Iodine concentrations in Danish groundwater: historical data assessment 1933-2011.

In areas where water is a major source of dietary iodine (I), the I concentration in drinking water is an important factor for public health and epidemiological understandings. In Denmark, almost all of the drinking water is originating from groundwater. Therefore, understanding the I variation in groundwater and governing factors and processes are crucial. In this study, we perform uni- and multivariate analyses of all available historical Danish I groundwater data from 1933 to 2011 (n = 2,562) to give an overview on the I variability for first time and to discover possible geochemical associations between I and twenty other elements and parameters. Special attention is paid on the description and the quality assurance of this complex compilation of historical data. The high variability of I in Danish groundwater (<d.l. to 1,220 µg/l) is characterised by both small-scale heterogeneity and large-scale spatial trends, e.g. higher concentrations observed in the eastern part of Denmark. Significant trends are observed also with respect to the depth of extraction and geology, indicating the importance of older marine limestone and chalk deposits. A principal component analysis on centred log-ratio-transformed data (clr) revealed associations between I, Li, B, Ba, Br implying saline water influence. High I is also associated with reduced and alkaline groundwaters for this data set, dominated by Ca-HCO3 water type.