Zinc concentrations in groundwater at different scales

An extensive database of the concentrations of heavy metals in phreatic groundwater was used to study the spatial variability of Zn. This paper focused on the Zn concentrations in relation to soil survey variables. Three scales of observation were distinguished: provincial level (1:1 000 000), city level (1:100 000), and parcel level (1:10 000). Use of classification of the study area based on digitized maps was investigated. A Geographic Information System (GIS) was used to overlay and investigate different sources of information. The variance of Zn in groundwater decreased by 50% when going from provincial to parcel scale. The results indicate that Zn variation is dominated by short-range variation. Significant differences in Zn contents were found for Zn grouped by seepage (infiltration vs. deep seepage) and geology. Classification according to seepage yielded reduced coefficients of variation within several seepage class. At the city level, significant differences in Zn were found for soil and land use. The sample semivariogram showed that Zn exhibits a spatial dependence different from that of electrical conductivity (EC) and pH. This may be explained by the distribution of soil groups on the soil map of scale 1:50 000. At the most detailed level, no significant differences in Zn were found between soil units or land use categories. The distinction of significantly different concentrations allows a flexible setting of background threshold values.