Evaluating the impact of muon-induced cosmogenic 39Ar and 37Ar underground production on groundwater dating with field observations and numerical modeling.

Groundwater dating by radioactive cosmogenic tracers such as 39Ar relies on the decay rate from a known initial atmospheric activity (100%modern). Thereby, it is assumed that cosmogenic 39Ar production in the subsurface is negligible at depths below the water table and that contributions from natural rock radioactivity are minor or missing. Here we present 39Ar data from aquifers located in quaternary glacial sediments and tertiary limestones in Denmark, which unequivocally demonstrate that cosmogenic production can induce considerable age biases. 39Ar values larger than 100%modern are observed at relatively shallow groundwater depths in non-radiogenic rocks. These activities are compared to calculations based on previously assessed depth-dependent production rates in rocks and realistic estimates of the emanated fractions to the water phase. The water residence time distribution with depth, which was determined by numerical flow modeling and particle tracking, underpinned the significance of muon-induced 39Ar production. The short-lived isotope 37Ar is produced by similar processes as 39Ar and demonstrated its usefulness as an indicator of local underground production in an aquifer. The significance of cosmogenic underground production in other possible recharge scenarios was then assessed by explicitly simulating the radioargon accumulation and decay in a 2D synthetical numerical model. These simulations demonstrated that underground production is negligible when the water infiltrates freely in a porous aquifer. However, in the presence of a confining layer impeding the infiltration at shallow depths (<30 m), as is the case in our study site in Denmark for instance, over-modern 39Ar activities (>100%modern) may occur. The age concluded from the dissolved activities is then possibly biased towards young values. Special attention should thus be paid to the recharge rates when using 39Ar for dating groundwater. 37Ar activities provide complementary information about the strength and mechanisms of underground production.

Analysis of the geological control on the spatial distribution of potentially toxic concentrations of As and F- in groundwater on a Pan-European scale.

The distribution of the high concentrations of arsenic (As) and fluoride (F-) in groundwater on a Pan-European scale could be explained by the geological European context (lithology and structural faults). To test this hypothesis, seventeen countries and eighteen geological survey organizations (GSOs) have participated in the dataset. The methodology has used the HydroGeoToxicity (HGT) and the Baseline Concentration (BLC) index. The results prove that most of the waters considered in this study are in good conditions for drinking water consumption, in terms of As and/or F- content. A low proportion of the analysed samples present HGT≥ 1 levels (4% and 7% for As and F-, respectively). The spatial distribution of the highest As and/or F- concentrations (via BLC values) has been analysed using GIS tools. The highest values are identified associated with fissured hard rock outcrops (crystalline rocks) or Cenozoic sedimentary zones, where basement fractures seems to have an obvious control on the distribution of maximum concentrations of these elements in groundwaters.

Degradation potential of MCPA, metolachlor and propiconazole in the hyporheic sediments of an agriculturally impacted river.

Hyporheic sediments are influenced by physical, biological, and chemical processes due to the interactions with river water and has been shown to play an important role in the environmental fate of pesticides. Therefore, this study evaluated the bacterial degradation potential of MCPA, metolachlor and propiconazole in hyporheic sediments sampled along a 20 km long stretch of an agriculturally impacted river dominated primarily by water losing conditions. Water physicochemical parameters in the river and nearby groundwater wells were assessed along with pesticide sorption to sediments and bacterial community composition. Degradation and mineralisation batch experiments were set up from six locations (five water losing, one water gaining) using environmentally relevant concentrations of pesticides (10 µg kg-1). Highly variable DT50 values from 11 to 44 days for MCPA, 11-27 days for metolachlor (MTC) and 60-147 days for propiconazole were calculated based on ~140 day studies. Degradation of MTC led to accumulation of the transformation products MOA and MESA in batch experiments. Noteworthy, MESA was detected in the groundwater wells adjacent to the part of the river impacted by losing conditions suggesting that degradation processes in hyporheic sediments may lead to the formation of transformation products (TP) leaching towards groundwater. Further, from propiconazole was identified a persistent transformation product being different from 1,2,4-triazole. Specific calculated DT50 values could not the linked to bacterial diversity. However, generally all sediment samples were characterised by high bacterial diversity, where approximately 80% of the relative sequence abundances were < 1%, which may increase the likelihood of finding contaminant-degrading genes, thereby explaining the general high contaminant-degrading activity. The studied sediments revealed a high potential to degrade pesticides despite only being exposed to low diffuse pollutant concentrations that is similar to calculated DT50 values in agricultural soils.

History and Sources of Co-Occurring Pesticides in an Abstraction Well Unraveled by Age Distributions of Depth-Specific Groundwater Samples.

When groundwater-based drinking water supply becomes contaminated, the timing and source of contamination are obvious questions. However, contaminants often have diffuse sources and different contaminants may have different sources even in a single groundwater well, making these questions complicated to answer. Age dating of groundwater has been used to reconstruct contaminant travel times to wells; however, critics have highlighted that groundwater flow is often complex with mixing of groundwater of different ages. In drinking water wells, where water is typically abstracted from a large depth interval, such mixing is even more problematic. We present a way to overcome some of the obstacles in identifying the source and age of contaminants in drinking water wells by combining depth-specific sampling with age tracer modeling, particle tracking simulations, geological characterization, and contaminant properties. This multitool approach was applied to a drinking water well, where bentazon and dichlorprop contamination was found to have different pollutant sources and release histories, even though both pesticides can be associated with the same land use. Bentazon was derived from recent application to a golf course, while dichlorprop was derived from agricultural use more than 30 years ago. The advantages, limitations, and pitfalls of the proposed course of action are then further discussed.

Reactive nitrogen in a clay till hill slope field system.

To assess the contribution of reactive nitrogen from groundwater to surface waters, we need more knowledge on how reactive nitrogen behaves in the glacial till systems underlying many agricultural fields. Groundwater sampled from suction cups and piezometers placed in the glacial till underlying a winter wheat field shows the nitrate concentration in water leaching to deeper than 2 m below ground surface (mbg) is ~ 60 mg L-1. Within 5 mbg, all of the nitrate is removed and this appears to take place within a redox zone rather than at a sharp redox front. Ammonium released from the till is negligible. A 2D dataset reveals that the depth to the redox zone undulates between 3 and 5 mbg, perhaps a result of local variations in infiltration. It appears that the nitrate is generally reduced by the oxidation of pyrite and locally by organic matter in lenses within the till.

The Baltic Sea as a time machine for the future coastal ocean.

Coastal global oceans are expected to undergo drastic changes driven by climate change and increasing anthropogenic pressures in coming decades. Predicting specific future conditions and assessing the best management strategies to maintain ecosystem integrity and sustainable resource use are difficult, because of multiple interacting pressures, uncertain projections, and a lack of test cases for management. We argue that the Baltic Sea can serve as a time machine to study consequences and mitigation of future coastal perturbations, due to its unique combination of an early history of multistressor disturbance and ecosystem deterioration and early implementation of cross-border environmental management to address these problems. The Baltic Sea also stands out in providing a strong scientific foundation and accessibility to long-term data series that provide a unique opportunity to assess the efficacy of management actions to address the breakdown of ecosystem functions. Trend reversals such as the return of top predators, recovering fish stocks, and reduced input of nutrient and harmful substances could be achieved only by implementing an international, cooperative governance structure transcending its complex multistate policy setting, with integrated management of watershed and sea. The Baltic Sea also demonstrates how rapidly progressing global pressures, particularly warming of Baltic waters and the surrounding catchment area, can offset the efficacy of current management approaches. This situation calls for management that is (i) conservative to provide a buffer against regionally unmanageable global perturbations, (ii) adaptive to react to new management challenges, and, ultimately, (iii) multisectorial and integrative to address conflicts associated with economic trade-offs.

Aquifer Vulnerability Assessment Based on Sequence Stratigraphic and ³9Ar Transport Modeling.

A large-scale groundwater flow and transport model is developed for a deep-seated (100 to 300 m below ground surface) sedimentary aquifer system. The model is based on a three-dimensional (3D) hydrostratigraphic model, building on a sequence stratigraphic approach. The flow model is calibrated against observations of hydraulic head and stream discharge while the credibility of the transport model is evaluated against measurements of (39)Ar from deep wells using alternative parameterizations of dispersivity and effective porosity. The directly simulated 3D mean age distributions and vertical fluxes are used to visualize the two-dimensional (2D)/3D age and flux distribution along transects and at the top plane of individual aquifers. The simulation results are used to assess the vulnerability of the aquifer system that generally has been assumed to be protected by thick overlaying clayey units and therefore proposed as future reservoirs for drinking water supply. The results indicate that on a regional scale these deep-seated aquifers are not as protected from modern surface water contamination as expected because significant leakage to the deeper aquifers occurs. The complex distribution of local and intermediate groundwater flow systems controlled by the distribution of the river network as well as the topographical variation (Tóth 1963) provides the possibility for modern water to be found in even the deepest aquifers.

European case studies supporting the derivation of natural background levels and groundwater threshold values for the protection of dependent ecosystems and human health.

The EU Water Framework and Groundwater directives stipulate that EU member states have to assess groundwater chemical status by the use of groundwater threshold values derived for the protection of dependent ecosystems and human health. This paper presents a synthesis of main results of 14 European case studies evaluating a methodology for derivation of natural background levels (NBLs) and groundwater threshold values (TVs) proposed by the EU research project "BRIDGE". The 14 investigated groundwater bodies were selected to represent as many aquifer types, climate settings and European ecoregions as possible within the project group that included partners from 17 EU countries. The selected case study sites include transboundary groundwater bodies, EU Pilot River Basins and other important groundwater systems. Some are known to interact with associated ecosystems, while others do not. The proposed method derives groundwater threshold values based on environmental objectives for dependent ecosystems or groundwater "itself" using relevant reference criteria such as natural background levels, environmental quality standards and drinking water standards. Derived groundwater threshold values for dissolved Cl and As applying drinking water standards as examples of reference values, are compared for all 14 case studies. Additionally, more detailed analyses are conducted for the Odense Pilot River Basin and the Vouga River Basin, where groundwater threshold values for N and P and Cl, As, Cd, Ni, Pb, and Zn, respectively, are derived based on environmental objectives and quality standards for groundwater dependent ecosystems. Results demonstrate that the proposed methodology is operational and may be used to protect human health and the environment. Further they show that groundwater threshold values derived from environmental objectives and environmental quality standards for dependent ecosystems in some cases may be significantly lower than drinking water standards, e.g. for nitrate.