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Xiahong Feng - Current Research Interests in Stable Isotope Geochemistry

Isotope hydrology and hydrochemistry

There has been an increasing recognition of a variety of hydrochemical problems at the watershed scale, including long-term acidification of surface and subsurface waters, sensitivity of aquatic biota to episodic pulse of pH depression associated with large runoff events, impact of land use on aquatic lives, and transport of pollutants from nonpoint sources. One of the most important approaches to studying these problems is to identify the pathways of water and water-soluble components by linking hydrological models to chemical transport mechanisms. This requires specifying the water sources that contribute to stream discharge, and understanding the nature and rates of the chemical reactions (e.g., adsorption, reduction, dissolution and cation exchange) that occur in each of these source reservoirs and along each pathway to the stream. In collaboration with a number of investigators, I am involved in several projects related to watershed hydrology and hydrochemistry.

1) Studies of Stable Isotopes and Chemistry of Meltwater from Seasonal Snowpacks: Snowmelt is one of the major water sources in drainage basins and may dominate hydrology in many areas. Seasonal snowpacks may also be sources of surface and groundwater pollution because they accumulate atmospherically derived contaminates through the winter and release them to catchments often in more concentrated pulses. This project involves both cold room experiments and field investigations in a well-instrumented snow laboratory, the Central Sierra Snow Laboratory in California. The ultimate objective of this project is to predict the isotopic and chemical compositions of snowmelt given a set of input compositions of snowstorms and climatic conditions at watershed scale.

2) Studies of time series of precipitation and stream chemistry: The travel time of water through a catchment -- that is, the time it takes for rainfall to reach the stream -- is a fundamental hydraulic parameter controlling the retention of soluble contaminants, and thus the downstream consequences of pollution episodes. In collaboration with Dr. James Kirchner at the University of California at Berkeley, U.S.A. and Colin Neal at the Institute of Hydrology in England, we are studying travel time (or residence time) distributions in a number of watersheds using the relationship between time series of precipitation and stream chemistry. This study has important implications for transport of soluble contaminants through watersheds.

3) Multi-Tracer Approach to Hydrochemistry: Hydrochemistry studies chemical interactions of water with the soil and rocks of the earth’s crust when water moves through them, which determines the chemical character of groundwater or surface water. Chemical compositions of groundwater water are essential measures of water quality. These compositions are shaped by many intricate connections among hydrological, chemical and biological processes in catchments. Where water enters a catchment, what pathway it takes, how it recharges, how long it stays in the subsurface, what materials and organisms it encounters on its journey – all interact to determine its chemical properties. Different pathways have different chemical interactions between water and the solid medium. For example, within soil layers, the interactions are typically cation exchange, while in the deep bedrock, chemical weathering reactions dominant the water chemistry. Our approach to understanding the chemistry of groundwater and surface water is to use different type of tracers to study water having different flow history. One on-going project, in collaboration with Bill McDowell at UNH and Mukul Sharma and Eric Posmentier at Dartmouth, is to study water chemistry in a granite catchment in Puerto Rico.

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