BASELINE TRACE ELEMENT OCCURRENCE IN THE PRINCIPAL AQUIFERS OF THE UK
Feb 1988SUMMARY AND CONCLUSIONS 14.1 Detailed results have been obtained for the occurrence and distribution of 26 minor and trace elements (Si, Sr, Ba, F, Br, I, Li, B, Fe, Mn, Cu, Zn, Al, As, Be, Cd, Co, Cr, La, Mo, Ni Pb, Sc, V, Y, Zr) in groundwaters from ten regions of the UK. These regions include representatives of all the major aquifers of the country including the Chalk, Permo-Triassic sandstones, Carboniferous sandstones, Millstone Grit, Old Red Sandstone, Lower Greensand, Wealden sands, Lincolnshire Limestone and the Carboniferous Limestone. In general the geochemistry of these aquifers is well understood and serves as a framework for interpretation. 14.2 Reconnaissance data are given for the occurrence of a further three elements (Sn, Bi, U) in some of these aquifers. 14.3 Special care has been taken to ensure that minimal or zero contamination has taken place during the sampling procedures so that reported data closely reflect the true "in situ" geochemical conditions. The data are thus representative of 'raw waters' and not treated waters that might be delivered at the tap. 14.4 Chemical analysis of the groundwaters has included measurement of physicochemical parameters (EH, pH, T, dissolved oxygen) at the well head so that full geochemical interpretations can be made. All samples were filtered (0.45Ám) and acidified to provide a standard reference condition for dissolved species, although it is concluded that for certain elements, e.g. Fe, Mn and Al, the results reflect the presence of some colloidal material. 14.5 Analysis has been carried out by ICP-OES for a majority of elements (remainder by automated colorimetry). A concentration step (x20) has enabled a high degree of sensitivity to be obtained using this technique. The results presented here have been carefully checked against appropriate standards and wavelength scans have been carried out to ensure the highest level of accuracy and precision of the data. Additional reconnaissance analysis has also been carried out for certain elements by ICP-MS. 14.6 The data for each region have been summarised in a series of tables and diagrams in individual chapters. Element summaries are provided as a series of cumulative frequency diagrams in Chapter 13. 14.7 Baseline values are defined for each aquifer corresponding to the median concentrations and these are summarised in Table 14.1. For several elements the baseline values are below the detection limits, although for each of these elements some positive data are recorded in the majority of aquifers. The baseline values quoted can be regarded as the best single value of the natural unpolluted background for an element in a given aquifer. 14.8 The baseline concentrations for most elements show significant variation between geological formations and also within each aquifer which indicates the importance of the lithology, hydrogeology and ambient hydrogeochemical conditions as controls on trace element occurrence. 14.9 Within each aquifer, a range of concentrations for any element is found. This range is considered to reflect natural conditions, resulting from reaction between water and rock and not to indicate pollution, except in a few extreme cases. For some elements including the halogens F, Br, I and boron, atmospheric sources are important in the groundwater budget. 14.10 For many trace elements, the soil and upper few metres of the unsaturated zone represent a primary source for the groundwater composition, mainly in response to carbonate dissolution (e.g. Sr, Ba, Mn, Cu, Zn) or silicate mineral hydrolysis (e.g. AL, Li, Y). 14.11 The soil may also act as an important sink or buffer zone for heavy metals, so that their deep percolation is not observed. 14.12 Once water has entered the groundwater system the presence of oxygen is paramount in controlling the mobility of many elements. Many elements (e.g. Cu, V, U and Cr) are shown to be mobile under oxidising conditions but occur at exceedingly low concentrations under anaerobic conditions, because of redox controls and mineral solubility. Other elements (e.g. Fe) are mobile only under reducing conditions at the pH of most groundwaters. Several elements (e.g. Mn, Zn) are relatively insensitive to redox changes. 14.13 The concentrations of many trace elements in groundwaters increase with acidity. In general terms it has been found that in the carbonate aquifers the concentrations of most of the toxic metals are low due to the buffering properties of the carbonate system; the exceptions to this would be the higher concentrations of F, Sr, I and B. In the sandstone lithologies, regardless of pH, there tend to be higher concentrations of a number of transition metals. However as pH decreases to <= 6.0, several metals, (e.g. AL, Zn, Be, Co, Cr, Ni, V) increase significantly due to dissolution from the sandstone matrix. 14.14 The concentrations of certain trace elements increase with increasing residence time in the groundwater system. In particular an increase can be seen where samples have been taken along flow paths. This is the case with Sr, F, Li, B, I and Mn and is due to the incongruent reaction of carbonate and other minerals. 14.15 Some elements are buffered by the solubility limits of certain minerals and an upper limit may be reached, e.g. Si (chalcedony, quartz), F (fluorite), Ba (barite). It is unlikely that values in excess of those indicated by solubility products would be reached in the normal range of aquifer conditions (unless, e.g. Ca or S04 were completely absent). 14.16 Several elements serve as useful natural tracers which can be used to label groundwaters from specific formations. For example lithium has a high baseline in Carboniferous sandstones and may be used to distinguish groundwater from the adjacent Permo-Trias as in Shropshire. Similarly elements such as Zn, F and Sr serve as pathfinder elements of mineralised areas as in Derbyshire. 14.17 The results of the present survey provide a useful reference in relation to potability and as a baseline for defining anthropogenic pollution against the natural background. 14.18 Many elements occur at natural concentrations above the CEC guide values for drinking water (Ba, Cu, FeT, Mn, AL, Zn and Ni) in at least some of the samples analysed. A number of elements (F, Fe, Mn, AL, As) are found at levels in excess of the maximum admissible concentrations (MAC). However only one element (Mn in the Moray Basin) has a median value above the MAC value, but for several elements the Guide Level is reached even in the median value (see Ba, Fe, Mn, Al, Zn). Many problems connected with these high values are removed during the treatment processes but it is important to recognise that geochemical conditions are often responsible for the problems.
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