Cécile Dagès

Managing ditches for agroecological engineering of landscape. A review

Agriculture must now feed the planet with the lowest environmental impact. Landscape management is a means to protect natural resources from the adverse impacts. In particular, the adequate management of ditches could improve crop quality. Here, we review ditch design and maintenance. We found the following major points: (1) ditch networks have been primarily designed for waterlogging control and erosion prevention. Nonetheless, when properly managed, farm ditches provide other important ecosystem services, namely groundwater recharge, flood attenuation, water purification, or biodiversity conservation. (2) All ditch ecosystem services depend on many geochemical, geophysical, and biological processes, whose occurrence and intensity vary largely with ditch characteristics. (3) The major ruling characteristics are vegetative cover; ditch morphology; slope orientation; reach connections such as piped sections and weirs, soil, sediment and litter properties, biota, and biofilms; and network topology. (4) Ditch maintenance is an efficient engineering tool to optimize ecosystem services because several ditch characteristics change widely with ditch maintenance. For instance, maintenance operations, dredging, chemical weeding, and burning improve waterlogging and soil erosion control, but they are negative for biodiversity conservation. Mowing has low adverse effects on biodiversity conservation and water purification when mowing is performed at an adequate season. The effects of burning have been poorly investigated.

Variability of glyphosate and diuron sorption capacities of ditch beds determined using new indicator-based methods

Pesticide sorption to ditch-bed materials can efficiently decrease pesticide concentrations in the flowing water. Pesticide sorption depends on flood characteristics and the nature and abundance of ditch-bed materials, such as soils, living and decaying vegetation and ash. However, the affinities of pesticides for various ditch-bed materials have rarely been investigated, and variations in the global sorption capacity of ditch beds resulting from their heterogeneous compositions and variable flood characteristics have not been determined. Thus, we studied the variability of sorption capacities of ditch-bed materials for glyphosate and diuron in three catchments in France and propose a method for calculating global sorption processes in heterogeneous ditch beds. The methodology consists in estimating a global sorption coefficient for the composite ditch-bed materials (Kdditch) and an indicator of the amount of pesticide potentially retained by sorption during a flood event (SPRI). Furthermore, we computed the Kdditch and SPRI of glyphosate and diuron for 8 ditches subjected to 3h flood events with water levels varying from 0.5 to 15cm. Our results show that increasing the water level from 0.5 to 15cm resulted in a 90% decrease in the sorption capacities of the ditch beds for both pesticides. At a medium water depth of 5cm, SPRI varied from 25 to 51% and from 7 to 35% among the ditches for glyphosate and diuron, respectively. The variabilities of the glyphosate and diuron sorption capacities among the ditches were mainly driven by the nature and abundance of soil and ash. As the management of farm ditches, performed to maintain their hydraulic performance, modifies the abundances of various ditch-bed materials, it constitutes a potential lever of action for water quality improvement. Thus, Kdditch and SPRI could serve as rapid and cost-effective tools for optimizing ditch network management strategies to improve water quality in cropped catchments.

Seepage patterns of Diuron in a ditch bed during a sequence of flood events.

Although ditches limit surface water contamination, groundwater recharge through ditches in Mediterranean catchments may result in groundwater contamination. We analysed the dynamics of pesticide percolation in ditches by conducting an original lab experiment that mimicked the successive percolation processes that occur during a flood season. Nine successive percolation events were operated on an undisturbed soil column collected from a ditch bed. The infiltrating water was doped with (14)C-Diuron at concentrations that were chosen to decrease between the events so as to correspond to values observed during actual flood events. The water and solute fluxes were monitored during each event, and the final extractable and non-extractable Diuron residues in the column were determined. Two main observations were made. First, a high leaching potential was observed through the ditch bed over a succession of infiltrating flood events, with 58.9% of the infiltrated Diuron and its metabolites leaching. Second, compared with the contamination of surface water circulating in the ditches, the contamination of seepage water exhibited smaller peak values and persisted much longer because of the desorption of Diuron residues stored in the ditch bed. Thus, ditches serve as buffering zones between surface and groundwater. However, compared with field plots, ditches appear to be a preferential location for the percolation of pesticides into groundwater at the catchment scale.

Using fluorescent dyes as proxies to study herbicide removal by sorption in buffer zones

The performance of buffer zones for removing pesticides from runoff water varies greatly according to landscape settings, hydraulic regime, and system design. Evaluating the performance of buffers for a range of pesticides and environmental conditions can be very expensive. Recent studies suggested that the fluorescent dyes uranine and sulforhodamine B could be used as cost-effective surrogates of herbicides to evaluate buffer performance. However, while transformation mechanisms in buffers have been extensively documented, sorption processes of both dyes have rarely been investigated. In this study, we measured the adsorption, desorption, and kinetic sorption coefficients of uranine and sulforhodamine B for a diverse range of buffer zone materials (soils, litters, plants) and compared the adsorption coefficients (Kd) to those of selected herbicides. We also compared the global sorption capacity of 6 ditches, characterized by varying proportions of the aforementioned materials, between both dyes and a set of four herbicides using the sorption-induced pesticide retention indicator (SPRI). We found that both the individual Kd of uranine for the diverse buffer materials and the global sorption capacity of the ditches are equivalent to those of the herbicides diuron, isoproturon, and metolachlor. The Kd of sulforhodamine B on plants and soils are equivalent to those of glyphosate, and the global sorption capacities of the ditches are equivalent for both molecules. Hence, we demonstrate for the first time that uranine can be used as a proxy of moderately hydrophobic herbicides to evaluate the performance of buffer systems, whereas sulforhodamine B can serve as a proxy for more strongly sorbing herbicides.

Reconstruction of the three-dimensional Darcy velocity in a small catchment using self-potential, electrical resistivity, and induced polarization data

One of the ultimate goals of hydrogeophysics is to map the groundwater table throughout a catchment and together with information about hydrogeological properties of the saturated zone, derived from geophysical and hydrogeological data, estimate the Darcy velocity in three dimensions throughout the saturated zone. In this work, we present our first attempts to achieve this goal in the small (~1 km2) catchment area of Roujan (Herault, France), which is a well characterized hydrogeological research site. Two mutually exclusive conceptual models to relate self-potential data to the determination of the water table were employed using a maximum likelihood approach, where the resulting models were averaged according to Bayesian model averaging. The estimated electrical resistivity structure derived from resistivity surveys was used together with laboratory measurements to estimate the streaming potential coupling coefficient and its spatial variation. Resistivity and induced polarization surveys were employed to determine the spatial variation of the thickness of the aquifer material (clayey silts) overlying the electrically more conductive aquitard (Miocene marls). Induced Polarization measurements were also performed to estimate the specific surface area and its variation in the aquifer. The electrical conductivity of the pore water was sampled throughout the aquifer and the formation factor was estimated in the laboratory. This allowed us to use both the Kozeny and Carman model and site-specific relationships to relate electrical resistivity and normalized chargeability with permeability. A cokriging method was applied to estimate large scale variations in the three-dimensional permeability structure by conditioning to available permeability data and several collocated two-dimensional models of electrical resistivity and normalized chargeability. Our estimates of the variations of the depth to the aquitard, depth to the water table, and the large-scale variations in the permeability structure allowed us to estimate the Darcy velocity throughout the saturated aquifer in the Roujan catchment.