Field Scale Variation in Soil Structure, Soil Hydraulic Properties and Leaching of Colloids.

Soil is a highly heterogeneous system in which the movement of water and colloids is far from fully understood. To predict the leaching risk for colloids and strongly sorbing contaminants from agricultural fields, knowledge about field scale variations of soil structure, water and colloid transport is essential.

Intact soil cores were sampled in Fårdrup, Denmark, at a sandy loam field (70-m by 50-m field) that is part of the PLAP (Danish Pesticide Leaching Assessment) programme. In total 92 cores (20-cm height by 20-cm diameter) were collected across the field. The cores were saturated and drained to a matric potential of -20cm H₂O before being irrigated with artificial rainwater at a rate of 10 mm h^-1. The effluent was collected to determine colloid concentration, electric conductivity, and pH. An inert tracer (Tritium) was applied as a pulse to give additional information about soil structure and flow properties. After the flow event, the cores were incubated for a given time interval (stop-flow) ranging between 1h and 5 months after which the irrigation and measurements were re-initiated. In addition, saturated hydraulic conductivity was measured on the intact cores after the last leaching event.

Preliminary results showed a high initial leaching of particles during the first 30 minutes followed by lower and relatively constant particle concentrations in the effluent. After stop flow, the initial leaching was again high and depending on duration of stop flow, suggesting diffusion limited release of colloids from matrix to conducting pores. No correlation between hydraulic conductivity and clay or organic carbon was found, which may indicate a poorly developed macropore structure, resulting from the relatively low clay content. This was supported by the tracer breakthrough curves which hint a high degree of matrix flow. Further, correlations between flow properties and particle leaching properties will be investigated to be used in risk assessment analysis.