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Reactive Solute Transport in Streams: 2. Simulation of a p H Modification Experiment
Author(s) -
Runkel Robert L.,
McKnight Diane M.,
Bencala Kenneth E.,
Chapra Steven C.
Publication year - 1996
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/95wr03107
Subject(s) - dissolution , hydrous ferric oxides , solubility , tracer , ferric , iron oxide , metal , chemistry , environmental chemistry , environmental science , mineralogy , inorganic chemistry , sorption , physics , organic chemistry , adsorption , nuclear physics
We present an application of an equilibrium‐based solute transport model to a p H‐modification experiment conducted on the Snake River, an acidic, metal‐rich stream located in the Rocky Mountains of Colorado. During the experiment, instream p H decreased from 4.2 to 3.2, causing a marked increase in dissolved iron concentrations. Model application requires specification of several parameters that are estimated using tracer techniques, mass balance calculations, and geochemical data. Two basic questions are addressed through model application: (1) What are the processes responsible for the observed increase in dissolved iron concentrations? (2) Can the identified processes be represented within the equilibrium‐based transport model? Simulation results indicate that the increase in iron was due to the dissolution of hydrous iron oxides and the photoreduction of ferric iron. Dissolution from the streambed is represented by considering a trace compartment consisting of freshly precipitated hydrous iron oxide and an abundant compartment consisting of aged precipitates that are less soluble. Spatial variability in the solubility of hydrous iron oxide is attributed to heterogeneity in the streambed sediments, temperature effects, and/or variability in the effects of photoreduction. Solubility products estimated via simulation fall within a narrow range ( pK sp from 40.2 to 40.8) relative to the 6 order of magnitude variation reported for laboratory experiments ( pK sp from 37.3 to 43.3). Results also support the use of an equilibrium‐based transport model as the predominate features of the iron and p H profiles are reproduced. The model provides a valuable tool for quantifying the nature and extent of p H‐dependent processes within the context of hydrologic transport.