Hydrogeological investigations at Morestead, Twyford, 2008-2009 (preliminary observations)

This report describes work undertaken at Morestead, Twyford as part of a BGS research project “Nitrate Fluctuations in Groundwater”. The project uses the same site as that described in Stuart et al. (2008a) for the project “Nitrate Mass Balance in the Saturated Zone”. A new borehole (Borehole B) was drilled in October 2008, located 10 m to the west of the existing one (Borehole A) to complement the existing infrastructure. During drilling the cuttings were inspected but no systematic recording of lithology was undertaken, as this information was already available in detail from the adjacent borehole. Borehole B was completed to a depth of 70 m and was cased to a depth of 12.7 mbgl; remaining unsupported below this. It should be stated that the casing protruded approximately 0.16 m above ground level. Initial geophysical characterisation was undertaken to evaluate the hole and its suitability for further testing. A multi-level sampler was installed in Borehole B during autumn 2008 when the groundwater level was low. The aim was to capture samples as the water table rose towards the anticipated spring 2009 high. The sampler worked well for most of the period, but there were issues with bottles not filling correctly, and the sampler length was insufficient to cope with rapid water level rise following heavy rainfall. A selection of the samples retrieved were analysed for nitrate, and provisional results ranged between 26.3 and 45.3 mg/l, with the exception of anomalous results. Concentrations of nitrate appeared to increase as the water table rose above 20.5 mbgl. However below this, nitrate concentration appeared to decrease with depth. Concentrations of sulphate and chloride both increased as the water table rose. Gamma-ray and resistivity logging results obtained in March 2009 identified marl horizons which correlated with the core from Borehole A, as reported in Stuart et al. (2008a). Fluid and flow logs indicated hydraulic layering within the Chalk. Temperature and conductivity logs suggested downward flow below 26 m below datum (m bd where datum = casing top). Heat Pulse Flow Metre (HPFM) results also indicated downward flow between 28 and 45 m bd. During pumping at 0.5 l/s the HPFM data showed downward flow was still present between 24.8 and 26.3 m bd. A nitrate sonde was also run before and during pumping. The results are currently awaiting calibration and are not available at the time of reporting. Groundwater levels were monitored in Borehole A and fluctuated 13.5 m, from around 10 to 23.5 mbgl, between December 2006 and the beginning of March 2009. However, numerous gaps in the data remain due to water levels exceeding the range of the pressure transducer initially installed in the borehole. Water levels were notably responsive to rainfall between December and February, for example between 25th January 2009 and 23rd February 2009 the water table rose by 8.7 m. The aquifer appears less responsive during summer and autumn when there is likely to be a greater soil moisture deficit.