Mapping groundwater vulnerability to pesticide contamination through fractured clays: CLAYFRAC

This report describes results from the CLAYFRAC project investigating possibilities to map or rank clay localities according to their vulnerability to pesticide leaching. The project entailed a combined and multiscale experimental and modelling approach starting from the polymorphological (PM) concept for ranking the heterogeneity of clay-till deposits. The PM concept separate clay-till deposits in landforms differing by their pattern of superimposed geomorphological units. Four large excavations have been established representing different PM landforms. The Havdrup-Salløv excavation was established in a PM landform named MML meaning a landform where basement marine clay (L) is overlaid by a moraine plain (M), overlaid by another moraine plain (M). MML landforms have previously been studied in detail why the other excavations were carried out at less studied landforms. The Farum excavation was on the PM-landform DMS being an outwash plain (S), overlaid by a moraine plain, overlaid by a hummocky moraine (D); the Holbæk excavation was on the PM-form DM being a moraine plain overlaid by a hummocky moraine, and finally the Tokkekøb excavation was on the PM-form DR being a marginal moraine (R), overlaid by a hummocky moraine. The lithology and presence of preferential flow paths, including macropores (wormholes, root channels, burrows etc), tectonic and desiccation fractures, and sand lenses were characterized at each excavation. All sites were geologically highly heterogeneous with different clay units being mixed with sand lenses and sand layers. This was most pronounced at Holbæk where half of the excavation showed exposure of sandy deposits.

The Havdrup-Salløv excavation was characterized by having three till units separated by sand layers. Macropores dominated in the upper till with many wormholes present down to one m depth. Grey fractures were present to a depth of 1-3 meter below surface (mbs), below which the fracture surface linings became red due to iron precipitation. Macropores suggested to be relic root channels were observed to more than 5.9 m depth. At Farum the excavation revealed two till units separated by sand layers. Macropores, but not fractures were present in the upper till and there were many sand layers and sand lenses that may as well serve as preferential water transport routes. In contrast, in the lower till many large connected vertical and horizontal fractures with iron precipitation were present. At Holbæk one part of the excavation was dominated by sandy lake deposits, while the other part was primarily clayey, but with dispersed sand lenses. Wormholes were present in large numbers penetrating to greatest depth at the sand deposits (2 mbs). Furthermore, the Holbæk LUC-middle (2.6-3.1 mbs) revealed macropores probably being relic root channels. Fractures were present from about one mbs being first grey and at below 3 mbs red. The Tokkekøb excavation was characterized by two till units above sandy lake deposits present at 3-4.5 mbs. No fractures were seen in the upper till, but instead this had the highest number of wormholes (up to 1048 per m²). Fractures with iron precipitates were present in the lower till, some being slanted and more than 6 m long, but their density decreased with depth.

Overall, all the sites were very heterogeneous with many geological units scattered among each other, implying that it may be difficult to attribute defined geological characteristics to PM types. The geological characterization, though, showed the following general trends: 1) Macropores, including wormholes were present in all upper tills where they probably are the most dominant preferential waterflow routes, 2) fractures changed colour from grey to red due to iron precipitation at greater depths, and 3) sand deposits were present at all locations in the form of sand layers and sand lenses of different thicknesses.

Six large undisturbed columns (LUCs) were sampled at different depths of the excavations; three in Havdrup and three in Holbæk. The LUC is a unique setup to determine preferential flow properties of clayey tills, including hydraulic conductivities, fracture apertures, water flow, and pesticide transport. Hydraulic characterization and transport experiments were performed in the LUC setups. Solute transport was investigated using tracers (e.g., bromide and the colour dye brilliant blue) as well as a mixture of pesticides (MCPA, bentazon and tebuconazole) with different properties. The LUC experiments showed that the hydraulic conductivity in the upper tills with grey fractures were significantly higher than observed in the deeper layers dominated by fractures with iron precipitations. The highest conductivity was measured in the Holbæk LUC-top (2.0-2.4 mbs) where macropores in the form of root channels were also present (> 200/m²). The root channel density then decreased to 5/m² at Holbæk LUC-middle and no root channels were observed in the deepest LUC. The hydraulic conductivity was several orders of magnitude lower in the other LUCs that all had fractures with iron precipitates at their surfaces and except for the Holbæk LUC-top and middle and Havdrup LUC-bottom (5.4-5.9 mbs) there were no open macropores present. The lower overall hydraulic conductivity at increasing depth was attributed the fracture aperture also decreasing with depth. This was also reflected in the transport of MCPA and bentazon as very rapid breakthrough of these pesticides, following the added bromide tracer was observed in the Holbæk LUC-top. Breakthrough of tebuconazole was retarded compared to the tracer in accordance with the higher sorption coefficient of this pesticide. In contrast, the breakthrough concentrations of MCPA and bentazon were about 50 % lower in the Havdrup LUC-bottom than Holbæk LUC-top probably due to the longer residence time and higher amounts of the pesticides being accumulated in the LUC due to diffusion into the matrix. This retardation was even higher for tebuconazole where only trace concentrations were measured in the outlet.

The influence of different geological features on the waterflow, observed either by field observations or in the LUC experiments was simulated using process-based numerical modelling. These features included fracture aperture, spacing (fracture density), and orientation as well as presence of sand layers. The fracture aperture was shown to be a strong determinant of pesticide leaching as the water flux through fractures scales with the third power of the hydraulic fracture aperture according to the cubic law. This may be even more important for the cylindrical macropores as their cross-sectional area scales with the fourth power of the hydraulic pore radius. Decreased fracture density and appearance of dead-end fractures were also shown to limit waterflow. Partially iron oxide filled root cannels were present in Havdrup LUC-bottom, shown to be prehistoric tees (Jørgensen et al., 2020). Simulations of such root cannels in a fracture plane were also modelled and shown to strongly influence the breakthrough time, reducing it from decades to a few days. Sand lenses and sand layers were present at all sites contributing to their high heterogeneity. Model simulations showed that if e.g. macropores are connected to such sand layers they may have a huge effect on waterflow. This indicates that when the macropores and fractures lose significance with depth, then sand layers may take over becoming the preferential water transport routes.

In conclusion, although we know that fractures and biopores are important pathways for pesticide transport through clayey tills, our understanding of how and where this occurs is still fragmented. We need a more solid understanding of the hydraulic properties of the deeper anoxic clay-tills, and why different clay locations apparently have different capacities to retain pesticides. The inherent properties of microbiological processes in clay-tills at depths below the plough zone must be understood if we are to provide realistic simulations of transport of pesticides at a larger scale in both oxidised and anoxic environments. The experimental and modelling investigation of CLAYFRAC illuminated the combined role of fractures and macropores as key preferential flow paths in clayey tills at depths covered by the performed investigation. It is suggested that below the depths of current or ancient root channels, waterflow and pesticide migration in the glacial tills are to a high degree controlled by textual heterogeneities e.g. embedded sand lenses, sand layers, micro-layers together with textural variation in clay matrix. The multi-scale investigations performed in CLAYFRAC highlighted different features of four field sites with different PM types. All four sites showed a strong local variability of the properties of the clayey tills and were ranked as potentially vulnerable. We showed that the Holbæk side had a high sedimentological variability with sandy units and fractures/macropores in the upper clayey tills, which suggests a high potential vulnerability for this PM type. However, in order to develop comprehensive vulnerability mapping methods knowledge about heterogeneities at greater depths below the ‘fracture zone’ investigated in CLAYFRAC is needed as well as a systematic verification with pesticides contamination of groundwater aquifers underlying the clayey till aquitards.