Stream Water Chemistry and Quality

Objectives

The SRBEX stream water chemistry component focuses on the roles that watershed scale, lithology, climate, and landuse/landcover play in controlling weathering rates and stream water quality.

This investigation supports the SRBEX modeling activities in two ways: by obtaining insight into how hydrological processes should be modeled at increasing scales, and by helping to validate the models.

Current Activities

During the initial phase of this study, we have compiled a water-quality database for 59 sampling stations in the Susquehanna River Basin (SRB) in Pennsylvania and Maryland. Catchments associated with the stations form a nested hierarchy of watersheds representing a suite of scales, lithologies and land uses. The database consists of all the available stream chemistry for each station, its location, the boundaries of its catchment area (digitized from 1:250,000 scale topographic maps), and, within the drainage basin, the lithology, stream network, and landuse/landcover. The data products are in Arc/Info export format and are available to other EOS and scientific investigators.

Using this database, we have completed a series of mass balance calculations of weathering fluxes of magnesium and silica(Richards and Kump, 1994b). These species have been chosen as a proxy for weathering over calcium and alkalinity becuase they come primarily from silicate minerals, at least in carbonate-poor units; agricultural practices commonly introduce large quantities of lime to watersheds which can greatly increse the alkalinity and Ca in runoff.

We have also developed a function relating concentration to discharge rate (Richards and Kump,1994a) that we have linked to the terrestrial hydrology model.

Results

The major findings from these studies include
  1. Weathering fluxes in the SRB are a strong function of the lithology and the extent of mining in a watershed.

  2. Mg fluxes are controlled by the amount of carbonate in the watershed.

  3. Mg fluxes from watersheds with abundant mining operations were found to be 6 to 10 times higher than fluxes from comparable pristine watersheds. Mining practices are believed to enhance chemical weathering by increasing the surface area of unweathered rock accessible to water.

  4. Weathering fluxes were found to increase with watershed scale. This scale dependance is believed to be caused by the inclusion of small amounts of carbonate at larger scales that are not taken into account in our lithology estimates.

  5. Flux-concentration-discharge relationships for the stations suggest that elemental fluxes in the SRB vary primarily with discharge and that export rates can be accurately modelled with a simple power function. The empirical constants for this function were found to vary with scale and dissolved species but not land use or lithology. The results indicate that small catchments, typical of the watersheds for which most hydrological models are developed, differ fundamentally from large watersheds. Fluxes from small watersheds are controlled by the chemistry and water residence time of flow paths in soils and fractured bedrock, whereas fluxes from larger scale watersheds represent the mixing of fluxes from the a number of sub-watersheds.

Last change: 9 May 1995, R. A. White / raw@essc.psu.edu