On the potential of on-line free-surface constructed wetlands for attenuating pesticide losses from agricultural land to surface waters

Abstract

Environmental contextPesticide losses from land to surface waters have the potential to cause ecological damage. Furthermore, pesticides in surface waters present a major challenge for water companies accessing these waters for the domestic supply, in terms of complying with water quality regulations. Here, we evaluate the potential of field- and ditch-scale free-surface constructed wetland systems for reducing pesticide transfers from land to surface waters. AbstractPesticides make important contributions to agriculture but losses from land to water can present problems for environmental management, particularly in catchments where surface waters are abstracted for drinking water. ‘On-line’ constructed wetlands have been proposed as a potential means of reducing pesticide fluxes in drainage ditches and headwater streams. Here, we evaluate the potential of two free-surface constructed wetland systems to reduce pesticide concentrations in surface waters using a combination of field monitoring and dynamic fugacity modelling. We specifically focus on metaldehyde, a commonly used molluscicide that is moderately mobile and has been regularly detected at high concentrations in drinking water supply catchments in the UK over the past few years. We also present data for the herbicide metazachlor. Metaldehyde losses from the upstream catchment were significant, with peak concentrations occurring in the first storm events in early autumn, soon after application. Concentrations and loads appeared to be minimally affected by transit through the monitored wetlands over a range of flow conditions. This was probably due to short solute residence times (quantified via several tracing experiments employing rhodamine WT – a fluorescent dye) exacerbated by solute exclusion phenomena resulting from patchy vegetation. Model analyses of different scenarios suggested that, even for pesticides with short aquatic half-lives, wetland systems would need to exhibit much longer residence times (RTs) than those studied here in order to deliver any appreciable attenuation. If the ratio of wetland surface area to the area of the contributing catchment is assumed to be a surrogate for RT (i.e. not accounting for solute exclusion), then model predictions suggest that this needs to be greater than 1% to yield load reductions of 3 and 7% for metaldehyde and metazachlor respectively.

Pesticide peak concentration reduction in a small vegetated treatment system controlled by chemograph shape

“…In fact, other studies reported limited mass removal in wetlands with comparable residence times. Ramos et al (2019) did not find relevant R M in two surface flow wetlands, with residence times between 45 min and 6 h in England. In contrast, Passeport et al (2013) found R M between 45 % and 96 % in a constructed wetland with a residence time of 66.5 h. However, their contaminant mass loss coincided with loss of water (45 %).…”

“…Such systems temporally retain polluted waters and thus provide space, time, and favorable conditions for degradation processes. VTSs studied in literature include very different types of systems (Lange et al, 2011), including vegetated ditches or detention ponds with hydraulic residence times (HRT) ranging in the order of minutes to several hours (Bundschuh et al, 2016;Elsaesser et al, 2011;Ramos et al, 2019) or constructed wetlands in which HRT may reach several weeks (Maillard and Imfeld, 2014), particularly when operated in batch mode (Tournebize et al, 2017;Maillard et al, 2016;Moore et al, 2000). The term "pesticide mitigation" can refer to contaminant mass removal (R M ) or peak concentration reduction (R C ).…”

Knowledge Atlas on the Relationship between Water Management and Constructed Wetlands—A Bibliometric Analysis Based on CiteSpace

“…The importance of CWs in eliminating nitrogen and phosphorous, the selection of plants, and CW monitoring that uses multiple models are all research themes in this cluster. For example, the potential of two freesurface CW systems to reduce pesticide concentrations in surface water was evaluated by combining in-field monitoring and dynamic fugacity modelling [67]. Recently, research on CWs has focused on complex agroecosystems [68], and nitrogen and phosphorus removal is no longer a frontier or hot spot.…”

A new conceptual model of pesticide transfers from agricultural land to surface waters with a specific focus on metaldehyde

“…Our assumption is that all the arable land in the catchment is under-drained. Details of the monitoring set up, sampling protocol and analytical methods employed are given in 14 Where possible, samples were analysed within a week of sample collection. In this case, samples were kept in the dark at 4°C before analysis.…”