The Second Las Cruces Trench Experiment: Experimental Results and Two‐Dimensional Flow Predictions

Abstract

As part of a comprehensive field study designed to provide data to test stochastic and deterministic models of water flow and contaminant transport in the vadose zone, several trench experiments were performed in the semiarid region of southern New Mexico. The first trench experiment is discussed by Wierenga et al. (this issue). During the second trench experiment, a 1.2 m wide by 12 m long area on the north side of and parallel to a 26.4 m long by 4.8 m wide by 6m deep trench was irrigated with water containing tracers using a carefully controlled drip irrigation system. The irrigated area was heavily instrumented with tensiometers and neutron probe access tubes to monitor water movement, and with suction samplers to monitor solute transport. Water containing tritium and bromide was. applied during the first 11.5 days of the study. Thereafter, water was applied without tracers for an additional 64 days. Both water movement and tracer movement were monitored in the subsoil during infiltration and redistribution. The experimental results indicate that water and bromide moved fairly uniformly during infiltration and the bromide moved ahead of the tritium due to anion exclusion during redistribution. Comparisons between measurements and predictions made with a two‐dimensional model show qualitative agreement for two of the three water content measurement planes. Model predictions of tritium and bromide transport were not as satisfactory. Measurements of both tritium and bromide show localized areas of high relative concentrations and a large downward motion of bromide relative to tritium during redistribution. While the simple deterministic model does show larger downward motions for bromide than for tritium during redistribution, it does not predict the high concentrations of solute observed during infiltration, nor can it predict the heterogeneous behavior observed for tritium during infiltration and for bromide during redistribution.

Vadose Zone Transport Field Study: Summary Report

“…Important vadose zone transport processes have been identified in the few large-scale transport studies conducted internationally during the last decade. Studies include transport tests in the United States at Las Cruces, New Mexico (Wierenga et al 1990;Hills et al 1991;Rockhold et al 1996); at Idaho National Engineering Laboratory in Idaho Falls, Idaho (Wood and Norrell 1996); and at Maricopa, Arizona (Young et al 1996). Studies outside of the United States include studies in Canada (Kachanoski et al 1990;van Wesenbeeck and Kachanoski 1994) and in Europe (Schulin et al 1987;Hammel and Roth 1998).…”

Steady state lateral water flow through unsaturated soil layers

“…Berino loamy fine sand [Hills et al, 1991] of the system, the decrease in the relative head is nearly linear. For the largest a(H 1 À H 2 ) = 1, there is an increase in the normalized head above the 1:1 line as x/L increases and then a return back to the 1:1 line at the outflow end of the system (x/L = 1).…”

“…Table 1. The van Genuchten model parameters are from Hills et al [1991] and the equivalent Gardner alpha parameter was computed by numerical integration of the van Genuchten hydraulic conductivity function between h = 0 and h = À1 m:…”

Site S-7 VOC Transport modeling for the Vadose Zone Monitoring System (VZMS), McClellan AFB - 1999 Semi-Annual Report

“…First, the simpler single-continuum model seems inadequate to model the observed VOC concentration data, and second, the last decade of research has shown that, with a few exceptions (e.g., Wierenga, 1991;Hills et al, 1991), preferential flow is pervasive in the vadose zone (Kung, 1990a,b;Ghodrati and Jury, 1990;1992;Li and Ghodrati, 1994;Flury et al, 1994Flury et al, , 1995McCord et al, 1997).…”