Mobilization and Transport of Colloids and Phosphorus from Undisturbed Soil Columns Sampled along a Natural Clay Gradient

Particle-facilitated transport of strongly sorbing compounds is an important factor in phosphorus leaching to groundwater and freshwater streams. The aim of this study is to describe colloid and phosphorus leaching from macropores in soil columns sampled along a natural clay gradient in Aarup, Denmark.

Undisturbed soil columns (20 cm height by 20 cm inner diameter) were sampled along a clay gradient ranging from 11 % to 23 % clay. After saturation and subsequent draining to -20 cm H2O the samples were irrigated at a constant rate (~5 ml/min.) for four hours using artificial rain water. Before the start of irrigation a bromide tracer was added manually. Air permeability was measured before saturation and after drainage. The lower boundary was free drainage in order to promote macropore flow. Breakthrough times of both water and tracer were recorded and the accumulated effluent was collected approximately every 20 min. Electrical conductivity, pH, turbidity, total phosphorus, total dissolved phosphorus, and dissolved organic matter were measured.

There was a clear positive correlation between the amount of leached particles and the clay content. All samples showed a high “first flush” of colloids and phosphorus followed by a lower and almost constant leaching rate. After the first flush, data implied a linear correlation between the leaching rate and the square root of the accumulated outflow as expressed by ACaff = H*sqrt(AOaff) (ACaff accumulated mass of colloids; AOaff accumulated outflow, both after first flush), with the slope H related to the clay content.

Relationships between clay content and phosphorus leaching, and a number of relevant parameters – including the possible use of macropore flow velocity and air permeability as functional structure and pore connectivity indexes for colloid and phosphorus leaching – will also be discussed.