Hydrogeological investigations at Morestead, Twyford, 2009-2010

This report describes work undertaken at Morestead, Twyford during the 2009/10 recharge season as part of a BGS research project “Nitrate Fluctuations in Groundwater”. The previous year’s work had been reported earlier in Sorensen et al. (2010). The project uses the same site as that described in Stuart et al. (2008a) for the project “Nitrate Mass Balance in the Saturated Zone”. During the 2009/10 recharge season a second year’s set of data were obtained from the multilevel sampler. The sampler was modified to allow sample collection at a greater resolution of 0.05 m over the first 2.05 m and at 0.1 m resolution up to 3.9 m. This was deployed in association with a GPRS telemetry system. This allowed the visualisation of water levels in the office which facilitated targeted, more efficient site visits. The telemetered Druck PDCR 1830 also provided superior quality water level data in comparison with the CTD Diver. During the recharge season water levels varied between 26 m below datum (m bd) in November 2009 and 12.3 m bd in February 2010. The groundwater hydrograph was characterised by an initial rapid rise in water level to 17 m bd, which was followed by four successive peaks. All water level peaks could be correlated to specific rainfall events. A total of 252 groundwater samples were collected between 25.73 and 12.34 m bd at a typical depth resolution of 0.05 m. These indicated concentrations of chloride, sulphate and nitrate (as NO3) range between 14.6 and 39.5 mg/l, 9.9 and 37.3 mg/l, and 21.8 and 62.2 mg/l, respectively. Additionally a discoloured sample was retrieved from above the water table which contained a chloride concentration of 85.3 mg/l, sulphate of 104 mg/l and nitrate of 123.7 mg/l. The high levels of H2PO4 2- indicated that this sample was derived from the near surface suggesting that pollution of the borehole from the surface is possible at the site. Analysis of the groundwater samples indicated:  A general rising trend in groundwater nitrate concentration with water level towards a maximum around 14.3 m bd, although concentrations decreased between 17.5 and 16.5 m bd and also 19.5 and 18 m bd.  Sudden increases in concentration at several depths, notably at around 20 m bd and 15.5 m bd corresponding to site visits and/or open fractures.  Fractures could be delivering high levels (e.g. 20 m bd) or low levels (16.8 m bd) of nitrate to the water table.  A potential correlation with an estimated current nitrate porewater profile with concurrent peaks at 14.5-15.0, 19.5 and 24.0 m bd.  Concentrations below the adjacent porewater concentration. Several recommendations regarding the improvement of the sampling procedure and future work have also been made.