CHAPTER 12 - DISCHARGE MEASUREMENTS USING TRACERS
2. Kinds of Tracers Used
Basically, a tracer is considered anything that mixes with or travels with the flow and is detectable. A detectable tracer can be timed as it passes through a reach, or tracer concentration profiles can be measured in a reach.
Some tracers that have been used are:
For irrigation measurements, salts and dyes are the most convenient and commonly used tracers. Salt tracers are sensed and quantified by measuring evaporated dry weight, chemical titration, or by measuring electrical conductivity. Dye concentrations are measured by fluorimetry or color comparison standards. Sometimes, visual observation of an exiting dye cloud is used, but considerable loss of accuracy occurs.
Fluorescein, Rhodamine B, Rhodamine WT, or Pontacyl Pink B dyes have been used because they are easily visible in very dilute solutions. Rhodamine B and Rhodamine WT have been cleared as nontoxic by the U.S. Food and Drug Administration. Rhodamine and Pontacyl Pink B are also quite stable with respect to fading by sunlight and to changes caused by waterborne chemicals. They do not tend to deposit on flow surfaces, sediments, or weeds. These dyes are usually available in powder form, and solutions are easily prepared. Before conducting a discharge measurement program, selected dyes should be tested with water samples or earth canal embankment material samples and exposed to check for possible adsorption, chemical reaction, and fading effects on dye stability.
Less frequently used methods involve measuring temperature upstream and downstream from a heat source and electronic cross correlations of trains of turbulent pulsations using acoustic methods discussed in chapter 11. The use of surface floats is discussed in chapter 13. Neutrally buoyant beads are usually used in laboratory work to track flow. Mixtures of beads with different specific gravities can also detect and measure fluid density profiles and stratification.
Radioisotopes are now rarely used because of their safety and pollution risks. In addition, isotope handlers must be licensed. However, use of any chemical or anything that can affect ecological characteristics of the water or conveyance boundaries may require clearance from several Federal and State authorities such as the U.S. Food and Drug Administration, the U.S. Environmental Protection Agency, and State fish and wildlife and natural resource departments. Government regulations and limits change with time and should be checked prior to a measurement program. However, even when operating within government regulations, public complaints related to taste and color and particles in the resulting water may occur.