This report presents a summary of the groundwater chemistry of the Devonian sedimentary
aquifer in Strathmore, eastern Scotland. The area covered by this study extends from Perth in
the southwest to Stonehaven in the northeast. The survey forms part of the ongoing Baseline
Scotland project.
The Devonian sedimentary rocks of Strathmore form an important regional aquifer in an area
of some of the most fertile agricultural land in Scotland, with a number of major urban
settlements. The aquifer provides water for agriculture, industry, recreation and domestic use.
The aquifer can be divided into six main geological units, largely sandstones but with
significant conglomerates and, less commonly, mudstones. All of these units are classed as
moderately or highly productive aquifers, but too few data are available to allow a detailed
analysis of the hydrogeological variations between the formations.
A total of 35 new groundwater samples were collected during this project, and the resulting
chemistry data combined with data from 13 additional samples collected during a BGS
sampling programme in 2001. The samples were analysed at BGS laboratories for a wide
range of chemical constituents.
The collection and interpretation of new groundwater chemistry data for the Strathmore area
has led to the following conclusions.
• The groundwaters of the Devonian aquifer in Strathmore are mainly weakly
mineralised, with TDS concentrations mostly less than 400 mg l–1. Groundwaters have
near-neutral to slightly alkaline pH values and are for the most part oxygenated, with
detectable dissolved oxygen and high redox potentials. As a result, dissolved iron,
manganese and ammonium (NH4-N) concentrations are usually low. Water from
shallow boreholes and springs is often undersaturated with calcite, but in deeper
boreholes, reaction with carbonate minerals in the aquifer is more usual and most of
these are saturated with respect to calcite.
• Nitrate concentrations are often high, with an interquartile range of 2.6 mg l–1 to
11.7 mg l–1 as NO3-N. Nearly one third of the samples exceeded the EC drinkingwater
limit for nitrate of 11.4 mg l-1 as NO3-N. An anomalously high nitrate
concentration of 81 mg l–1 as NO3-N was observed in one sample, and appears to
result from direct downhole contamination by nitrogen fertiliser. Under the oxidising
conditions, nitrate is a stable solute species across the aquifer and is found at depths in
excess of 100 m.
• Concentrations of phosphorous in groundwater across the aquifer are typically less
than 0.1 mg l-1 (the 90th percentile), with a median of 0.03 mg l-1. Given the
importance of P in controlling eutrophication in surface water, these concentrations in
groundwater may be significant.
• Increased salinity occurs in groundwater in some near-coastal boreholes, most likely
as a result of mixing with seawater.
• Most cationic trace elements have low concentrations, in accordance with the neutralpH
groundwater conditions. Concentrations of arsenic are relatively high in some
groundwaters (up to 8.8 μg l-1) though none exceeds the EC maximum permissible
value for drinking water of 10 μg l-1. Concentrations of uranium reach up to
15.4 μg l–1, with the highest concentration just exceeding the WHO provisional
guideline value for drinking water of 15 μg l–1. Two other exceedances above
maximum permissible values for drinking were observed, for nitrite (highest
concentration 0.194 mg l-1) and fluoride (highest concentration 3.7 mg l-1).
The chemistry and residence time indicators (CFC and stable isotopes) indicate that
the groundwaters are largely of young age, being mostly recharged within the last 40
years, with mixing throughout the top 100 m of the aqufier. The samples show little
evidence of the presence of palaeowaters. The young age of the groundwaters means
they are vulnerable to contamination.
• A first estimate of the baseline groundwater chemistry conditions in the Strathmore
Lower Devonian aquifer can be given by the statistical summary of the data collected
in this study (with the exception of nitrogen (N) and phosphorous (P), where the
influence of anthropogenic activity is likely to have affected concentrations across the
aquifer). To compliment this approach, six good quality sites have been chosen which
represent the majority of groundwaters found in Strathmore.
• The most significant groundwater-quality problems identified by this survey are:
o the widespread presence of elevated nitrate concentrations in groundwater,
which is strongly linked to agricultural activity;
o elevated phosphate concentrations which may affect the quality of surface water
when discharged to rivers as baseflow.
o the presence of saline water in some near-coastal boreholes, indicating localised
saline intrusion. It is not clear to what degree the saline intrusion is natural and
to what degree it has been enhanced by over-pumping
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