Evaluation of Sampling Strategies for Pesticides in a Macroporous Sandy Loam Soil

Abstract

It is not straightforward to sample and demonstrate the presence and transport of pesticides in heterogeneous soil. Following leaching experiments with four differently structured 50-cm-long soil columns (tilled and untilled soil), the objective of this study was to investigate the extent that visual tracing of the dye Brilliant Blue could support in soil sampling for two strongly sorbing pesticides ( 14 C-labeled glyphosate and pendimethalin). About 830 samples were collected. No pesticide was found below 10-25 cm depth by random sampling, even though 0.21-0.31% of the applied amounts were leached, and 0.18% of the soil volume was sampled. With similar sampling efforts, the pesticides could generally be traced throughout the columns by sampling from stained soil volumes, only. None of the two particular sampling strategies for pesticides produced accurate mass balances or balances that were obviously better than the other. No pesticide was detected outside stained soil volumes, except for glyphosate in one sample. Below 30 cm, stained soil comprized on average 5% of the total soil volume, leaving 95% as expectedly pesticide-free. The results suggest that much more efficient sampling for sorbing pesticides can be obtained by using the dye and focusing on stained soil volumes.

Macroscopic Evidence of Sources of Particles for Facilitated Transport during Intensive Rain

“…As described in Gjettermann et al (2011b), the tops of all columns were completely blue; however, the depth of the blue layer was a few centimeters for the plowed columns and only about half a centimeter for the minimally disturbed columns. Furthermore, the plowed columns had a larger volume of colored soil (macropores and volumes acting as “internal catchments”) within the plow layer, consistent with the later occurrence of leaching on these columns.…”

“…In Kim et al (2005), the mixing zone of Steenhuis et al (1994) was named the “distribution zone.” This zone is saturated and water is released from here into macropores and through the “conveyance zone,” which is unsaturated. This picture was further documented by Gjettermann et al (2011b), who studied the distribution of Brilliant Blue and glyphosate in soil columns. Pesticides could only be traced efficiently all the way through the columns in the blue samples.…”

“…Below this depth, blue color was distributed around deeply penetrating macropores (mainly earthworm channels) and locally at the column wall where the wall intersected blue‐stained macropores on both A and B columns. The distribution of glyphosate concentrations was described in Gjettermann et al (2011b), but the findings of relevance for the discussion are summarized here. By far the largest average glyphosate concentrations were found in the uppermost 5 mm of soil, which was completely dyed in both treatments (8.6 and 10.5 mg kg −1 in Columns GA2 and GB1, respectively).…”

“…As described in Gjettermann et al (2011b), soil samples were obtained from nine (Treatment B) or 10 (Treatment A) separate layers 1 to 2 d after dye application. Initially, the columns were sectioned into seven or eight depth intervals (cylindrical slices).…”

“…This study aimed to show that particles generated in the very surface of the soil, probably aided by the splash process, dominate the particle‐facilitated transport of glyphosate. The study built on experiments and findings from Gjettermann et al (2009) describing the leaching experiments, Gjettermann et al (2011a) describing desorption–sorption properties of the leached and splashed particles, and Gjettermann et al (2011b) describing the distribution of surface‐applied glyphosate, pendimethalin [ N ‐(1‐ethylpropyl)‐3,4‐dimethyl‐2,6‐dinitrobenzenamine], and Brilliant Blue dye tracer throughout the soil columns after three irrigation experiments, as well as data not formerly presented.…”