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.

Abstract Since the 1980s, nitrate has been shown to be present in soils and the vadose zone of various types of geological materials years after fertilizer application. In chalk, where the vadose zone is thick, nitrate storage can be considerable and its transport time towards groundwater can be lengthy. In this context, evaluation of the impact of changes in agricultural practices on groundwater quality remains a major question. Improvement of groundwater quality can in certain cases be greatly delayed after the implementation of environmental agricultural practices. The principal objective of this study is to improve our knowledge of when changes in agricultural practices will have a noticeable effect on groundwater quality. To meet this objective, nitrate concentration profiles were performed in agricultural plots in Picardy (France). A crop marker event was used to calculate the transport velocity of water and associated solutes. This method is useful when other tracers (such as tritium or chloride) cannot be used. Estimated velocities range from 0.51 to 0.54 m a −1 ; these values are similar to those described in similar chalk aquifers.

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.

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.

ABSTRACT The concept of biostratigraphy was a significant step in the evolution of geoscience. Alexandre Brongniart (1770–1847) and Georges Cuvier (1769–1832) were key contributors to developing the subdiscipline as they worked to decode the stratigraphy of the Paris Basin in the first decades of the nineteenth century. Their illustrations of fossils, local geologic columns, and a regional geologic map played a decisive role in furthering an understanding of the value of paleontology in the service of illuminating Earth history.

Abstract The Toarcian Oceanic Anoxic Event (T-OAE) is marked by major palaeoenvironmental and palaeoceanographical changes on a global scale, associated with a severe disturbance of the global carbon cycle and organic-rich facies deposition. Here, a multiproxy approach (petrographic and geochemical techniques) was applied to the study of the organic content of the T-OAE of the Paris Basin, whose phytoplanktonic origin has been previously inferred by its geochemical signature. The top of the tenuicostatum Zone is characterized by palynomorphs and marine phytoplankton-derived amorphous organic matter (AOM), representing a proximal marine environment with emplacement of euxinic conditions at the top (total organic carbon/sulfur content and increase in AOM). At the base of the serpentinum Zone the proliferation of bacterial biomass begins, with phytoplankton playing a secondary role. This indicates the development of stagnant and restrictive conditions in a proximal environment, with water column stratification (neohop-13(18)-ene). The majority of the serpentinum Zone is dominated by bacterial biomass, suggesting a marine environment with bottom-water stagnation, possibly related to basin palaeogeomorphology and circulation patterns, with episodic euxinia. This therefore suggests that the T-OAE organic fraction is dominated by bacterial biomass, not phytoplankton, showing the importance of an integrated approach to the determination of the organic facies.