Controls on solute transport in large spring-fed karst rivers

Abstract

The spring-fed rivers of North Florida flow across a karst plain, leading us to predict significant hyporheic transient storage within the secondary porosity of the carbonate media. They also span a gradient of submerged macrophyte cover and density, offering an opportunity to investigate reach-scale vegetation effects on dispersion and transient storage. Conservative tracer tests using Rhodamine WT were conducted on nine rivers spanning the extant range of vegetative and geomorphologic characteristics. A one-dimensional advection-dispersion-storage model with variable configurations of storage zones was fit to tracer breakthrough curves to determine optimal model coefficients as well as storage-zone configuration (i.e., single, two in parallel, two in series). In most cases, two zones in series best fit the observed breakthrough curves. Moreover, fitted storage-zone attributes were significantly correlated across springs with field-measured analogs (i.e., vegetation area for storage zone 1, sediment area for zone 2), supporting inference that this model configuration physically represents the river systems. While the hyporheic zone area was large, its effect on transient storage was not, presumably because low-hydraulic-conductivity sediments and weak hydraulic gradients limit exchange with the secondary porosity of karst matrix; tracer mass recovery uniformly near 100% underscores the absence of significant hydraulic turnover. Vegetation was a significant predictor of transient storage, mean velocity, and mean residence time, suggesting that plant beds exert important controls on reach-scale hydraulics in these systems. © 2012, by the Association for the Sciences of Limnology and Oceanography, Inc.