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

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. A summary of the conclusions arising from this study follows. 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). 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). 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. 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