Natural attenuation along subsurface flow paths based on modeling and monitoring of a pesticide metabolite from three case studies

Abstract

Background Groundwater—especially for the use as drinking water—is a strictly protected resource in the existing guidelines for pesticide registration and drinking water protection in the EU. One aspect that has hardly played a role in this context so far is the attenuation of pesticide concentrations along the flow path from the regulatory leaching concentration at a depth of 1 m below the applied field to raw water abstraction systems. The soil metabolite N,N-dimethylsulfamide (DMS) is formed from two fungicidal substances: tolylfluanid and dichlofluanid. According to the EU guidance document on relevant metabolites in groundwater, DMS is a “non-relevant” metabolite. However, long-term application of the two active substances on permanent crops has resulted in elevated and quantifiable amounts of DMS in groundwater catchment areas of water supplying plants. Therefore, in the case of DMS, substantial monitoring data is available. This enables in combination with groundwater modeling, a quantitative analysis of the natural attenuation of DMS concentrations over time and distance. To this end, extensive real-world data from three case studies of drinking water catchment areas in Germany were analyzed. Results The environmental fate of DMS in soil and groundwater was evaluated according to the respective data determined at the study sites. Analyses using monitoring data and combined modeling approaches as well, were performed to obtain comparable results. These merged outcomes from monitoring and modeling show total attenuation factors of 12–93 from leachate at 1 m depth down to monitoring wells—close to raw water collection. If concentration attenuation further downwards to collected raw water is considered, the overall attenuation factor is even higher (40–246). Conclusions The conditions at the catchment areas of the three case studies are very diverse, thus providing a wide range of attenuating conditions. When following the path of DMS from its formation in soil below a treated field, to its leaching into the aquifer, and within the aquifer down to the raw water collection site, its concentration in water is continuously and consistently decreasing. The results from DMS represent conservative estimations due to non-sorptive, quick transport processes. Extended to other sorptive solutes, it represents the lower end of expected attenuation. Therefore, natural pesticide concentration attenuation processes are suggested for the consideration in regulatory pesticide risk assessments for a more realistic yet still protective evaluation of expected concentrations in raw water.

Statement of the Scientific Panel on Plant Protection Products and their Residues (PPR Panel) on the design and conduct of groundwater monitoring studies supporting groundwater exposure assessments of pesticides

“…Processes that result in lower concentrations are for example hydrodynamic dispersion, diffusion and degradation in the saturated zone (Tiktak et al., 2005 ). Herrmann and Sur ( 2021 ) demonstrated that the concentration of the metabolite N,N‐dimethylsulfamide (DMS) decreased continuously and consistently when the substance was followed from its formation in soil below a treated field, to its leaching into the aquifer, and within the aquifer down to the raw water collection site. Preferential flow is one of the known processes that could make the assessment at Tier‐4 more conservative.…”

Proposal for a tiered approach to evaluate the risk of transformation products formed from pesticide residues during drinking water treatment

“…The surface water compartment serves as a first example of concept and methodology. In principle, the concept can be transferred to groundwater as well, i.e., starting with regulatory PEC gw values and consideration of dilution factors until the point of abstraction [29] by additionally taking geometric configuration of groundwater aquifers into account.…”

Proposal for a tiered approach to evaluate the risk of transformation products formed from pesticides residues during drinking water treatment

“…The surface water compartment serves as a rst example of concept and methodology. In principle, the concept can be transferred to groundwater as well, i.e., starting with regulatory PEC gw values and consideration of dilution factors until the point of abstraction [29] by additionally taking geometric con guration of groundwater aquifers into account.…”