Tracer Injection

Streamflow can be determined by tracer injection in a gaining stream reach where dilution of the tracer concentration by surface or subsurface inflows can be translated to a quantifiable increase in flow. Streamflow cannot be determined in a losing stream reach because the tracer concentration remains constant even though flow decreases. No indications of losing mainstem reaches were observed during this study.

Before the start of the tracer injection, an ample volume of tracer solution was prepared in a polyethylene tank by mixing 325 pounds of sodium chloride (NaCl) with 175 gallons of stream water collected from site 0. This mixture ratio produced a chloride concentration much higher than ambient concentrations in the stream, but less than the concentration at which the solution would be saturated with respect to sodium chloride. The tracer solution was injected continuously into Daisy Creek at a rate of 160 mL/min for 47 hours. The injection was started at 1703 hours on August 24 using a positive-displacement pump system controlled and monitored by an electronic data logger. This pump system failed at 1205 hours on August 25. Starting at 1215 hours on August 25, the tracer solution was manually added to the stream for 10 minutes until a backup constant-head metering system with manual control and monitoring was started to continue injection of the tracer solution. The tracer injection was stopped at 1600 hours on August 26 after all synoptic water samples had been collected. Three samples of the tracer solution were collected periodically during the injection period to document the tracer concentration. The chloride concentration of each tracer-solution sample was determined by measuring the solution density with volumetric glassware and an analytical balance and then converting density to concentration using data in Weast and Astle (1981). The chloride concentration ranged from 128.4 g/L to 130.8 g/L; the average concentration (129.2 g/L) was used for calculating streamflow.

Water samples for chloride analysis were collected at four mainstem sites, referred to as tracer-monitoring sites (fig. 2), to document the downstream movement of the injected tracer. Each tracer-monitoring site was sampled prior to the arrival of the tracer to determine the natural pre-injection chloride concentration and sampled multiple times during and after the tracer injection. These samples were collected manually or with automatic pumping samplers at a single point near midstream and were filtered though 0.45-μm capsule filters.

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