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