Abstract The Chalk is an unusual karst aquifer with limited cave development, but extensive networks of smaller solutional conduits and fissures enabling rapid groundwater flow. Small-scale karst features (stream sinks, dolines, dissolution pipes, and springs) are common, with hundreds of stream sinks recorded. Tracer velocities from 27 connections between stream sinks and springs have median and mean velocities of 4700 and 4600 m d −1 . Tests to abstraction boreholes also demonstrate very rapid velocities of thousands of metres per day. Natural gradient tests from observation boreholes have rapid velocities of hundreds of metres per day. There is strong geological control on karst with dissolution focused on stratigraphical inception horizons. Surface karst features are concentrated near the Paleogene boundary, or where thin superficial cover occurs, but rapid groundwater flow is also common in other areas. The Chalk has higher storage and contaminant attenuation than classical karst, but recharge, storage and flow are influenced by karst. Point recharge through stream sinks, dolines, losing rivers, vertical solutional fissures, and soakaways enables rapid unsaturated zone flow. Saturated zone networks of solutional fissures and conduits create vulnerability to subsurface activities, and enable long distance transport of point source and diffuse pollutants, which may be derived from outside modelled catchment areas and source protection zones.

Abstract The conceptualizations of matrix, fracture and fissure porosity are important for understanding relative controls on storage and flow of groundwater, and the transport of solutes (and non-aqueous phase liquids) within chalk aquifers. However, these different types of porosity, rather than being entirely distinct, represent elements in a continuum of void sizes contributing to the total porosity of the aquifer. Here we define such a continuum and critically examine the selection of appropriate values of effective porosity, a widely used parameter for mass transport modelling in aquifers. Effective porosity is a transient phenomenon, related to the porosity continuum by the timescales under which mass transport occurs. An analysis of 55 tracer tests and 20 well inflow tests in English Chalk aquifers identifies spatial scaling in groundwater velocity and groundwater flow respectively, which are interpreted within the context of the wider literature on carbonate aquifers globally. We advance transport modelling in the Chalk by developing a fissure aperture velocity mapping method using transmissivity data from existing regional groundwater models, together with the identified transient and spatial scaling phenomena. The results show that chalk aquifers exhibit widespread rapid groundwater flow which may transport contaminants rapidly in almost any setting.

Abstract The small town of Yport in the northern part of the Pays de Caux, Normandy, is located on the shore of the English Channel between Étretat and Fécamp, at the outlet of a long dry valley that incises the chalk plateau. Eight hundred metres to the east, at the base of a cliff, is a cluster of springs, the ‘Fontaines d'Yport’, that emerge on the foreshore. Given their high discharge, measured at between 1 and 2.5 m 3 s −1 , the Le Havre authorities have been interested in these springs since the 1960s, in determining their origin, locating the karst conduit, developing them, and investigating the boundaries and vulnerability of the groundwater basin. Today, one third of the drinking water that supplies the Le Havre conurbation comes from the Yport wells, and investigations into their vulnerability and protection of their resource continues.

Abstract Chalk porosity plays a decisive role in the transport of solutes and heat in saturated chalk. From a geological point of view, there are at least two types of porosity: the porosity of pores corresponding to the micro-spaces between the fossil coccoliths that form the chalk matrix and the porosity owing to the micro- and macro-fractures (i.e. secondary porosity). For groundwater flow, the fracture porosity is a determining factor at the macroscopic scale. The multiscale heterogeneity of the porous/fractured chalk induces different effects on solute and heat transport. For solute transport considered at the macroscopic scale, tracer tests have shown that the ‘effective transport porosity’ is substantially lower than the ‘effective drainable porosity’. Moreover, breakthrough curves of tracer tests show an important influence of diffusion in a large portion of the ‘immobile water’ (‘matrix diffusion’) together with rapid preferential advection through the fractures. For heat transport, the matrix diffusion in the ‘immobile water’ of the chalk is hard to distinguish from conduction within the saturated chalk.

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.