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Title: SEASONAL OXYGEN-18 VARIATIONS AND GROUNDWATER RECHARGE FOR THREE LANDSCAPE TYPES IN CENTRAL PENNSYLVANIA, USA

Author
item O'DRISCOLL, M - PENN STATE UNIV.
item DEWALLE, D - PENN STATE UNIV.
item MCGUIRE, K - ORGEGON STATE UNIV.
item Gburek, William

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2004
Publication Date: 3/20/2005
Citation: O'Driscoll, M.A., Dewalle, D.R., Mcguire, K.J., Gburek, W.J. 2005. Seasonal oxygen-18 variations and groundwater recharge for three landscape types in central Pennsylvania, USA. Journal of Hydrology. 303:108-124.

Interpretive Summary: Environmental isotopes, such as oxygen-18 (O-18), have been used to study hydrologic processes in a variety of settings. The seasonal variations in isotopic composition of stream baseflow at a catchment outlet are often used to estimate the residence time of groundwater within a catchment. Further, mathematical models used to portray of residence time can be improved with information related to spatial variability of baseflow isotopic composition within a catchment. This study aimed to quantify the annual variations in O-18 composition of waters within several Appalachian catchments representative of three common landscape types in central Pennsylvania: the Valley and Ridge-shale (Mahantango Creek); Valley and Ridge-carbonate (Buffalo Run); and Appalachian Plateau-sandstone (Benner Run). Bi-weekly precipitation, snowmelt, soil water, and baseflow isotopic composition data were collected within these catchments over one year (May 1999 - May 2000). Results suggest that soils at these sites have effectively damped the seasonal patterns in precipitation O-18 by the time recharge has reached the 3-meter depth in the subsurface. This suggests that seasonal isotopic composition variations observed in baseflow leaving the catchment are due to the variety of flowpaths draining the shallower soils within these catchments. The presence of springs was also found to exert an influence on baseflow isotopic composition within each of the catchments. In total, the seasonal variability of baseflow isotopic composition observed at the catchment outlets was a result of the combination of several distinct water sources: slow-draining groundwater; fast-draining near-channel groundwater; and spring discharges. An improvement in modeling of residence times of these catchments for the purposes of estimating timing of impact of land management on streamflow water quality may be achieved by incorporating these findings.

Technical Abstract: Seasonal oxygen-18 variations in precipitation, soil water, snowmelt, spring flow and stream baseflow were analyzed to characterize seasonal dynamics of groundwater recharge in three central-Pennsylvania catchments. The catchments were selected to represent three common landscape types: Valley and Ridge-shale (Mahantango Creek), Valley and Ridge-carbonate (Buffalo Run), and Appalachian Plateau-sandstone (Benner Run). Samples were collected on a bi-weekly basis from May 18, 1999 - May 9, 2000. Precipitation, soil water, and baseflow isotopic composition data indicate that a seasonal recharge bias exists for these catchments, with most recharge occurring in the fall, winter, and spring months. An altitude effect of -0.16' to -0.32' per 100 m change in elevation was discernible in precipitation/snowmelt, soil water (90 cm), and stream baseflow isotopic compositions. Soils effectively damped seasonal variations of recharge waters after depths of 1.62-2.85 meters. The greatest damping of the annual isotopic composition signal occurred in the shallow soil layers (0-15 cm). These data indicate that in upland settings with thicker soils, the annual isotopic composition signal may be completely damped prior to reaching the stream as baseflow. Isotopic variations measured in stream baseflow are more likely the result of the shallow flowpath water relatively close to the streams combined with spring flow contributions.