Statistical analysis to produce master hydrographs for aquifers in England and Wales

Groundwater level data are a primary resource for all hydrogeological work, from sitespecific enquiries to regional numerical modelling studies. However, groundwater level data held at the British Geological Survey is varied in detail. Information is not evenly distributed to cover major aquifers, but is often concentrated in limited areas, that have been mapped in detail or where boreholes have been drilled. The current water level database “Wellmaster”, includes rest water level data for more than 40 000 boreholes across the UK. The data ranges from high frequency measurements over long time periods (ca. 5% of the total data) to oneoff measurements. While these raw data are useful for point determination of water level for site-specific investigations, it is not an adequate basis for any spatial coverage. There is, though, a need to provide improved spatial coverage of water level data, allowing some estimate of the likely groundwater level across wide areas. In order to provide such coverage, interpolation of water level data, including one-off point level measurements, is needed. To include point water level measurements in interpolations, the data need to be categorised as to whether the given reading is likely to represent very low, low, average, high or very high levels for the respective aquifer. This is done by establishing long term hydrographs deemed typical for the aquifer on the basis of boreholes with long time series data, here called master hydrographs. Comparison of point data with the master hydrograph will allow the point data to be tagged within the database as belonging to one of the five categories ranging from “very low” to “very high”. This will allow the selection of, for example, “average” water levels before interpolation is carried out to obtain spatial coverage of level data for a wider area. This report outlines a basic methodology to process long term hydrographs in order to obtain master hydrographs for the various aquifers in England and Wales. Master hydrographs are established for aquifers with sufficient long-term water level records and findings are detailed in the respective aquifer sections. If, within one aquifer, different response patterns are observed, master hydrographs were developed if a majority of boreholes could be associated with a particular water level response. If this was not the case, it was deemed not feasible to establish a master hydrograph. It was beyond the scope of this present study, to investigate aquifers showing diverse water level responses in more detail to establish the likely causes for particular water level responses over time.