Electricity-based Study Of Solute Transport Processes In Fracture-matrix System

Bedrock fissures,as the main deposit medium of groundwater storage,are widely distributed in China,and their aquifers have various forms and complex hydrodynamic conditions influenced by geological and tectonic factors,which make the burial and distribution of fissure water uneven and somewhat directional,therefore,quantitative portrayal of fissure rock solute transport law plays a key role in groundwater pollution prevention.In this paper,we construct a physical model of the fissure-matrix system,carry out experimental studies on solute transport in the fissure-matrix system with different matrix porosity at different flow rates,and monitor the solute transport process by using electrical method,and quantify the degree of trailing of breakthrough curves in the matrix and fissure domains under different test conditions by constructing C/C0-t^-n power law functions,and use self-potential data to achieve the quantification of the flow rate provides a theoretical basis for groundwater pollution control and riskiness evaluation.The following main conclusions were obtained in this paper:（1）The water flow characteristics of the fracture-matrix system were analyzed.Under the same water pressure,the seepage velocity of the fracture-matrix system is positively correlated with the porosity of the matrix medium.When the difference between the fracture and matrix permeability coefficients is nearly 270 times,the water flow in the fracture-matrix system gradually differs from Darcy flow to non-Darcy flow,and the relationship curve between the hydraulic gradient and the flow velocity under the non-Darcy flow is fitted with the Forchheimer equation with good results.（2）The solute migration characteristics of the fracture-matrix system were clarified.Along the solute migration direction,the peak breakthrough curves of each electrode measurement point in the fracture-matrix system shows a decreasing trend and the peak breakthrough curves concentration in the matrix domain is basically higher than that in the fracture domain.With the increase of flow rate,the peak breakthrough curves of each electrode increases and the peak arrival time is earlier.With the decrease of matrix porosity,the peak arrival time of breakthrough curves in matrix and fissure domains became longer,and the degree of trailing of breakthrough curves was significantly enhanced,and the characteristics of early arrival,double peaks and trailing of breakthrough curves in non-Darcy flow were more obvious.（3）The degree of trailing of penetration curves for different flow rates in the fracture-matrix system with different matrix porosity was quantified.A C/C0-t-n power-law function was constructed to quantify the degree of trailing of the breakthrough curves in the fracture and matrix domains under different test conditions,and it was found that the fitted n values varied from 1.5156 to 2.1390 under Darcy flow and from0.4159 to 1.8283 under non-Darcy flow,i.e.,when the fitted n values were less than 1.8,the solute migration changed from Fick transport to non-Fickian transport.In the same fracture-matrix system,the n value increases with the increase of flow rate,and the fitted n value of the fracture domain penetration curve is larger than that of the matrix domain.In different fracture-matrix systems,the n-value decreases with decreasing matrix porosity.（4）The self-potential is revealed as a response law during solute migration.The change of SP peak reflects the change of concentration during solute migration,the SP peak at each electrode decreases and the peak arrival time becomes longer along the direction of solute migration,the SP peak at the fracture domain appears at a shorter time compared with the matrix domain.With the increase of the flow rate,the peak SP arrival time and the time required to return to the native value at the electrode points in the fracture and matrix domains are earlier and the peak SP at the electrode points in the matrix domain is larger than that in the fracture domain.As the matrix porosity decreases,the SP peaks in the fracture and matrix domains increase,and the self-potential curve takes longer to return to its native value,which is more obvious in the matrix domain.（5）The feasibility and accuracy of the self-potential in quantifying the flow rate during solute migration were verified.The relationship between SP peak arrival time and solute migration distance was calculated to quantify the flow rate in the fracture domain and matrix domain of the fracture-matrix system,and the calculated flow rate was smaller than the measured flow rate of the system,in which the error was controlled at about 7.8%using the self-potential values of the fracture domain electrode measurement points,which was more accurate than the matrix domain flow rate calculation.