Pollution of groundwater with plant protection products is presented by the share of sampling points at aquifers with intergranular and karst-fractured porosity where threshold concentration values of individual products and their degradation products or their totals in the samples taken were exceeded at all times, periodically or never. In Slovenia, threshold values concerning concentrations of the residues of plant protection products in groundwater are defined by the Degree on Groundwater Quality Standards (Official Gazette RS No. 100/2005) as 0.1 µg/l for individual active substances, and 0.5 µg/l for the total residues of plant protection products. Along with the entire legislation governing this field, these measures are in accordance with the Drinking Water Directive (98/83/EC), which applies to the entire European Union.

In 2004, arithmetic mean values for the total residues of the plant protection products did not exceed the 0.5 µg/l threshold value in groundwater in any of the observed aquifers. Between 1993 and 2004, a decrease in the total residues of the above-mentioned products and their degradation products was noted in the aquifers along the Mura River (Prekmursko-Mursko polje) and along the Savinja River (Lower Savinjska dolina valley – the Bolska River valley). In the 1993-2004 period, total residue contents of plant protection products at Ljubljansko polje and Krško-Brežiško polje were within the permissible values. In karst-fractured aquifers, these contents were below the identifiable levels for analytical methods.

At sampling points, threshold values were most frequently exceeded for atrazine and its metabolite desethylatrazine. In Slovenia, the share of sampling points where the concentrations of threshold values exceeded the threshold values determined in the legislation is above the European average. In spite of that, the monitoring results indicate a clear decrease in the concentration of atrazine and desethylatrazine residues at the sampling points most polluted in the past, such as Rakičan, Lipovci and Gotovlje. It should be noted that in most parts of Slovenia, atrazine values are already below the threshold value of 0.1 µg/l, while desethylatrazine values still exceed it in some places. Atrazine has been out of use for the last four years, but it will take a few more years for its concentration in the lower levels of groundwater to fall below the 0.1 µg/l threshold value. The reason for the slow decrease of desethylatrazine is its slow degradation.

In spite of the increasing number of active substances included in regular monitoring (27 in 1995 and 66 in 2004), it is encouraging to know that the number of active substances exceeding the 0.1 µg/l threshold value is actually decreasing. Between 1995 and 2004, we observed a decrease in the total residues of the plant protection products and their degradation products at the considerably polluted Brunšvik sampling point, but in spite of that, in 2004 the total value was still above the 0.5 µg/l threshold value. Between 1995 and 2004, the total value of residues of these products and their degradation products was also on the decrease at the Sobetinci sampling point. In the last two years, it was considerably below the 0.5 µg/l threshold value, in spite of its high values in the past.