions, MP particle release location and timing, as well as particle...

Riparian zones are known to modulate water quality in stream corridors. They can act as buffers for groundwater-borne solutes before they enter the stream at harmful, high concentrations or facilitate solute turnover and attenuation in zones where stream water (SW) and groundwater (GW) mix. This natural attenuation capacity is strongly controlled b...

Microplastic (MP) particles are commonly found in freshwater environments such as rivers and lakes, negatively affecting aquatic organisms and potentially causing water quality issues. Understanding the transport and fate of MP particles in these environments is a key prerequisite to mitigate the problem. For standing water bodies (lakes, ponds) th...

Riparian zones are known to modulate water quality in stream-corridors. They can act as buffers for groundwater borne solutes before they enter the stream at harmful, high concentrations, or facilitate solute turnover and attenuation in zones where stream water (SW) and groundwater (GW) mix. This natural attenuation capacity is strongly controlled...

Riparian zones are highly‐dynamic transition zones between surface water (SW) and groundwater (GW) and function as key biogeochemical‐reactors for solutes transitioning between both compartments. Infiltration of SW rich in dissolved oxygen (DO) into the riparian aquifer can supress removal processes of redox sensitive compounds like NO3‐, a nutrien...

Rivers play a major role in transport of plastic debris from inland sources like urban areas into the marine environment. The present study examines plastic particle concentrations and loads (> 500 µm) upstream and downstream of an urban subcatchment over 15 months and investigated the relationship between river water discharge (Q) and plastic conc...

Stream-groundwater mixing zones are well known for their role in facilitating ecosystem metabolism which also results in enhanced water quality (e.g. by denitrification). However, due to their highly dynamic biogeophysical characteristics (i.e. temperature, flow directions, residence times), a simple and general quantification of the reactivity pot...

Gravel bars contribute to carbon dioxide (CO2) emissions from stream corridors, with CO2 concentrations and emissions dependent on prevailing hydraulic, biochemical and physicochemical conditions. We investigated CO2 concentrations and fluxes across a gravel bar (GB) in a pre‐alpine stream over three different discharge‐temperature conditions. By c...

A systematic understanding of hyporheic flux (HF) and residence times (RT) is important as they are a major control of biogeochemical processing in streambeds. Previous studies addressing the effect of heterogeneity in streambed hydraulic conductivity (K) on HF and RT have come to deviating conclusions depending on the specific study design and the...

Besides gas-water-exchange in surface waters, respiratory consumption of dissolved oxygen (DO) in adjacent riparian groundwater may trigger the addition of so far hardly explored sources from the unsaturated zone. These processes also systematically influence stable isotope ratios of DO and were investigated together with Cl− as a conservative trac...

The biogeochemical composition of stream water and the surrounding riparian water is mainly defined by the exchange of water and solutes between the stream and the riparian zone. Short term fluctuations in near stream hydraulic head gradients (e.g. during stream flow events) can significantly influence the extent and rate of exchange processes. In...

Rivers in climatic zones characterized by dry and wet seasons often experience periodic transitions between losing and gaining conditions across the river-aquifer continuum. Infiltration shifts can stimulate hyporheic microbial biomass growth and cycling of riverine carbon and nitrogen leading to major exports of biogenic CO2 and N2 to rivers. In t...

Streams are significant sources of CO2 to the atmosphere. Estimates of CO2 evasion fluxes (fCO2) from streams typically relate to the free flowing water but exclude geomorphological structures within the stream corridor. We found that gravel bars (GBs) are important sources of CO2 to the atmosphere, with on average more than twice as high fCO2 as t...

A substantial fraction of marine plastic debris originates from land-based sources and rivers potentially act as a major transport pathway for all sizes of plastic debris. We analyzed a global compilation of data on plastic debris in the water column across a wide range of river sizes. Plastic debris loads, both microplastic (particles <5 mm) and m...

Flow patterns in conjunction with seasonal and diurnal temperature variations control ecological and biogeochemical conditions in hyporheic sediments. In particular, hyporheic temperatures have a great impact on many temperature-sensitive microbial processes. In this study, we used 3-D coupled water flow and heat transport simulations applying the...

Dissolved oxygen (DO) is one of the most commonly measured parameters in aqueous studies, however its sources and sinks such as atmospheric exchange, photosynthesis or respiration often remain unknown. A large number of studies have combined concentration measurements with investigations of stable isotope ratios (18O/16O and 17O/16O) of DO that are...

Optical sensing technologies provide opportunities for in situ oxygen sensing capable of capturing the whole range of spatial and temporal variability. We developed a miniaturized Distributed Oxygen Sensor (‘mDOS’) specifically for long-term in situ application in soil and sediment. The mDOS sensor system enables the unattended, repeated acquisitio...

Dissolved oxygen (DO) is one of the strongest oxidation agents in aquatic environments. Besides gas-water- exchange, mixing and mineral oxidation, it is a key player in fundamental biogeochemical processes such as respiration and photosynthesis. These processes also systematically influence stable isotope ratios of DO and of dissolved inorganic car...

The application of heat as a hydrological tracer has become a standard method for quantifying water fluxes between groundwater and surface water. Typically, time series of temperatures in the surface water and in the sediment are observed and are subsequently evaluated by a vertical 1D representation of heat transport by advection and dispersion. S...

Flow pattern and seasonal as well as diurnal temperature variations control ecological and biogeochemical condi-tions in hyporheic sediments. In particular, hyporheic temperatures have a great impact on many microbial pro-cesses. In this study we used 3-D coupled water flow and heat transport simulations applying the HydroGeoSpherecode in combinati...

The application of heat as a hydrological tracer has become a standard method for quantifying water fluxes between groundwater and surface water. The typical application is to estimate vertical water fluxes in the shallow subsurface beneath streams or lakes. For this purpose, time series of temperatures in the surface water and in the sediment are...

The movement of water, matter, organisms, and energy can be altered substantially at ecohydrological interfaces, the dynamic transition zones that often develop within ecotones or boundaries between adjacent ecosystems. Interdisciplinary research over the last two decades has indicated that ecohydrological interfaces are often “hotspots” of ecologi...

Intermittent streams are increasingly recognized as a factor for underestimating potential CO2 emissions of aquatic ecosystems, because they are neglected during their dry phase. This can be partly attributed to poor understanding of dissolved organic matter (DOM) processing at highly reactive interfaces such as the hyporheic zone (HZ). Here, hydro...

Bed form-induced hyporheic exchange flux (qH) is increasingly viewed as a key process controlling water fluxes and biogeochemical processes in river networks. Despite the fact that streambeds are inherently heterogeneous, the majority of bed form flume scale studies were done on homogeneous systems. We conducted salt and dye tracer experiments to s...

We introduce LPMLE3, a new 1D approach to quantify vertical water flow components at streambeds using temperature data collected in different depths. LPMLE3 solves the partial differential equation for coupled water flow and heat transport in the frequency domain. Unlike other 1D approaches it does not assume a semi-infinite halfspace with the loca...

Aerobic respiration is an important component of in-stream metabolism. The larger part occurs in the streambed, where it is difficult to directly determine actual respiration rates. Existing methods for determining respiration are based on indirect estimates from whole-stream metabolism or provide time invariant results estimated from oxygen consum...

Dissolved oxygen concentrations are amongst the most fundamental and frequently measured parameters in aqueous sciences including fresh-, marine-, waste-, and industrial water systems. New knowledge about dissolved oxygen concentration changes in time and space is critically important for most aqueous processes. One key reason for this need is that...

The analytical evaluation of diurnal temperature variation in riverbed sediments provides detailed information on exchange fluxes between rivers and groundwater. The underlying assumption of the stationary, one-dimensional vertical flow field is frequently violated in natural systems where subsurface water flow often has a significant horizontal co...

Improved understanding of stream solute transport requires meaningful comparison of processes across a wide range of discharge conditions and spatial scales. At reach scales where solute tracer tests are commonly used to assess transport behavior, such comparison is still confounded due to the challenge of separating dispersive and transient storag...

Bioclogging in rivers can detrimentally impact aquifer recharge. This is particularly so in dry regions, where losing rivers are common, and where disconnection between surface water and groundwater (leading to the development of an unsaturated zone) can occur. Reduction in riverbed permeability due to biomass growth is a time-variable parameter th...

The hyporheic zone is the site of intensive biogeochemical cycling in streams, however, the controls on spatio-temporal variability in hyporheic processing are largely unknown. Here, the distribution of dissolved oxygen (DO) is of particular interest since it serves as primary indicator of redox and interlinked biogeochemical zonation. Continuous m...

Improving predictive capabilities of solute transport through stream systems requires meaningful comparison of dominant transport controls across different discharge conditions and spatial scales. While in-stream tracer tests are commonly used to assess transport behavior at reach scales (e.g., transient storage and dispersion), a main challenge is...

1. Das Lückensystem der Sohlsedimente und Uferbereiche von Fließgewässern wird als das hyporheische Interstitial bezeichnet. Es steht in einem hydrologischen und stofflichen Austausch zum Oberflächenwasser und häufig auch zum anliegenden Grundwasser. Es ist die Übergangszone zwischen Fließgewässern und Grundwasser, die den Austausch zwischen beiden...

1. Das Lückensystem der Sohlsedimente und Uferbereiche von Fließgewässern wird als das hyporheische Interstitial bezeichnet. Es steht in einem hydrologischen und stofflichen Austausch zum Oberflächenwasser und häufig auch zum anliegenden Grundwasser. Es ist die Übergangszone zwischen Fließgewässern und Grundwasser, die den Austausch zwischen beiden...

Transient storage zones can be locations of intensive biogeochemical processing in streams, enhancing reach-scale nutrient uptake and metabolism. Despite this, the relationship between stream morphology, solute transport, nutrient cycling, and ecosystem functioning remains largely unresolved. We investigate the influence of stream channel morpholog...

Hyporheic exchange transports solutes into the subsurface where they can undergo biogeochemical transformations, affecting fluvial water quality and ecology. A three-dimensional numerical model of a natural in-stream gravel bar (20 m x 6 m) is presented. Multiple steady state stream flow is simulated with a computational fluid dynamics code that is...

The use of temperature-time series measured in streambed sediments as input to coupled water flow and heat transport models has become standard when quantifying vertical groundwater-surface water exchange fluxes. We develop a novel methodology, called LPML, to estimate the parameters for 1D water flow and heat transport by combining a local polynom...

Heat is increasingly used as a natural tracer to quantify water fluxes at the groundwater-surface water-interface. We present a systematic approach to monitor and evaluate stream and streambed temperatures to derive daily-updated temperature-based water exchange fluxes between the stream and the streambed. Specifically designed multi-level temperat...

At the interface between stream water, groundwater and the hyporheic zone (HZ), important biogeochemical processes occur, that play a crucial role in fluvial ecology. Solutes that infiltrate into the HZ can react with each other and possibly also with upwelling solutes from the groundwater. In this study, we systematically evaluate how variations o...

Determining oxygen consumption (respiration) rates is important for characterizing the ecological functioning of a stream. It is known, that respiration is strongly temperature dependent, but the variability over time and the effects of changing hydrologic conditions are still scarce. Existing respiration measuring methods mostly utilize ex situ re...

While there has been substantial improvement of understanding hyporheic exchange flow and residence time controls on biogeochemical turnover rates, there is little knowledge of the actual drivers of the spatial and temporal variability of interlinked biogeochemical cycles. Previous research has mainly focused on bedform controlled hyporheic exchang...

The hyporheic zone is the site of intensive biogeochemical cycling in streams. However, the controls on spatio-temporal variability in hyporheic processing, and the impact of this hyporheic processing on reach-scale processing, are largely unknown. We aimed to evaluate spatial variability in hyporheic respiration along an upland river over the cour...

Hyporheic zones (HZ) are typically characterized by steep gradients of dissolved oxygen and nutrients which form a highly reactive biogeochemical zone. Under the complex conditions of natural environments, the existing flow field is difficult to characterize, and temporal changes of the hydrological conditions induce variable flow paths and travel...

Exchange of water and solutes across the stream-sediment interface is an important control for biogeochemical transformations in the hyporheic zone (HZ). In this paper, we investigate the interplay between turbulent stream flow and HZ flow in pool-riffle streams under various ambient groundwater flow conditions. Streambed pressures, derived from a...

One of the key parameters controlling biogeochemical reactions in aquatic sediments like streambeds is the distribution of dissolved oxygen. We present a novel approach for the in situ measurement of vertical oxygen profiles using a planar luminescence-based optical sensor. The instrument consists of a transparent acrylic tube with the oxygen-sensi...

Micro‐organisms are known to degrade a wide range of toxic substances. How the environment shapes microbial communities in polluted ecosystems and thus influences degradation capabilities is not yet fully understood. In this study, we investigated microbial communities in a highly complex environment: the capillary fringe and subjacent sediments in...

In this study we used the deterministic, fully-integrated surface-subsurface flow and heat transport model (HydroGeoSphere) to investigate the spatial and temporal variability of surface water-groundwater (SFW-GW) interaction along a lowland river reach. The model incorporates the hydrological as well as the heat transport processes including (1) r...

Spatial patterns of water flux in the stream bed are controlled by the distribution of hydraulic conductivity, bedform-induced head gradients and the connectivity to the adjoining groundwater system. The water fluxes vary over time driven by short-term flood events or seasonal variations in stream flow and groundwater level. Variations of electrica...

Magnitudes and directions of water flux in the streambed are controlled by hydraulic gradients between the groundwater and the stream and by bedform-induced hyporheic exchange flows. These water fluxes vary over time driven by for instance by short term flood events or seasonal variations in stream flow and groundwater level. Variations of electric...

On of the key parameters, controlling biogeochemical reactions in the hyporheic zone (HZ) is the distribution of oxygen. A reliable measurement of the vertical oxygen distribution is an important tool to understand the dynamic fluctuations of the aerobic zone within the HZ. With repeated measurements of continuous profiles, mixing of surface water...

The interaction between stream water, groundwater and the hyporheic zone is important for many hydrogeological processes and solute transport. In the hyporheic zone, where stream water mixes with groundwater, biogeochemical reactions and transformations occur and are responsible for self-cleaning mechanisms of stream systems. The extent of the hyp...

Observing flow pattern and directions in the streambed is a challenging yet important task to understand many hyporheic zone processes. Our approach is tracing a salt (NaCl) plume with geoelectrical methods. The salt solution was injected into the sediment at the upstream end of the gravel bar and its temporal and spatial spreading was detected by...

Magnitudes and directions of water flux in the streambed are controlled by hydraulic gradients between the aquifer and the stream and by bedforms which induce hyporheic exchange flows. Varying hydrologic conditions such as short term flood events or seasonal variations in groundwater flow typically change the flow regime in the streambed. Consequen...

Quantification of subsurface water fluxes based on the one dimensional solution to the heat transport equation depends on the accuracy of measured subsurface tem-peratures. The influence of temperature probe setup on the accuracy of vertical water flux calculation was systematically evaluated in this experimental study. Four temper-5 ature probe se...

Population growth coupled with industrialization, increasing effects of climate change, and increasingly stringent water management regulations regarding the conservation of aquatic life are resulting in previously unknown agricultural water shortages in Padana Valley, Italy. To mitigate water shortage, it was recently proposed to use the water sto...

Quantification of subsurface water fluxes based on the one dimensional solution to the heat transport equation depends on the accuracy of measured subsurface temperatures. The influence of temperature probe setup on the accuracy of vertical water flux calculation was systematically evaluated in this experimental study. Four temperature probe setups...

Significant natural attenuation may occur on the passage of groundwater plumes through streambed sediments because of the transition from anaerobic to aerobic conditions and an increased microbial activity. Varying directions and magnitudes of water flow in the streambed may enhance or inhibit the supply of oxygen to the streambed and thus influenc...

Significant natural attenuation of organic pollutants may occur in streambed sediments because of the transition from anaerobic to aerobic conditions and an increased microbial activity. Alternating directions and magnitudes of water flow in the streambed may enhance or inhibit the supply of oxygen to the streambed and thus influence the redox zona...

In industrialized countries the majority of streams and rivers have been subject to changes in the hydrological regime and alteration of the channel morphology. Urban streams are typically characterized by ``flashier’’ hydrographs as a result of more direct runoff from impervious surfaces. Channel structure and complexity are often impaired compare...

Analytical solutions to the one-dimensional heat transport equation for steady-state conditions can provide simple means to quantify groundwater surface water exchange. The errors in exchange flux calculations that are introduced when the underlying assumptions of homogeneous sediments and constant temperature boundary conditions are violated were...

The transition zone between surface waters like streams and rivers and the adjacent aquifers is a zone of paramount ecological importance. Due to the complexity of exchange processes, their temporal variability, and the spatial heterogeneity of the ecotone, the understanding and ability to modell the hydrodynamic, hydrochemical, and (micro-)biologi...

The spatial distribution of groundwater fluxes through a streambed can be highly variable, most often resulting from a heterogeneous distribution of aquifer and streambed permeabilities along the flow pathways. Using a groundwater flow and heat transport model, we defined four scenarios of aquifer and streambed permeability distributions to simulat...

Groundwater discharge to a stream may show small-scale heterogeneities caused by the structure of the connected aquifer. The spatial pattern of the groundwater discharge can be investigated by temperature measurements in the streambed. Thus, the heterogeneity of the hydraulic conductivity (K) of an aquifer can be inferred from measured streambed te...

As a result of intensive industrial, mining, and urban development, numerous large-scale contaminated areas exist in Germany. These so-called megasites represent a challenge to risk assessment and remediation strategies. At the Bitterfeld megasite, the contaminated groundwater interacts with the local streams. Along a stream reach 280 m long, the m...

Streambed sediments can act as long-term storage zones for organic contaminants originating from the stream water. Until the early 1990s, the small man-made stream, subject of our study, in the industrial area of Bitterfeld (Germany), was used for waste water discharge from the chemical industry nearby. The occurrence of contaminants in the streamb...

More than a century of mining and industrial activities has resulted in large-scale groundwater contamination at the mega-site Bitterfeld, Germany. Contaminated groundwater is discharging to the local streams and poses a long-term threat to the stream ecosystems. A small man-made stream was investigated in detail to determine water and contaminant...

Leaky sewers affect urban groundwater by the exfiltration of untreated wastewater. However, the impact of sewer exfiltration on the groundwater is poorly understood. Most studies on sewer exfiltration focus on water exfiltration, but not on the impact on groundwater quality. In this paper we present a new monitoring approach to estimate mass flow r...

A method for calculating groundwater discharge through a streambed on a sub-reach to a reach scale has been developed using data from plan-view mapping of streambed temperatures at a uniform depth along a reach of a river or stream. An analytical solution of the one-dimensional steady-state heat-diffusion–advection equation was used to determine fl...

The groundwater discharge to a stream may show small-scale heterogeneities caused by the structure of the connected aquifer. Traditional subsurface investigation techniques are often not capable of providing data in sufficient resolution to capture these small-scale variations in aquifer properties. In the streambed, the spatial pattern of the grou...

Streambed sediments can act as long-term storage zones for organic contaminants originating from the stream water. Although contamination levels in German rivers have generally declined in recent years, streambed sediments might still be considerably contaminated. Groundwater discharge through the sediments can induce an advective contaminant trans...

Several local groundwater studies within the EU project AquaTerra in the Basins of the Meuse, Elbe point at significant influences of groundwater on surface water, while the Brévilles Catchment shows a distinct problematic of pesticide loading to groundwater. Further modeling studies are currently being developed. In the Danube Basin no specific gr...

Several local groundwater studies within the EU project AquaTerra in the Basins of the Meuse, Elbe point at significant influences of groundwater on surface water, while the Brévilles Catchment shows a distinct problematic of pesticide loading to groundwater. Further modeling studies are currently being developed. In the Danube Basin no specific gr...

The Bitterfeld/Wolfen region is a megasite with multiple contaminant sources from more than a century of industrial activity, which have a considerable impact on the environment. At present, the contaminated groundwater covers an area of about 25km(2) and poses a threat for the surrounding aquifers and the Mulde River. This study focuses on the Sch...

The spatial pattern and magnitude of mass fluxes at the stream-aquifer interface have important implications for the fate and transport of contaminants in river basins. Integral pumping tests were performed to quantify average concentrations of chlorinated benzenes in an unconfined aquifer partially penetrated by a stream. Four pumping wells were o...

Streambed temperatures can be easily, accurately and inexpensively measured at many locations. To characterize patterns of groundwater-stream water interaction with a high spatial resolution, we measured 140 vertical streambed temperature profiles along a 220 m section of a small man-made stream. Groundwater temperature at a sufficient depth remain...