Uncertainties in Pesticide Monitoring Using Suction Cups: Evidences From Numerical Simulations.

Porous cups provide a simple and direct method for solute monitoring in the vadose zone and are widely used e.g. in pesticide leaching studies. A serious limitation, however, resides in the fact that the suction cups act like a small point sink and therefore, soil heterogeneity plays an important role in the amount of extracted water and solute concentration. On the other hand, nearly all experiments for the characterization of the suction cup influence were performed under steady-state flow conditions. Over the last years various authors investigated the reactive solute transport in heterogeneous media under transient conditions, whereby large differences occurred between transient and steady-state flow.

Up to now there is no detailed information in literature if suction cup sampling is suitable to predict accurately field-scale leaching of pesticides in heterogeneous soils. Therefore, we performed a numerical study to evaluate the uncertainty associated with suction cup sampling in heterogeneous soils under atmospheric boundary conditions. With regard to the two main operation methods of suction cup sampling either a variable constant or a constant applied suction once a week were analyzed. In a first step, a single pesticide application of two different compounds was performed in a number of realizations of a heterogeneous soil profile to analyze the variability in breakthrough and pore water velocity as well as sampled mass. In a second step, a time series of 10 years was chosen to analyze repeated application and estimate the variability in leached mass fraction and mean concentration observed. In both cases pesticide breakthrough was compared to an inert tracer. The results indicate that for a single application pore water velocities varies less than 13 % for all operation modes and compounds. On the other hand, total extracted mass varied much stronger with 32, 79, and 160 % for bromide, and the two pesticides for daily sampling and 35, 70, and 130 % for weekly sampling. The calculated mean concentration for a yearly repeated application varies with 5.5 % for bromide and 42.9 and 53.3 % for the two different pesticides.