WaterSubject

Solute and colloid transport in karst aquifers under low and high flows was investigated by tracer tests using fluorescent dyes (uranine) and microspheres of the size of pathogenic bacteria (1 ?m) and Cryptosporidium cysts (5 ?m), which were injected into a cave stream and sampled at a spring 2.5 km away. The two types of microspheres were analyzed using an epifluorescence microscope or a novel fluorescence particle counter, respectively. Uranine breakthrough curves (BTCs) were regular shaped and recovery approached 100%. Microsphere recoveries ranged between 27% and 75%. During low flow, the 1-?m spheres displayed an irregular BTC preceding the uranine peak. Only a very few 5-?m spheres were recovered. During high flow, the 1-?m-sphere BTC was regular and more similar to the uranine curve. BTCs were modeled analytically with CXTFIT using a conventional advection dispersion model (ADM) and a two-region nonequilibrium model (2RNE). The results show that (1) colloids travel at higher velocities than solutes during low flow; (2) colloids and solutes travel at similar velocities during high flow; (3) higher maximum concentrations occur during high flow; and (4) the 2RNE achieves a better fit, while the ADM is more robust, as it requires less parameters.

Karst aquifers are important fresh water resources but highly vulnerable to contamination, which can easily enter the aquifer through thin soils or via swallow holes, and may be rapidly transported in a network of fractures and conduits. Microbial contamination from agriculture and domestic waste water often constitutes the most serious water quality problem Karst aquifers often show strong hydraulic and hydrochemical responses to rainfall, which also result in high variability of suspended matter (turbidity) and bacteria levels. Suspended matter can be classified into colloids (less than 1 ?m) and larger particles. found that a large proportion of fecal bacteria in karst ground water are associated with suspended sediments. These findings underscore the importance of event-based monitoring of bacterial water quality and illustrate the necessity to better understand the processes governing particle transport under different hydrologic conditions.

Tracing techniques are powerful tools for this type of investigations. Solute tracers, such as fluorescent dyes, can be used to study ground water flow and the migration of dissolved contaminants; particulate tracers are surrogates for particle-bound chemical contaminants and pathogens. Lanthanide-labeled clay is an effective but relatively rarely used method of tracing sediment transport. Specific bacteriophages and bacteria can be used to simulate pathogen transport but require analysis within 24 h and in some cases legal permission. Some bacteria that were often used as tracers are now classified as pathogens, for example, Serratia marcescens. Therefore, fluorescent microspheres are increasingly used as particulate tracers. They are available at different diameters and physicochemical properties and can be considered as surrogates for bacteria (1 ?m) and Cryptosporidium cysts (3 to 7 ?m) note that most previous microsphere traces were done in porous media and at a lab to local field scale, while more experiments would be required in karst aquifers and at larger field scales (km flow distance). The few published examples include multitracer tests in the karst aquifer of the mineral springs of Stuttgart, Germany, where low numbers of 1-?m spheres were detected 6 km downstream of the injection well.

In order to obtain further insight into colloidal transport in karst ground water, the Hölloch cave in the Austro-German Alps has been selected as a test site. The primary goals are (1) to compare the transport of colloids/particles and solutes and (2) to assess the impact of different hydrologic conditions on these transport processes. The study also presents the first application of a relatively new analytical modeling approach for the simulation of tracer breakthrough curves (BTCs) to colloids, and the first application of a fluorescence particle counter for the detection of fluorescent microspheres.

The tracer results demonstrate that colloids/particles of the same size as pathogenic bacteria (1 ?m) and Cryptosporidium cysts (5 ?m) are transported in this karst conduit system over a 2.5-km distance. The 5-?m spheres were injected only during low flow, in a relatively low quantity, and arrived at the spring at low concentration levels—yet sufficiently high to cause illness if it were infectious cysts. The 1-?m spheres were used during both low and high flow and were effectively transported. Attenuation processes removed a part of the colloids, but their normalized maximum concentrations were similar than those for solutes. During low flow, colloids were found to travel at higher velocities than solutes. During high flow, all velocities increase, but colloids and solutes show similar velocities.

These findings underscore the vulnerability of karst aquifers to contamination and the need of adequate protection strategies. Karst spring monitoring often reveals high levels of fecal bacteria after storm rainfall, but good water quality during low-flow periods. This observation mainly reflects the fact that fecal bacteria often originate from agricultural activities at the land surface. During rainfall events, they are flushed into the aquifer and typically arrive at the springs together with organic carbon and secondary turbidity (a primary turbidity peak may occur due to remobilization of sediments in the . However, the present study demonstrated that particle transport, and thus pathogen transport, can also occur during low flow. Therefore, waste water releases into karst aquifers are generally problematic, even at great distances to a drinking water source.