Abstract The Upper Cretaceous Chalk Group is renowned as a major aquifer, but the development of secondary porosity due to karstic conduits is poorly understood. Hydrogeological data and evidence from boreholes, sections, and tracer tests indicate that dissolutional conduits occur throughout the Chalk aquifer. Here, we assess the evidence for Chalk karst, and combine it with theoretical models of dissolution and cave formation to produce a conceptual model for the development of karstic conduits. Dissolution due to the mixing of saturated waters of contrasting chemistry along key lithostratigraphical inception horizons form extensive but isolated conduit networks. These form a significant proportion of the secondary porosity and enhance permeability. They prime the aquifer for the development of more integrated conduit networks formed by focused recharge of unsaturated surface derived water. However, the porous, well-fractured nature of the Chalk means that the time needed to form large integrated cave systems is often longer than timescales of landscape change. Continued landscape evolution and water table lowering halts conduit development before they can enlarge into cave systems except where geological and geomorphological settings are favourable. Groundwater models need to consider the formation of secondary karst permeability as this has a major influence on groundwater flow.

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 Cretaceous Chalk in England forms dual-porosity aquifers, with low-permeability matrix and high-permeability networks of fissures, which are predominantly stress-relief fractures that have been enlarged by dissolution. This enlargement is a function of the volume of water that has passed along a fracture (the flowrate effect) and its degree of chemical undersaturation. Feedback effects result in the development of a distinctive permeability structure, with four particular characteristics: (i) troughs in the water table with high transmissivity and convergent groundwater flow; (ii) substantial increases in transmissivities in a downgradient direction; (iii) downgradient decreases in hydraulic gradient; and (iv) discharge from the high-transmissivity zones to the surface commonly at substantial springs. This distinctive self-organized permeability structure occurs throughout unconfined chalk aquifers. Early enlargement of fissures at a depth of 50–100 m below the water table is slow, but is much more rapid close to the water table and in the uppermost bedrock due to non-linear dissolution kinetics. A modelled dissolution profile shows that more than 95% of dissolution takes place in the top 1 m of bedrock, and that enlargement of fissures in the saturated zone results from progressive dissolution occurring over a period of a million years or more.

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 Rivers in karstic environments are known to be greatly influenced by surface water–groundwater interactions, with significant localized inflows during floods from springs, or with losses that can dry up rivers. The Middle Risle River is frequently affected by the development of sinkholes in a chalk karst area (Normandy, France). In the 2010s, two new major sinkholes in the riverbed caused a complete loss of water into the underlying phreatic aquifer, causing the river to dry up over several kilometres. The resulting changes in hydrogeological processes and surface water–groundwater interaction greatly affected water quality, water use and water-dependent ecosystems, causing a political crisis in this river-dependent touristic valley. To understand these phenomena and improve crisis management, the Middle Risle Critical Zone Observatory was set up to enhance monitoring, surveying and/or modelling of groundwater and river levels, river and spring flow, water temperature and conductivity, and ecosystem characteristics (fish, macro-invertebrates and vegetation). The results showed notable impacts on fish, macro-invertebrates and vegetation, some plants proving to be reliable indicators of surface-water–groundwater interaction. The dynamics of local hydrogeological processes were assessed and linked to the measured effects on ecosystems and water supply. Inverse modelling based on an analytical solution of the diffusive wave equation assessed lateral flow during floods, quantifying the spatial–temporal variability of surface-water and groundwater exchanges. It also highlighted the important role of karst zones in both storage and flood-peak attenuation processes, thereby protecting downstream villages against floods.

Abstract The Chalk is a principal aquifer which provides an important resource in SE England. For two centuries, it allowed the establishment of a thriving watercress-growing industry, indirectly through diverted stream flow and directly through the drilling of flowing artesian boreholes. The distribution of artesian boreholes across different catchments, suggests a regional control of vertical groundwater flow within the New Pit and Lewes Chalk units. Interrogation of location-specific information points to the confining role of a few key marls within the New Pit Chalk Formation, which can be traced up-catchment to where they naturally outcrop or have been exposed by quarrying. Evidence is found in geophysical logging of a number of boreholes across catchments, confirming a consistent pattern of the spatial distribution of such key markers. When tectonic stress was applied to the various Chalk formations, the marl bands would have reacted, producing more plastic deformation and less fractures in comparison with rigid rock strata. Such a scenario would have created the conditions for secondary aquifer units, giving the Chalk confining or semi-confining hydraulic characteristics on a regional scale. This conceptual understanding helps explain why the river flow response to reductions in groundwater abstraction varies across the flow duration curve.

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.

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.

Abstract The implementation of new groundwater tracer tests to the chalky karst plateau of the northwestern end of the Paris Basin, combined with a critical review of previous tracer test investigations, makes it possible to characterize the role of karsts in relation to many structural features. The impact of tectonic structures and lithology on the development and evolution of the karst networks is analysed. The consequences for drinking-water supplies and its protection are examined.

Abstract The Cam and Ely Ouse Chalk aquifer has been an important source of public water supply for over 100 years. In response to growing demand for water in the area in the 1970s and 1980 s, the National Rivers Authority developed the Lodes–Granta scheme to provide augmentation water to key rivers, subject to low flows. However, during the droughts in the late 1990s, the River Granta, which derives baseflow from the Chalk aquifer, was dry in some locations for several months. In response, the Environment Agency and Cambridge Water carried out investigations into the impacts of abstraction on the flow and ecology of the Granta and agreed to restrict abstraction from two operational groundwater sources during low flow periods. However, these abstraction restrictions could potentially result in a shortfall within the relevant public water supply zone under some climatic conditions and so Cambridge Water was considering increasing abstraction from an alternative source of groundwater within the catchment to retain the level of resilience of its supply. The Environment Agency was concerned that use of this abstraction could pose a risk of deterioration of the ecological status of the water body under the EU Water Framework Directive. This paper describes the investigations undertaken to assess the risk of deterioration and shows how these are being used to manage this risk going forward.

Abstract The UK, and England in particular, is the stronghold for chalk rivers, streams and wetlands in Europe. A number of sites are recognized as being important for nature conservation and have been designated as such under UK and European legislation. However, as the chalk is also an important aquifer for southern and eastern England, there have been significant impacts on these groundwater-dependent ecosystems from abstraction. Chalk rivers and streams have been used for centuries for mills and water meadows, so impacts have not just occurred in recent times. Intensification of agriculture in the twentieth century has added to the pressure by increasing levels of pollution, especially nitrates, with significant levels now being recorded. However, moves have been made to resolve some of these issues, with investigations into the effects of abstraction and options for reducing these impacts, research into the nature of the chalk aquifer so that it can be modelled more accurately, and assessment made of pollution pathways and their timescales. Associated projects have characterized the ecosystems associated with the chalk in more detail, enabling the mechanism for impacts to be better understood. While the extent of impacts is increasingly understood, action is also being taken to reduce their effects and restore chalk ecosystems.

Abstract In Groundwater-Dependent Terrestrial Ecosystems (GWDTEs), atmospheric nitrogen (N) inputs have often been studied in isolation from terrestrial groundwater and surface water inputs. We describe for the first time the development and application of a combined atmospheric and terrestrial N source apportionment methodology, able to identify contributing catchment and N loadings to GWDTEs. We combined all N inputs using a site-specific conceptual model supported by 12 months’ monitoring for a Chalk-fed GWDTE at Newbald Becksies, East Yorkshire. We discuss implications for effective catchment management, wetland protection and development of a source apportionment methodology. Potential sources of nitrate include: atmospheric deposition, mineralization, leaching from agricultural soils, manure heaps, septic tanks, sewer and mains water leakage. Atmospheric deposition was calculated from measurements of ammonia and nitrogen dioxide concentrations together with rainfall inputs of ammonium and nitrate. Quantification of agricultural sources used the FarmScoper modelling tool to estimate nitrate leaching in the groundwater catchment. Comparison between modelled nitrate concentrations in leachate (15–17 mg N l −1 ) and observed groundwater nitrate concentrations (12.3–19.8 mg N l −1 ) are good. The majority of nitrate is leached from arable land. FarmScoper allows mitigation scenarios to be tested, supporting measures to reduce nitrate within a groundwater catchment.

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.

Abstract Nitrate concentrations in groundwater abstracted from the Hampshire Chalk have increased over the last 20 years. Concentrations at a public water supply are now close to or above the drinking water standard and additional treatment will be required. Investigations of land use and nitrate source apportionment indicate that the largest contributor of nitrate entering the chalk aquifer comes from arable agriculture. Modelling has shown that, under present land use conditions, nitrate concentrations will continue to rise until the latter half of the twenty-first century. South East Water have successfully engaged with land managers and have trialled the use of cover crops at two sites close to the public water supply. Cover crops retain nutrients in the soil and improve soil condition as an integral part of crop rotation. Trials included different crop mixtures such as a variety of Raphanus sativus (oil radish), Vicia sativa (common vetch) and Trifolium alexandrinum/Trifolium incarnatum (clover). Trials were calibrated using porous pot installations and water extracted from the pots was analysed throughout the cover crop growing season. Results indicate cover crops can reduce nitrate concentration losses to the subsurface by up to 80%. Widespread use of cover crops could reduce nutrient leaching to the aquifer and provide a sustainable solution to current groundwater quality issues.

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 The Chalk is a strategically important aquifer in the UK, but it is at risk of contamination, with impacts on abstractions, because of extensive outcrop. Assessments of the risk of contamination impacting water supply are commonly based on bulk aquifer parameters, usually reported for abstraction wells, and do not consider the macro- and micro-scale variation at the site and within the Chalk succession. The presence of less permeable layers is significant in restricting contaminant entry into the aquifer, pushing the contaminants into fractures and voids, where their flow is limited by density differences that cause hydrocarbons to smear into micro-fractures. Dense solvents flowing to depth are further impacted by the presence of lower permeability layers within the Chalk, while horizontal flow is affected by fractures and faults. Porewater concentrations of contaminants are limited, by pore throat size and limited connectivity, to low concentrations of the most soluble compounds, but are characterized by electron acceptors that are observed as reduced and oxidized species. The combination of macro- and micro-scale physical structures, combined with natural attenuation, means that initial risk assessments for hydrocarbons and solvents in Chalk can be highly conservative and should be supported by further targeted investigation.

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.