Hydrological Assessment of Star Carr and the Hertford Catchment, Yorkshire, UK

Abstract The survival of one of Europe's most important Mesolithic sites, Star Carr, has been threatened owing to recent changes in its hydrology and associated changes in groundwater geochemistry. Before this study it was unclear what was controlling these changes, but possible causes were changes in precipitation regime (notably the frequency of drought), changes in groundwater abstraction, recent agricultural drainage or a combination of these factors. This paper evaluates the hydrology of Star Carr and its environs within the River Hertford sub-catchment of the River Derwent. Available hydrological data (precipitation, evapotranspiration, river flow and groundwater levels) were collated and used to characterise the River Hertford catchment and provide a hydrological and hydrogeological context for the site. The data were augmented by the insertion of 12 dipwells into the site which were used for both site monitoring and model testing. The monitoring included water abstraction for isotopic analyses (δ2H v δ18O). Two modelling approaches were undertaken: firstly, calculation of the Palmer Hydrological Drought Index (PHDI); and secondly, development of a three layer, finite difference groundwater model of the site using Modflow. The results strongly suggest that hydrological changes at Star Carr have not been caused by changes in precipitation patterns, although the water flowing through the site is largely of meteoric (rainfall) origin. Groundwater abstraction has also not had any impact upon the site. However, results from the groundwater model suggest that the dominant factor has been the insertion of under-drainage, which has lowered the water-table by >0.5 m into the archaeologically sensitive zone. The study reported here shows how high the sensitivity of a small wetland can be to local land management practices, and, significantly, illustrates how a hydrological assessment of wetland archaeological sites can be made even without a history of on-site monitoring.