Abstract

Shales are important in determining global fluxes of C, P and Pt-group elements as they are widely exposed at the Earth's surfaces. A critical zone observatory (CZO) to study the hydrology, geochemistry, ecology, and geomorphology is established at the Shale Hills catchment in central Pennsylvania to complete such intensive investigations on the Rose Hill shale. The Shale Hills is V-shaped and forested catchment, with slopes around 16-18%. The parent shales are comprised of primarily illite, quartz, and chlorite. The dominant chemical reactions in the soil profiles are dissolution of chlorite and illite to more stable kaolinite, with vermiculite and hydroxy interlayered vermiculite as intermediate phases. Depth to the bedrock in the catchment depends mainly on the landscape positions, with thinner soils observed at the ridge tops, and much thicker soils at the valley floors and topographically depressional areas. Previous hydrologic studies have included monitoring the soil moisture contents and modeling the water flow dynamics in the soil zones. Study areas were selected to investigate the propagation rates of the weathering front (the interface between intact bedrock and weathered material) with increasing complexity: fluid flow above the bedrock interface is largely 1D and vertical at ridgetop, downslope and 2D along a planar transect, and convergent downslope and 3D along a swale transect. Weekly soil waters were collected at these sites. Chemistry of soil waters and first-order streams is controlled by chemical weathering reactions with only little contribution from rainfall. Soil waters become more concentrated from the ridge top to valley floor, as mineral dissolution progresses. The depth variation of water chemistry is distinctively different among the sites, which is closely related to soil- texture controlled water flowpaths. The stream reflects mixing among soil waters of different chemistry (shallow versus deep), and also mixing of soil waters and groundwaters. Temporal variations of stream chemistry are related not only to the temperature-dependent mineral weathering rates, but also the different proportions of these sources during high-flow and low-flow seasons. Mineral weathering rates are calculated from water chemistry on both catchment scale and pedon scale. These data and those laboratory-derived dissolution rates differ, reflecting different field and experimental conditions.

Citation

Jin, L. and Brantley, S.L., (2008): Using Water Chemistry to Characterize Chemical Weathering in the Critical Zone Observatory: Shale Hills Catchment (Central Pennsylvania, USA) . American Geophysical Union fall meeting, San Francisco, CA..