Trend of herbicide loads in the river Rhine and its tributaries

Many policies have been established and actions have been taken to reduce pesticide pollution of surface waters. However, the effectiveness of these initiatives was rarely tested on an empirical basis. This study suggests that evidence on the policy effects can be evaluated by analyzing the temporal changes in the loads rather than the concentrations of active ingredients in rivers. It is shown that the long-term change of pesticide emissions into surface waters can be tested statistically by the number of upward versus downward trends in river load. To evaluate the situation in Germany, 57 concentration time series of 14 herbicide substances at 7 river monitoring stations with a minimum of 24 analyses per year were assembled and annual substance river loads were calculated. The longest time course was 17 years (1990-2006). The significance of trends of data rows was analyzed by an univariate Mann-Kendall test that evaluates 27 (47.4%) of the 57 time series as statistically significant downward trended. It took a period of 10 years and longer before the high annual atrazine and simazine loads measured in the years 1990-1991 had been diminished to a drastically lowered level after ban of the herbicides. Data are available on the yearly consumption of 8 substances used in German agriculture. A total of 36 time series for this subset were tested with a partial Mann-Kendall test with the consumption as covariance factor, which reduces the number of significant trends in river load noticeably. Based on this test, only 7 (19.4% of 36) declining time series remain. As a result, the intended effect of measures to reduce surface water contamination by the use of pesticides seems to be only partially successful, however, the database to justify this statement is small. For the water monitoring strategies in Germany, it is recommend to enhance the sampling frequency at river stations. A minimum of a semimonthly sampling interval would facilitate the calculation of valuable annual river loads and would therefore allow a pronounced validation of the long-term change of pesticide emission into rivers.