| Abstract |
This report describes work undertaken at Morestead, Twyford during the 2010/11 recharge season as part of a BGS research project NEE 3344S 'Nitrate Fluctuations in Groundwater'. The previous recharge seasons have been reported earlier in Sorensen et al. (2010a; 2010b). The project uses the same site as that described in Stuart et al. (2008a) for the project 'Nitrate Mass Balance in the Saturated Zone'. This year a third set of data were obtained from the multi-level sampler. Additional procedures were also introduced, as recommended in Sorensen et al. (2010b), to further avoid the potential for sample contamination. Samples were also bailed from the water table on each visit and discrete constant depth samples were taken from Borehole A. During the 2010/11 recharge season groundwater levels varied between 26.1 m bd in October 2010 and only 19.4 m bd in February 2011. The groundwater hydrograph was characterised by an initial rise in water level to 19.4 m bd in early February, a small decline and then a second peak in mid-March. Groundwater levels then declined following an unusually dry March and April and therefore failed to reach the typical maximum levels. A total of 140 samples were collected from Borehole B between 25.9 and 19.4 m bd at a typical depth resolution of 0.05 m. These indicated concentrations of chloride, sulphate and nitrate (as NO3) range between 11.6 and 27.8 mg/l, 7.3 and 24.8 mg/l, and 19.7 and 50.4 mg/l, respectively. Additionally 11 samples were retrieved in the multi-level sampler above the water table. Many of these samples were so dilute that they tend towards expected rainfall composition. Other samples which contained typical groundwater concentrations of chloride are depleted in sulphate and usually nitrate. This could be indicative of these waters been reduced at some point or possibly of an alternative contaminant source which is relatively enriched in chloride such as road salt. One sample collected above the water table contained elevated concentrations of nitrate and this has occurred in both previous recharge seasons. Analysis of the groundwater samples indicated: A general rising trend in nitrate groundwater concentration with water levels between 26 and 19.4 m bd, although reductions in concentration do often follow peaks. Sudden increases in concentration at 24.2, 21.6, 20.7, 20.1 m bd which sometimes correspond with open fractures. This trend is similar to the 2009/10 recharge season. Concentrations always below the porewater concentration at the estimated same depth. No correlation between the observed trends and site visits indicating there are no issues with the current sampling procedure. Constant depth samples in Borehole A suggested initial concentrations of chloride, sulphate and nitrate were similar to those recorded 26-25 m bd in Borehole B but final concentrations were lower. This is indicative of some, but not complete, mixing between shallower and deeper groundwaters. Several recommendations to tackle remaining areas of uncertainty have also been suggested. |
