Sediment Transport in an Urbanizing Karst Aquifer-Watershed

University Of Texas At Austin, Austin TX

Investigators

Abstract

0001197 Bennett The turbulent flow of groundwater through karst-aquifers effectively transports sediment and colloids through the subsurface and may provide a pathway for the rapid transport of organic contaminants. Sediment storage in karst aquifers, in contrast, may sequester contaminants for long time periods while altering the flow properties of the aquifer. The availability and transport of sediment in a karst aquifer are in turn profoundly influenced by changes in land-use and the associated changes in runoff and stream discharge. While there has been extensive research into the hydrodynamics of sediment transport in surface water, little is known about karst hydrodynamics, less about the connection between the surface and subsurface domains, and almost nothing about the dynamics of sediment transport in karst aquifers. The proposed study will investigate the transport of sediment in the urbanizing Barton Creek segment of the Edwards aquifer watershed of central Texas. Sediment characteristics, sediment transport, and sediment chemistry will be examined in Barton Creek across the primary recharge area in SW Austin, into the aquifer, and out at the watershed's only discharge points at Barton and Cold Springs. The goals of this project are to 1) Characterize the sediment chemistry in the stream at the recharge point and compare that to the discharged sediment; 2) quantify the transport of sediment through this section of the aquifer using natural and introduced tracers; and 3) quantify the sediment budget in urbanizing and agricultural watersheds under different flow regimes. We will characterize the sediment in the Barton Creek runoff at base and stormflow stages, at the Barton and Cold Springs discharge points, and directly in the aquifer using several monitoring wells between the recharge and discharge points. A sediment mass-balance will be developed by comparing sediment 'recharge' into the aquifer to sediment discharge at the springs. Samples will be collected during baseflow and stormflow conditions to examine sediment loading, size distribution, and mineralogy as a function of discharge. Tracing of water flow using fluorescent dyes in cooperation with the City of Austin, and both natural and introduced sediment tracers, will be used to characterize sediment transport in the karst aquifer. A novel sediment tracer using DNA-labeled clay will be used to quantify transport time from the recharge feature and the loss of sediment to storage in the aquifer. The potential for contaminant transport by suspended sediment will be characterized by analyzing surface area sediment organic carbon, and contaminant loading will be characterized by analysis of total petroleum hydrocarbons on selected sediment samples. This project will provide fundamental insight into sediment transport processes into and through karst aquifers, the mobility of sediment and sediment-bound contaminants, and the sources of mobile sediment in urban and rural watersheds.