Is Rhodamine WT conservative in small stream studies? Results from upland and lowland reaches of a tropical stream.
Abstract:
Soluble chemical species are widely used as tracers in stream studies.
They can be used to determine stream travel time and flow rates,
longitudinal and transverse dispersion coefficients, groundwater
discharge rates and transient storage. Conservative tracers are also
used in conjunction with reaeration estimates in whole-stream
metabolism studies.
A tracer is defined as a quantifiable chemical species whose movement mimics that of water itself. An ideal tracer species must not react with the water or other chemical constituents in the water, including itself. It must not react with sediments or be selectively taken up or rejected by biota in contact with the water, either in the stream or hyporheic zone. Finally, it must not be volatile or photoreactive. Species meeting these criteria are referred to as "conservative". Deuterated and tritiated water come very close to satisfying all of the criteria. Chloride and bromide ions are regarded as nearly perfect tracers for most surface water and groundwater applications.
Water-soluble fluorescent dyes can be desirable as tracers because they can be detected at very low concentrations, greatly reducing the amount of tracer used. They are also easily and accurately measured in the lab, can be measured in the field using hand-held fluorometers, and are comparatively inexpensive. Numerous studies have investigated potential fluorescent tracer species (FEUERSTEIN & SELLECK 1963, SMART & LAIDLAW 1977, SABATINI & AUSTIN 1991). Although not perfectly conservative, they are generally acceptable in cases where the residence time of the tracer is short.
The fluorescent dye Rhodamine WT (RhWT), has been widely used as a tracer in both surface water and groundwater studies (COBB & BAILEY 1965, BENCALA et al. 1983). RhWT was developed to be more weakly sorbed to sediments than other fluorescent dyes. RhWT is less conservative than chloride and bromide because of potential losses due to sorption and photodecay.
The first objective of this study was to determine whether RhWT was conservative over a 3.7-km reach of Quebrada Salto in Costa Rica including both upland and lowland settings. The second objective was to assess the transient storage characteristics of the stream, which can be critical to lotic ecologists in interpreting data from ecosystem process studies (e.g. nutrient uptake, metabolism). To achieve these objectives, we conducted one long-term and several short-term continuous RhWT injections.
A tracer is defined as a quantifiable chemical species whose movement mimics that of water itself. An ideal tracer species must not react with the water or other chemical constituents in the water, including itself. It must not react with sediments or be selectively taken up or rejected by biota in contact with the water, either in the stream or hyporheic zone. Finally, it must not be volatile or photoreactive. Species meeting these criteria are referred to as "conservative". Deuterated and tritiated water come very close to satisfying all of the criteria. Chloride and bromide ions are regarded as nearly perfect tracers for most surface water and groundwater applications.
Water-soluble fluorescent dyes can be desirable as tracers because they can be detected at very low concentrations, greatly reducing the amount of tracer used. They are also easily and accurately measured in the lab, can be measured in the field using hand-held fluorometers, and are comparatively inexpensive. Numerous studies have investigated potential fluorescent tracer species (FEUERSTEIN & SELLECK 1963, SMART & LAIDLAW 1977, SABATINI & AUSTIN 1991). Although not perfectly conservative, they are generally acceptable in cases where the residence time of the tracer is short.
The fluorescent dye Rhodamine WT (RhWT), has been widely used as a tracer in both surface water and groundwater studies (COBB & BAILEY 1965, BENCALA et al. 1983). RhWT was developed to be more weakly sorbed to sediments than other fluorescent dyes. RhWT is less conservative than chloride and bromide because of potential losses due to sorption and photodecay.
The first objective of this study was to determine whether RhWT was conservative over a 3.7-km reach of Quebrada Salto in Costa Rica including both upland and lowland settings. The second objective was to assess the transient storage characteristics of the stream, which can be critical to lotic ecologists in interpreting data from ecosystem process studies (e.g. nutrient uptake, metabolism). To achieve these objectives, we conducted one long-term and several short-term continuous RhWT injections.
Jackman, , A. P., F. J. Triska, J. H. Duff, and C. M. Pringle. 2006. Is Rhodamine WT conservative in small stream studies? Results from upland and lowland reaches of a tropical stream. Verhandlungen Internationale Verein. Limnol. 29:1645-1650.
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