This report is an output from the Baseline Scotland project,
which ran from 2005 to 2014. It provides a summary of
data on the chemistry of groundwater from the eleven main
bedrock aquifer groups in Scotland.
Groundwater is an important natural resource for Scotland.
It provides drinking water, supports agriculture, and is
fundamental to the nation’s mineral water and whisky
industries. Groundwater also plays a vital role in sustaining
the flow of rivers and supporting many of Scotland’s fragile
ecosystems. The naturally high quality of groundwater in
Scotland is an important part of why it provides so many
benefits. However, groundwater is not invulnerable, and it
needs to be protected and managed to preserve it.
This report presents a synthesis of the results of the Baseline
Scotland project, which mapped the natural chemistry of
groundwater in Scotland’s aquifers. The project ran from 2005
to 2014, funded mainly by the British Geological Survey with
additional support from the Scottish Environment Protection
Agency, and included ten regional surveys that covered much
of Scotland. In this overview, the results of the surveys are
combined to produce a summary of the baseline chemistry
of groundwater in the eleven main bedrock aquifer groups of
Scotland.
These aquifer groups represent a range of hydrogeological
environments with differing geological controls on both
physical aquifer properties and natural groundwater
chemistry. They were primarily divided according to rock
type: sedimentary (indurated sedimentary or calcareous),
metamorphic or igneous; and secondarily according to
geological age. The aquifers are: Permo-Triassic; Carboniferous
sedimentary rocks (not extensively mined for coal);
Carboniferous sedimentary rocks (extensively mined for coal);
Old Red Sandstone North; Old Red Sandstone South; Silurian-
Ordovician; Precambrian North, Precambrian South; Igneous
Volcanics; Igneous Intrusive rocks; and Highland Calcareous
rocks.
The chemistry of groundwater in Scotland’s bedrock aquifers
is highly variable, reflecting a combination of lithology,
mineral reactions, redox conditions, groundwater flow paths
and residence times. Major ion water types include Ca-HCO
3
,
Na-HCO
3
, Na-SO
4
and Na-Cl, with no single type dominating
across Scotland. Total dissolved solid (TDS) concentrations in
groundwater are typically between 54 and 520 mg/L (10–90th
percentile; median 150 mg/L). Some of the highest values of
TDS (up to 5000 mg/L) are seen in Carboniferous sedimentary
aquifers in central Scotland, particularly where mining has
occurred. Elevated TDS also affects some groundwaters in
coastal areas. Mineralised springs (e.g. Na-Cl, Na-SO
4
types)
occur rarely.
Median pH values for each of the aquifers are near neutral,
in the range 6.5 to 7.5 (overall median 7.2). However, acidic
groundwater (pH<6) occurs in most of the aquifers, reflecting
an absence of carbonate minerals and/or oxidation of pyrite
and other metal sulphides. More strongly acidic conditions
can give rise to the presence of dissolved Al, Fe, Mn and REE in
some groundwaters. Oxic groundwater conditions dominate in
most aquifers in Scotland, consistent with dominantly shallow
groundwater flowlines in fractured bedrock. Local exceptions
occur with mildly reducing zones in several aquifers, but the
only regionally extensive reducing conditions are in the Old
Red Sandstone North aquifer, particularly in Moray. Within
the Carboniferous sedimentary aquifers, and in localised
mineralised springs in the Ordovician–Silurian aquifer,
conditions can be more strongly reducing, causing SO
4
and
NH
4
reduction and even methanogenesis at some locations.
The main impact of land use on groundwater chemistry is
the common occurrence of high concentrations of NO3 in
groundwater, which correlate reasonably well with the areal
extent of agricultural land use. Occurrences of high P and K
are also seen, but are more sporadic in distribution, reflecting
the more complex transport properties of these elements in
soils and aquifers.
Not all the groundwaters sampled are used for drinking
water, and it is not appropriate to assess the state of drinking
water quality in Scotland on the basis of the data presented
here. These data nonetheless show the typical chemical
compositions of raw groundwaters from the sampled aquifers,
and indicate the general state of groundwater quality and
any potential problems that may be encountered within
each aquifer. On this basis, the most frequent exceedances
of drinking water limits in the groundwaters are for Fe and
Mn (21% and 27% of samples respectively). These elements,
together with NH
4
(6.7% exceedance), are largely naturally
derived, linked to reducing aquifer conditions. Exceedances
for NO
3
(11%) occur in oxic aquifer conditions, and are linked
to pollutant inputs, particularly from agriculture. Exceedances
for trace elements are less common but do occur locally.
In this report the study methodology is described along with
some of the main factors controlling groundwater chemistry.
The inorganic chemistry of Scotland’s groundwater is then
summarised and put in context, before the baseline chemistry
for each aquifer is presented