Evaluation of Some New Tracers for Soil Water Studies

Six anions which showed potential as tracers of soil water movement were evaluated in laboratory and field experiments. The anions included iodide (I ‐ ), thiocyanate (SCN ‐ ), and four fluorobenzoates: m ‐(trifluoromethyl)benzoic acid ( m ‐TFMBA), o ‐(trifluoromethyl)benzoic acid ( o ‐TFMBA), pentafluorobenzoic acid (PFBA) and 2,6‐difluorobenzoic acid (2,6‐DFBA). The sorption and mobility characteristics of the six potential tracers were compared to those of bromide (Br ‐ ), which served as an index tracer in all experiments. Initial tracer concentrations ranged from 20 to 100 mg L −1 . The anions were quantified simultaneously in sub‐mL samples of soil solution using a high performance liquid chromatography technique. Batch equilibrations (28 d), and breakthrough curves (7 to 10 d) in a 2.5‐m column of loamy sand, under both ponded and trickle‐irrigated conditions, indicated that all the tracers except SCN ‐ were conservative and were not sorbed. SCN ‐ was not retarded in the column studies but did show a negative mass balance, most likely due to chemical and/or biological transformation. Distribution of a tracer solution applied to a field soil was monitored for 69 d in soil water from suction samplers (1.0‐ and 1.8‐m depth) and in extracts from core samples. SCN ‐ , I ‐ , and m ‐TFMBA showed reduced concentrations relative to Br ‐ at all sampling times and depths; o ‐TFMBA, PFBA, and 2,6‐DFBA had distributions similar to that of Br ‐ . These latter three fluorobenzoates show promise as laboratory and field tracers of soil water, particularly where multiple tracing tests or totally exotic tracers are required.