Abstract

Adataset of well and spring locations in NW Anglesey with accompanying geological observations was recently discovered at the British Geological Survey. The data are likely to have been recorded as part of an environmental survey in the late 1960s or early 1970s. The area comprises Palaeozoic and Precambrian rocks partly covered by moderately permeable till. The bedrock forms a shallow fractured aquifer, which offers individual well and spring discharges up to 2 l s−1. The newly discovered data provide an indicator of the hydraulic characteristics of each bedrock type such that the Holyhead Quartzite Formation offers the least favourable hydraulic conditions, whereas a small area of Silurian (Llandovery) strata at Parys Mountain offers the most favourable hydraulic characteristics. The occurrence of discontinuities caused by dyke intrusion and metamorphic foliation make some Precambrian strata better disposed to successful well digging than others. Base flow index data indicate that storativity is greatest in the Precambrian and the Carboniferous Limestone Formation. Groundwater chemistry reflects shallow, short groundwater flowpaths.

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