Difference between revisions of "Hydrogeology of Wales: Precambrian and Cambrian aquifers - groundwater occurrence in the Cambrian"
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Revision as of 11:02, 22 August 2013
|This page is part of a category of pages that provides an updated review of the occurrence of groundwater throughout Wales.
Author(s): N S Robins and J Davies, British Geological Survey
Contributor(s): D A Jones, Natural Resources Wales and G Farr, British Geological Survey
Groundwater occurrence and circulation in the Cambrian sedimentary rocks is much the same as it is in the crystalline basement rocks described above. Groundwater is contained largely in the near-surface weathered and fractured zone of bedrock which offers little storage potential with transport via dilated fissures. Again, flow paths are typically short and shallow and are within catchment scale — usually down hill slopes towards valley bottoms. Spring discharges occur where fracture systems intercept the ground surface and along valley bottoms to provide base flow to surface waters. Steep topography over much of the Cambrian outcrop provides additional transport of ‘groundwater’, via soil or scree interflow, to discharge into valley bottom streams ( Shand et al. 2001).
|Llanberis slate quarry. P802416.|
Although there are numerous springs associated with the Cambrian outcrops some are sourced partly, if not entirely, from overlying drift deposits. Collections from groups of bedrock springs have in the past been used for public supply. One example was the Nine Wells springs [SM 787 248] which were operated by the St Davids Water and Gas Company. The nine springs drained directly from bedrock slates into a chamber and delivered up to 225 m3 d-1. In the same area of Pembrokeshire at Solva [SM 7862 2507] a 10 m-deep, 1 m-wide shaft penetrating a clearly visible fault in well-jointed fine-grained sandstone belonging to the Menevian Group, was capable of yielding up to 35 m3 d-1. The static water level ranged between 1 m and 8 m below ground level depending on season.
In north Wales, detailed engineering investigations were carried out in exploratory boreholes in the Llanberis Slate Formation (Plate P802416) during the construction of the Central Electricity Generating Board’s Dinorwic Pumped Storage Scheme in the 1970s (Robertson, 1974). Heat-pulse flow logs of two of the boreholes in the valley bottom (see Dinorwic exploratory boreholes table ), measured under non-pumping conditions, are shown in FIGURE 2.3. These show upward movement of groundwater from the interception of the lowest active fracture in each of the boreholes EP4 and EP9. The upward flow continues to a point near the top of the water column in both boreholes. It demonstrates the increasing head with depth on active fractures in valley bottoms, and reflects the interception of successively longer flow paths, each upwelling along the valley bottom and derived from a higher recharge elevation on the valley side (FIGURE 2.4). By contrast, exploratory boreholes EP5 and EP7 in the same vicinity were static throughout the borehole column indicating constant head in the fractures penetrated reflecting poor contact with the overall fracture system.
|Borehole||Grid Reference||Depth (m)||Rest water level on 5 March 1974|
|EP4||SH 5995 5875||36.8||1.2|
|EP5||SH 6007 5882||33.8||1.6|
|EP7||SH 6000 5881||52.3||1.8|
|EP9||SH 5983 5879||38.0||0.9|
Boreholes EP4 and EP7 were also flow logged during pumping at 2 l s-1. The flow logging was carried out with an impeller and fluctuating pumping rates caused some error to arise. However, the work showed that all the pumped water in EP4 derived from the uppermost 15 m of the borehole, reflecting the location of active fractures seen in the static log (FIGURE 2.3). Borehole EP7 which showed no upward transport of water in the non-pumped state revealed its production zone to be located between 4 and 10 m below ground level, again demonstrating the shallow nature of active groundwater flow in these rocks.
Pumping was only maintained for brief periods at Dinorwic and sustainable yields are likely to be smaller than the recorded 2 l s-1. Typical yields are about 0.5 l s-1, for example from a 24 m deep borehole in the Ffestiniog Flag Formation at Criccieth [SH 5266 3991]. The static water table in this borehole is 1.3 m below ground level. However, it should be remembered that the distribution of springs and wells is significantly greater than that shown from the records in FIGURE 1.2 as many sources have not been recorded.