Abstract
In intensive agricultural systems runoff is one of the major potential diffuse pollution pathways for pesticides and poses a risk to surface water. Ditches are common in the Po Valley and can potentially provide runoff mitigation for the protection of watercourses. The effectiveness depends on ditch characteristics, so there is an urgent need for site-specific field trials. The use of a fugacity model (multimedia model) can allows recognition of the mitigation main processes. A field experiment was conducted in order to evaluate the mitigation capacity of a typical vegetated ditch, and results were compared with predictions by a fugacity model. To evaluate herbicide mitigation after an extreme runoff, the ditch was flooded with water containing mesotrione, S-metolachlor and terbuthylazine. Two other subsequent floods with uncontaminated water were applied 27 and 82 days later to evaluate herbicides release. Results show that the ditch can immediately reduce runoff concentration of herbicides by at least 50% even in extreme flooding conditions. The half-distances were about 250 m. As a general rule, a runoff of 1 mm from 5 ha is mitigated by 99% in 100 m of vegetated ditch. Herbicides retention in the vegetated ditch was reversible, and the second flood mobilized 0.03-0.2% of the previous one, with a concentration below the drinking water limit of 0.1 μg L-1. No herbicide was detected in the third flood, because the residual amount in the ditch was too low. Fugacity model results show that specific physical-chemical parameters may be used and a specific soil-sediment-plant compartment included for modelling herbicides behaviour in a vegetated ditch, and confirm that accumulation is low or negligible for herbicides with a half-life of 40 days or less. Shallow vegetated ditches can thus be included in a general agri-environment scheme for the mitigation of pesticides runoff together with wetlands and linear buffer strips. These structures are present in the landscape, and their environmental role can be exploited by proper management.
Highlights
Risk mitigation measures for pesticides are increasingly important [1]
Hereafter ditches, are common in the Po Valley landscape, being a traditional part of field margins [3], and even if they are mainly designed for drainage purposes, they can provide two important ecosystem services: 1) habitats and green corridors for wildlife and wild plants [4], and 2) runoff mitigation for the protection of watercourses [5]
This highlights that ditches can be very effective for the mitigation of ordinary runoff, and that mitigation is similar to that obtained with vegetative filter strips (86–88% for S-metolachlor and terbuthylazine [33])
Summary
Risk mitigation measures for pesticides are increasingly important [1]. Previous research [2] showed that a constructed surface flow wetland can reduce the pollution of watercourses from a watershed of hundreds of hectares in Northern Italy agro-systems. Hereafter ditches, are common in the Po Valley landscape, being a traditional part of field margins [3], and even if they are mainly designed for drainage purposes, they can provide two important ecosystem services: 1) habitats and green corridors for wildlife and wild plants [4], and 2) runoff mitigation for the protection of watercourses [5]. The ditch is an “off-field mitigation measure” for runoff as it can reduce flow velocity, intercept and remove sediment, organic material, nutrients and chemicals carried in runoff water. This has been shown in general [6], and for some studies the basic mitigation effectiveness is about 50% [7]. Mitigation depends strictly on ditch characteristics, i.e. size, length, slope, vegetation cover [8,9], macrophyte adsorption [10,11], and a great variability exists in ditch types and effectiveness
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