Including mixtures in the determination of water quality criteria for herbicides in surface water

Monitoring programs throughout America and Europe have demonstrated the common occurrence of herbicides in surface water. Nevertheless, mixtures are rarely taken into account in water quality regulation. Taking mixtures into account is only feasible if the water quality criteria (WQC) of the single compounds are derived by a common and consistent methodology, which overcomes differences in data quality without settling on the lowest common denominator but making best use of all available data. In this paper, we present a method of defining a risk quotient for mixtures of herbicides with a similar mode of action (RQm). Consistent and comparable WQC are defined for single herbicides as a basis for the calculation of the RQm. Derived from the concentration addition model, the RQm can be expressed as the sum of the ratios of the measured environmental concentration and the WQC for each herbicide. The RQm should be less than one to ensure an acceptable risk to aquatic life. This approach has the advantage of being easy to calculate and communicate, and is proposed as a replacement for the current limit of 0.1 microg/L for herbicides in Switzerland. We illustrate the proposed approach on the example of five commonly applied herbicides (atrazine, simazine, terbuthylazine, isoproturon, and diuron). Their risk profile, i.e., the RQm as a function of time for one exemplary river, clearly shows that the single compounds rarely exceeded their individual WQC. However, the contribution of peaks of different seasonally applied herbicides, whose application periods partially overlap, together with the continuously emitted herbicides from nonagricultural use, results in the exceedance of the RQm threshold value of one upon several occasions.     

Input dynamics and fate in surface water of the herbicide metolachlor and of its highly mobile transformation product metolachlor

A large number of herbicide transformation products has been detected in surface waters and groundwaters of agricultural areas, often even in higher concentrations and more frequently than their parent compounds. However, their input dynamics and fate in surface waters are still rather poorly understood. This study compares the aquatic fate, concentration levels, and dynamics of the transformation product metolachlor ethanesulfonic acid (metolachlor ESA) and its parent compound metolachlor, an often-used corn herbicide. To this end, laboratory photolysis studies were combined with highly temporally resolved concentration measurements and lake mass balance modeling in the study area of Lake Greifensee (Switzerland). It is found that the two compounds show distinctly different concentration dynamics in the lake tributaries. Concentration-discharge relationships for metolachlor ESA in the main tributary showed a high baseflow concentration and increasing discharge dependence during harvest season, whereas baseflow concentrations of metolachlor were negligible and the discharge dependence was restricted to the period immediately following application. From this it was estimated that 70% of the yearly load of metolachlor ESA to the lake was due to groundwater recharge, whereas, for metolachlor, the bigger part of the load, 50-80%, stemmed from event-driven runoff. Lake mass balance modeling showed that the input dynamics of metolachlor and metolachlor ESA are reflected in their concentration dynamics in the lake's epilimnion and that both compounds show a similar fate in the epilimnion of Lake Greifensee during the summer months with half-lives on the order of 100-200 days, attributable to photolysis and another loss process of similar magnitude, potentially biodegradation. The behavior of metolachlor ESA can likely be generalized to other persistent and highly mobile transformation products. In the future, this distinctly different behavior of mobile pesticide transformation products should find a more appropriate reflection in exposure models used in chemical risk assessment and in pesticide risk management.     

Frequency dependence of the electrochemical activity contrast in AC-scanning electrochemical microscopy and atomic force microscopy-AC-scanning electrochemical microscopy imaging

Alternating current mode scanning electrochemical microscopy (AC-SECM) enables local detection of electrochemical surface activity without any redox mediator present in solution. Z-approach curves toward the substrate result in a negative feedback curve of the ac signal for insulating samples. On conducting samples, however, the shape of the feedback curve was found to be dependent on the ac perturbation frequency. Approach curves over a wide range of frequencies were performed, and the results were applied to interpret laterally resolved frequency-dependent measurements obtained with combined atomic force microscopy-AC-SECM (AFM-AC-SECM). For the first time, this frequency dependence of the signal was utilized to fine-tune the electrochemical contrast in lateral imaging in AC-SECM. An array of gold microelectrodes embedded in silicon nitride displaying significant changes in electrochemical activity as well as in topography was investigated using a bifunctional AFM-SECM tip with an integrated recessed ring microelectrode. Due to the unique geometrical conditions the electrochemical contrast between the conducting gold spots and the insulating SixNy is reversed, crosses zero, and inverts as a function of the applied ac frequency.     

Cooperative Catalysis with Iron and a Chiral Brønsted Acid for Asymmetric Reductive Amination of Ketones

The direct asymmetric reductive amination (DARA) of ketones with anilines is described by combining a chiral Brønsted acid (TRIP) and the non‐chiral Knölker iron complex as the catalyst system. In situ‐formed imines are reduced with molecular hydrogen to give chiral amines in high yields (90%) and enantioselectivities of up to 99% ee.