Measure of mobility and persistence

The measure of mobility and persistence indicates the potential of a PAI to move from farmland into the aquatic environment via runoff and to persist in that environment. K_OC was used as an indicator of mobility. To evaluate the suitability of K_OC for predicting mobility, experimental runoff loss data from field trials from the Wet Tropics region (Fillols et al. 2018) were compared with K_OC. The experimental runoff data were for two different soil types, a well-drained deep sandy soil and a poorly drained hydrosol soil, both with a green cane trash blanket with simulated rainfall applied 2 days after pesticide application. The PAIs were ranked in terms of their mobility (e.g., proportion of PAI loss) in the sandy soil and hydrosol, and each was then compared with the rank of the corresponding K_OC values using a Spearman rank correlation coefficient test. For both soil types, there was a negative trend between ranked experimental PAI loss and ranked K_OC (Figure S1), but the correlation was only statistically significant for the hydrosol (r = − 0.592, P = 0.017). However, when negatively charged MCPA was removed from the sandy soil comparison, the correlation coefficient between runoff loss and K_OC became significant (r = − 0.607, P = 0.019). Weak linear relationships, with R² values of 0.432 and 0.232, were observed between log K_OC values and PAI losses in hydrosols and sandy soil, respectively (Fig. ​(Fig.1).1). The relationship between log K_OC and experimental runoff loss from the hydrosol soil was used to predict the percent of pesticide transported from soil for each PAI (Eq. 2):

Linear regression of log K_OC and experimental percent of PAI transported for hydrosol and sandy soil

Percent of PAI transported was converted to proportion of PAI transported by dividing by 100. The relationship between K_OC and proportion of PAI transported is curvilinear and does not assume that the gradient equals 1.

Degradation half-life in soil and water was used as a surrogate for persistence. The measure of mobility and persistence was calculated as the product of the soil half-life (t_{1/2 soil}), the aqueous phase half-life (t_{1/2 water}), and the proportion of pesticide transported from soil (Eq. 3):