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Baseline Scotland : the Lower Devonian aquifer of Strathmore

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

Abstract

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

Topics: Hydrology, Earth Sciences
Publisher: British Geological Survey
Year: 2006
OAI identifier: oai:nora.nerc.ac.uk:10299

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