Chalk Groundwater Research Articles

Long term changes in Chalk groundwater quality in East Anglia, UK

Groundwater abstraction quantity from the Chalk is limited to what is sustainable. Water quality data collected in the 1970s and 1980s have been compared with modern water quality monitoring data for 25 public supply abstraction sites. This comparison has established that there has been both substantial (magnitude) and significant (statistical) change in composition at all sites. Identified changes generally represent real changes in groundwater composition. At some sites in the south of the area considered, water that is not the result of present-day recharge processes is being replaced by water from present-day recharge. It is also clear that, at those sites where ancient brackish water is known to be present at depth, it is generally not being mobilised by pumping. In the north of the area, changes are more subtle but there is a tendency of groundwater composition to approach the regional average, apart from calcium which is trending higher. Nitrate has not increased here, suggesting that reduction is a still-active process. Thus, although quantity has been sustained, quality is changing but not to a degree that impacts potable use.

Predicted variable river response at high and low flows due to groundwater abstraction changes in Chalk catchments

Improving low flows in Chalk streams by relocating groundwater abstraction further downstream is an idea that has become popular in the UK in recent years. Simulations using the Environment Agency's Hertfordshire Chalk Groundwater Model predict that reducing groundwater abstraction close to ephemeral or intermittent Chalk streams is more likely to increase river flows during high-flow conditions than during low flows. This finding helps to explain the apparent lack of observed benefit to river flows during drought periods in catchments where abstractions have been reduced. If abstraction reductions are predicted to result in more of an increase to high river flows than to low flows, they may risk contributing to increased flood likelihood downstream, without providing significant habitat protection during low flows. Conversely, it has also been found that, under certain circumstances, preferential benefits can be predicted for low flows. Simulations show that such benefits are most likely to manifest by relocating abstraction to downstream locations where groundwater levels are already below the base of the riverbed (e.g. due to existing abstraction or artificial channel modifications). Here, the varying degree of hydraulic connection between groundwater and river can result in preferential benefits to downstream low flows and reduced downstream flood risk. Thematic collection: This article is part of the Karst: Characterization, Hazards & Hydrogeology collection available at: https://www.lyellcollection.org/topic/collections/karst

The Environment Agency Chalk groundwater level monitoring network in England

Abstract This paper reviews the Environment Agency chalk groundwater level monitoring network. The network has evolved over many years to enable management of the resource and to assess the impact of abstractions on the environment. The paper considers the strengths and weaknesses of the network, the use and accessibility of the data and how the network supports, and works with, the Environment Agency's regional groundwater models. It concludes with the suggestion that the network is suffering from a degree of lack of maintenance and that there is a disparity between the ambitions of the modelling programme with its Modflow6-driven shift to multi-layer conceptualization and a largely open-hole, single-layer monitoring installation.

Contribution of stratigraphy to groundwater motion understanding in chalk: examples of karstogenic horizons of the Pointe de Caux, France

Abstract Chalk groundwater is the main renewable drinking water resource for many cities of the Paris–London Basin. Understanding karst groundwater motion enhancement appears to be a major issue in order to better protect drinking water, to define hydrogeological surveys and to explore the aquifer. In Normandy, the stratigraphy of chalk was investigated in the 1970s and 1980s but this newly developed stratigraphy was not introduced to hydrogeology where chalk aquifers are studied without considering the sequence boundaries and key event surfaces. Upper Normandy is a unique hydrogeological region where both stratigraphy and hydrogeology can be studied together. In this article we focus on field observations and their direct application to scientific theory. Eight hydrogeological surfaces, linked to sequence boundaries or key event surfaces, are identified. They increase porosity and permeability sufficiently to develop karstic features, hereafter called karstogenic horizons. These field observations lead us to propose a new stratified chalk groundwater model. Palaeokarsts and perched springs not aligned to the current base level can be explained from a geodynamic perspective. Global eustatism and regional uplift during the Quaternary Period have to be taken into account with the hydrogeological stratified model, as the controlling factors of the groundwater motion and the karstogenic horizon development. This theory will help hydrogeologists to determine the probability of encountering palaeokarsts above the piezometric level and thereby define well locations with a greater degree of confidence according to the karstogenic horizon drilled. Chemical studies may also be applied to show if this stratified model can enhance water quality by a new well design.

Groundwater control for construction projects in chalk

Abstract Construction excavations and tunnels in chalk can encounter groundwater challenges, including high water flow rates, instability of excavations in weathered chalk and basal instability in overlying aquitards caused by high groundwater pressures in deeper chalk aquifers. In hydrogeological settings where the chalk has been exposed to periglacial weathering during the Quaternary Period the upper zones may be degraded to structureless chalk which can potentially be of very low hydraulic conductivity (putty chalk) or very high hydraulic conductivity (chalk bearings). In deeper, structured chalk groundwater flow tends to be concentrated along fissures associated with pre-existing geological structures such as bedding planes, flint beds or faults. A range of groundwater control strategies can be deployed, including open pumping, pre-drainage pumping, shallow and deep cut-off walls, ground treatment and, for tunnels and shafts, application of fluid counter pressures to exclude groundwater. The strategy appropriate to a given site must be selected based on a thorough understanding of the hydrogeological setting and chalk weathering profile. This requires a ground investigation of appropriate scope, using suitable techniques to characterize the chalk. Borehole geophysics can play a key role in identifying discrete zones of inflow.

Transfer of water and contaminants in the Chalk unsaturated zone – underground quarry of Saint-Martin-le-Nœud

Abstract The aim of this study is to understand the water and contaminant (nitrate and atrazine) transfer in the unsaturated zone (UZ) of Chalk. For this, the underground quarry of Saint-Martin-le-Nœud is an exceptional site because it permits entry to the aquifer at the limit between the UZ and the saturated zone (SZ). It provides direct access to the water table: underground lakes and the output of the UZ (percolation water at the ceiling). The thicknesses of the UZ and the clay-with-flints (CwF) layer that overlie the Chalk, vary along the 1200 m length of the quarry. From 2012, the chemical evolution and the flow variability of groundwater are characterized for 16 sites with different UZ properties. Chalk groundwater has highly spatially variable hydrodynamic behaviour and geochemical properties. A peak of contaminants is observed in the UZ around 15–20 m depth, with differing behaviours of nitrate and atrazine. The downward matrix water velocity is estimated to be from 0.3 to over 0.72 m a −1 , and the water table is mainly composed of ‘old’ water resulting from transfer through the matrix. A thick CwF layer modifies (1) the transfer processes: surface water is stored in a sort of ‘near-surface perched groundwater’, the infiltration is concentrated by preferential pathways; and (2) water quality: pesticides degradation processes occur in the perched groundwater.

Characteristics of the chalk groundwater along the Picardy coast and its relationship with wetlands

Abstract Water resource management is a major concern in Marquenterre, a maritime plain located in the western part of the Somme department. Water management is particularly indispensable for protecting wetlands in Marquenterre, regulating water usage and avoiding saline intrusion into aquifers. Various approaches including geological modelling and hydrogeological and hydrogeochemical studies were used to prepare a conceptual model of the Marquenterre hydrosystem and to provide better water management in this sector. The conceptual model shows that the chalk aquifer and the various Quaternary deposit aquifers are in hydrogeological continuity. No seawater intrusion has been discovered in groundwater. Salinization discovered at depth is a result of the most recent marine transgressions. Finally, wetlands are primarily supplied by the chalk groundwater or by rainfall. The study results are used to direct policies for surface and subsurface water resource management. The proposed conceptual model may be useful for other coastal aquifers along the English Channel in order to address challenges of managing the chalk groundwater and backshore swamps.

Selenium mobility in a major Chalk aquifer (Lille metropolis, northern France): Contaminants cycles driven by geology, redox processes and pumping

Chalk groundwater in northern France presents selenium (Se) concentrations from <10 to 70 μg L−1, partly exceeding the latest European Framework Directive's drinking-water limit value of 20 μg L−1. Better to understand the heterogeneous dynamics of Se, we used a combination of geochemical, isotopic and geophysical tools on a study site belonging to the Metropolis of Lille, France. This approach provided a fine understanding of the Se fate in a dynamic redox system constrained by geology, related redox processes and pumping, illustrating how local versus global controls affect the Se cycling. A remarkable redox sequence controls element transfers in groundwater with a progressive creation of reductive conditions. Highlighted by geophysical tools, a wide fracture corridor in the Chalk formation disrupts the geological setting of this redox sequence. Under pumping, this corridor allows the mixing of oxygenated groundwater with groundwater under reducing conditions. The evolution of isotopic compositions of sulphate molecules confirms the global reduction trend of sulphates, while pyrite oxidation occurs very locally, together with high Se concentrations. Pyrite is expected to play a predominant role in Se mobility in the Chalk aquifer. Se and other redox sensitive elements (Fe, Mn, N and S) undergo multiple redox cycles, resulting in a Se-rich redox front that migrates downward over time within the water-level fluctuation zone of the porous Chalk. With the decreasing trend of water levels caused by global changes, a Se stock could be immobilized in the unsaturated zone, but nitrate content and redox conditions in the saturated zone will be major drivers for Se mobility.

A conceptual model of hydrogeological function and perchlorate transfer in the unconfined Champagne Chalk aquifer (NE France)

Abstract Groundwater of the unconfined Chalk aquifer in Champagne-Ardenne (NE France) is contaminated by perchlorate ( ClO 4 − ), a persistent water-soluble anion. The Chalk aquifer is a crucial water resource of the region, with complex hydraulic properties. The presence of ClO 4 − is of concern due to its potential adverse human health effects. In France, three sources of ClO 4 − contamination are suspected: industrial, military and agricultural. Both a comprehensive understanding of hydrogeological characteristics of the aquifer, and a sound knowledge of sources and behaviour of ClO 4 − in groundwater are required to allow the sustainable use of this groundwater resource. From data acquired during hydrogeological, geochemical, isotopic ( ClO 4 − ) and groundwater age (CFCs and SF 6 ) studies, and historical investigations in a study area located east of Reims, a conceptual model of Chalk aquifer function and ClO 4 − transfer has been established. High spatio-temporal heterogeneities in the unconfined Chalk aquifer are discussed. Different correlations between ClO 4 − , major ions and groundwater-level fluctuations are shown and interpreted, highlighting main factors governing the Chalk groundwater geochemistry and ClO 4 − transfer mechanisms, including water-level fluctuation, groundwater residence time, thickness of the unsaturated zone, superficial formations, distribution of fissure networks, aquifer–river relationships, origin and location of ClO 4 − in soil and human activity.

Uncertainty Estimation in Hydrogeological Forecasting with Neural Networks: Impact of Spatial Distribution of Rainfalls and Random Initialization of the Model

Neural networks are used to forecast hydrogeological risks, such as droughts and floods. However, uncertainties generated by these models are difficult to assess, possibly leading to a low use of these solutions by water managers. These uncertainties are the result of three sources: input data, model architecture and parameters and their initialization. The aim of the study is, first, to calibrate a model to predict Champagne chalk groundwater level at Vailly (Grand-Est, France), and, second, to estimate related uncertainties, linked both to the spatial distribution of rainfalls and to the parameter initialization. The parameter uncertainties are assessed following a previous methodology, using nine mixed probability density functions (pdf), thus creating models of correctness. Spatial distribution of rainfall uncertainty is generated by swapping three rainfall inputs and then observing dispersion of 27 model outputs. This uncertainty is incorporated into models of correctness. We show that, in this case study, an ensemble model of 40 different initializations is sufficient to estimate parameter uncertainty while preserving quality. Logistic, Gumbel and Raised Cosine laws fit the distribution of increasing and decreasing groundwater levels well, which then allows the establishment of models of correctness. These models of correctness provide a confidence interval associated with the forecasts, with an arbitrary degree of confidence chosen by the user. These methodologies have proved to have significant advantages: the rigorous design of the neural network model has allowed the realisation of models able to generalize outside of the range of the data used for training. Furthermore, it is possible to flexibly choose the confidence index according to the hydrological configuration (e.g., recession or rising water table).

Electrical resistivity monitoring of river–groundwater interactions in a Chalk river and neighbouring riparian zone

ABSTRACTIn the past several decades, there has been considerable interest in groundwater–surface water interactions and their ability to regulate and cycle nutrients and pollutants. These interactions are spatially and temporally complex, but electrical resistivity imaging can be a useful tool for their characterization. Here, an electrical resistivity imaging monitoring array was installed laterally across a groundwater‐dominated Chalk river and into the adjacent riparian wetland; data were collected over a period of 1 year. Independent inversions of data from the entire transect were performed to account for the changing river stage and river water conductivity. Additionally, data from just the riparian zone were inverted using a temporally constrained inversion, and the correlation between the riparian zone resistivity patterns and river stage was assessed using time‐series analysis. The river stage and the Chalk groundwater level followed similar patterns throughout the year, and both exhibited a sharp drop following cutting of in‐stream vegetation. For the independent inversions, fixing the river resistivity led to artifacts, which prevented reliable interpretation of dynamics in the riverbed. However, the resistivity structure of the riparian zone coincided well with the intrusively derived boundary between the peat and the gravel present at the field site. Time‐series analysis of the inverted riparian zone models permitted identification of seven units with distinct hydrological resistivity dynamics (five zones within the peat and two within the gravel). The resistivity patterns in the gravel were predominantly controlled by up‐welling of resistive groundwater and the down‐welling of more conductive peat waters following the river vegetation cutting event. In comparison, although the vegetation cutting influenced the resistivity dynamics in the peat zones, the resistivity dynamics were also influenced by precipitation events and increasing pore‐water conductivity, likely arising from biological processes. It is evident that such approaches combining electrical resistivity imaging and time‐series analysis are useful for understanding the spatial extent and timing of hydrological processes to aid in the better characterization of groundwater‐surface water interactions.

Heterogeneous behaviour of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools

Chalk groundwater is an important aquifer resource. It is intensively exploited for human use, with a large proportion utilized for drinking water. The improvement of the knowledge on Chalk aquifer hydrogeological functioning is essential for the management of this resource. Here, we developed a methodology based on a combination of hydrodynamic, hydrochemistry and groundwater dating tools. A study site with Chalk outcrops was selected in NE France where groundwater geochemistry and water level were monitored continuously for 2 years from 2017 to 2019 and groundwater residence time was estimated using CFCs and SF6. Overall, the aquifer has an inertial behaviour with respect to recharge. Nevertheless, a rapid water level response following rainfall events was observed at one site, suggesting the presence of highly developed fracture network at the local scale. According to the mixing process (piston flow, exponential or binary mixing) defined at each sampling site, groundwater dating indicated rather heterogeneous ages ranging from modern to about 50 years. High spatial and temporal heterogeneities were observed and interpreted by a combination of hydrogeological setting, residence time and land use information, highlighting main factors governing the Chalk groundwater geochemistry including water level fluctuation, thickness of the Unsaturated Zone (UZ), superficial formations, distribution of fracture network, aquifer-river relations and human activities. A conceptual model was proposed accordingly to explain the hydrogeological functioning of unconfined Chalk aquifers.

Improved understanding of spatio‐temporal controls on regional scale groundwater flooding using hydrograph analysis and impulse response functions

AbstractControls on the spatio‐temporal extent of groundwater flooding are poorly understood, despite the long duration of groundwater flood events and distinct social and economic impacts. We developed a novel approach using statistical analysis of groundwater level hydrographs and impulse response functions (IRFs) and applied it to the 2013/2014 Chalk groundwater flooding in the English Lowlands. We proposed a standardized index of groundwater flooding which we calculated for monthly groundwater levels for 26 boreholes in the Chalk. We grouped these standardized series using k‐means cluster analysis and cross‐correlated the cluster centroids with the Standardized Precipitation Index accumulated over time intervals between 1 and 60 months. This analysis reveals 2 spatially coherent groups of standardized hydrographs that responded to precipitation over different timescales. We estimated IRF models of the groundwater level response to effective precipitation for 3 boreholes in each group. The IRF models corroborate the Standardized Precipitation Index analysis showing different response functions between the groups. We applied identical effective precipitation inputs to each of the IRF models and observed differences between the hydrographs from each group. It is suggested this is due to the hydrogeological properties of the Chalk and of overlying relatively low permeability superficial deposits (recent unconsolidated sediments overlying the bedrock, such as clays and tills), which are extensive over 1 of the groups. The overarching controls on groundwater flood response are concluded to be a complex combination of antecedent conditions, rainfall, and catchment hydrogeological properties. These controls should be taken into consideration when anticipating and managing future groundwater flood events. The approach presented is generic and parsimonious and can be easily applied where sufficient groundwater level and rainfall data are available.

Temporal variability of micro-organic contaminants in lowland chalk catchments: New insights into contaminant sources and hydrological processes

This paper explores the temporal variation of a broad suite of micro organic (MO) compounds within hydrologically linked compartments of a lowland Chalk catchment, the most important drinking water aquifer in the UK. It presents an assessment of results from relatively high frequency monitoring at a well-characterised site, including the type and concentrations of compounds detected and how they change under different hydrological conditions including exceptionally high groundwater levels and river flow conditions during 2014 and subsequent recovery. This study shows for the first time that within the Chalk groundwater there can be a greater diversity of the MOs compared to surface waters. Within the Chalk 26 different compounds were detected over the duration of the study compared to 17 in the surface water. Plasticisers (0.06–39μg/L) were found to dominate in the Chalk groundwater on 5 visits (38.4%) accounting for 14.5% of detections but contributing highest concentrations whilst other compounds dominated in the surface water. Trichloroethene and atrazine were among the most frequently detected compounds. The limit for the total pesticide concentration detected did not exceed EU/UK prescribed concentration values for drinking water. Emerging organic compounds such as caffeine, which currently do not have water quality limits, were also detected. The low numbers of compounds found within the hyporheic zone highlight the role of this transient interface in the attenuation and breakdown of the MOs, and provision of an important ecosystem service.

Persistent and emerging micro-organic contaminants in Chalk groundwater of England and France

The Chalk aquifer of Northern Europe is an internationally important source of drinking water and sustains baseflow for surface water ecosystems. The areal distribution of microorganic (MO) contaminants, particularly non-regulated emerging MOs, in this aquifer is poorly understood. This study presents results from a reconnaissance survey of MOs in Chalk groundwater, including pharmaceuticals, personal care products and pesticides and their transformation products, conducted across the major Chalk aquifers of England and France. Data from a total of 345 sites collected during 2011 were included in this study to provide a representative baseline assessment of MO occurrence in groundwater. A suite of 42 MOs were analysed for at each site including industrial compounds (n=16), pesticides (n=14) and pharmaceuticals, personal care and lifestyle products (n=12). Occurrence data is evaluated in relation to land use, aquifer exposure, well depth and depth to groundwater to provide an understanding of vulnerable groundwater settings.

Spatial heterogeneity of high-resolution Chalk groundwater geochemistry – Underground quarry at Saint Martin-le-Noeud, France

Chalk groundwater is an important aquifer resource in France because it accounts for a production of 12 million m3 y-1 with a large proportion reserved for drinking water. Processes occurring in the unsaturated zone (UZ) and the overlying superficial formations have a high impact on Chalk groundwater geochemistry and require better understanding. The study site is a former underground Chalk quarry located near Beauvais (France) that extends over 1200 m in length, at a depth ranging from 20 to 30 m. The water table intersects the cavity creating 15 underground that give access to the Chalk groundwater. Lakes geochemistry has been studied: water samples were collected in July 2013 and major ion concentrations were analyzed. UZ and clay-with-flints thickness above each lake were estimated qualitatively using an electromagnetic sensor (EM31) and Underground GPS. The results unexpectedly showed that groundwater quality varied widely in spatial terms for both allochthonous and autochthonous ions (e.g., HCO3- ranged from 2.03 to 4.43 meq L-1, NO3- ranged from 0.21 to 1.33 meq L-1). Principal component analysis indicated the impact of agricultural land use on water quality, with the intake of NO3- as well as SO42-, Cl- and Ca2+. Chalk groundwater geochemistry is compared with the nature and structure of the UZ. We highlight correlations (1) between thick clay-with-flints layers and the ions Mg2+ and K+, and (2) between UZ thickness and Na+. In conclusion, this paper identifies various ion sources (agriculture, clay-with-flints and Chalk) and demonstrates different processes in the UZ: dissolution, ionic exchange and solute storage.

Tracking selenium in the Chalk aquifer of northern France: Sr isotope constraints

Groundwater at the southern and eastern edges of France’s Paris Basin has a selenium content that at times exceeds the European Framework Directive’s drinking-water limit value of 10μg/L. To better understand the dynamics of the Chalk groundwater being tapped to supply the city of Lille and the Se origins, we used a combination of geochemical and isotopic tools. Strontium isotopes, coupled with Ca/Sr, Mg/Sr and Se/Sr ratios, were used to identify the main groundwater bodies and their mixings, with the Mg/Sr and Se/Sr ratios constraining a ternary system. Groundwater in the agricultural aquifer-recharge zone represents a first end-member and displays the youngest water ages of the catchment along with the highest Sr isotopic signature (0.70842) and low Se contents. Anaerobic groundwater constitutes a second major end-member affected by water-rock interactions over a long residence time, with the lowest Sr isotopic signature (0.70789) and the lowest Se content, its low SF6 content confirming the contribution of old water. Se-rich groundwater containing up to 30μg/L of Se represents a third major end-member, with an intermediate Sr isotopic ratio (0.70826), and is mainly constrained by the clayey Se-rich formation overlying the Chalk aquifer. The spatial and temporal Se variability in the groundwater is clearly linked to the presence of this formation identified as Tertiary and also to the hydrological conditions; saturation of the Se-rich clays by oxygenated groundwater enhances Se mobility and also Sr adsorption onto the clays. This multi-tool study including Sr isotopes successfully identified the Se origins in the aquifer and has led to a better understanding of the regional mixing and processes affecting the Chalk groundwater.

Molybdenum distributions and variability in drinking water from England and Wales.

An investigation has been carried out of molybdenum in drinking water from a selection of public supply sources and domestic taps across England and Wales. This was to assess concentrations in relation to the World Health Organization (WHO) health-based value for Mo in drinking water of 70 μg/l and the decision to remove the element from the list of formal guideline values. Samples of treated drinking water from 12 water supply works were monitored up to four times over an 18-month period, and 24 domestic taps were sampled from three of their supply areas. Significant (p < 0.05) differences were apparent in Mo concentration between sources. Highest concentrations were derived from groundwater from a sulphide-mineralised catchment, although concentrations were only 1.5 μg/l. Temporal variability within sites was small, and no seasonal effects (p > 0.05) were detected. Tap water samples collected from three towns (North Wales, the English Midlands, and South East England) supplied uniquely by upland reservoir water, river water, and Chalk groundwater, respectively, also showed a remarkable uniformity in Mo concentrations at each location. Within each, the variability was very small between houses (old and new), between pre-flush and post-flush samples, and between the tap water and respective source water samples. The results indicate that water distribution pipework has a negligible effect on supplied tap water Mo concentrations. The findings contrast with those for Cu, Zn, Ni, Pb, and Cd, which showed significant differences (p < 0.05) in concentrations between pre-flush and post-flush tap water samples. In two pre-flush samples, concentrations of Ni or Pb were above drinking water limits, although in all cases, post-flush waters were compliant. The high concentrations, most likely derived from metal pipework in the domestic distribution system, accumulated during overnight stagnation. The concentrations of Mo observed in British drinking water, in all cases less than 2 μg/l, were more than an order of magnitude below the WHO health-based value and suggest that Mo is unlikely to pose a significant health or water supply problem in England and Wales.

Infiltration processes in karstic chalk investigated through a spatial analysis of the geochemical properties of the groundwater: The effect of the superficial layer of clay-with-flints

Summary In the Paris Basin in Upper Normandy (France), the chalk plateaus are covered with thick deposits of loess and clay-with-flints, from a few meters to approximately 40 m thick locally. A perched groundwater is sometimes observed in the superficial layers in which evapotranspiration processes seem to occur. This study’s objective was to understand the effects of the thick clay-with-flints layers on the infiltration processes. To achieve this, we adopted a spatial approach comparing the maps of the geochemical properties of the Chalk groundwater and the maps of the thickness of clay-with-flints. The French national groundwater database, ADES (Acces aux Donnees des Eaux, BRGM), provided the mean geochemical properties in the Chalk aquifer of Upper Normandy. This database was used to prepare maps of the environmental tracers: Ca 2+ , HCO 3 − , Mg 2+ , Cl − , Na + , NO 3 − , and SO 4 2 . The data are spatially well organized. Using principal component analysis (PCA), these maps were compared with the maps of the thickness of clay-with-flints. A focus on the coastal basins (northern Upper Normandy) shows a very strong spatial correlation between the maps of clay-with-flints thickness and all of the maps of the major ions. The thickness of clay-with-flints is negatively correlated with the autochthonous ions (HCO 3 − and Ca 2+ ) and is positively correlated with the allochthonous ions (Cl − , Na + , SO 4 2− , and NO 3 − ). These results highlight that the thickness of clay-with-flints controls recharge. Two types of infiltration processes are proposed: (1) Thicker clay-with-flints allows storage in the perched groundwater, which allows evapotranspiration, resulting in high concentrations of allochthonous ions and a decrease in the dissolution potential of water and low concentrations of autochthonous ions. The infiltration of the perched groundwater is thus delayed and concentrated. (2) Thinner clay-with-flints causes the infiltration to be more diffuse, with low evapotranspiration and thus low concentrations of allochthonous ions in the Chalk groundwater; more, there is more dissolution and higher concentrations of autochthonous ions in the Chalk groundwater.

Using δ15N and δ18O values to identify sources of nitrate in karstic springs in the Paris basin (France)

In many parts of the world, groundwater quality deterioration is occurring often due to changes in land use along with overexploitation of some aquifers. Some of the drinking water distributed in Paris (France) is collected from a karstic Cretaceous chalk aquifer at two groups of springs located 80km south of Paris, in the Lunain catchment. Although geographically close to each other, these springs (Sp1 and Sp2) have quite different hydrogeochemical characteristics. In Sp1, an increase in the NO3- concentrations of 18mgL−1 (from 16mgL−1 in 1965 to 34mgL−1 in 2009) has been observed and in Sp2, NO3- concentrations have increased by 26mgL−1 (from 20mgL−1 in 1965 to 46mgL−1 in 2009). The origin of the NO3- was, however, unclear. The stable isotopic composition of NO3- was used to identify the sources causing the increase of NO3- concentrations and to assess seasonal variations. The isotopic composition of two potential sources including agricultural effluents and wastewater was determined. Likewise, two additional NO3- sources, the chalk groundwater and the Lunain River water, were considered, following the hydrogeological context of the Lunain catchment. Determination the concentrations and the δ15N and δ18O values of spring water NO3- and the use of a Bayesian isotopic mixing model SIAR (Stable Isotope Analyses in R) allowed determination of the proportional contribution of respective NO3- sources in the springs. In spring Sp1, all the potential NO3- sources (4) contributed almost equally during the low flow period, but higher NO3- contributions from the agricultural source and the chalk groundwater were observed during the high flow period. For the Sp2 group of springs, elevated contributions of NO3- from the agricultural source and from the chalk aquifer were observed during the high flow period as well as during the low flow period. Knowledge of NO3- sources contributing to these springs is important for the implementation of remediation measures to protect the water resource and avoid further water quality deterioration.