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Summary A multi-layered aquifer system (eastern Po plain, northern Italy) was investigated by means of isotopic data, with the goal of quantifying groundwater recharge from different sources and assessing the intrinsic vulnerability of aquifers to surface sources of contamination. The geology of the area is based on a stratigraphic alternation of several sandy aquifers and silty–clayey aquitards, down to a maximum depth of 200 m b.g.s. Water isotopes (δ 18 O, δ 2 H), and hydrochemical analyses were performed on groundwater samples collected from a regional network of 70 boreholes distributed on an area of almost 2000 km 2 . In addition to the regional sampling, detailed vertical isotopic profiling was performed in one location by means of groundwater and sediment samples collected through the whole sequence of aquifers and aquitards. Water isotopes indicated mixing from different sources of recharge (i.e., vertical recharge, Po river, deeper aquifers). Mixing calculations were used to quantify the contributions to the aquifers from the different sources. The vertical profiling allowed for integrating and validating the interpretations at a regional scale. The recharge pattern defined for the different aquifers was translated into an index of hydrogeologic interconnections with the surface, which represents a physically based proxy of the intrinsic vulnerability of the aquifers to surficial sources of contamination. The investigated setting can be considered to be representative of many other anthropized and groundwater demanding plain settings around the world. Thus, the proposed method represents a valuable approach for such settings both for recharge quantification (e.g., to be used as input for numerical modeling) and for a physically based assessment of the intrinsic vulnerability.

Anaerobic microbial degradation of hydrocarbons, typically occurring at the oil-water transition zone, influences the quality of oil reservoirs. In Pitch Lake, Trinidad and Tobago--the world's largest asphalt lake--we found that microorganisms are metabolically active in minuscule water droplets (1 to 3 microliters) entrapped in oil. Pyrotag sequencing of individual droplet microbiomes revealed complex methanogenic microbial communities actively degrading the oil into a diverse range of metabolites, as shown by nuclear magnetic resonance and Fourier transform ion cyclotron resonance mass spectrometry. High salinity and water-stable isotopes of the droplets indicate a deep subsurface origin. The 13.5% water content and the large surface area of the droplets represent an underestimated potential for biodegradation of oil away from the oil-water transition zone.

Lagoons are important ecosystems occupying large coastal areas worldwide. Lagoons contain various mixtures of marine and freshwater sources which are highly dynamic in time. However, it often remains a challenge to identify and quantify dynamic changes of water sources, particularly in heterogeneous lagoon systems like the Köycegiz-Dalyan Lagoon (KDL), which is located at the southwest of Turkey on the Mediterranean Sea coast. The objective of this study was to quantify different contributions of potential water sources i.e. surface water, groundwater and seawater in the lagoon and how these water sources changed over time and space. In the wet and dry season stable isotopes of water, chloride concentration (Cl-) and salinity were measured in two depths in the lagoon and surrounding water bodies (sea, lake, groundwater). Different components of water sources were quantified with a three component endmember mixing analysis. Differences in Cl- and stable isotopes over time indicated the dynamic behaviour of the system. Generally, none of the groundwater samples was impacted by water of the Mediterranean Sea. During the wet season, most of the lagoon water (>95%) was influenced by freshwater and vertically well mixed. During the dry season, high Cl- in the deeper sampling locations indicated a high contribution of marine water throughout the entire lagoon system due to salt water intrusion. However, a distinct layering in the lagoon was obvious from low Cl- and depleted isotope contents close to the surface supporting freshwater inflow into the system even during the dry season. Besides temporal dynamics also spatial heterogeneities were identified. Changes in water sources were most evident in the main lagoon channel compared to more isolate lagoon lakes, which were influenced by marine water even in the wet season, and compared to side branches indicating slower turnover times. We found that environmental tracers helped to quantify contributions of different water sources in the Köycegiz-Dalyan Lagoon which is a highly dynamic and heterogeneous groundwater dependent ecosystem.

Summary The Diass horst aquifer system located 50 km east of Dakar (Senegal) is exploited in two main aquifers covered by a sandy superficial aquifer: the confined/unconfined Palaeocene karstic limestone and the confined Maastrichtian sandstone aquifer underneath. This system has experienced intensive groundwater abstraction during the last 50 years to supply increasing water demand, agricultural and industrial needs. The high abstraction rate from 1989 to 2009 (about 109,000 m 3 /d) has caused a continuous groundwater level decline (up to 30 m), a modification of the groundwater flow and salinization in parts of the aquifers. The objective of the study is to improve our understanding of the system functioning with regards to high pumping, identify the geochemical reactions that take place in the system, infer origin and timing of recharge by using mainly stable (δ 18 O, δ 2 H, 13 C) and radioactive ( 3 H and 14 C) isotopes. Water types defined in the Piper diagram vary in order of abundance from Ca–HCO 3 (65%), Ca/Na–Cl (20%), Na–HCO 3 (3%) and Na–Cl (12%). Values of δ 18 O and δ 2 H for the superficial aquifer range between −5.8 and −4.2‰ and between −42 and −31‰, respectively. For the Palaeocene aquifer they range from −5.8 to −5.0‰ and from −38 to −31‰, respectively; values in the Maastrichtian aquifer are between −5.9 and −4.3‰ for δ 18 O and −38 to −26‰ for δ 2 H. Plotted against the conventional δ 18 O vs δ 2 H diagram, data from the upper aquifer exhibit a dispersed distribution with respect to isotopic fractionation while those of the Palaeocene and Maastrichtian aquifers are aligned parallel and slightly below/or on the Global Meteoric Water Line (GMWL) evidencing ancient waters which had evaporated during infiltration. The low tritium (generally 14 C (0.7–57.2 pmc) contents indicate predominance of older water being recharged during the Pleistocene and Holocene periods. However, few boreholes which exhibit high tritium (1.2–4.3 TU) and 14 C (65.7–70.8 pmc) values indicate some mixture with recent water likely through faulting and vertical drainage from the upper to deeper aquifers as well as lateral flow along flow paths to the piezometric depressions created by pumping.

Microbial life thrives in an oily bubble Microorganisms can break down hydrocarbons in oil reservoirs. Microbes grow primarily at the interface between oil and water, where they can find nutrients and dispose of metabolites. Meckenstock et al. now show that tiny water droplets can also provide a suitable home for hydrocarbon-degrading microorganisms. The authors examined oil from Pitch Lake, Trinidad and Tobago, and found that diverse microorganisms thrived in these tiny isolated microhabitats. Science , this issue p. 673

The abstraction of groundwater is a global phenomenon that directly threatens groundwater ecosystems. Despite the global significance of this issue, the impact of groundwater abstraction and the lowering of groundwater tables on biota is poorly known. The aim of this study is to determine the impacts of groundwater drawdown in unconfined aquifers on the distribution of fauna close to the water table, and the tolerance of groundwater fauna to sediment drying once water levels have declined. A series of column experiments were conducted to investigate the depth distribution of different stygofauna (Syncarida and Copepoda) under saturated conditions and after fast and slow water table declines. Further, the survival of stygofauna under conditions of reduced sediment water content was tested. The distribution and response of stygofauna to water drawdown was taxon specific, but with the common response of some fauna being stranded by water level decline. So too, the survival of stygofauna under different levels of sediment saturation was variable. Syncarida were better able to tolerate drying conditions than the Copepoda, but mortality of all groups increased with decreasing sediment water content. The results of this work provide new understanding of the response of fauna to water table drawdown. Such improved understanding is necessary for sustainable use of groundwater, and allows for targeted strategies to better manage groundwater abstraction and maintain groundwater biodiversity.

Land cover and agricultural management pracƟ ces can signifi cantly infl uence soil structure. However, liƩ le is known about how ferƟ lizer applicaƟ ons and land cover aff ect soil hydrology and groundwater recharge over long Ɵ me periods. The objecƟ ve of this study was to use stable water isotopes as environmental tracers to provide addiƟ onal informaƟ on required for beƩ er understanding of water fl ow and solute transport processes in the unsaturated zone infl uenced by land cover and type of ferƟ lizer applicaƟ ons. Five lysimeters containing undisturbed soil monoliths from the same agricultural fi eld site were invesƟ gated over a period of 5 yr. Liquid caƩ le slurry and solid animal manure were applied to the maize (Zea mays L.) and winter rye (Secale cereale L.) lysimeters. The grass–clover lysimeter was treated with mineral ferƟ lizer. QuanƟ taƟ ve infl uence of land cover and type of ferƟ lizer applicaƟ on on water fl ow and solute transport was evaluated for all lysimeters using a modifi ed version of HYDRUS-1D. The highest drainage was observed in the maize lysimeter treated with caƩ le slurry, and the lowest in the grass lysimeter treated with mineral ferƟ lizer. Pronounced diff erences in water contents and esƟ mated saturated hydraulic conducƟ viƟ es between the lysimeters were restricted to the upper 25 to 30 cm of the soil. In parƟ cular, the lysimeters treated with animal manure had higher porosiƟ es, indicaƟ ng a higher content of organic maƩ er. Main diff erences in discharge between the lysimeters were observed in spring and during the plant growth periods, indicaƟ ng the importance of nonuniform, patchy infi ltraƟ on paƩ erns during snow melt and of root water uptake, respecƟ vely. Mean water fl ow velociƟ es, transit Ɵ mes, and eff ecƟ ve water contents were esƟ mated from the stable water isotope data, providing evidence on the impact of land cover and type of ferƟ lizer applicaƟ on. We found smaller mean transit Ɵ mes in the maize lysimeters and for soils with liquid caƩ le slurry applicaƟ ons. SimulaƟ ons indicate that numerical modeling can reproduce the general trend of water fl ow and isotope transport. Despite diff erences in mean transit Ɵ mes, fi Ʃ ed dispersiviƟ es were all in the same range, suggesƟ ng similar soil structures in the fi ve lysimeters. However, more data for calibraƟ on and more informaƟ on about heterogeneous infi ltraƟ on would be required to improve the model accuracy. In general, stable water isotopes clearly contributed an added value, elucidaƟ ng diff erences in mean fl ow parameters between the lysimeters. Thus, they provided evidence of the impacts of land cover and ferƟ lizer applicaƟ ons, which are not obvious from water balance and mean discharge rates alone.

The impact of bacteria-solid and bacteria-bacteria interactions on the transport of Klebsiella oxytoca, Burkholderia cepacia G4PR1, and Pseudomonas sp. #5 was investigated in saturated sand column experiments (L = 114 mm; o = 33 mm) under constant water velocities (∼ 5 cm · h(-1)). Bacterial strains were injected into the columns as pulses either individually, simultaneously, or successively. A one-dimensional mathematical model for advective-dispersive transport and for irreversible and reversible bacterial kinetic sorption was used to analyze the bacterial breakthrough curves. Different sorption parameters were obtained for each strain in each of the three experimental setups. In the presence of other bacteria, sorption parameters for B. cepacia G4PR1 remained similar to results from individual experiments, indicating the presence of other bacteria generally had a lesser influence on its migration than for the other bacteria. K. oxytoca is more competitive for the sorption sites when simultaneously injected with the other bacteria. Ps. sp. #5 generally yielded the greatest detachment rates and the least affinity to attach to the sand, indicative of its mobility in groundwater systems. The results of this study clearly indicate both bacteria-solid and bacteria-bacteria interactions influence the migration of bacteria. A more complete understanding of such interactions is necessary to determine potential migration in groundwater systems.

An approach is presented which enables the quantification, in integrative form, of the preferential flow and the heterogeneity in water transport through the unsaturated zone under natural atmospheric conditions. For these purposes, deuterium transport was estimated in seven lysimeters filled with different sediment materials by applying a conceptual model, which consists of preferential and matrix flow paths for each using the lumped parameter approach. The piston flow and dispersion transit time distribution functions were assumed, for preferential and matrix flow paths, respectively. Weekly 2H data measured in precipitation (1984–1991) were taken as the input function. Combining modelling of isotopes and hydrological data enabled the quantification of preferential flow rates. The fraction of preferential flow directly appearing in the outflow within one week varied between 17 and 30%. The fraction of preferential flow was practically independent of the soil texture and flow rates. The crucial parameter influencing the fraction of direct flow was found to be the saturated hydraulic conductivity (Ks). In the matrix flow, it was found that the apparent dispersion parameter (PD)* is indirectly proportional to the mean water content (). This relationship shows that the heterogeneity of the water flux in the matrix is higher for lower water contents. Finally, the transit time distribution functions, determined for both flow paths and the fractions of preferential flow, were used to construct vulnerability diagrams. Such a diagram gives the amounts of tracer mass (conservative pollutant), which appear weekly in the outflow between 0 and 60 weeks. The vulnerability diagrams showed different patterns for different soils, depending on the saturated conductivity and mean water content. Coarser material with low and high Ks showed a short mean transit time for the matrix flow (about 10 weeks) and mean preferential flow equal to or higher than 20%. Finer sand, with lower Ks and higher , resulted in mean transit times of approximately 30 weeks and preferential flow of 17–21%. These diagrams can be used to estimate the vulnerability of groundwater to pollution for different soil materials of the unsaturated zone.

Groundwater table fluctuations create a dynamic interface between the unsaturated and saturated zone with spatially as well as temporally changing water content and water fluxes. Our objective was to investigate the impact of periodic water table fluctuations on transport parameters, geochemical gradients, and bacterial abundance and community structure in groundwater sediments. We conducted tracer experiments in one-dimensional column systems investigating vertical flow with static or fluctuating water tables, constantly infiltrated with groundwater and model dissolved organic carbon (DOC). Pore water sampling was performed throughout the experiments, and sediment bacteria of both systems were characterized after completion of the experiment. Our results indicate that neither the longitudinal dispersivity nor column geochemistry was affected by the applied water table fluctuations. However, water content and flow velocities varied dynamically over time and depth in the fluctuation zone. Depth-resolved bacterial community composition was clearly distinct, with additional populations detected especially in the fluctuation zone. These were absent in the static column, while overall bacterial diversity was similar over depth in both columns. We assume that spatial and temporal changes in water content and/or flow velocities were driving these microbial distinctions. This implies that complex feedback mechanisms between local hydrology and sedimentary bacterial communities in porous media exist. These results are important for the understanding of transformation and biodegradation processes at groundwater tables under increased hydraulic forcing.

Seven experimental lysimeters were filled with different soil materials and used to study water flow and tracer transport through the unsaturated zone under natural atmospheric conditions during an eight year period (1984–1991). The study of soil characteristics and water flow was presented in Part 1. The present paper focuses on environmental deuterium transport. The concentrations of environmental deuterium were measured in both precipitation and the water outflowing from the lysimeters. Two model approaches were used to describe tracer transport. The numerical solution of water flow and two-phase mass transport equations, referred to here as the variable flow dispersion model (VFDM), can exactly simulate spatial and temporal distribution of flow and transport parameters. The fitting parameters of VFDM are the soil characteristics, the longitudinal dispersivity, the fraction of immobile water and the transfer constant between mobile and immobile water. However, the application of the VFDM for d...

Over an eight-year period (1984–1991), seven experimental lysimeters filled with different soil materials from the State of Bavaria (quartz and tertiary sands, quartz and fluvioglacial gravels) were used to study water flow and tracer transport through the unsaturated zone under natural atmospheric conditions. The experiments were performed in the input—output mode using weekly water flow and tracer contents (environmental deuterium) in the precipitation and in the outflow in order to estimate the unsaturated hydraulic and transport parameters of the soils with limited measured data. This paper (Part 1) presents the evaluation of soil hydraulic parameters for tested soils, while the second paper (Part 2) is concerned with the estimation of transport parameters (dispersion, water content and possible zones with stagnant water). The estimation of the soil physical characteristics, i.e. the relationship between water pressure head and water content, performed by applying the van Genuchten model, sho...

Column experiments are often used to study transport properties of sediments to predict the fate of contaminants in the subsurface. However, water flow in column experiments is rarely monitored over the entire length of experiments, despite the known impact of biogeochemical processes on pore structures. To study if flow path changes occur over time, water flow and solute transport were investigated in a homogeneous, nearly saturated column by conducting three tracer experiments over 7 mo. Water flow and transport parameters were determined from tracer breakthrough curves using different model approaches. The initial homogeneous transport was adequately described by a simple advection-dispersion model. However, after some months, the porous medium changed from being uniform to non-uniform. The observed bimodal breakthrough curves were simulated with a simple multi-flow advection-dispersion and a dual-permeability model. Both models supported the development of non-uniform water flow and solute transport due to flow paths changes. Those changes were attributed to clogging of small pores near the column inlet due to microbial growth and calcite precipitation leading to a heterogeneous infiltration front. The results highlight the need to determine transport properties continuously over the course of column experiments to reliably predict parameters for groundwater flow and understand contaminant transport into aquifers.

Project Co-ordinators: Dr. Jose Alfonso Gómez Calero (Instituto de Agricultura Sostenible (IAS-CISC), Dr. Weifeng Xu (Fujian Agriculture and Forest University, FAFU)., Knowledge of crop water requirements and the effects of management practices on the amounts of water used for crop transpiration and that lost through soil evaporation is essential for efficient agricultural water management. Therefore, this study investigated the temporal evolution of weekly evaporation and transpiration rates under varying soil water conditions in a conventionally managed soybean field by partitioning evapotranspiration based on a water and δ18O-stable isotope mass balance. The estimated rates were considered in combination with vertical soil water distribution, atmospheric demand (based on crop evapotranspiration), actual evapotranspiration, and the plant development stage. This allowed for the weekly rates to be compared to the current conditions resulting from dry periods, rain or irrigation events, and the extent of the canopy. The range of weekly transpiration/evapotranspiration, from blossom to maturation, was between 0.60 (±0.11) and 0.82 (±0.10). Within this range, transpiration/evapotranspiration shifted depending on the vertical soil water distribution and meteorological conditions. During dry soil surface periods, evaporation dropped to almost zero, whereas a wet surface layer substantially increased evaporation/evapotranspiration, even under a closed canopy. Under given conditions, the application of a few intense irrigations before the drying of the soil surface is recommended., This work was supported by the European Union’s Horizon 2020 Research and Innovation Programme (Grant number 773903)(2018-2022).

We address transport by transit times (i.e., the age of water parcels leaving a storage as discharge, deep loss, or evapotranspiration) in subvertical soil systems, key to our understanding of water quality in catchments and streams. While the use of field and lysimeter observations to constrain and validate modeling approaches is generally accepted, different views exist on the relative ranges of applicability of spatially integrated or spatially explicit approaches. This study specifically illustrates how one class of spatially integrated models of transport, based on StorAge Selection (SAS) functions, fares with respect to spatially explicit hydrologic models in a case where detailed tracer data from experimental lysimeters exist and for which both approaches are viable. Data from two lysimeters experiments that differ in atmospheric conditions, size of the installation, tracer type, soil texture, and vegetation are used to contrast results from two transport models: tran-SAS (space implicit) and HYDRUS-1D (space explicit). Results suggest that although the two lysimeters are characterized by different transit time distributions, their underlying transport mechanisms are similar and represented well by both models. The comparison between the two models results in robust estimates of the transport timescales and clearly shows that percolation fluxes at the bottom of a lysimeter tend to drain the relatively old components of the soil storage. We conclude that the convergence of the approaches in a geomorphologically simple and data-rich problem supports extensive uses of the spatially integrated approach in cases where the scale of the problem, or subgrid parameterization needs, may limit the applicability of detailed modeling approaches.

Biodegradation is one of the most favored and sustainable means of removing organic pollutants from contaminated aquifers but the major steering factors are still surprisingly poorly understood. Growing evidence questions some of the established concepts for control of biodegradation. Here, we critically discuss classical concepts such as the thermodynamic redox zonation, or the use of steady state transport scenarios for assessing biodegradation rates. Furthermore, we discuss if the absence of specific degrader populations can explain poor biodegradation. We propose updated perspectives on the controls of biodegradation in contaminant plumes. These include the plume fringe concept, transport limitations, and transient conditions as currently underestimated processes affecting biodegradation.