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Characterizing Storm Response in an Urban Karst Aquifer at Valley Creek, Chester County, Pennsylvania
Yang, Youa
Yang, Youa
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Thesis/Dissertation
Date
2006
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Earth and Environmental Science
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Yang-Thesis-2006.pdf
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https://doi.org/10.34944/9ynd-1e54
Abstract
Karst aquifers respond more quickly to precipitation events than other types of aquifers. However, continuous monitoring and geochemical analysis can provide the necessary information to better understand aquifer recharge pathways and sources during precipitation events. From May 2005 to March 2006 continuous data and water samples were collected from four springs in the Valley Creek Watershed, Chester County Pennsylvania. The springs selected for this study are in commercial or residential land use in an urban setting. This area is underlain by carbonate units that trend east-northeast to west-southwest bounded on the north and south by crystalline igneous and metamorphic units. All springs were in the Elbrook Formation, which is a fractured dolomite unit of Upper Cambrian age.
The goal of this study was to look at the effects of urbanization in a karst setting by monitoring conductivity, water level, and temperature at several springs with different land-use and use halides to distinguish possible source of anthropogenic contamination of the groundwater. Storm response was rapid, on the order of 1-3 hours. Recovery to baseflow conditions of both conductivity and water level was usually within 3-6 hours. This was in contrast to storm response measured in a previous study in a rural setting showing 1 -2 days response time from the start of the precipitation event to increases in water stage and to decreases in conductivity at the monitored sites. The water stage response was also on the order of 10-100 centimeters higher in the rural setting than in the urban setting. The maximum water stage increases at the urban springs were typically less than 10 centimeters greater than baseflow conditions, with the more commercialized springs showing only < 5 cm increase. The fast response and lower water stages at these urban springs were due to smaller capture areas for the springs compared to those in the rural settings.
Conductivity responses were small during storms similar to water level responses. The timing of the initial conductivity responses corresponded also with the timing of the initial water level responses. In general a decrease in conductivity occurred during storm responses, this indicated that recharge moves through quickly. There were slight variations in storm responses between the different springs. Overall however, each spring showed little variation from storm to storm. The consistent and fast response indicated short, fast flow paths.
Geochemical analysis of halide concentrations (chlorides and bromide) in the water samples from the commercial spring site shows contamination by road salt. High chloride concentrations (150 mg/L -280 mg/L) were measured throughout the year. The excess application of road salt during deicing periods has led to accumulation of road salt in the capture area of these springs and has become a continuous anthropogenic halide contamination source.
The urban setting thus influenced spring response by limiting the capture area and reducing infiltration. The reduced infiltration led to smaller storm response and the smaller capture area led to rapid storm response. The presence of high chloride concentrations, concluded to be contributions from road salts also was further indication of shallow, rapid flow paths in this urban setting.
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