Characteristics on water-body solute migration of gravel clay in mine subsidence area

Subsidence area is easily caused by coal mining,where surface water affects shallow groundwater through permeable medium,and shallow groundwater solute changes. In the coal-mine subsidence of the western Jiaozuo,the permeable medium between surface water and shallow groundwater is gravel clay. Surface water easily permeates and pollutes shallow groundwater,which seriously influences groundwater function. In the test area,eight surface water monitoring points and thirteen shallow groundwater ones were arranged,and TDS,pH,EC Na⁺,Cl^-,SO^2-₄,Ca²⁺,Mg²⁺,NO^-₃and in surface water and shallow groundwater were monitored. Based on these monitoring data,the differences of chemical components and types between surface water and shallow groundwater were carefully analyzed,the retardation effect of permeable medium on the chemical components in surface water were explored,and the characteristics on the chemical-difference relationship between surface water and shallow groundwater was studied. The results showed that the differences of TDS,pH,EC were obvious between two-river surface water and shallow groundwater,the TDS,pH,EC of surface water in Baimamen river and Dashi river were -23. 31%,9. 65%,-23. 58% and 377.30%,19. 86%,378.60% of shallow groundwater respectively. The transitions of chemical type from surface water to shallow groundwater were different between two rivers,the Cl·SO₄-Na·Ca type of surface water in Baimamen river was transformed to Cl·SO₄-Mg·Ca type of shallow groundwater through Cl·SO₄-Ca·Na type in permeable medium,whereas the Cl·SO₄-Na and Cl·SO₄-Na·Ca types of surface water in Dashi river was transformed to Cl·SO₄-Mg·Ca type of shallow groundwater through Cl·SO₄-Ca·Na and Cl·SO₄-Mg·Na types in permeable medium. The concentrations of Na⁺,Mg²⁺,Ca²⁺,Cl^-,SO^2-₄in Dashi river were higher than that in Baimamen river,and the differences of these ions weren't obvious between surface water and shallow groundwater in Baimamen river. With the increase of distance to Dashi river,the concentrations of Na⁺,Mg²⁺,Ca²⁺,Cl^-,SO^2-₄ showed the trend from sharp decrease to slow decrease to stable along the direction from southwest to northeast. The degradation ratios of chemical components in gravel clay were direct proportion to surface water and inverse proportion to shallow groundwater respectively,and the orders of degradation ratios and unit ones were Na⁺>Cl^->SO^2-₄>Ca²⁺>Mg²⁺. When the concentration differences of Na⁺,Cl^-,SO^2-₄,Ca²⁺ and Mg²⁺between surface water and groundwater were less than 1 478. 71,1 325. 09,2 177.54,308.71 and 39.52 mg/L respectively,their unit degradation ratios were positively correlated with the concentration difference.