Reconnaissance hydrogeological survey of Guernsey

The key findings from this study are: • There is no evidence of any old waters or of any mineralisation indicative of long or deep flow paths from an off-island source; • The unpredictable nature of the so called “deep aquifer” (25 m plus beneath the water table) does not warrant its further exploitation; • The uppermost “shallow aquifer” can best be exploited by dispersed small volume sources • The groundwater body and the surface water body are part of a single water system; • Reduction in aquifer yield with depth provides a self protection mechanism to the aquifer; • Based on the survey conducted there is a consensus on Guernsey that the renewable water resource is finite and that it needs safeguarding. The annual volume of water supplied by the States of Guernsey Water Board is approximately 5 Mm3. Additional abstraction from private sources may amount to a further 1.5 Mm3, particularly in a relatively dry year. The public water supply derives from surface water courses and is stored in surface water reservoirs. The private abstractions draw on groundwater (wells, boreholes and springs), roof top collection and surface water abstraction. The groundwater body and the surface water body are part of a single water system. Groundwater discharge (or baseflow) from springs and seepages into streams maintains the surface water low-flows during prolonged dry weather. The groundwater body is itself divisible into three contiguous levels. Where present, there is an upper granular aquifer within superficial deposits of alluvium and raised beach material. Beneath this is the main aquifer which is contained within the shallow weathered zone of the bedrock, which is underlain by a deeper aquifer with groundwater flow restricted to occasional dilated fractures. Bedrock mainly consists of ancient crystalline metamorphic rocks. Generally, the water table lies within 3 to 8 metres of the ground surface, and the main aquifer, in which the majority of groundwater flow takes place, is situated in a 25 m zone immediately below the water table. Beneath this depth there is some groundwater flow in deeper fractures, but borehole yields from the greater depths are commonly less than those from the shallow weathered zone. This reduction in aquifer yield with depth provides an element of self protection, whereby baseflow discharge from the aquifer and abstraction from boreholes is automatically reduced as the water table falls. Groundwater recharge occurs in the High Parishes and flows south to the coast and north-west to the coast. There is little potential for groundwater flow beneath the low-lying land towards the northern part of the island. Using meteorological and catchment recharge calculations from Jersey as an analogue, the normal average annual water budget for the island can be assigned: Rainfall = 831 mm = Potential evapotranspiration = 613 mm + Streamflow = 226 mm (of which nearly 60% derive from groundwater recharge as baseflow) + Groundwater recharge = 128 mm iv These figures do not reflect a poor rainfall year in which resource renewal may be small and infiltration may be zero. Annual variation in rainfall from the long-term mean is significant, and annual rainfall has been declining since the 1940s over Guernsey. The groundwater contains an element of salinity derived from rainfall and sea spray, although there is little evidence of actual physical marine invasion of the aquifer. The groundwater samples were otherwise moderately mineralised with specific electrical conductance in the range 427 to 1578 μS cm-1. Over half the 21 samples collected contained NO3-N (nitrate) at concentrations greater than the EC maximum admissible concentration. Some of the NO3-N may be derived from leaking cess pits, but past application of nitrogen fertilizer to cultivated land probably accounts for the majority. The average pH of the samples was 6.4. This acidity reflects the lack of carbonate material in the aquifer, other than shelly debris present in the superficial deposits. Attempts at groundwater dating by analysis of CFC species at a small number of sites was hindered by local contamination, particularly in the vicinity of the airport. It is likely that the pumped groundwaters are mixtures of young and older water. If this is the case, then at three of the four sites sampled, between 75 and 100% of the water can be identified as young, recently recharged water, in keeping with shallow, short flow path groundwater circulation in the main weathered aquifer. At the fourth, only between 40 and 50% of the sample consisted of the young component. However, there was no evidence of any old waters or of any mineralisation indicative of long or deep flow paths from an off-island source. There is an urgent need to create a small groundwater level and groundwater quality monitoring network. This would provide early warning of any change that may be occurring to the groundwater body. A catalogue of point source pollution risk activities is also recommended. Given consumption of about one third of the overall water resource it is also recommended that a comprehensive evaluation of the groundwater system would assist the overall management of the resource

Mine water rebound in South Nottinghamshire : risk evaluation using 3-D visualization and predictive modelling

Progressive abandonment of the South Nottinghamshire Coalfield raises concerns over the security of the Permo–Triassic Sherwood Sandstone aquifer which overlies the concealed part of the coalfield. A 3-D digital visualization package has been used to assemble and display the complex and diverse data-sets of relevance. Predictive scenarios have been run from these data using the University of Newcastle program GRAM (Groundwater Rebound in Abandoned Mineworkings). The work comprised three phases: (i) confirmation of the geological framework for the so-called ‘Pond 3’ area (southernmost part of the coalfield) and establishment of a water balance along with an outline groundwater flow path system for the Coal Measures and adjacent strata; (ii) the collation of detailed geometric information on the spatial distribution of discrete geological layers that are considered to have hydrogeological significance, the distribution of mineworkings within key horizons, and the locations of boreholes, shafts and pumping stations (both in the Coal Measures and within overlying strata). Possible flooding configurations have been assessed geometrically to identify ‘hot spots’ where mine water discharge to surface may occur, and areas where the piezometric level of the rising mine water might promote upward fluxes into the Permo–Triassic Sherwood Sandstone aquifer. In addition, critical areas where coal has been worked close to the base of the Permian and where hydraulic continuity may occur between the Sherwood Sandstone and Coal Measures have been identified; (iii) the GRAM model used data held in the 3-D visualization package VULCAN to define discrete ‘ponds’ within the coalfield. Recharge to the system allows each pond to fill until overflow pathways are reached, when the adjacent pond may start to fill. A variety of such scenarios have been completed and predictive data generated, which suggest that possible discharge to surface and into the Sherwood Sandstone might occur about 20 years after the end of dewatering

The arsenic concentration in groundwater from the Abbey Arms Wood observation borehole, Delamere, Cheshire, UK:

A 150 m observation borehole was drilled in Abbey Arms Wood, Delamere, Cheshire, UK in order to explore the local hydrogeological conditions and to understand better the source of the high concentrations of arsenic in some of the local groundwaters. The borehole was located on an outcrop of the Helsby Sandstone Formation (part of the Sherwood Sandstone Group) and was cored into the underlying Wilmslow Sandstone Formation. The aquifers in the area are unconfined and give rise to low-Fe groundwaters with As concentrations in the 10–50 µg l–1 range. The chemical composition of the sediments is quite uniform down to 150 m. The total arsenic content is in the range from 5 to 15 mg kg–1 and averaged 8 mg kg–1 (n = 60). There is no trend in sediment As concentration with depth, but pore water centrifuged from the core steadily increased in As concentration with depth. The As concentration ranges from 8 µg l–1 at 10 m (unsaturated zone) to 30 µg l–1 at 150 m. The source of the dissolved As remains unclear but the lack of evidence for discrete high-As minerals or zones of mineralization suggests that it is probably derived by desorption from rock-forming minerals in the sandstones, e.g. iron oxides. This may be in response to slightly higher pH (up to 8.0 at depth). If this trend applies throughout the area, restricting the screened interval for abstraction boreholes to the uppermost parts of the saturated zone may reduce As concentrations, but is likely to reduce yields and may also risk encountering groundwaters with high nitrate concentrations

The physical properties of minor aquifers in England and Wales

This report is the result of a three-year collaborative project between the British Geological Survey and the Environment Agency. The aim of the project has been to collect, collate and present information concerning the physical hydraulic properties of the minor aquifers in England and Wales. These properties include hydraulic conductivity, porosity, transmissivity and storage coefficient. In addition, specific capacity (yield per unit drawdown) values are included for many of the formations described, together with yields for those formations where aquifer properties data are sparse. Although the parameters studied were limited in number, the study has proven to be complex for several reasons. Firstly the aquifers themselves are hydraulically complicated. They are bodies of rock, sometimes with indeterminate boundaries, which are heterogeneous either because of sedimentological factors in the case of the Cainozoic aquifers, or because of the effects of fracturing in older formations. This heterogeneity presents several problems. Firstly, hydraulic tests on such materials often violate the classical assumptions used in the test analysis, and the complexity of the aquifers makes interpolation between data points difficult. Secondly, the physical properties of the aquifers are often scale dependent, so that the value of a parameter at one scale may not be appropriate for use at a larger or smaller scale. Thirdly, there are problems of data quality and quantity which are particularly significant for these smaller aquifers. The quality of the pumping tests is variable and many results are from short duration pumping tests which are designed more to assess the yields of boreholes than to examine the properties of the aquifer. Also, data can be very irregularly distributed, being a product mainly of the evolving requirements of groundwater users and not of well-planned resource assessments. This irregular spacing can be both vertical as well as lateral, as in the case of thick structurally complex sequences with only scattered productive horizons. Awareness of these inherent hydrogeological factors dictated the project’s approach, which was to collect both data and knowledge about the aquifers. This permits the report to describe not only the magnitudes and variability of the aquifer parameters at a given tested locality, but also to provide some insight into factors controlling the properties, so that the results can be more confidently extrapolated. Project resources were therefore initially employed in data collection. This involved a detailed search through Agency records, with additional information from BGS, published and unpublished literature. Most of the data obtained were from analysed pumping tests, the results of which were entered in a database. The latter originally housed data on the major aquifers, collected under a preceding project, but the database needed to be significantly altered and expanded so as to manage efficiently the much larger number of aquifers involved. It was also linked with the BGS Core Analysis Database. The result comprises the National Aquifer Properties Database which is now a major UK geoscience resource, with data from more than 8000 pumping test analyses at over 8250 sites. The second main strand of the project was the collection and summarising of knowledge about the aquifers. In addition to the collection of reports of hydrogeological studies and a literature survey, expert opinion was canvassed. The latter is a vital source of information that is not often published. The results of these two approaches are synthesised in this report. After the introductory sections each chapter takes the form of a detailed review of the physical properties of a group of minor aquifers, subdivided as appropriate on stratigraphic or geographical grounds. The chapters are arranged in order of increasing age. The purpose of the review is to present the magnitudes and variability of the data (mainly from the database, but with other examples) in the context of current understanding of the aquifer systems involved and the controls on the data. To that end the review includes geological, geographical and physical hydrogeological aspects of the aquifers. Useful summaries of data from the database are included on the accompanying CD-ROM. The intention of the report is therefore to acquaint the reader with the aquifer properties data values that characterise the aquifers in the context of what is known about the complexities of their hydraulic structure and the physical controls on the data. The reader is specifically dissuaded from taking raw values out of context. A further purpose of the report is provide a comprehensive set of references by which the reader can obtain more detailed information about particular areas of interest in an aquifer. As a result of the collection and review of information about the physical properties of the minor aquifers in England and Wales, it is apparent that there are many areas in which knowledge is inadequate. For example, a critical comparison of the equivalent aquifer systems in the London and Hampshire basins was not possible in other than the most general terms. Similarly, the lateral variability in aquifer properties in the Lower Cretaceous aquifers of the Weald is suspected to arise partly from fault-controlled compartmentalisation, but the role of the faults is not well enough understood for predictive purposes. For all the effort expended on geological characterisation over almost two centuries of detailed study of English Jurassic rocks, the flow systems of the numerous arenaceous and carbonate minor aquifers of that system are in general poorly characterised. Very localised borehole development and the effects of tapping complex multi-aquifer sequences mean that the fracture-dominant, structurally-affected systems of older rocks of Palaeozoic age are in many cases barely conceptualised. Such gaps in our knowledge are inevitable considering the paucity of data. Nevertheless, the project has provided the first opportunity to review comprehensively the aquifer properties of this second rank of British aquifers whose role is so important in providing local sources of water supply for both private and public use