Dye solution experiments for determining solute transport behaviour in degraded wetland soils of the Yellow River Delta

Solute transport behaviour can indicate the efficiency of eco-hydrological responses. However, this behaviour has not been consider for use in assessing the health of wetland ecosystems. We assessed solute transport behaviour in degraded wetland soils using 72 undisturbed soil columns located in Robinia pseudoacacia (tree) and Tamarix chinensis (bush) communities. Soil column tracer experiments were conducted under the same initial and boundary conditions using Brilliant Blue FCF as a conservative tracer. Solute transport behaviour at a soil column scale was described by four measures: steady infiltration rate of effluents, concentration of effluents, dye staining patterns and cumulative effluents leaching. Numerical modelling by the dual-permeability model in HYDRUS-1D was used to simulate the proportion of cumulative effluents leaching from soil macropores. These data showed that steady infiltration rate of effluents at the tree site decreased with increasing soil depth (5.742, 0.716, 0.657, and 0.458 ml/min) but at the bush site first increased and then subsequently decreased with increasing soil depth (0.336, 0.548, 0.974 and 0.379 ml/min). The concentration of effluents at the tree site (0.583, 0.149, 0.147 and 0.067 g/L) and the bush site (0.281, 0.109, 0.161 and 0.053 g/L) decreased nonlinearly with increasing soil depth. Dye staining patterns were significantly different between different soil locations. The largest dark blue staining domains were from 0 to 10 cm soil depths at the tree site and 20-40 cm depths at the bush site. The proportion of cumulative effluents leaching from soil macropores ranged from 37.6% to 61.1% at the tree site and ranged from 0% to 99.9% at the bush site. Understanding the solute transport behaviour in the Yellow River Delta is the first step towards the restoration of degraded wetlands.