Karst aquifers are highly productive groundwater systems often associated with conduit flow. These systems can be highly vulnerable to contamination, resulting in a high potential for contaminant exposure to humans and ecosystems. This work develops statistical models to spatially characterize flow and transport patterns in karstified limestone and determines the effect of aquifer flow rates on these patterns. A laboratory-scale Geo-HydroBed model is used to simulate flow and transport processes in a karstic limestone unit. The model consists of stainless-steel tanks containing a karstified limestone block collected from a karst aquifer formation in northern Puerto Rico. Experimental work involves making a series of flow and tracer injections, while monitoring hydraulic and tracer response spatially and temporally. Statistical mixed models are applied to hydraulic data to determine likely pathways of preferential flow in the limestone units. The models indicate a highly heterogeneous system with dominant, flow-dependent preferential flow regions. Results indicate that regions of preferential flow tend to expand at higher groundwater flow rates, suggesting a greater volume of the system being flushed by flowing water at higher rates. Spatial and temporal distribution of tracer concentrations indicates the presence of conduit-like and diffuse flow transport in the system, supporting the notion of both combined transport mechanisms in the limestone unit. The temporal response of tracer concentrations at different locations in the model coincide with, and confirms the preferential flow distribution generated with the statistical mixed models used in the study.
This paper presents the development and testing of a three-dimensional laboratory-scale soil tank system for modeling ERC fate and transport under controlled, but variable environmental conditions in partially saturated soil. The system incorporates a rainfall simulator, variable light (visible and UV), temperature and relative humidity components, and a 3D SoilBed capable of simulating several boundary and initial conditions. Experimental work indicate that water and solute transport is highly influenced by interrelated environmental and boundary conditions. The presence of light and higher system temperatures induces greater water drainage and solute fluxes. During infiltration, hydraulic heads increase at faster rates under no light exposure suggesting greater water and solute retention. The spatial and temporal distribution of hydraulic heads during rainfall events is not uniform and flow patterns reflect preferential paths. Transport of conservative solutes closely follows water flow patterns, and reflects the influence of variable and interrelated environmental factors on spatial and temporal concentration distribution. These experiments show that interrelated environmental factors must be taken into account to accurately predict the distribution of chemicals near the soilatmosphere surface. They demonstrate that non-reactive solutes are highly influenced by variation in hydraulic, advective, and dispersive processes induced by changes in environmental conditions. Greater impacts are expected for reactive and semi-volatile solutes such as ERCs. In such case, fate and transport will also be affected by variations in soptive, gas transport, and degradation processes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.