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Burd Run Interdisciplinary
Watershed Research Laboratory

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1871 Old Main Dr.
Shippensburg, PA 17257

Email: cjwolt@ship.edu

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Grote, T. D. 2001. Baseflow geochemistry of a fluviokarst watershed: Burd Run, south-central Pennsylvania. M.S. Thesis, Shippensburg University, 106pp.

Abstract

This study examines the relationship of local bedrock geology to groundwater and surface water interactions and its influence on the base flow geochemistry throughout the 20.5 square mile (53 km2) Burd Run watershed, south-central Pennsylvania. The watershed consists of meta-sedimentary rocks at higher elevations and carbonate rocks downstream. The middle portion of the watershed is covered by varying amounts of colluvium, estimated to be greater than 400 feet thick in some areas. Surface and sub-surface features found within the watershed are controlled by fluvial and karst geomorphic processes.

By relating the base flow geochemistry at various locations to several geologic and hydrologic factors, the degree of chemical weathering occurring within three sub-regions of the watershed can be determined. These sub-regions are defined by mapped geologic variations in bedrock and surficial features.

Data collected and analyzed in this study were subjected to a charge balance analysis for quality assurance. Samples within the carbonate sub-region typically fall between +/- 20% of the theoretical charge balance while values for samples in other areas of the watershed are typically greater than +/- 45% charge imbalances. These regions can not accurately be balanced against bicarbonate (anion) due to their lithologic character and should not be classified as erroneous.

Data presented in this study indicate that variable climatologic and hydrologic conditions play a key role on the base flow chemical composition of the watershed. Strong seasonal variations in chemical data are noticed in all regions of the watershed. However, the analysis of winter and spring/summer samples at different laboratories may explain some of the seasonal variation. Headwater areas of the watershed are underlain by resistant quartzite bedrock that produce acidic waters (pH 4.4-5.5 and HCO3 0-2.98 mg/L) capable of aggressively dissolving carbonate bedrock in down-gradient localities. As water travels from the headwaters into the area of carbonate bedrock buried beneath colluvial sediment, water is quickly lost to the sub-surface due to the karstic nature of the underlying rock and the dry, droughty conditions in-place during the study. Highly variable geochemical data (pH 4.85-7.9; HCO31.49-89.33 mg/L; pCO2 5.31e-4-1.01e-3) from this region indicate that water that emerges at the surface in numerous areas was still acidic and had not been in contact with carbonate material. As water flows out of the area covered by colluvium, increased contact with carbonate strata occurs. The geochemistry of the water quickly changes to an alkaline state (pH 7-8.4; HCO3 74.74-506.22 mg/L) as water flows out of this region into the last region of the watershed that is predominantly carbonate bedrock with only thin amounts of colluvium. Water samples collected in the lower reaches of the watershed are close to or saturated with respect to calcite and dolomite and showed decreased carbon dioxide concentrations due to chemical weathering processes.

Overall, this watershed is controlled by the hydrologic response of the water table to the prevailing climate of the area, the amount of contact water has with sub-surface materials, the amount of carbon dioxide available in the water to readily dissolve carbonate bedrock and the location and time of year at which the sampling is conducted. Further investigation over longer time intervals may help explain these controls as well as the seasonal variations presented in this study.