Groundwater NO-3 concentrations decrease along the hydraulic gradient in the Lincolnshire Limestone (66mg1-1 NO-3) and the Berkshire Chalk (21mgl-1NO-3) to trace values at the interface between the oxidized and reduced aquifer zone. The NO-3 is initially diluted by stored pore water, but further down gradient denitrification results in enhanced dissolved N2 and the appearance of NO-2. N2 is then reduced to NH+4 in the deep aquifer. Semi-quantitative determinations of the N2/Ar ratio were made by mass spectrometry (+/-4.9 ratio units, 20 = 2.32) to detect N2 gas enhancement by NO-3 denitrification. 15N/14N ratios of nitrogen species were used as provenance indicators and incorporated into a model of the s15N of dissolved N2. Stable Isotope Dilution Analysis, gas chromatography and absolute gas pressure measurements were extensively tested but proved inaccurate in the determination of N2 concentrations. In the Lincolnshire Limestone the s15N of dissolved NO-3 (+2.770/00 and +7.590/00 +/-0.650/00) is typical of the soil. The 15N/14N ratios for dissolved N2 shows that it had a lower 15N content than the dissolved NO-3. The 15N/14N ratios decrease with depth in the aquifer and this is shown to be the result of isotopic fractionation produced by the diffusion of NO-3 from fissure water into the pore waters and its subsequent bacterial denitrification. NO-3 reducing bacteria were identified in the Lincolnshire Limestone groundwaters. Quantitative determination of Ar and other inert gases were used to establish palaeo recharge temperatures and in the Berkshire Chalk the oldest groundwaters were shown to be permafrost meltwater. 4He and Cl- concentrations in dicated the presence of older groundwaters and the extent of mixing between modern fissure water and stored fluids. The downdip decrease in NO-3 concentrations is greater in the Lincolnshire Limestone (4.1 and 1.6mgl-1 NO-3 km-1) than in Berkshire Chalk (0.66mgl-1 NO-3 km-1) where the shape of the NO-3 concentration profile indicated that contaminated pore waters are present. The continued removal of NO-3 by dilution with stored pore water is less effective in the Chalk than in the Lincolnshire Limestone.
|Date of Award||1982|