Search

Non-Darcian flow has been well documented for fractured media, while the potential non-Darcian flow and its driven factors in field-scale discrete fracture networks (DFNs) remain obscure. This study conducts Monte Carlo simulations of water flow through DFNs to identify non-Darcian flow and non-Fickian pressure propagation in field-scale DFNs, by adjusting fracture density, matrix hydraulic conductivity, and the general hydraulic gradient. Numerical simulations and analyses show that interactions of the fracture architecture with the hydraulic gradient affect non-Darcian flow in DFNs, by generating and adjusting complex pathways for water. The fracture density affects significantly the propagation of hydraulic head/pressure in the DFN, likely due to fracture connectivity and flow channeling. The non-Darcian flow pattern may not be directly correlated to the non-Fickian pressure propagation process in the regional-scale DFNs, because they refer to different states of water flow and their controlling factors may not be the same. Findings of this study improve our understanding of the nature of flow in DFNs.

Biogenic nano-hydroxyapatite (bio-nHAP) has recently gained great interest in many domains, especially in the remediation of heavy metal-contaminated soil, due to its high reactivity, low cost, and eco-friendly nature. The co-transport and reaction of bio-nHAP with Pb(II) in saturated porous media, however, are not well understood. This work investigated the effects of ionic strength (IS), ionic composition (IC), dissolved organic matter (DOM), and flow velocity on transport-reaction dynamics of Pb(II) and bio-nHAP by combining column breakthrough experiments and model simulations. Results showed that the mobility of Pb(II) was significantly enhanced with increasing IS/IC but less affected by flow velocity during the transport-reaction process of bio-nHAP and Pb(II) in the saturated sand column; while the transport of bio-nHAP was restricted by increasing IS/IC but facilitated by increasing velocity. IC, IS, and velocity only slightly affected the reaction kinetics between Pb(II) and bio-nHAP, likely due to the fast reaction rate between Pb(II) and bio-nHAP and precipitation of pyromorphite. The transport dynamics of bio-nHAP and Pb(II) were significantly changed by DOM, and this effect depended strongly on the type of DOM with different molecular weights. Breakthrough curves of Pb(II) and bio-nHAP exhibited apparent "anomalous", sub-diffusive transport behaviors, which could be well quantified by a novel tempered fractional derivative bimolecular reaction equation (T-FBRE). Our findings highlighted the accurate simulation of the co-transport and reaction of bio-nHAP with Pb(II) using T-FBRE and had a great benefit for risk assessment and remediation strategy development for Pb(II) contaminated soil.

Contaminant transport in a single fracture cannot be reliably captured by the classical advection-dispersion equation model, due to the heterogeneity nature of the fracture. This study applied a time fractional advection-dispersion equation (ADE) model to quantify chloride ion transport in a single fracture observed in the lab. We further explored the solute transport characteristics by comparing them with the simulation results using a classical ADE model and a space-time fractional ADE model. Comparison results show that the time fractional derivative model is better than the classical ADE model in describing the blocking process of solute transport in the fracture (the tailing phenomenon of the breakthrough curve or BTC). The space-time fractional ADE model is not needed to describe the observed BTC, because only time evolution of solute concentration is considered in this case. Experimental analysis also indicates that the order of the time fractional derivative increases with the fracture aperture, and sub-diffusion is enhanced by a slower flow velocity.

The transport behavior of arsenic (As(V)) loaded by ferric humate (HA-Fe) colloid, denoted as HA-Fe/As(V), moving in a saturated quartz sand column, was tested in the laboratory under varying pH values, ionic strengths, and HA and Fe(III) content. The time-fractional advection-dispersion equation (fADE) model was then employed to analyze the observed migration of HA-Fe/As(V). Results showed that the stability of the HA-Fe colloid exhibited an upward trend with an increasing pH and HA content. An increasing HA content led to a decrease in the particle size of the HA-Fe colloid. However, the effect of Fe(III) concentration on colloidal particle size exhibited the opposite phenomenon. The ability of the HA-Fe colloid to load As(V) gradually increased with the increase of the Fe(III) concentration. During the co-transport of the HA-Fe/As(V) colloid, transport of As(V) was promoted with increasing pH, increasing HA and Fe(III) content, and decreasing ionic strength in the saturated porous medium. The transport behavior of As(V) can be well fitted by the fADE model. The model analysis revealed that sub-diffusion of As(V) was weakened in the HA-Fe/As(V) colloid with high HA content. Sub-diffusion of As(V) in the low pH colloid was stronger than that of the high-pH colloid, and the molecular diffusion and mechanical dispersion were more weakened in the high-pH colloid than that of the low-pH colloid. When observing varying ionic strengths, As(V) exhibited stronger sub-diffusion in the HA-Fe/As(V) colloid with a higher ionic strength. As for the Fe(III) content, transport of As(V) was mainly affected by sub-diffusion in the HA-Fe/As(V) colloid with a low Fe(III) content. These findings provided direct and necessary insights into the effects of the HA-Fe colloid on the migration of As(V) throughout saturated porous media under different hydrochemical conditions found in natural environments.