Tracer breakthrough curves (BTCs) provide information about preferential flow and the risk of accelerated leaching of dissolved and particle-associated contaminants in soil. Obtaining BTCs in the laboratory is time and labor consuming. Visible near-infrared (vis-NIR) spectroscopy has been revealed to give rapid and accurate predictions of soil functional properties related to texture and organic matter, such as water retention and compaction.

We evaluated vis-NIR spectroscopy as a rapid and indirect method for predicting selected BTC tracer mass arrival times (TMATs) and the volumetric contents of clay and mineral fines (< 0.05-mm, clay and silt), based on only a single vis-NIR measurement. The BTC TMATs were obtained using a conservative tracer pulse on 181 intact soil columns collected from six Danish fields, exhibiting wide ranges in texture and organic carbon (clay: 0.033 to 0.41 kg kg^-1, organic carbon: 0.011 to 0.084 kg kg^-1). The time required to recover selected 5, 10, 15, 20, 25, 30, 40 and 50% of the applied tracer mass were obtained from BTCs. Recently, 5% TMAT was found to correlate to the volumetric content of mineral fines, and the correlation was stronger compared to using gravimetric content of mineral fines. Structured clayey soils typically exhibit relatively fast 5% TMATs compared to sandy soils, due to preferential flow in well-connected macropores.

The 181 samples were analyzed with a vis-NIR spectrometer (DS2500, Foss, Hillerød, Denmark) from 400 to 2500 nm. Selected TMATs and volumetric contents of clay and mineral fines were correlated to spectral data with partial least squares regression on a calibration set (121 samples) and then tested on a validation set (60 samples). We obtained accurate vis-NIR predictions of the 5% TMAT and volumetric contents of clay and mineral fines. Vis-NIR predictions of later TMATs of 10, 15, 20, 25, 30, 40 and 50% showed decreasing accuracy with increasing TMAT, which probably reflect decreasing correlation with soil texture.