Baseline Groundwater Quality : a comparison of selected British and Norwegian aquifers

The aim of this work is to present a discussion on the concept of baseline and to compare the natural variations in inorganic water quality present in a selection of British and Norwegian groundwaters. The use of boxplots and cumulative frequency plots facilitates comparison between individual rock aquifers, different regions with divergent geological and climatic records, and between various elements and parameters. The range of baseline concentrations is often large; e.g. uranium concentrations in Precambrian granitic groundwaters in Norway spans almost 4 orders of magnitude. Baseline values are useful as a means to assess pollution or to set a realistic base for remediation. The EU Maximum Admissible Concentrations (MAC values) of drinking water should be set on toxicological criteria only, as natural unpolluted groundwater sometimes contain elements in concentrations deemed to be harmful. Most of the hard rock groundwaters in Norway have relatively high pH compared with those of the UK. Na-HCO3 type waters seem to be much more common in Norway than in the UK where Ca-HCO3 type water dominate. High F, U and Rn concentrations are found in many granitic and sedimentary groundwaters in Norway, while Ba concentrations tend to be higher in the UK sedimentary aquifers. Universal baseline values do not exist for any element and statistical representative sampling from all aquifers is necessary to establish reliable knowledge about the natural groundwater quality in each area. A suggested series of methodologies are suggested which can be applied to aquifers where the effects of anthropogenic pollution are present. There is a strong need for timeseries data on a wide range of parameters to ascertain the long-term effects of human activity on groundwater quality. The trends of groundwater quality with depth should also be studied more thoroughly. In order for a European wide policy to be implemented it is necessary to establish protocols for criteria related to data quality, sampling and analytical wor

Evolution of groundwater chemical composition by plagioclase hydrolysis in Norwegian anorthosites

The Precambrian Egersund anorthosites exhibit a wide range of groundwater chemical composition (pH 5.40–9.93, Ca2+ 1.5–41 mg/L, Na+ 12.3–103 mg/L). They also exhibit an evolutionary trend, culminating in high pH, Na-rich, low-Ca groundwaters, that is broadly representative of Norwegian crystalline bedrock aquifers in general. Simple PHREEQC modelling of monomineralic plagioclase–CO2–H2O systems demonstrates that the evolution of such waters can be explained solely by plagioclase weathering, coupled with calcite precipitation, without invoking cation exchange. Some degree of reaction in open CO2 systems seems necessary to generate the observed maximum solute concentrations, while subsequent system closure can be invoked to explain high observed pH values. Empirical data provide observations required or predicted by such a model: (i) the presence of secondary calcite in silicate aquifer systems, (ii) the buffering of pH at around 8.0–8.3 by calcite precipitation, (iii) significant soil gas CO2 concentrations (PCO2 > 10−2 atm) even in poorly vegetated sub-arctic catchments, and (iv) the eventual re-accumulation of calcium in highly evolved, high pH waters

The natural inorganic chemical quality of crystalline bedrock groundwaters of Norway

One thousand, six hundred and four groundwater samples from crystalline bedrock aquifers in Norway have been analysed for pH, alkalinity, radon, major ions, and metals. A subset of 476 samples was further analysed for a range of trace elements by ICP-MS. For most major ion parameters, including pH, the median values display modest dependence on lithology, while there is a large variation in hydrochemistry within each lithological subset. Three main water types can be distinguished: Na–Cl waters, Ca–HCO3 waters and Na–HCO3 waters, of which the latter group comprises 25% of the samples. Many trace elements show a clear dependence on lithology with enrichments in aquifers of Precambrian granites relative to Precambrian anorthosites. Concentrations of fluoride, radon and uranium are above given drinking water limits for a significant part of the samples, while elevated concentrations of, for example, arsenic and nitrate seem to be rare. A comparison of two granitic aquifers in southern Britain and Norway, respectively, illustrates the importance of glacial and postglacial weathering history for the hydrochemistry. PHREEQC modelling reveals that a common hydrochemical evolutionary pathway exists for most silicate crystalline rock aquifers, independent of lithology. The main controlling factors are the initial PCO2 of groundwater recharge, the plagioclase composition of the host rock, the extent to which initial CO2 has been converted to HCO3 by hydrolysis, whether the system is ‘open’ with regards to CO2, and to which extent plagioclase hydrolysis has progressed after calcite saturation has been reached

The chemistry of Norwegian groundwaters: IV. The pH-dependence of element concentrations in crystalline bedrock groundwaters

A total of 1604 samples of crystalline bedrock groundwaters in Norway have been analysed for pH, major and minor elements. A subset of 476 samples were also analysed for a wide range of trace elements by ICP-MS. The pH of the samples ranges from 5.4 to 9.8, with a predominance of pH values between 8.0 and 8.2. The data-set is divided into five 20-percentile groups according to increasing pH. The concentrations of 60 elements are then displayed as box-plots for each pH group. A line through the five medians yields a visual demonstration of the relationship with, and sensitivity to, pH variations for concentrations of each element. Twelve characteristic trends are distinguishable, from which some of the main hydrogeochemical processes related to pH and groundwater evolution can be inferred