Characterising variations in the salinity of deep groundwater systems: A case study from Great Britain (GB)

A.J. Newell,S.E. Loveless,M.E. Stuart,J.P. Bloomfield,M.A. Lewis

doi:10.1016/j.ejrh.2020.100684

Abstract

Study regionThe study region is Great Britain (GB), a small non-continental island landmass in North West Europe Study focusData for Total Dissolved Solids (TDS) from groundwater samples can be used to characterise regional-scale variations in the quality of deep groundwater systems. Combined with information about typical well-depths, TDS data can be used to identify the presence of currently undeveloped fresh or brackish groundwater at depth that may require protection. This study considers the distribution of TDS with depth relative to sea level in the main GB aquifers and selected other key hydrogeological units, and demonstrates how useful insights can be obtained from data-led analyses of depth variations in groundwater chemistry if the regional context of hydrogeological systems is taken into account. New hydrogeological insightsIn GB, TDS varies over about five orders of magnitude, up to about 330,000 mg/L, with a general increase in mineralisation with depth. Overall, there is a transition from fresh <1625 mg/L to brackish <10,000 mg/L groundwater at about 500 m below surface, and from brackish to saline >10,000 mg/L groundwater at about 700 m. Given that the 95 %tile depth of water wells is about 200 m, it is evident that there is currently undeveloped fresh groundwater at depth across large parts of the study area that may require protection, although it is inferred that TDS is not the only factor limiting exploitation and use of these deeper resources. As in this study, previous data-led analyses of fresh groundwater at depth have typically analysed TDS as depth below surface. However, if TDS data is analysed relative to sea level and in the context of regional hydrogeological information or models, additional insights can be gained on the distribution and controls on fresh groundwater at depth. Projecting TDS data into a 3D hydrogeological model of the study area shows that fresh groundwater at depth exhibits spatial coherence and is generally associated with relatively deep sedimentary basins overlying older, less permeable basement.

Highlights

Groundwater provides of the order of one third of all freshwater supplies (Doll et al, 2012, 2014)
This study considers the distribution of Total Dissolved Solids (TDS) with depth relative to sea level in the main Great Britain (GB) aquifers and selected other key hydrogeological units, and demonstrates how useful insights can be obtained from data-led analyses of depth variations in groundwater chemistry if the regional context of hydrogeological systems is taken into account
There is a general increase in mineralisation of groundwater with depth defined by a general increase in the minimum value of TDS for a given depth interval

Summary

Introduction

Groundwater provides of the order of one third of all freshwater supplies (Doll et al, 2012, 2014). Fan et al (2013) have estimated that about one third of the global land surface area has a water table depth or capillary fringe within 3 m of the ground surface, and Journal of Hydrology: Regional Studies 28 (2020) 100684 much of the groundwater that is abstracted is from relatively shallow, unconfined groundwater systems of only a few 10 s of metres in depth. Due to pressure on groundwater resources (Wada et al, 2010; MacDonald et al, 2016) as well as the growing use of the subsurface for other activities, there is an increasing focus on deep groundwater systems for potable water as well as for agricultural irrigation and industrial purposes. Across large parts of Africa the need for water security in relation to growing populations and the need to develop resilience to the effects of climate change has led to continental-scale assessments of deep groundwater systems (MacDonald et al, 2012). In the USA, the drive to find groundwater to augment supplies for irrigation and potable use has resulted in an interest in deep groundwater, including brackish groundwater (Kang and Jackson, 2016; Stanton et al, 2017; Ferguson et al, 2018a; Kang et al, 2019; Perrone and Jasschko, 2019)

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