The Las Cruces Trench Site: Characterization, Experimental Results, and One‐Dimensional Flow Predictions

Abstract

A comprehensive field trench study was conducted in a semiarid area of southern New Mexico to provide data to test deterministic and stochastic models of vadose zone flow and transport. A 4 m by 9 m area was irrigated with water containing a tracer using a carefully controlled drip irrigation system. The area was heavily instrumented with tensiometers and neutron probe access tubes to monitor water movement and with suction tubes to monitor solute transport. Approximately 600 disturbed and 600 core samples of soil were taken to support deterministic and stochastic characterization of the soil water hydraulic parameters. The core sample‐based saturated hydraulic conductivities ranged from 1.4 to 6731 cm/d with a mean of 533 cm/d and a standard deviation of 647 cm/d, indicating significant spatial variability. However, visual observation of the wetting front on the trench wall shows no indication of preferential flow or water flow through visible root channels and cracks. The tensiometer readings and the neutron probe measurements also suggest that the wetting front moves in a fairly homogeneous fashion despite the significant spatial variability of the saturated hydraulic conductivity. In addition to the description of the experiment and the presentation of the experimental results, predictions of simple one‐dimensional uniform and layered soil deterministic models for infiltration are presented and compared to field observations. These models are presented here to provide a base case against which more sophisticated deterministic and stochastic models can be compared in the future. The results indicate that the simple models give adequate predictions of the overall movement of the wetting front through the soil during infiltration. However, the models give poor predictions of point values for water content due to the spatial variability of the soil. Comparisons between the one‐dimensional infiltration model predictions and field observations show that the use of the layered soil model rather than the uniform soil model does not consistently improve the accuracy of the predictions for this particular field application. This result illustrates that increasing the spatial resolution of the deterministic characterization of the site in the vertical direction does not always improve the model predictions. Uncertainties due to horizontal spatial variability and due to other difficulties associated with experimental characterization appear to be more significant.

Vadose Zone Transport Field Study: Status Report

“…It is well known that laboratory-determined parameters are usually not directly applicable to the field scale (e.g., Wierenga et al 1991). Thus, there is a need for techniques capable of estimating hydraulic properties applicable to the field scale.…”

Vadose Zone Transport Field Study: Detailed Test Plan for Simulated Leak Tests

“…Bromide was selected because of its stability, low background concentrations at Hanford, and the relative ease of chemical analysis. Bromide has been used successfully in other vadose zone studies as a non-reactive tracer to study flow paths and arrival times (Wierenga et al 1991;Killey and Moltyaner 1988). Tracer distributions will be determined from core and pore water samples.…”

“…Tritium was used at the Las Cruces trench experiment (Wierenga et al 1991), and Killey andMoltyaner(1988) used tritium to compare to the iodine measured by the NaI detector. Other radionuclides such as strontium or cesium can be cored or used with solution samplers, but the reactive properties of these radionuclides with materials that comprise the sampling device area concern.…”

“…The occurrence and impact of spatial and temporal variability in hydrologic and chemical properties of natural soils is well established (Roth et al 1991;Wierenga et al 1991;Russo and Bouton 1992). Information on variability is critical to the prediction of contaminant spreading (Russo 1993) and the effectiveness of remediation (Berglund and Cvetkovic 1996).…”

An alternative deterministic method for the spatial interpolation of water retention parameters

“…The database was generated as part of a comprehensive field study conducted in southern New Mexico, near Las Cruces, for validating and testing numerical models of water flow and solute transport in the unsaturated zone (Wierenga et al, 1991). A 24.6-m long by 6.0-m deep trench wall was excavated, and a total of 450 undisturbed samples, 7.6 cm internal diameter and 7.6 cm long, were taken from nine layers, 50 samples were taken from each layer, for every 0.5 m. Soil samples (undisturbed and disturbed) were then analyzed in the laboratory for soil properties, such as bulk density, saturated hydraulic conductivity, and soil water retention curve.…”