Baseline groundwater chemistry in Scotland's aquifers

MacDonald, A.M. ORCID: https://orcid.org/0000-0001-6636-1499; O Dochartaigh, B.E.; Smedley, P.L.. 2017 Baseline groundwater chemistry in Scotland's aquifers. British Geological Survey, 77pp. (OR/17/030)

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Abstract/Summary

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.

Item Type:	Publication - Report
ISBN:	9780852728697
Funders/Sponsors:	British Geological Survey, SEPA
Additional Information. Not used in RCUK Gateway to Research.:	This item has been internally reviewed but not externally peer-reviewed
Additional Keywords:	GroundwaterBGS, Groundwater, Hydrogeological data, Groundwater quality
Date made live:	30 Jan 2018 14:29 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/519084

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