Conservative and reactive mass transport in homogeneous and heterogeneous groundwater flow systems

Experimental studies have investigated the influences of varied coupling processes on the flow and mass transport. Coupling developed as a consequence of difference in density, sorption/desorption, plume size/density/loadings, dissolution/precipitation, compositions of tracer and medium, and medium heterogeneity.; Plumes with large density differences as compared to the ambient fluid or a relatively large initial size sink faster and deform less, as compared to plumes with relatively small density differences or a smaller initial size, respectively. Increasing the density during loading forms a plume with a thick tail and a thin head. Reducing the density during loading causes the nose of the plume to move downward much faster than the tail and to form a vertical lobe at the nose. This lobe can actually separate from the rest of the plume. Both loadings lead to the formation of instabilities along the bottom of the plumes.; The transport of reactive contaminants in a plume is influenced by coupling related to the retardation factor and density. With pulse loading, contaminants with large retardation factors can separate from the dense plume at early time and follow their own pathways. However, contaminants with low retardation factors are coupled with the density-determining component and affect each other in that both plumes follow similar pathways.; Experiments were conducted to evaluate how reaction zones developed as a reactive tracer (precipitating) was introduced to the flow tank. A high Fe3+ concentration in the tracer solution makes the reaction zone grow faster, enhances the downward movement but hinders the horizontal movement of the dissolved plume. A high calcite content, which enhanced the rapid precipitation, resulted in a slow growth rate, no preferential direction of growth of the reaction zone, and finger structures. A low calcite content resulted in a somewhat larger reaction zone but a fewer number of finger structures, with faster growth rates and preferential growth direction for both reaction zone and finger structures.; Heterogeneity in the original hydraulic conductivity distribution also controlled the evolution of the reaction zone and the dissolved plume. High permeable lenses convey most of the flow and tracers. As a result, both the reaction zone and the dissolved plume developed along more permeable lenses first. Finger structures are developed along different high permeability lenses, forming separate pathways. Heterogeneity in hydraulic conductivity also causes irregularities in the reaction zone and dissolved plume.; Dissolution and precipitation change both the hydraulic conductivity and the solution chemistry. Some constituents such as pH, Ca2+, $HCO-3$, and $H2CO03$ are affected but not totally controlled by the reaction zone. Others are totally associated with the reaction zone, such as Fe3+. Overall, this work has shown that coupling in mass transport can make the processes behave in a much more complicated way, as compared to simple uncoupled systems. These results have implications to a variety of practical problems.