Fluid and Solute Transport from a Conduit to the Matrix in a Carbonate Aquifer System

Within carbonate systems, the working hypothesis suggests that when a conduit is flooded fluid and solute migrate from the conduit into the matrix. This flux of fluid and solute into the matrix creates a reservoir that can be slowly released once the flooding recedes. Although hypothesized, these fluxes have never been measured. To quantify the distance that a fluid and solute would move into a matrix, the fluxes of fluid and solute from a conduit into a matrix were simulated for nine different carbonate aquifer systems. Two independent numerical approaches were used to simulate (1) fluid flux into the matrix and (2) solute flux into the matrix during a flooding event. When flooding occurs within the conduit, the volume of water transported into and stored in the matrix with a high porosity and high hydraulic conductivity (Floridan Aquifer) was less than 0.34 m³ along a 1 m length of conduit, resulting in a penetration depth of 7.2×10⁻² m into the matrix. In a low porosity and low hydraulic conductivity matrix (Ozark Plateau), the volume of water transported into and stored in the matrix was less than 6.85×10⁻⁵ m³ along a 1 m length of conduit, resulting in a penetration depth of 2.0×10⁻⁴ m into the matrix. Simulated solute flow shows that less than 0.1% of the solute moves in to the matrix. The two approaches demonstrate that during high flow conditions fluid and solute are forced through the conduits, with very little moving into the carbonate matrix. Once the fluid and solute enter a conduit and are moving, they will remain in the conduit until they are discharged at an outlet. Thus, a carbonate matrix does not become a reservoir for solute and fluid during a high-flow event.