Effects of water-retaining agent content on the water-holding characteristics of soil in waste dump

Background  The waste dump soil(muck)produced by the open-pit coal mines is an artificial loose accumulation with poor water-holding characteristics and uneven pore distribution, which is not conducive to vegetation growth. This phenomenon is more serious in arid and semi-arid areas and has become an important factor restricting the use of vegetation for ecological restoration in mining areas. Based on this, different water-retaining agent content were applied to the muck of Ningxia open-pit coal mine to improve the water-holding characteristics, water availability and pore environment of the muck, and the optimal content was selected.

Methods  We set up 3 sampling points on the upslope, mid-slope, and downslope at the dumping site of Dafeng Mine in Rujigou, Shizuishan city, Ningxia. We then collected and mixed 0-20 cm muck, then passed them through a 2 mm soil sieve and determined its density and particle composition. The soil high-speed centrifuge was used to simulate the dehumidification process of the muck containing water-retaining agent, and Van Genuchten model was used to analyze the effect of the soil water characteristic curve, fitting parameters, water availability and pore distribution of the muck with different water-retaining agent content.

Results  1) Muck's water content decreased sharply with the increase of soil water suction at low suction stage, and it was positively correlated with water-retaining agent content, but the downward trend slowed down at low suction stage and had no significant change at high suction stage. 2) Saturated water content(θ_s)increased with the increase of water-retaining agent content. The inverse of air-entry value (α) reduced siginificantly by the use of water-retaining agent, but it increased first and then decreased, and the peak appeared at 1.0% treatment. Residual water content(θ_r)had no significant difference, and the value of shape parameter(n) had no obvious change rule.3) Water-retaining agent increased the total available water and easily available water content of muck by 1.12-3.31 times and 1.11-3.29 times respectively. The high water-retaining agent content(1.0%, 1.5% and 2.0%)significantly increased slowly available water content by 3.00-4.38 times, and the unavailable water content only significantly reduced in 0.2% and 0.5% treatment. 4) The water-retaining agent significantly increased the mesopore by 1.2-2.6 times, the micropore by 1.8-3.8 times, and the super micropore by 1.2-4.7 times.

Conclusions  Our results reveal that water-retaining agent relies on its capillary pore to improve the water-holding characteristics of muck. The increase in its content may significantly increase the total available water content, easily available water content, and the ratio of mesopore, micropore, and super micropore. In order to be suitable for vegetation growth, water-retaining agent content applied to the muck should be < 1.0%, and 0.5% is the optimal one.