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 application of production geochemistry techniques has been shown to provide abundant and often low-cost high-value fluid information that helps to maximize and safeguard production. Critical aspects to providing successful data relate to the appropriate sampling strategy and sampling selection which are generally project-aim-specific. In addition, the continuous direct integration of the production geochemistry data with subsurface and surface understanding is pivotal. Examples from two specific areas have been presented including: (a) the effective use of IsoTubes in the production realm; and (b) the application of geochemical fingerprinting primarily based on multidimensional gas chromatography. Mud gas stable carbon isotopes from low-cost IsoTubes have been shown to be very effective in recognizing within-well fluid compartments, as well as recognizing specific hydrocarbon seals in overburden section, including the selective partial seal for only C 2+ gas species. With respect to geochemical fingerprinting, examples have been presented related to reservoir surveillance including compartmentalization, lateral and vertical connectivity, as well as fluid movements and fault/baffle breakthrough. The production-related examples focus on fluid allocation within a single well, as well as on its application for pipeline residence times, fluid identification and well testing.

Abstract Randomness of fracture networks still makes channelized flow a challenge to track in hard-rock aquifers. While not underestimating geological and hydrological criteria that are also handled here through mapping exercises, this study raises an issue of water quality encountered in lifelong boreholes. Chemical classification checked against a recent conceptual model of bedrock aquifers gives birth to a new typology of groundwater in a complex granitic aquifer system located in the SW of Ivory Coast (West Africa). Major ion chemistry, borehole completion data, digital elevation model and satellite images are used to interpret the geochemical water facies as an expression of connexions between the saprolite and the saprock, or transient insulation. From major ions ratios, cumulate mineralization, carbonate equilibrium, stable isotopes, the maturation of ground waters and mixing between bedrock layers are described at seasonal and local scales. The results highlight some vertical feeding of the water table into the main saprock aquifer owing to shortcuts through the saprolite, along with the existence of dead-ends in the hydraulically active fracture network. Also, some influence of fault zones, either drain or barrier, is confirmed on the (Ca, Mg) bicarbonate water facies within the saprock.

Abstract Groundwater pathways and residence times are controlled by aquifer flow and storage properties, which, in weathered/fractured hard rock aquifers, are characterized by high spatial heterogeneity. Building on earlier work in a metamorphic aquifer in NW Ireland, new clay mineralogy and analyses of geophysical data provided high spatial resolution constraints on the variations in aquifer properties. Groundwater storage values derived from magnetic resonance sounding and electrical resistivity tomography were found to largely vary laterally and with depth, by orders of magnitude. The subsequent implementation of hillslope, two-dimensional numerical groundwater models showed that incorporating heterogeneity from geophysical data in model parametrization led to the best fit to observations compared with a reference model based on borehole data alone. Model simulations further revealed that (1) strong spatial heterogeneity produces deeper, longer groundwater flow paths and higher age mixing, in agreement with the mixed sub-modern/modern ages (mostly <50 years) provided by independent tritium data, and (2) areas with extensive weathering/fracturing are correlated with seepage zones of older groundwater resulting from changes in the flow directions and are likely to act as drainage structures for younger groundwater on a catchment or regional scale. Implications for groundwater resilience to climate extremes and surface pollution are discussed together with recommendations for further research.