Halite karst geohazards
Colin Serridge discusses halite dissolution subsidence in the Knutsford Seven, a group of meres in north Cheshire, UK
Halite (salt) dissolves rapidly in the presence of circulating groundwater and subsidence – both natural and anthropogenic – has impacted Cheshire.
Here, extensive Triassic-age halite deposits are present in two units: the lower, Northwich Halite Member and the upper, Wilkesley Halite Member. Dissolution of the lower unit in north Cheshire has created subsidence features, including water-filled depressions that are referred to as the ‘Knutsford Group’ of meres (lakes). Whilst six of the meres are natural and largely of post-glacial age, the seventh, Melchett Mere, is completely anthropogenic. It is attributed to historical uncontrolled (wild) brine pumping, principally around Plumley and Northwich, located 7.3 and 9.0 km, respectively, south of Melchett Mere. Wild brine pumping has also accentuated many of the natural meres (Serridge & Cooper, 2022).
Natural halite dissolution subsidence features in north Cheshire relate mainly to advance and retreat of the Devensian ice sheet, followed by the establishment of the natural pre-anthropogenic hydrogeological regime. This caused brine movement towards low areas and the development of brine springs (Cooper, 2020), locally referred to as ‘wiches’ (with the term captured in local place names, such as Nantwich, Northwich and Middlewich). Exploited since pre-Roman times, these natural brine springs later became the main salt-producing areas in Cheshire, particularly in the late 19th to early 20th centuries, when an expanding salt and chemicals industry developed.
Dissolution largely occurs at the top of the halite deposits, where collapsed insoluble residues along with collapsed strata remain as a collapse breccia. The basal contact between these superficial collapse breccia structures and the underlying halite is referred to as ‘wet rockhead’ (Bell, 1975; Cooper, 2020), while ‘dry rockhead’ refers to the dry contact between halite and overlying dry mudstone. Historic, uncontrolled wild brine pumping in ‘wet rockhead’ areas, including from abandoned unstable flooded mines in and around Northwich, for example, led to severe subsidence problems. Brine dissolution channels (brine runs) can extend many kilometres away from the abstraction points; freshwater drawn into the system to replace the removed brine accentuated the dissolution, creating artificial halite karst.
The mechanism of subsidence that is attributed to salt dissolution and brine removal has been described as analagous to that of longwall coal mining subsidence (Evans et al, 1968). As the subsidence develops, a wave of deformation progresses around the subsiding land, firstly causing extension, followed by compression. Surface fractures and subsidence slip scars can be reactivated into compression features with the intervening areas pushed up in the opposite direction. Such features are observed at Melchett Mere (Serridge & Cooper, 2022).
Exploited since pre-Roman times, these natural brine springs later became the main salt-producing areas in Cheshire, particularly in the late 19th to early 20th centuries
The morphological and geomorphological characteristics of both natural and anthropogenic-induced halite dissolution subsidence features are similar, meaning the cause can only be attributed using temporal evidence for their formation, such as historical maps and aerial photographs, as well as airborne light detection and ranging (LiDAR) data (which provides more recent high-resolution information about subsidence features and slip scars).
Wild brine pumping in Cheshire halted in 2006. However, the possibility of continued natural dissolution and metastable cavities remains. Further research is needed to ascertain the precise routes for brine in the subsurface, particularly given the implications for infrastructure development, such as HS2 in north Cheshire (Serridge & Cooper, 2022). Future changes to the groundwater regime may also impact the long-term stability of these features, particularly if the groundwater cannot be satisfactorily controlled.
Dr Colin J. Serridge is an Engineering Geologist in the Specialist Ground Improvement industry and a Senior Lecturer in Earth Sciences at Edge Hill University, UK.
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