Aberdeenshire

A total of 37 samples from Precambrian and igneous aquifers in Aberdeenshire are included in the Baseline Scotland dataset, which were collected between 2004 and 2006 (see map). The sites for samples were chosen so that the baseline data would be representative of groundwater across Aberdeenshire, within each of the studied aquifer types.

Four aquifer groups have been identified: three divisions of metamorphic aquifers of Dalradian age — the Appin, Argyll and Southern Highland groups — and a single group incorporating all the intrusive rocks (largely granitic) across the region.

Main findings

Groundwater pH

Most of the groundwater pH values in the region were slightly acidic (5.5 to 7.2). Groundwaters from the Appin Group tended to have slightly higher pH (median 6.7) than those from the other aquifer groups (median pH 6.2 to 6.3).

Bicarbonate alkalinity

Most groundwaters had relatively low bicarbonate concentrations, with a range from 11 to 127 mg/L (10th to 90th percentiles), and a median of 34 mg/L. There were similar patterns to pH between the aquifer groups, with HCO₃ generally higher (median 80 mg/L) in Appin Group groundwaters, than in the others (medians 19 to 34 mg/L).

Chloride and sulfate

Cl and SO₄ concentrations were generally low but, because of the relatively low mineralisation of the Aberdeenshire groundwaters, Cl constituted a relatively large proportion of the total dissolved anions, particularly in the Argyll and Southern Highland groups and in the igneous aquifers. The most significant source of Cl was likely to be maritime rainfall.

Major cations

Concentrations of the major cations reflected a combination of rainfall input, mineral weathering and anthropogenic inputs derived from the use of fertilisers and liming, or from other sources of pollution. Most notable was a relative enrichment in bicarbonate and calcium and more generally alkaline conditions in groundwaters from the Appin Group compared to the other aquifer groups.

All the groundwaters were undersaturated with respect to calcite, reflecting the general lack of carbonate minerals in the bedrock. The few samples close to saturation were derived from the Appin and Argyll groups, which both contain calcareous bands.

Dissolved oxygen

All of the groundwaters in the study area were oxidising, with dissolved oxygen concentrations generally in the range 3.9 to 9.6 mg/l and a redox potential (Eh) greater than 300 mV. This reflected the unconfined nature of the aquifers and the generally rapid groundwater flow.

Nitrate

Nitrate concentrations were highly variable: the median concentration of nitrate in groundwater was 7.15 mg/l NO₃-N across the aquifers. Concentrations were related to land use, with groundwaters beneath agricultural land showing higher concentrations than those beneath non-agricultural land. The land-use class showing the highest median NO3 concentration (13.6 mg/L NO₃-N, compared to less than 7.6 mg/L NO3-N for other land-use classes) was land used for rearing dairy cattle, pigs or poultry.

Phosphorus

The concentration of phosphorous in groundwater can be an important influence on surface water eutrophication if present in baseflow to streams and rivers. Concentrations in Aberdeenshire generally ranged from 20 to 80 mg/L-P. Although a slightly higher proportion of relatively low phosphorus concentrations occurred in samples from areas of woodland and natural land use than from agricultural land, there was not a strong relationship between phosphorus and land use, illustrating the complexity of phosphorus geochemistry and the important role that soil geochemistry plays in the mobilisation of phosphorus into groundwater.

Interpretations in terms of groundwater flow

The hydrochemical data and information on groundwater residence times can help give an insight into groundwater flow in aquifers across Aberdeenshire. The best interpretation of the groundwater flow system within all of the aquifers was that it was largely through fractures, except in areas of enhanced, deep weathering, where intergranular permeability was likely to have been enhanced and intergranular groundwater flow may dominate.

Groundwater was well mixed in the top 50 m or so. There was no evidence of palaeowater in the area and all samples (where CFCs and SF₆ have been measured) recorded some proportion of water less than 50 years old. However, groundwater can be resident for several decades, even in  fractured, low-permeability aquifers.

Summary statistics

These tables provide a statistical summary of the natural variation in groundwater chemistry in all aquifers across Aberdeenshire: the Dalradian metamorphic aquifers (the Appin, Argyll and Southern Highland groups) and the igneous (mainly granitic) aquifers. Data between the 10th and 90th percentiles for each element or ion are presented, which allows the influence of outliers to be minimised. We would expect new data to plot within this range 80 per cent of the time.

Aquifer groundwater chemistry summaries

• Appin Group
• Argyll Group
• Southern Highland Group
• Intrusive rocks
• Piper diagram illustrating the distribution of groundwater chemistry data for the four main aquifers and indicating outliers

Maps of regional variation in selected ion concentrations

Full report

You can download the full Baseline Scotland: groundwater chemistry of Aberdeenshire report.