OR/14/041 Summary and conclusions

Lafare, A E A, Hughes, A G, and Peach, D W. 2014. Eden Valley observation boreholes: hydrogeological framework and groundwater level time series analysis. British Geological Survey Internal Report, OR/14/041.

The results of the time series decomposition, relationships between the hydrogeological setting and the characteristics of the hydrographs can be summarized as:

• The boreholes situated within the St Bees Sandstone generally display hydrographs characterized by a well identified seasonality.
• The hydrographs from the boreholes in the Penrith Sandstone are in general rather more influenced by the long term trend found in the data.
• Therefore, the hydrogeological differences between the Penrith and the St Bees sandstones aquifers are reflected in the differing responses (inferred porosity, storage, hydraulic conductivity and heterogeneity) (Allen et al. 2010; Allen et al. 1997; Seymour et al. 2008; Younger and Milne 1997).

As described in the Geology and hydrogeology of the Eden Valley section, the Penrith Sandstone is characterized by potentially important vertical and/or horizontal heterogeneities. The differences in groundwater level response emphasized by the results of the decomposition could be explained by such heterogeneities.

• A group of boreholes is, for example, characterized by a strong trend component (boreholes 8, 12, 19 and 24). They are located in the northern part of the Penrith Sandstone that is more likely to contain silicified beds that could prevent the aquifer to respond efficiently to localized recharge.
• The borehole at Great Musgrave (10) has an anomalous response not typical of the Penrith Sandstone, which could be explained by the presence of Brockram (see Geology and hydrogeology of the Eden Valley). These carbonate breccias and conglomerates, containing mainly carbonate clasts and calcite cement may function like other other carbonate rocks (Ford and Williams 2007) and could have been locally dissolved and respond rapidly to local recharge events.
• The influence of stream stage via river-aquifer interaction on the hydrograph response is clearly demonstrated by the great importance of the signal remainder after removal of seasonality and long term trends. The best examples are the boreholes situated in the region of Cliburn, near to river Leith (1, 2, 17, 18, 20, 21).
• The depth of the borehole has a distinct influence on the response (paired boreholes 1–2, 17–18 and 20–21) suggesting potential vertical heterogeneity mainly within the Penrith Sandstone (Seymour et al. 2008), which is consistent with presence of beds characterized by different grain size and sorting, as well a secondary cemented beds.

Other potential factors, such as the characteristics of the superficial deposits or the influence of important geological features (faults, Armathwaite dykes, relationships with the carboniferous limestone) require further investigation. Indeed a significant degree of structural control on groundwater flow has for example already been demonstrated within the Permo-Triassic sandstone aquifers of North-West England (Seymour et al. 2006). The dominant faulting appears to be able in this case to divide the aquifers into a series of interconnected blocks.