A Comparison Of The Stormflow Response Of Four Zero Order Watersheds In Western Maryland
Sloan, James McAlpine
Eshleman, Keith N.
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Surface mining and reclamation activities have been shown to significantly affect watershed hydrologic processes. Using natural isotopic and chemical tracers, the present study sought to quantify the impacts of surface mining and reclamation on old water and new water contributions to stormflow hydrographs for four zero order watersheds in western Maryland. The primary goal of the study was to determine whether results from the hydroisotopic and straight line hydrograph separations could be used to assess the success of reclamation at three zero order mined watersheds of various reclamation ages. Similarly, a secondary goal was to determine whether such conventional methods could be employed as successfully at zero order watersheds in the mid-Atlantic as they are at the small watershed and river basin scale. A final goal was to determine the effectiveness of the three natural tracers used in this study: (1) 18O; (2) dissolved silica; and (3) specific conductance. To achieve these goals, 13 storms occurring between September 2004 and June 2006 were analyzed and used to characterize the rainfall-runof response of each of the four watersheds via the straightline hydrograph separation technique. Likewise, three storms occurring on September 17, 2004, April 22, 2006 and May 14, 2006 were sampled and hydrographs were separated by the hydroisotopic method to generate new water contributions to hydrographs at the four watersheds. This study found that 18O, silica and specific conductance time series data to be valuable, as dilutions in stream water values of the tracers were obvious and timing of the dilutions generally corresponded to peak discharge. Likewise, all three tracers estimated that old water provided over 50% of total runoff at the forested TNEF, which was encouraging, as these results were similar to the results from other studies in similar ecological settings. Despite these encouraging results, all tracers exhibited serious limitations for the three storms examined in this study. All three tracers were thought to violate critical assumptions of the mixing theory of the hydroisotopic hydrograph separation technique. Despite the problems encountered with each of the tracers, data from the hydroisotopic separations provided insight into how each of the watersheds generated runoff. Results from the hydroisotopic and straightline separation techniques illustrated that two of the three mined watersheds (denoted TMAT and TSSR) exhibited considerably higher peak runoff, total runoff, and new water percentages when compared to the other mined watershed (TSNR) and TNEF. Therefore, surface mining and reclamation impacted the hydrologic response of the mined TMAT and TSSR watersheds more than the mined TSNR watershed. The differences in stormflow response of the four watersheds were thought to be explained by soil compaction and the mixing of clays into the topsoil during reapplication of the overburden during reclamation activities. The limited infiltration capacities observed at TMAT and TSSR, combined with the results from the hydroisotopic and straightline separations, indicate that the likely runoff mechanism at these sites is Hortonian overland flow. The moderate infiltration capacity and the large storage capacity of soils at the TSNR site are thought to explain, in part, the dampened hydrologic response of this watershed. Runoff at the TSNR site may be dominated by saturation overland flow, specifically by precipitation onto saturated areas near the stream channel. As hypothesized, the forested watershed, TNEF, exhibited the lowest runoff volumes, runoff ratios and the lowest estimates of peak and total new water for two of the three tracers. The primary runoff mechanism at the TNEF site, like other forested watersheds, is likely shallow subsurface flow. While it may be erroneous to depend on the hydroisotopic results alone, by integrating the results from the three independent field measurements (hydroisotopic, straightline and infiltration results) one is able to quantify and assess the impacts of surface mining and reclamation.