Passive Pan Sampler for Vadose Zone Leachate Collection

Abstract

Sampling agrochemicals in the vadose zone provides an early warning system for ground water pollution. However, accurate in situ determination of agrochemicals in the vadose zone is often not feasible because of lack of appropriate instrumentation. This paper presents a new passive soil water sampler to effectively collect vadose zone leachate. The passive pan sampler (PPS), and a suction cup lysimeter (SCL) were positioned in soil columns. Two soil types were used. Vacuums applied to both PPS and SCL were adjusted to tensiometer readings in each soil column. Measurements included volumetric soil water content by time domain reflectometry (TDR), soil water potentials by tensiometers at three positions in each soil column, and the cumulative leachate collection (CLC) for the PPS, SCL, and free drainage (FD). Measurements were made at two initial soil water potentials of less than −10 kPa and −20 to −25 kPa, referred to high (HSWP) and low (LSWP) soil water potentials. Results indicated that the PPS performed well in both soil types irrespective of the initial soil water potential. Leachate collected by SCLs in the soils was lower than that collected by PPSs. The maximum CLC for silt loam soil ranged from 9.6 to 10.0 mL cm−2 at both water potentials for PPS, 1.6 to 2.1 mL cm−2 at both water potentials for SCL, and 0 to 2.0 mL cm−2 at HSWP for FD and 0 to 0.7 mL cm−2 at LSWP for FD, respectively. The corresponding values for the sandy loam soil were 8.0, 2.7, and 5.2 mL cm−2, for PPS, SCL, and FD, respectively. The leachate collection efficiencies of PPS at HSWP (98%) and LSWP (99%) were significantly (P < 0.01) higher than for SCL at HSWP (59%) and LSWP (31%), indicating greater performance of the PPS.