Seasonal distribution of multiclass pesticide residues in the surface waters of northwest Croatia

River/stream water samples were collected in 2015 with a telescopic pole for grab sampling from the bank at a depth of about 0.5 m as described earlier (9). Figure 1 shows the sampling locations: two at the Sava River a few kilometres upstream and downstream of the Zagreb city centre, two at the mouths of its tributaries Krapina (in Jablanovec) and Sutla (in Harmica), one at the Drava River in the city of Varaždin, two at its tributaries Bednja (in Stažnjevec) and Plitvica (in Greda), and one at the Bednja tributary stream Bistrica (in Ivanec). Of the two major rivers, the Sava springs in Slovenia and flows through rural and urban areas for about 200 km before it reaches the Croatian capital Zagreb, and the Drava springs in Italy to flow through rural and urban regions in Italy, Austria, and Slovenia for about 420 km before it reaches Varaždin. The minor rivers Krapina, Sutla, Bednja, Plitvica, and the Bistrica stream flow through semirural/semiurban areas of northwest Croatia dominated by corn fields, orchards, and vineyards.

Figure 1

Sampling was organised around seasonal pesticide application to coincide with its beginning (March), mid-season (June), and end (August). The only exception was the Sava 2 location, where we did not collect the August sample.

The samples were stored in 2.5-L tinted glass bottles at 4 °C until analysis. Water pH ranged from 6.7 to 7.3. Samples were analysed for 23 pesticides (16 herbicides and seven insecticides) (Table 1), eight of which are listed as priority pollutants (1): alachlor, atrazine, chlorfenvinphos, chlorpyrifos-ethyl, diuron, isoproturon, trifluralin, and cypermethrin. Some of the remaining 15 compounds are allowed for use and are common environmental contaminants in Croatia.

Of the 23 analysed pesticide pollutants, eight were not detected in any water samples, namely trifluralin, alachlor, malathion, chlortoluron, linuron, chlorpyrifos-ethyl, chlorfenvinphos, and α-cypermethrin. Negative findings for the first three pesticides are in line with their ban in Croatia since 2009, but the other five were still legal to use back in 2015 (18).

Among the 15 pesticide compounds detected in our river/stream water samples, 12 were herbicides and their degradation products and only three were insecticides. The number of positive water samples in all three sampling rounds and mass concentrations of detected pesticide residues are shown in Table 3. Six pesticide compounds were detected in more than half of the analysed samples: metolachlor in 91 %, DEA and terbuthylazine in 70 %, atrazine in 65 %, DIA in 57 %, and chlorpyrifos-methyl-oxon in 52 %.

Atrazine and acetochlor were also found, although their use has been banned in Croatia since 2009 and 2014, respectively. While the possibility of illegal leftover use cannot be completely ruled out, as may be the case with acetochlor found in only one sample in June (see Table 3), a more likely explanation for the relatively high incidence of atrazine and its degradation products DEA and DIA is their persistence in treated agricultural soils resulting in prolonged water pollution in agricultural catchments (8, 19, 20).

Most pesticides were identified in the height of the application season (14 in June as opposed to nine in March and eight in August) (Table 3).

Of the priority water pollutants, we found only atrazine, isoproturon, and diuron, and their peak mass concentrations were one to two orders of magnitude lower than the maximum allowable concentrations defined by the Directive as Environmental Quality Standards for surface waters (1). As there are no limits for pesticide compounds not considered priority pollutants, their mass concentrations in surface waters are often considered in terms of maximum permissible mass concentrations in water intended for human consumption of 100 ng/L for single pesticides and 500 ng/L for total pesticides (21). In this regard, Figure 2 shows that the 100 ng/L limit was mostly exceeded in the middle of the application season.

Figure 2

Table 4 also clearly illustrates seasonal variations in pesticide load by sampling locations. The sum of mass concentrations of detected pesticides was the lowest at the beginning (March) and the highest in the peak of the pesticide application season (June). As expected, the lowest sums of pesticide mass concentrations were measured in the Drava River sampled in the urban area and the highest in the Krapina and Sutla rivers sampled in rural areas with intensive agricultural production. In the peak of the pesticide season, it exceeded the 500 ng/L limit for drinking water (21) in the samples taken from the Sutla, Krapina, Sava, and Bednja and the Bistrica stream. Metolachlor and terbuthylazine accounted for the largest shares in total mass concentrations in the Krapina and Sutla, metolachlor with 76 % and 74 % and terbuthylazine with 10 % and 12 %, respectively (see Figure 2, June). In the Sava, the highest shares were those of metolachlor and AMBA, ranging between the two sampling locations with 26–30 % and 20–24 %, respectively. The same pollutants were also dominant in the Bednja, where AMBA and metolachlor accounted for 51 % and 17 %, respectively. The high shares of AMBA point to recent application and subsequent degradation of a triketone herbicide mesotrione, which is less stable in soil and water than its degradation product (8, 22). As for the Bistrica stream, the highest shares in the sum of pesticide mass concentrations were those of tembotrione and mesotrione with 37 % and 15 %, respectively. All these findings point to the prevalence of herbicides metolachlor, terbuthylazine, tembotrione, and mesotrione in pesticide application in northwest Croatia.

The prevalence of metolachlor and terbuthylazine and their degradation products has already been noted in our earlier study of the Sava samples taken the year before (9) and earlier studies across Europe (2, 23–25).

As for insecticides analysed herein, all water samples collected in the height of the pesticide application season (June) contained trace concentrations of imidacloprid and chlorpyrifos-methyl-oxon (Table 3 and Figure 2). The finding of the latter points to a recent use of its parent compound chloropyrifos-methyl. Not only is it more persistent than the parent pesticide [over four weeks, according to Lacorte and Barceló (26)] but it is also more toxic to aquatic organisms and the environment (4) and should be regarded as pollutant of concern. As for the low imidacloprid concentrations, they are likely the consequence of restricted use in Croatia since 2013.

The limitation of our study is that our current data are limited to the measurements taken in 2015, while further data from the OPENTOX project (concluded with 2018) are still pending. Despite this limitation, the study has confirmed the persistence of atrazine in water environment and the domination of herbicide over insecticide use in urban and agricultural areas of northwest Croatia. Our findings further emphasise the need to extend regular monitoring of pesticides in surface waters to herbicide and insecticide compounds that are not considered priority water pollutants at the moment but are frequently used in larger amounts in regions with intensive agriculture. A systematic and regular monitoring of frequently used pesticides in surface waters can inform pesticide users and authorities and help to prevent or at least reduce their adverse effects on aquatic organisms and environment in a timely manner.