| Abstract: | Pesticide residuals after point‐source pesticide spills in clay‐rich aquitards may potentially affect underlying groundwater for many decades due to slow release of accumulated pollution in the clayey matrix material of the aquitard. In this study, we evaluated factors behind different degrees of accumulation of phenoxy acids (MCPP, dichlorprop, 2,4‐D MCPA) and triazines (simazine and terbutylazine) observed in an old pesticide pollution described by Jørgensen et al. (2016a, this issue). By using leaching experiments, it was shown that a zone of maximum concentrations of MCPP and dichlorprop (mg/L) observed by Jørgensen et al. (2016a, this issue) represented accumulated potentially mobile pollution in anaerobic, however largely immobile pore water of the clayey matrix in the upper few meters of the unoxidized aquitard zone. By using sorption experiments, we determined 9 to16 times higher mobility by diffusion and flow for the phenoxy acids (R = 1 to 2) than for the triazines (R = 9 to 16) in the clayey matrix material of the aquitard. This indicated that more rapid and greater accumulation could occur for the phenoxy acids in the clayey matrix than for the triazines. In contrast, the relative mobility of the phenoxy acids and triazines was much closer in sand‐filled fractures and thin sand layers/lamina in the clay, suggesting that the migration of the same compounds along these textural preferential flow paths into the underlying aquifer was less different. Despite that a greater mass had originally been spilled of 2,4‐D and MCPA having similar mobility as the accumulated MCPP and dichlorprop, these compounds were not accumulated in the zone of maximum concentrations. It is suggested that the controls, which initially allowed for the observed separate accumulation of MCPP and dichlorprop as a zone of maximum pollution, were due to the combination of high persistence and high mobility for these specific pesticides in the clayey till matrix material of the aquitard. The investigation showed that over time the initial high concentrations of the accumulated phenoxy acids (MCPP, dichlorprop) transformed into high concentrations of related mobile degradation products (e.g., 4‐CPP and 2‐CPP), which extended the total time of groundwater pollution beyond the disappearance of the original phenoxyacids. |
