Hydrological Monitoring of an Alluviated Landscape in the Lower Great Ouse Valley at Over, Cambridgeshire: Results of the Gravel Extraction Phase

Abstract

This paper sets out the dramatic results of the hydrological monitoring programme conducted during the first 2.5 year phase of gravel extraction at the Hanson Over quarry in Cambridgeshire. It follows on from the study of the pre-extraction phase detailed in Environmental Archaeology 4 (French et al. 1999). Less than two months after the start of gravel extraction and pumping, the groundwater table within the extraction area had dropped to 5m+ below the modern ground surface and has since been maintained at that level. This is 3.5–4m lower than the norm in the pre-extraction phase, and has been accompanied by marked increases in pH and dissolved oxygen values. Specific monitoring of a Bronze Age barrow within the extraction area has demonstrated significant dewatering and lowering of soil moisture contents in the mound, ditch and buried soil to minimal levels within six months. Moreover, the groundwater table downstream and beyond the extraction area gradually dropped to between 2m and 5m below the modern ground surface, at a distance of up to 500m from the quarry face. Beyond this 'halo' effect, the hydrological regime is apparently unaffected. Nonetheless, once the southern edge of the extraction area was bunded with impermeable clay, there was a rapid (within the month) restoration of groundwater levels to the south/upstream of the extraction area. But, the groundwater table to the north/downstream of the extraction area has continued to fall over a distance of 500m from the quarry face. These results are of wide applicability to lowland English river terrace valleys. To ensure the future curation of alluviated landscapes that are threatened with water and gravel extraction, groundwater and water quality monitoring programmes should be part of the archaeological brief in response to Policy Planning Guidance 16 (DoE 1990).