Point and non-point source leaching of pesticides in a till groundwater catchment

6. Conclusion

In a clayey till groundwater catchment at Skælskør, pesticide leaching from the Skælskør orchard has been compared with leaching from a pesticide source spill in the machinery yard belonging to the orchard.

From approximately 1960 to 1992 phenoxyherbicides were applied in Skælskør orchard in amounts corresponding with average application on farm land. Also simazine, atrazine, terbuthylazine and amitrol have been used in significant amounts.

Groundwater samples from the till sequence were collected six times over a period of one year from five 15-25 m deep monitoring wells (14 screens). The samples were analyzed for dichlorprop, mecoprop, MCPA, 2,4-D, (phenoxyherbicides) simazine, atrazine and terbuthylazine (trazines).

In the point source site, very high pehnoxyherbicide contrations (mainly dichlorprop and mecoprop) and lower simazine concentrations, were found in a "hot spot" zone down to 5-7 m depth in the geological profile (sum contrations up to 1-2 mg/l). Below the "hot spot" zone, pesticide contrations decreased several orders of magnitude.

In the sand aquifer at the point source site (underlying the till sequence), sum contrations of phenoxyherbicides and triazines were below detection and up to 1 mg/l. All pesticide findings, except for the "hot spot" contamination in the point source site, were erratic in time and space.

Pesticide application in the orchard exclusively occur in spring and summer, while on farm land additional application in autumn (primary period of precipitation surplus) of mecoprop has been common since the beginning of the 1980'ies.

In the orchard, the only phenoxyherbicide identified repeatedly was MCPA (in 24% of water samples at 0.01-4.8 mg/l). Simazine occurred in 17% of water samples collected (0.04-1.0 mg/l). In laboratory leaching experiments also terbuthylazin and mecoprop were identified. Findings of MCPA may occur both as a primary product and as a metabolity of mecoprop degradation. No other metabolites has been analyzed for.

Applied primary phenoxyherbicide products (MCPA, dichlorprop and 2,4-D) appear to undergo extensive immobilisation i.e. degradation, near the ground surface in the orchard. A significant capacity for uptake of simazine in the clay minerals of the till was measured.

Field scala tritium profiles and laboratory experiments indicate that the influence of fractures on solute contamination transport is small or absent at depths below the upper few metres at the field sites. The distribution of tritium in the till profiles is consistent with numerical modelling of transport in the till by piston flow.

Sensitivity analyses carried out by numerical modelling of fracture flow indicate that in case the shallow hydraulically conductive fractures are extended through the till and degradation of the pesticides is ignored, a breakthrough in the main aquifer at a concentration level at 1 ‰ of the input concentration should be expected within a few years.

Inclining sand layers and sand streaks observed in the till may constitute alternative transport paths for pesticide leaching. Transport along these paths may account for some of the deep pesticide findings in the till and the sand aquifer below the point source spill.6.

In the point source site, very high phenoxyherbicide concentrations (mainly dichlorprop and mecoprop) and lower simazine concentrations, were found in a "hot spot" zone down to 5-7 m depth in the geological profile (sum concentrations of approximately 1 mg/l). Below the "hot spot" zone, pesticide contrations decreased several orders of magnitude.

In the sand aquifer at the point source site (underlying the till sequence), sum concentrations of phenoxyherbicide and trazine were below detection to approximately 1 mg/l.

All pesticide findings, expect for the "hot spot" contamination in the point source site, were erratic in time and space (total of 87 water samples).

Sensitively analyses by numerical modelling of fracture flow indicate the following: in case the shallow zone fractures (fracture hydraulic conductivities determined by the large column experiments) are extended through the aquitard and degradation of the pesticides ignored, a breakthrough at a concentration level at 1 ‰ of the input concentration should be expected within a few years.

Fracture transport simulations can not, at the present level of knowledge, be used as a quantitative prediction tool. It is, however, possible to use the models to analyze effect of the different uncertain parameters and perform "worst case" simulations.