Tracer estimation of localized flow loss from a stream gaining water at the catchment scale
Kenneth E. Bencala1, Briant A. Kimball1, and Michael N. Gooseff2. (1) Water Research MS 439, US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, (2) Civil & Environmental Engineering Department, Pennsylvania State University, 212 Sackett Bldg., University Park, PA 16802
Mineral Creek (Silverton, Colorado) predominantly gains water while losing water in localized segments. The flow losses, although a small part of total flow, are significant in estimating solute budgets for the stream and in understanding the connections of the stream to its catchment. In August 2005, flow increased one order-of-magnitude (2 to 20 L/s) over a 2 Km study reach. Over a 400 m headwater segment, gains and losses of 30 to 50% were estimated from analysis of injected bromide and ambient sulfate tracer concentrations. Using solute budget calculations we demonstrate inherent uncertainty in the use of tracer (injected or ambient) concentrations for flow estimation in losing stream reaches. In-stream tracer experiments are a well-established method for providing (1) spatial flow data at scales of minutes and tens of meters without physical disturbance to the flow of water, the streambed, or biota and (2) an approach to characterize hyporheic exchange time-scales for a stream with hyporheic exchange flowpaths (HEFs) that are short relative to the distance over which the stream gains water. Analysis of tracer data, to estimate flow and solute budgets, needs to be interpreted with an understanding of the likely concurrent subsurface inflows, outflows, and HEFs.