Breakthrough dynamics of s-metolachlor metabolites in drinking water wells: Transport pathways and time to trend reversal.

We present the results of a two years study on the contamination of the Luxembourg Sandstone aquifer by metolachlor-ESA and metolachlor-OXA, two major transformation products of s-metolachlor. The aim of the study was twofold: (i) assess whether elevated concentrations of both transformation products (up to 1000 ng/l) were due to fast flow breakthough events of short duration or the signs of a contamination of the entire aquifer and (ii) estimate the time to trend reversal once the parent compound was withdrawn from the market. These two questions were addressed by a combined use of groundwater monitoring, laboratory experiments and numerical simulations of the fate of the degradation products in the subsurface. Twelve springs were sampled weekly over an eighteen month period, and the degradation rates of both the parent compound and its transformation products were measured on a representative soil in the laboratory using a radiolabeled precursor. Modelling with the numeric code PEARL simulating pesticide fate in soil coupled to a simple transfer function model for the aquifer compartment, and calibrated from the field and laboratory data, predicts a significant damping by the aquifer of the peaks of concentration of both metolachlor-ESA and -OXA leached from the soil. The time to trend reversal following the ban of s-metolachlor in spring protection zones should be observed before the end of the decade, while the return of contaminant concentrations below the drinking water limit of 100 ng/l however is expected to last up to twelve years. The calculated contribution to total water discharge of the fast-flow component from cropland and short-circuiting the aquifer was small in most springs (median of 1.2%), but sufficient to cause additional peaks of concentration of several hundred nanograms per litre in spring water. These peaks are superimposed on the more steady contamination sustained by the base flow, and should cease immediately once application of the parent compound stops.
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