The Sherwood Sandstone Group of Yorkshire and the East Midlands is the second most important aquifer in the UK. It provides a source of groundwater for industry, agriculture, and the home, especially in northern and central England.
Increasing demands on this natural resource, and corresponding legal processes including the EU Water Framework Directive require that the Environment Agency, the authority responsible for licensing the way in which the aquifer is used, enhance their understanding of the region's water by producing a groundwater model.
The Fylde Aquifer
Fig 10.30: Schematic cross-section through the Fylde Aquifer (source: Groundwater hydrology, Fig 10.30, p330)
The Sherwood Sandstone of the Fylde Aquifer is bounded in the east and underlain by Carboniferous strata, comprising interbedded mudstones, shales, sandstones and limestone (fig. 10.30, above). Permo-triassic sediments were deposited on the Carboniferous; the Sherwood Sandstone Group is the main aquifer. From it's contact with the Carboniferous in the east, the thickness increases to over 500m; it is then downthrown by up to 600m beneath the siltstones and mudstones of the Mercia Mudstone Group.
The Sherwood Sandstone Group is predominantly a fine-to-medium-grained sandstone with occasionally inter-bedded mudstone (marl) beds. The sandstone aquifer is almost entirely covered by drift deposits, which are mainly inter-bedded boulder clay (till), with sands and gravels of glacial origin. The thickness of the drift various from 5m to 30m.
Since the 1970s, the Fylde aquifer has been part of the Lancashire Conjunctive Use Scheme, in which upland reservoirs, river abstractions, river transfers and borehole sources are used conjunctively to utilise the cheaper and more abundant surface water when available, but relying on groundwater to meet the shortfalls, especially in times of drought (Walsh 1976). Since the groundwater sources are only called upon in drought years, skill is required in utilising these resources.
Abstractions are licensed according to three-year 'rolling' totals; abstraction is spread over the whole aquifer by using groups of boreholes. to avoid the risk of saline intrusion, groundwater gradients must be positive towards the boundaries. River augmentation is used to lessen the impact of groundwater abstraction with 'hands-off' conditions enforced at certain observation boreholes to prevent derogation of other groundwater sources.
Due to the extensive low-permeability drift cover, the original conceptual model of the Fylde acquifer was that the vertical flow through the drift would be neglible but that pumping would draw water from the Carboniferous strata to the east; this is indicated on Figure 10.30 by arrows with a question mark. A mathematical model was derived in which the Carboniferous strata provided most of the water pumped from the sandstone aquifer.
This initial analysis proved to be unreliable. A careful study of the Carboniferous strata shows that, due to faulting and the low permeability of many of the strata, little water can be drawn into the sandstone aquifer from the Carboniferous. Further more, it is possible to draw water downward through the low permeability drift in a similar manner to the Mehsana aquifer (Section 10.2). In a recent study (Seymour et al. 1998), conceptual and mathematical models of the Sherwood sandstone aquifer are developed with an upper layer representing the drift. As abstraction occurs from the sandstone aquifer, water is drawn down through the drift. These flows are represented in Figure 10.30 by small arrows at the top and bottom of the drift.
However, certain observation borehole responses at some distance from the major abstraction sites were not reproduced adequately by the model. When a single layer is used to represent the vertical leakage through the drift, the storage properties of the drift are ignored.
As shown in Section 7.5.4, failure to represent the storage qualities of the drift can lead to incorrect simulations. A more detailed investigation showed that towards the bottom of the drift there are extensive sand and gravel deposits, as indicated in Figure 10.30. When substantial abstraction occurs from the sandstone aquifer, the source of water most easily accessible is from the sand and gravel deposits towards the bottom of the drift. This means that is easier to attract water than if it has to be drawn through the full thickness of the low permeability drift layer. Local dewatering of these sand and gravel deposits has subsequently occurred. Consequently, larger pumped drawdowns are now required to draw water through the drift into the sandstone aquifer and then into the pumped boreholes.
When the true nature of the drift is represented in the numerical groundwater model, improved simulations are achieved.
Source: Groundwater Hydrology: Conteptual and Computational Models by K. R. Rushton
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