Mike J. Streetly

In 1998, the Permo-Triassic Sandstone aquifer of the West Midlands–Worfe (WMW) area became the subject of one of the first major groundwater modelling projects initiated under the Environment Agency of England's national groundwater modelling framework. The 4R runoff–recharge code that allowed these models to apply a semi-integrated approach to simulating catchment hydrology was developed and trialled in parallel with the WMW project. This combined groundwater and surface water modelling approach has allowed the Environment Agency of England to make complex water resource management decisions in a wide range of aquifers and catchments based, in part, on outputs from such models. Since that time, the Environment Agency has used the framework to develop around a dozen further models of Permo-Triassic Sandstone aquifers, of varying levels of complexity. Many of the older models have subsequently been through one or more cycles of updating and/or recalibration. This paper provides a high-level review of these models based on the personal experience of the author and compares and contrasts their implementation in the US Geological Survey's groundwater modelling code MODFLOW. This includes a discussion of key conceptual issues that are common across the models, or unique to particular models, and the uses to which the models have been put. The paper concludes with a discussion of future opportunities and challenges for the programme. Thematic collection: This article is part of the Hydrogeology of Sandstone collection available at: https://www.lyellcollection.org/topic/collections/hydrogeology-of-sandstone

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.

Papers published in QJEGH illustrate advances in the monitoring and measurement of groundwater systems over the last 50 years. The emphasis of QJEGH over the years has tended to be towards practice in hydrogeology in the UK, rather than theory, and therefore we refer also to some seminal work published elsewhere. The range of measurements and monitoring reviewed includes from laboratory to regional scale, physical measurement of groundwater flow and transport within groundwater, geophysical, chemical and thermal properties, and applies both to resources and pollution. Several themes can be recognized: groundwater resource development (especially in the early days), groundwater quality protection, initially focused on landfill but now including industrial and diffuse pollution, groundwater management integrated into wider environmental management since the European Water Framework Directive in 2000, and latterly novel uses of groundwater, in part driven by a desire for ‘clean’ energy. We conclude by looking at future drivers and technologies to consider how the practice of monitoring and measurement of groundwater systems may evolve in coming years and some of the challenges that we will need to overcome.

Chalk has unique properties that affect how it stores and transmits groundwater. Understanding its dual porosity and dual permeability nature, the marked contrast between largely static water held in pore spaces and the more mobile water in fractures, has developed significantly over recent decades. More recently, the differing properties of the various formations within the Chalk Group have become more apparent, particularly on the back of detailed investigations in support of major tunnelling projects in London. This paper shows how recent developments in the conceptual understanding of the Chalk-Basal Sands aquifer in London (Streetly et al, 2018) were used as the basis of a new, detailed groundwater model of the aquifer. We discuss the challenges in developing this model and some of its subsequent uses in water resource management, climate change assessment and environmental impact assessment. The model was also the basis of more localised models developed to investigate the potential impacts of a significant increase in the exploitation of the Chalk -Basal Sands aquifer for heating and cooling in ground source energy schemes.

Chalk has unique properties that affect how it stores and transmits groundwater. Understanding its dual porosity/dual permeability nature (the marked contrast between largely static water held in pore spaces and the more mobile water in fractures) has developed significantly over recent decades. More recently, the differing properties of the various formations within the chalk have become more apparent, particularly on the back of detailed investigations in support of major tunnelling projects in London. This paper presents an outline of these developments in understanding of the chalk as an aquifer and explains how this was used to develop a comprehensive conceptual model of the chalk aquifer of the London Basin and North Downs. This conceptual understanding was then the basis of a new, detailed groundwater model of the aquifer (described in a separate paper in this conference set, Streetly et al. (2018)).

To ensure that the environment is adequately protected and abstractors are fairly regulated, hydroecological assessment tools are needed that give local interpretation, indicate where ecological communities might not be resilient to current or future abstraction pressures, and take account of the effect of other stressors. These tools should ideally be transferrable across different river catchments. This study presents a hydroecological model indicating that macro‐invertebrates were not resilient to long‐term, steady‐state levels of groundwater abstraction when flows at Q75 were reduced by more than 50% in unpolluted streams in the West Midlands of England. The proportion of silt and clay covering the substratum and distance from source were also significant predictors of ecological condition, but did not interact with the abstraction effect. Combinations of different biotic indices and supplementary field observations of catchment land use gave additional evidence of the impacts of excessive inputs of fine sediment overriding the effects of abstraction at some locations. This study has shown that regional hydroecological models can be used with existing environmental flow indicators and other local environmental information as a weight of evidence to identify where abstraction and excessive inputs of fine sediment need to be mitigated separately or together to achieve the full ecological benefits. Such regional hydroecological models can increase the certainty of regulatory decisions made at the local scale for licencing abstraction at current use and predicted future levels.

If management of water resources is to fully take into account the requirements of the environment, it will benefit from quantitative predictions of the ecological effects of river flow alterations. A significant relationship between flow reductions caused by groundwater abstraction and ecological conditions (as measured by relevant biotic indices) has been shown in streams in the midlands of England. In this article, we combine this relationship with hydrological indices derived from calibrated regional groundwater models to assess river reaches that are likely to be ecologically impacted by abstraction and might consequently be at risk of failing to meet EC Water Framework Directive standards. We demonstrate the application of this method within the framework of the Ecological Limits of Hydrologic Alteration (ELOHA) approach to making water resource decisions. We provide examples of how this approach can be used to assess the implications of different groundwater abstraction scenarios for river water bodies.

Despite the pressing environmental, economic and social issues surrounding water abstraction, scientific methods for managing its ecological impacts remain in their infancy. In this paper, we demonstrate statistically significant relationships between in‐stream ecological condition using macroinvertebrates and the hydrological effect of groundwater abstraction on surface water flows in streams originating from Permo‐Triassic sandstone aquifers in the English midlands. Ecological condition was most strongly correlated to the effect of abstraction on medium‐low flows (Q₇₅) compared with effects at other flows, water quality, habitat or seasonal effects. Ecological impacts occurred when the effect of abstraction on Q₇₅ flows exceeded 60%. The same relationships were shown among individual macroinvertebrate taxa, validating the biological responses. The hydroecological model has provided a scientific basis for making local decisions on investigation sites and has helped to focus resources to areas at risk of not meeting Good Ecological Status under the Water Framework Directive because of abstraction.