Date of Award

Fall 2005

Project Type

Dissertation

Program or Major

Natural Resources and Environmental Studies

Degree Name

Doctor of Philosophy

First Advisor

Bill McDowell

Abstract

Surface water chemistry from a tropical and temperate rivers system was studied in order to understand the controls on longitudinal variation of stream chemistry. In the Rio Icacos-Blanco system in Puerto Rico, I examined changes in stream chemistry associated with a change in bedrock composition from intrusive quartz diorite (upstream) to volcaniclasitc material (downstream). Overall, after sea-salt correction, most solutes decreased consistently with distance downstream. The silica to alumina ratio, however, changed sharply with changes in the underlying bedrock. Other indices of weathering rates and processes showed strong similarities despite the change in bedrock. The dominance of silica, alkalinity, calcium and sodium (after sea-salt correction) in surface waters suggests that anorthite and albite are the dominant minerals within the entire basin, and they are weathering rapidly (silicon to Na + K ≥4) with the bisiallitization type of weathering (silica to alumina >2). Human influences on river chemistry in the Icacos-Blanco system appear to be minor, as nutrients (N and P) show little change along the drainage network.

Sampling of tributaries and source points at high and low elevation within the uniform intrusive bedrock of the Icacos was undertaken to identify spatial variability in weathering processes. Landslides are frequent in the basin, and expose fresh mineral surfaces to weathering. Concentrations of weathering products were inversely related to pCO2, suggesting that the availability of primary reactive minerals, rather than carbonic acid concentrations, limits weathering.

In the temperate Bagmati drainage system in Kathmandu valley, Nepal, population density appears to be the most fundamental control on the chemistry of surface waters. Concentrations in the Bagmati were extraordinarily high for nitrogen and phosphorus, and both nutrients and major ions were found in proportions similar to those in raw domestic sewage. The contribution of chemical weathering processes to water quality of the Bagmati does not seem to be significant within the Kathmandu valley. Ammonium contributes almost all nitrogen in the total dissolved nitrogen and the concentration of nitrate is negligible, probably due to rapid denitrification and limited nitrification within the stream channel under relatively low oxygen conditions.

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