Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 8, 1994
Publication Date: N/A
Interpretive Summary: Chemicals become contaminants when they occur where they are unwanted. Many agricultural chemicals are carried through the environment dissolved in water. Consequently, if we can determine how fast and by what path water moves through the environment, we can establish bounds on chemical movement. Water movement in soil and geologic material is often traced with a substance added to the water to make it different from its surroundings. Bromide is commonly used hydrologic tracer because it generally doesn't occur in soils and it doesn't interact with the soil. The results reported here demonstrate that bromide is readily absorbed by ryegrass and cannot be viewed as unreactive when plants are growing. Ryegrass was found to take up nearly one-half of the applied bromide; an amount nearly equivalent to nitrogen uptake. This magnitude of plant uptake means that bromide can only be used with caution as a tracer in long-term field studies. The speed of contaminant movement will be underestimated from tracer studies if plant uptake occurs but is not accounted for in the analysis.
Technical Abstract: The reactivity of a hydrologic tracer (bromide) in a soil-plant environment was investigated at a well and poorly drained site. Ryegrass (Lolium perenne L.) accumulated an average 44% of applied bromide in aboveground biomass on both a well and a poorly drained soil. Bromide concentration, mass of bromide contained in aboveground biomass, and the fraction of applied bromide taken up by ryegrass were all linearly related to bromide application rates from 42 to 126 kg Br/ha. Greater quantities of bromide were taken up by ryegrass during spring growth compared to summer and fall growth and from the well drained soil compared to the poorly drained soil. The differences in bromide uptake between harvest dates and soil drainage were due to higher ryegrass yields early in the year and from the well drained soil rather than differences in bromide concentration in the plants. The large amount of bromide taken up by grasses greatly restricts the settings in which bromide is an appropriate hydrologic tracer.