York Historic Environment Record

Three boreholes and the installation of three instrumented water monitoring points that will record data over a six month period. Borehole 1 was drilled to a depth of 10.5m BGL, borehole 2 to 10.0m and borehole 3 was terminated at 11.0m BGL. Following a period of time to allow groundwater levels to recover, two pressure transducers were installed into the standpipes at BH 1 and BH2 on the 21st September 2019, and a water quality meter was installed on the 8th October 2019 into BH3. Water levels are logged using the Rugged TROLL™ 100 pressure transducer (from In-Situ Europe) suspended below the groundwater table. As these transducers are of the non-vented type, a BaroTROLL™ (recording barometric pressure) was deployed to enable compensation for localised changes in atmospheric pressures. This was installed in BH2 to a depth of circa 0.3m BGL hoping that it remained dry and continually above the groundwater table throughout the monitoring period. Water quality data including optically dissolved oxygen, pH, conductivity, temperature and redox (ORP) has been collected using the Aqua Probe™ 2000 sonde connected to an Aqualogger ™ R2000 datalogger (both retailed by Bell Flow Systems Ltd) installed at a depth of circa 4.0m BGL in BH 3. The groundwater level is strongly influenced by the River Ouse, and fluctuates as the river rises and falls. The lowest level for the water table has been between 5.29m AOD (BH1) and 5.51m AOD (BH2) indicating that the medieval (Phase 2) and lower alluvial silts (Phase 1) have remained fully saturated throughout this time. Post-medieval deposits (Phase 3) lie within the capillary zone, immediately above the water table. However, the nature of the sediments is such that they have the ability to retain water as the river level falls, and hence saturated reducing conditions have been maintained throughout this period, even during the recent flood event in March 2019 where it would appear that the potential impact from the sudden ingress of aerated flood water has been mitigated by the saturated deposits. In conclusion, all available evidence indicates that conditions within the sub-surface deposits below the water table can be described as highly reducing and hence favourable to the preservation of archaeological materials, especially organics.