Summary of the hydrogeology of Ince Marshes area

Schematic cross-section of the geology of the Ince Marshes area

The Cheshire energy research field site in the Ince Marshes area is one of two locations for the UK Geoenergy Observatories project. An initial hydrogeological conceptual model has been developed from existing hydrogeological and hydrochemical information. Information obtained from the proposed drilling and groundwater monitoring programme will enhance this understanding in the future.

Groundwater is very important for public water supply, industry and agriculture in this region, with large abstractions from the Permo-Triassic sandstone aquifer.

The research field site is in the Ince Marshes area. Much of this area is artificially drained reclaimed land, close to the River Mersey and the Manchester Ship Canal. The area is flat, much of it below 10 mOD, with some high elevations e.g. about 15 mOD at Thornton-le-Moors and reaching about 26 mOD in the Ince–Elton area. Land use includes large areas of present and past industrial development, agriculture, transport infrastructure, urbanisation and residential development. Industrial development (past and present) includes the Stanlow oil refinery and landfill sites.

A summary of the geology from a hydrogeological perspective is provided in a schematic cross-section (Figure 1), with a second schematic cross-section identifying some key hydrogeological features of the Permo-Triassic sandstone aquifer in the Ince Marshes area (Figure 2). These are based on initial findings and will be modified as our understanding develops, as the work progresses.

Figure 2: schematic cross-section f the hydrogeology of the Permo-Triassic sandstone aquifer

Hydrogeology of the superficial deposits

Superficial deposits are widespread and highly variable in composition and thickness, with low permeability tidal-flat deposits and glacial till dominating both in terms of their lateral extent and their impact on the hydrogeology. The superficial deposits are expected to be saturated within a few metres of the ground surface, the water level being dependent on factors such as the drainage regime. More permeable deposits, such as glaciofluvial sands and gravels, tend to be present in lenses of limited lateral extent, surrounded by lower-permeability deposits which will make any recharge or discharge (and therefore flow) very limited. It is therefore expected that little natural groundwater flow occurs within the superficial deposits, with the exception of buried channels. Buried channels are typically tens of metres deep and hundreds of metres wide, infilled with glacial deposits comprising mainly sands, gravels, clays and silts. Depending on the composition of the infill, these may influence groundwater flow in the area.

Permo-Triassic sandstones are present at rockhead over most of the area, with a thick, weathered zone reaching a maximum thickness of at least 20 m. The Permo-Triassic strata have been subjected to syndepositional faulting, leading to large displacements and lateral variation in the thickness of some units.

Hydrogeology of the bedrock

The Permo-Triassic sandstones are the second most important aquifer in the UK and have been well studied in the north-west of England. A key feature is their slow response to change, with observation wells showing a damped response to recharge and abstraction.

The Permo-Triassic sandstones have moderate matrix permeability with fractures providing secondary permeability. The hydraulic conductivity is highly anisotropic, with considerably higher hydraulic conductivity in the horizontal than the vertical, due mainly to the presence of marl horizons within the sandstones. The hydraulic conductivity of the weathered zone might be higher than the underlying sandstone, but we do not have any measurements. Bulk permeability declines with increasing depth, and salinity increases; thus the effective aquifer thickness is considered to be about 200 m. The aquifer has high storativity, which is why it responds relatively slowly to perturbations (such as abstraction) compared to other UK aquifers.

The regional groundwater head gradient would suggest flow in a westerly and north-westerly direction from the main recharge area in the east (the higher ground of the Mid-Cheshire Ridge) towards Ince Marshes and the Mersey Estuary. Faulting can affect the Permo-Triassic sandstone aquifer and groundwater flow in a range of ways, with faults sometimes acting as barriers to flow, or having a high permeability forming a preferential flow path. These are documented regionally, but the behaviour of the groundwater flow in the vicinity of the faults near the proposed site is not known.

Groundwater levels in the Permo-Triassic sandstones have been modified over time by large abstractions, e.g. for public water supply near the recharge area, and for industrial use in the Ellesmere Port and Stanlow areas. Over most of the area the Permo-Triassic sandstone aquifer is confined by low-permeability superficial deposits and the piezometric surface is above the top of the sandstone, but not above ground level in the area. The hydraulic gradient in the area is very low, and thus groundwater flow is expected to be very slow.

There is very little known about the hydrogeology of the Carboniferous strata beneath the Permo-Triassic sandstones in the study area.

Hydrochemistry

The past and present land uses in the area have had considerable effects on the groundwater chemistry, as evidenced by the available data. The natural hydrochemistry of the Permo-Triassic sandstone aquifer, where measured, also gives an insight into much older processes within the system.

The Permo-Triassic sandstone aquifer has zoned salinity: saline palaeowaters are found at the base, moving through a mixing zone to fresh waters at the surface. The saline palaeowaters are thought to be the result of halite dissolution and ponding of the resulting saline water during a warmer period (55 000 to 50 000 years ago) during the last ice age. The saline water was then pushed to depth due to sea-level fall and refreezing of the ground. Abstraction-related marine salinity is seen close to the Mersey Estuary within the upper aquifer, especially around the Stanlow oil refinery. A similar zonation is expected for the redox conditions of the aquifer and associated redox-sensitive hydrochemical parameters. Waters close to the surface are found to be oxic, with low concentrations of dissolved redox-sensitive ions such as iron and manganese, however, deeper groundwaters are reducing with high iron and manganese. The depth of the zones will be variable within the different areas. Faulting will affect zonation with low-permeability faults restricting water movement; this may result in saline palaeowaters being trapped within faulted blocks.

The historical and recent industrial activity in the area is a significant source of contamination of groundwater in the made ground, superficial deposits and the sandstone. Historical landfill sites used by industry including the refinery and the former Ince power station were often unlined and represent point sources of contamination. Run-off and effluent from historical industrial activity along the Mersey's banks flowed into the Mersey Estuary and the Manchester Ship Canal. Although the water quality within the estuary and canal's surface waters has improved, their sediments are still highly contaminated and are a potential source of contamination of surface water and groundwaters, especially where dredged material has been deposited locally on the land surface.

Nitrate contamination is also an issue within the Permo-Triassic sandstone aquifer. This is believed to be mostly agricultural in origin but leaking sewers may also make a contribution.

For more information on the hydrochemistry of the area:
Griffiths, K, Shand, P, and Ingram, J.  2002.  Baseline report series: 2. The Permo-Triassic sandstones of west Cheshire and the Wirral.  Environment Agency, 52pp. (CR/02/109N)

Contacts

For more information, please contact the UK Geoenergy Observatories project team.