Estimating hydraulic conductivities of the soil aggregates and their clay-organic coatings using numerical inversion of capillary rise data

SummarySoil aggregates are in some soils and their horizons covered by organomineral coatings, which may significantly influence water and solute transfer into the aggregates. Knowledge of a coating occurrence, their structure and hydraulic properties is required for a more precise description of water flow and contaminant transport in soils. The aim of this study was to describe hydraulic properties of clay and organic matter coatings in the iluvial (Bt2) horizon of Haplic Luvisol. Sets of 30 unsorted aggregates, 24 aggregates with mostly clay coatings and 24 aggregates with clay-organic coatings, respectively, were studied to evaluate an impact of various coating composition. The coatings were removed from a half of the aggregates of each set. First, the wetting soil–water retention curve was measured on all soil aggregates. Then the capillary rise from the saturation pan into the multiple aggregates (set of 14 or 15 aggregates) without and with coatings was measured. Numerical inversion of the measured cumulative capillary rise data using the HYDRUS-1D program were applied to estimate the saturated hydraulic conductivities of the aggregates, Ks,aggr, and their coatings, Ks,coat. Results were compared with saturated hydraulic conductivities evaluated analytically using the sorptivity method, which was proposed previously.Data of the soil–water retention curves, measured on aggregates with and without coatings, did not allow distinguishing between retention curve parameters of the soil aggregates and their coatings. Therefore the same parameters were evaluated for both. Capillary rise into the soil aggregates without coatings was always faster than into the aggregates with coatings. As result the optimized saturated hydraulic conductivities, Ks,coat, of the clay and the organic matter coatings (the average values for unsorted, mostly clay and clay-organic coatings were 3.69 × 10−7, 2.76 × 10−7 and 1.81 × 10−7 cm min−1, respectively) were one to two order of magnitude lower than the saturated hydraulic conductivities, Ks,aggr, of the aggregates (the average values for corresponding aggregates were 3.87 × 10−6, 6.52 × 10−6 and 1.11 × 10−5 cm min−1, respectively). Slightly variable Ks,aggr (or Ks,coat) values were obtained for different bottom pressure heads, and the lower Ks,aggr (or Ks,coat) values were estimated from the second run of the tests compare to the first run of tests for each set of aggregates. No statistically significant differences between Ks,coat values obtained for coatings of various compositions were found. Resulting Ks,aggr values were in the range of the values obtained analytically.

► Hydraulic properties of aggregates and their coatings were evaluated.
► Soil water retention and capillary rise into the aggregates was measured.
► Numerical inversion using HYDRUS-1D was used to estimate hydraulic conductivities, Ks.
► The Ks values of coatings were one to two order magnitude lower then Ks of aggregates.
► No differences were found between the Ks values of coatings of various compositions.