Determination of the natural background content of metals in ground waters of the Czech Republic

Knowledge of the natural background content of metals is important, but can be difficult to establish because the concentrations of substances dissolved in ground waters vary considerably with time and space. The main objective of the paper is to assess the natural background of five selected elements: As, Al, Cd, Pb and Hg. Each of these elements, with the exception of Al, is included in the Minimum list of pollutants and their indicators for which the EU Member States should establish threshold values (Daughter Groundwater Directive). The data of the Czech Hydrometeorological Institute which contains analyses obtained by regular monitoring of the quality of ground waters at intervals of 6 months has been used as the source information. This system incorporates ca. 450 monitoring sites which provide information about water in all the types of rocks penetrated by individual boreholes. Because of the low concentrations of certain elements (Hg, Pb and Cd in particular) a significant number of analytical results lie below the quantification limit of the analytical methods used. Therefore, conventional statistical methods for processing data were not applicable and alternative procedures were used. The Kaplan–Meier procedure was used within the NADA module for statistical analysis of data sets containing values below the quantification limit. The concentrations of monitored elements that can be considered natural background are suggested to be values of the third quartile, i.e. values that are less than or equal to 75% of analytical results in the assessed dataset. The remaining 25% of analytical results that exceed the proposed limit can be considered to be anomalies which may be natural or anthropogenic. Based on the statistical analysis of data specific values for the natural background content of elements in ground waters within particular types of lithology have been proposed. These can be considered the natural background values that apply within the whole of the territory of the Czech Republic.