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Analysis of Effects of Urbanization on Spring Geochemistry and Sediment for a Karst Aquifer in Valley Creek Basin, Chester County, Pennsylvania
Gross, Kathleen A.
Gross, Kathleen A.
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Thesis/Dissertation
Date
2007
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Earth and Environmental Science
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Gross-Thesis-2007.pdf
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https://doi.org/10.34944/e9vj-sg91
Abstract
Valley Creek Basin located in eastern Chester County, southeastern Pennsylvania, is an urban karst system. Four springs within the Elbrook Formation and two springs in the Ledger Dolomite have been monitored for suspended sediment, baseflow water chemistry, and storm response. Data from previous studies at two rural sites, Nolte in Lancaster and Bushkill in Easton Pennsylvania, were used for comparison to examine the effects of urbanization on spring chemistry. The flow paths were compared using hardness coefficient of variations (CV) to differentiate between matrix and conduit dominated springs. The urban springs CV values ranged from 14.5% to 8.2%, showing a mixture between conduit and matrix flow paths. The rural sites CV values place Nolte as a matrix dominated system (5.6%) and Bushkill as distinctly conduit (17.5%). The nitrate values for the urban sites were low (< 5 mg/L) and had little variation within a 1 to 2 mg/L range. The rural sites had higher nitrate concentrations that showed seasonal variations around 10 mg/L. The difference between the urban and the rural nitrate values is most likely due to the agricultural use of fertilizers in rural areas. The calcite and dolomite average Si’s were slightly undersaturated (-0.01 to -0.5), suggesting that these systems are most likely made of solutionally enlarged fractures, with mixing between conduit and matrix flow. Most of the urban log PCO2 values averaged around -1.9, indicating that the urban springs are open systems under the influence of soil gas. The Bushkill spring’s log PCO2 averaged -2.3 indicating that it is also an open system, while Nolte spring had slightly higher log PCO2 (around -1.5) indicating a closed system. The suspended sediment concentrations for the urban springs have shown fairly constant low levels. The concentrations mostly range from 0.5 to 4.0 mg/L. The low suspended sediment values suggest that the karst system is not receiving a heavy load of urban sediments in the recharge area. It is believed that urbanization and increased impervious surfaces have lead to a reduction of infiltrating waters, which has in turn limited the amount of sediment entering the system. Furthermore, trace metal analysis of baseflow water samples and spring mouth sediment showed low concentrations. The metal concentrations in the water samples were generally less than 10 ppb and below EPA drinking water standards. The metal concentrations in the sediment were higher than those in the water samples. Even so, two-thirds of the sites had no metal concentrations over typical background values. The low concentration of metals on the sediment suggests that there is a lack of build up of metals in the karst system. Karst aquifers are usually more vulnerable to contamination than other aquifers due to reduced filtration of infiltrating waters. However, urbanization in Valley Creek Basin’s karst system has not had the effect of exposing the karst system to metal or sediment contamination, although the other geochemical indicators are typical of karst with a mixture of matrix and conduit flow paths. The lack of metal and sediment contamination appears to be related to urbanization restricting the recharge area and thus the potential flux of sediment and metals.
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