An Alternative to the Advection Dispersion Model for Interpreting Dye Tracing Studies in Fractured-Rock and Karst Aquifers

Due to the complexity of groundwater fl ow in fractured-rock and karst aquifers, solute transport models for these aquifers are typically stochastic models based on tracer transport studies. Water and tracers do not fl ow at one single advective velocity but experience a wide range of velocities, from rapid fl ow in conduits to near stagnant conditions in adjacent voids. This variance of velocities is referred to as dispersion and is traditionally described mathematically by the advection-dispersion equation (ADE). Analytical solutions to the ADE are available and are referred to as advection-dispersion models (ADM).The ADM is fitted to the tracer data by varying the parameters until a best-fit is achieved between the experimental residence time distribution (RTD) and the model RTD. The major shortcomings of this approach are due to the symmetry of the ADM and its associated prediction of finite concentrations at zero time and its inability to reflect the long upper tail typical in experimental RTD data. This paper presents an alternative conceptual approach to the ADM for modeling solute transport in fractured-rock and karst aquifers. In this approach the variance in fl ow velocities and fl ow path lengths are addressed directly by treating them as random, gamma distributed variables and deriving the RTD from a transformation of random variables based on the ratio of length to velocity and representing the RTD as a conditional probability distribution of time. The resulting four parameter (Gamma-RTD) model is relatively easily parameterized since the fl ow path length is tightly distributed about the known straight line distance between the injection point and the effluent. The model is demonstrated and contrasted to the ADM below by applying it to tracer data from a quantitative tracer study at Mammoth Cave National Park. The results indicate that the Gamma-RTD is superior to the ADM in modeling the shape as well as the area of the experimental RTD

Evaluation of Stormwater Filters at Mammoth Cave National Park, Kentucky, 2011-12

Studies in the 1970s found potentially toxic levels of metals entering Mammoth Cave’s underground streams through storm recharge. Additional studies confirmed that stormwater from parking lots and buildings fl owed rapidly into critical cave habitats. The Park’s management responded to these findings by installing storm runoff filter systems on the most heavily used parking lots in 2001. The Park entered an agreement (2010-12) with Tennessee State University, the USGS, and WKU-Mammoth Cave International Center for Science and Learning to evaluate the filter systems to determine if they were removing hazardous compounds from stormwater runoff . The objective of this study was to evaluate stormwater filters before and after replacing 2-year-old ZPG cartridge filters. The study focused on the first-flush runoff waters during the storms. The filters were not effective at removing quaternary ammonia compounds (QACs), and moderately eff ective at removing zinc and copper. The filters were very effective at removing diesel-range aromatic ring compounds (fuels). Regression analyses were used to evaluate trends between parking lot size and filter efficiency. The efficiency of the filters to remove fuels improved with basin size. The efficiency to remove QACs decreased with basin size. Basin size did not appear to have any correlation to zinc or copper removal efficiency. Human activity, such as construction, probably played a role in the storm-water chemistry and the efficacy of the filters to remove certain contaminants

Three Examples of Chemical Transport in Storm Runoff at Mammoth Cave National Park, Kentucky

The karst landscape at Mammoth Cave National Park, Kentucky, was formed by water through the dissolution of soluble rocks forming sinkholes, disappearing streams, emerging springs, closed depressions, and a combination of wet and dry caves. The Park’s cave streams and pools provide a home to unique organisms. Surface waters in the Park tend to rapidly drain into subsurface geologic features and caves. This rapid infiltration makes the subsurface vulnerable to contamination. The objective of this investigation was to characterize chemical transport from the surface into the cave. The preliminary results were achieved by tracer studies and monitoring water chemistry along known flowpaths. The results presented in this paper are the outcome of several studies occurring between 2009-2012 in a partnership between Mammoth Cave National Park, Tennessee State University, Mammoth Cave International Center for Science and Learning, and U.S. Geological Survey. Processes that influenced chemical transport included storm intensity, time between storms, epikarst saturation, dispersion, dilution, and complex fl ow paths in the geology