| Abstract (english) | Groundwater quality is a consequence of cumulative eects of natural and anthropogenic
processes occurring in unsaturated and saturated zone, which, in certain conditions, can lead
to elevated concentrations of chemical substances in groundwater. In this paper, the concept of
determining the ambient background value of a chemical substance in groundwater was applied,
because the long-term eects of human activity influence the increase in concentrations of substances
in the environment. The upper limits of ranges of ambient background values were estimated
for targeted chemical substances in four groundwater bodies in the Pannonian region of Croatia,
according to the demands of the EU Groundwater Directive. The selected groundwater bodies are
typical, according to the aquifer typology, for the Pannonian region of Croatia. Probability plot (PP),
the modified Lepeltier method, as well as the simple pre-selection method, were used in this paper,
depending on a number of chemical data in analysed data sets and in relation to the proportion of
<limit of quantification (LOQ) values in a data set for each groundwater body. Estimates obtained
by using PP and the modified Lepeltier method are comparable when data variability is low to
moderate, otherwise dierences between estimates are notable. These methods should not be used if
the proportion of <LOQ values in a data set is higher than 30%; however, the integration of results of
both methods can increase the confidence of estimation. If the proportion of <LOQ values is higher
than 30%, it is recommended to use the robust pre-selection method with the adequate confidence
level. For highly skewed data, the 90th percentile of the pre-selected data set is comparable with
other methods and preferable over the 95th percentile. The estimates obtained for inert and mobile
substances are comparable on dierent scales. For highly redox-sensitive substances, estimates may
dier by one to two orders of magnitude, in relation to the observed heterogeneity of the aquifer
systems. The critical issue in the estimation process is the determination of hydrogeological and
geochemical homogeneous units within the heterogeneous aquifer system. |
