Bromide reactivity in topsoil: Implications for use as a “conservative” tracer in assessing quantity and quality of water

Bromide is a frequently used conservative tracer in soil leaching studies, including studies on contaminant leaching from arable topsoils. However, bromide often does not behave conservatively. Biogeochemists have known for many years that in natural soils, bromide is converted into organic bromine in a process known as bromination. However, bromination is seldom used as an explanation of non-conservative leaching behavior by soil hydrologists. In a controlled small-scale lysimeter study with arable soil we demonstrate such nonconservative behavior of bromide in opposition to control columns with fine gravel/coarse sand. By combining a literature review with the lysimeter study, we demonstrate the potential importance of bromination in topsoil and that bromination cannot be ignored, when interpreting bromide tracer experiments in arable soils. We also highlight the need for further studies on the processes of bromination and remineralization, to be able to account for these when conducting bromide leaching assessments.