Using Conservative and Biological Tracers to Better Understand the Transport of Agricultural Contaminants from Soil Water through the Epikarstic Zone
Agriculture contamination is very common in karst systems due to the vulnerability of these aquifers. Animal waste is often spread across crop land to enrich the soil with nitrates and phosphates. Herbicides and pesticides are also applied to the crops. The transport of these pollutants through the soil and epikarst is a difficult process to monitor due to the complex, heterogeneous behavior of the groundwater as it makes its way down to the aquifer below. An experimental site at Crumps Cave lended a unique opportunity to monitor the vadose zone at a waterfall in the cave below. A previous dye trace established the connection between an 11.15 m2 grass plot and this underground waterfall. The field design used here, utilizing a rainfall simulator, allowed control of the input of precipitation and tracers to understand more about the movement of stormwater infiltrating the soil and the differences in transport of solute particles and bacteria in the epikarstic zone. Two particle transport experiments were used to better understand these processes. The first trace involved injection of fluorescein dye and sodium chloride. The 2,650 liters of solution were injected over a period of 3.6 hours at a rate of 6.6 cm/hr. An electrical resistivity traverse, perpendicular to the hypothesized straight-line path of the established dye trace connection, showed a peak in lower resistance at the upper epikarst layers 4 hours and 15 minutes after the beginning of the injection. Dye concentrations reached a peak of 1,600 ppb 3 hours and 15 minutes after the beginning of the injection. The conductivity also peaked at this time with a value of 814 µS. This first trace showed that rapid transportation of solutes happened in localized conduits causing a peak of both solutes in the cave before the widespread mobilization of sodium chloride was seen in the epikarst by the resistivity images an hour later. In the second trace, 2,605 liters of a sulphorhodamine dye solution was injected over 180 kg of dairy cattle manure spread on the 11.15 m2 plot of grass for a period of 3.6 hours at a rate of 6.6 cm/hr. Dye concentrations reached a peak of 27 ppm 4 hours and 15 minutes after the beginning of the injection. Fecal coliform reached its first peak of 2,755 MPN (most probable number of viable cells per 100 mL of water) 90 minutes prior to the dye peak and a second peak of 2,481 MPN occurred 15 minutes prior to the dye peak. These results showed that solutes travelling through the soil and epikarst follow similar paths while bacteria prefer conduits that offer more rapid transmission to the underground waterfall.
Crumps Cave waterfall, particle transport, Agricultural and Resource Economics, Animal Sciences, Environmental Health and Protection, Environmental Monitoring, Geographic Information Sciences, Geography, Mineral Physics, Physical and Environmental Geography
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