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Baseline Scotland : groundwater chemistry of the Old Red Sandstone aquifers of the Moray Firth area

By B.E. O Dochartaigh, P.L. Smedley, A.M. MacDonald and W.G. Darling


The groundwater chemistry of Old Red Sandstone aquifers in the Moray Firth area has been characterised based on new chemistry analyses generated during the Baseline Scotland project, combined with existing analyses from earlier projects. A total of 39 groundwater sample analyses were interpreted for the purposes of this study. Of these, 17 were collected in 2007 specifically for the Baseline Scotland project. These were augmented with a further 22 samples collected during separate BGS projects since 2001. The sites were chosen to be representative of groundwater in the area, and sources that were poorly constructed were avoided.\ud A summary of the conclusions arising from this study follows.\ud 1. Groundwater in the Old Red Sandstone aquifers of the Moray Firth is generally moderately mineralised, with a median SEC of 469 μS/cm (interquartile range 341–591 μS/cm). The pH is variable: median pH is slightly alkaline (7.31), but values range from 5.33 to 8.06. The major ion chemistry appears to be dominated by the dissolution of carbonate cements within the aquifer and overlying deposits, and the variable influence of seawater (either directly as saline intrusion or as aerosols).\ud 2. The dominant cation is Ca, with a median concentration of 54.8 mg/L (interquartile range 42.5 – 74 mg/L). Mg and K concentrations are generally low (median 4.21 and 3.7 mg/L respectively). The median Na is 20.6 mg/L (interquartile range 12.6–28 mg/L); however, a few samples have been affected by proximity to the sea and have much higher concentrations, as illustrated by the 95th percentile (68 mg/L) and maximum concentration (153 mg/L).\ud 3. The dominant anion is bicarbonate, with a median concentration of 183 mg/L (interquartile range 183–230 mg/L). Around one third of the samples are saturated with respect to calcite. Sulphate concentrations are generally low (median 14.6 mg/L, interquartile range 7.7–36.4 mg/L), although higher concentrations are encountered in samples affected by seawater, and/or possibly by gypsum bands within the aquifer. Chloride concentrations follow broadly the same distribution as Na and have a median of 38.8 mg/L and interquartile range of 19.5–49.5 mg/L; the same few samples show high Cl concentrations as do Na.\ud 4. Concentrations of minor and trace elements in the groundwater are dominated by the redox conditions. Measured values of dissolved oxygen indicate a large range in redox conditions across the aquifer. Concentrations of DO close to 10 mg/L indicate fully oxic conditions. By contrast, groundwaters with DO concentrations <1 mg/L are indicative of sub-oxic or mildly reducing conditions, and appear to be prevalent in much of the Upper Old Red Sandstone outcrop, and parts of the Middle Old Red Sandstone. Reducing conditions may reflect the presence of low permeability layers (often marine in origin) within the thick superficial deposits overlying the Old Red Sandstone aquifer. The effect of reducing conditions is to increase concentrations of Fe and Mn, which show median concentrations of 38 and 43 μg/L respectively, and 75th percentile values of 354 and 227 μg/L respectively.\ud 5. The majority of samples (21) were collected from the Upper Old Red Sandstone aquifer; 14 samples were collected from the Middle Old Red Sandstone and only 4 samples from the Lower Old Red Sandstone. The samples show broadly similar chemistry across the three aquifer units; however, there are several notable differences. Samples from the Upper Old Sandstone aquifer show very similar cation distribution, dominated by Ca, while the Lower and Middle Old Red Sandstone aquifers show a wider cation distribution and appear less affected by calcite dissolution. The pH of the Lower and Middle Old Red Sandstone aquifers is slightly lower (more acidic), generally less than 7.0. Groundwaters within the Upper Old Red Sandstone aquifer are generally more reducing, probably reflecting their location close to the coast and hence the influence of the sea and/or overlying marine superficial deposits.\ud 6. Nitrate concentrations are variable across the aquifer units, although median concentrations are low (1.45 mg/L TON-N or less in each aquifer). The prevalence of low oxygen conditions in the sampled groundwaters has led to denitrification, which means the relationship between land use and nitrate concentrations is less obvious than for other parts of Scotland (MacDonald et al., 2005a). However, there is a clear relationship between nitrate concentrations and the Nitrate Vulnerable Zone (NVZ) that covers much of the study area, with the seven highest groundwater nitrate concentrations, ranging from 5.98 to 22.1 mg/L TON-N, all from samples taken within the NVZ. The highest median concentrations were from samples collected on land known to be used for dairy, pig or poultry farming.\ud 7. Phosphorus concentrations in Moray Firth groundwaters range from less than detection limit up to 172 μg P/L, with an overall median of 36 μg P/L, which is in the eutrophic range for surface waters. Concentrations are generally low in the western part of the study area, and an observed relationship with the spatial pattern of F suggests that both elements may be in part derived from the dissolution of phosphate minerals, such as apatite, from the aquifer rocks. Concentrations in the eastern part of the study area are generally higher, usually in the mesotrophic or eutrophic range for surface waters. The higher values may be related to land use, with P inputs from agricultural activity.\ud 8. An estimate of the baseline groundwater chemistry conditions in the Old Red Sandstone aquifers has been presented, based on a statistical summary of the chemical data. This represents data between the 10th and 90th percentiles, with the exception of NO3-N and P, where the influence of anthropogenic activity is likely to have distorted baseline conditions throughout much of the study area. This statistical approach to estimating baseline was complemented by selecting ten analyses of groundwater samples collected from high quality groundwater sources, which are unlikely to have been impacted by any agricultural contamination, and which represent the general the groundwater conditions in the Old Red Sandstone aquifers in the Moray Firth area

Topics: Earth Sciences, Hydrology
Publisher: British Geological Survey
Year: 2010
OAI identifier: oai:nora.nerc.ac.uk:10293

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