Do Solids Travel Faster Than Solutes along Karst Routes?

Particulate matter may pose significant aesthetic and public health problems for groundwater quality. This is particularly relevant for karst systems that contain highly transmissive conduits which contrast with lower permeability volumes favoring water storage. Variations in spring discharge and water quality in these systems arising from recharge events often display peak particle concentrations offset from discharge and solute maximums. The origin of this particulate matter is often ambiguous and may arise from direct transfer of extraneous matter and/or re-suspension of material previously deposited within a conduit system. Conversely, the surrounding lower permeability rock is often assumed less accessible to particles resulting in significant attenuation, particularly in overlying vadose zone.

Comparative tracer testing implemented at a vadose zone karst test site, located away from known conduit systems, permitted assumptions concerning mass transport through the uppermost part of a fissured limestone rock mass to be evaluated. Testing employed particle tracers with different properties, and compared their responses to a conservative solute. Particle and solute tracer breakthrough revealed that particles traveling through the epikarst can experience exclusion. This resulted in higher peak relative concentrations that arrived earlier than those of conservative solutes, despite being susceptible to attenuation. Moreover, differently-sized micro biological tracers had almost identical breakthrough curves suggesting that particle diffusion was not a significant process.

Experimental results indicate that particles can travel through the test site vadose zone with higher mean velocities than solutes, and reach receptors at higher relative concentrations. This in turn highlights the vulnerability of karst groundwater quality to micro organisms reaching groundwater, not only via conduits, but also through less karstified fissures. Such information indicates the need to consider exclusion-driven enhanced particle mobility when developing groundwater protection strategies against contamination by particulate matter.