Deep groundwater systems

Groundwater is potentially vulnerable to pollution from a wide and growing range of sub-surface activities, including the exploration for and production of hydrocarbons such as shale gas. In order to protect groundwater while also enabling the development of hydrocarbon resources there is a need to better understand the 3D nature and extent of groundwater systems and in particular to better understand deep groundwater systems above potential shale gas source rocks.

Definitions of aquifers and groundwater

The Environment Agency guidance titled "Groundwater protection guides covering: requirements, permissions, risk assessments and controls" defines groundwater as any "water stored below the water table in rocks or other geological strata which we call aquifers". It also defines three types of aquifers or strata as follows:

Principal Aquifer: Rocks that provide significant quantities of water for people and may also sustain rivers, lakes and wetlands. Formerly referred to as 'major aquifers'.
Secondary Aquifer: Rocks that provide modest amounts of water, but the nature of the rock or the aquifer's structure limits their use. They remain important for rivers, wetlands and lakes and private water supplies in rural areas. Formerly referred to as 'minor aquifers'.
Unproductive strata: These are rocks that are generally unable to provide usable water supplies and are unlikely to have surface water and wetland ecosystems dependent upon them. Formerly referred to as 'non-aquifers'.

Aquifers are designated initially on their rock type and aquifer designation data are available for England and Wales. These maps show the distribution of aquifers at the land surface. In a previous study of Aquifers and Shales, BGS also produced maps of the full extent and lower surfaces of all Principal Aquifers in England and Wales. The present 3DGWV project is producing an attributed 3D model of all Principal and Secondary Aquifers in England.

Groundwater protection and deep groundwater systems

One of the primary drivers of regulation and management of groundwater in England, from the Water Framework Directive and Groundwater Directive, is to "implement the measures necessary to prevent or limit the input of pollutants into groundwater and to prevent the deterioration of the status of all bodies of groundwater". In the UK, the UK Technical Advisory Group (UKTAG) has provided guidance on what the implications are for deep groundwater systems.

UKTAG state that "at some depth, depending on the nature of the aquifer, groundwater loses its value as a resource that can be either exploited for human activities or support surface flows and ecosystems". They note that the default maximum thickness for most aquifers in the UK should be 400 m. However, they also state that this value "should be amended using local information if available. This information should comprise hydrogeological and hydrochemical information to identify the resource boundaries". So understanding variations in aquifer characteristics, particularly the quality of the groundwater at depth, is an important factor in defining deep groundwater systems.

Variations in water quality in deep groundwater systems

There is almost no information on the physical aquifer properties of deep groundwater systems, but there is some limited information on water quality. As noted by UKTAG, the hydrogeochemistry or quality of deep groundwater systems is an important factor in the usability of groundwater. There are a wide range of chemical components that affect the quality of groundwater, however the degree of mineralisation of groundwater (expressed as total dissolved solids, TDS, in mg/litre or as electrical conductivity in μS/cm) is a good indicator of overall quality. The World Health Organisation (WHO) standards for drinking water, based on taste and palatability, suggest total dissolved solids (TDS) are acceptable in the range 300 to 1200 mg/litre, and the Water Supply Regulations for England (2016) give a maximum indicative value of electrical conductivity of 2500 μS/cm.

The TDS for water quality samples from deep groundwater systems in England shows a wide range of values from ~10 mg/L up to ~35,000 mg/l (the latter approximating to the TDS of seawater), although there is a general decrease in quality (increase in TDS) with depth. In the depth interval of potential shale gas development in England, between ~1 and 2 km below ground level, TDS is typically in the range from ~1000 to ~35000 mg/l. This is generally above the WHO recommended values for drinking water.