Vadose Techniques

Soil Physics

The Bureau has used a variety of instruments to monitor soil physics parameters, including water content, water pressure, and hydraulic conductivity. The Bureau has two neutron probes (Model 503DR Hydroprobe, CPN, Martinez, CA) and various types of time domain reflectometry systems to monitor soil water content. The neutron probe is used to monitor the vertical variation in water content in neutron probe access tubes. The probe is calibrated using water content data from soil samples based on oven-dried measurements of core samples. A 41 m deep access tube was installed to monitor water content in the Hueco Bolson in West Texas. Temporal variability in water content is used to monitor infiltration. Horizontal access tubes (aluminum and clay) were used to monitor water movement beneath a prototype engineered cover designed for waste containment at a site in West Texas. The SEAMIST system (FLUTe, Santa Fe, NM) is used to transport the neutron probe through the horizontal neutron probe access tubes.

Time domain reflectometry was used to monitor water content at many sites in Texas. Two different systems were used, a Tektronix system (Tektronix Inc. Model 1502c, Beaver, OR) and a TRASE system (Soilmoisture Equipment Corp, Santa Barbara, CA). Different types of probes were also used: 30 cm long uncoated probes (Campbell Scientific Inc. model 610), 20 cm uncoated probes (Soilmoisture Equipment Corp. model 6005), and 20 cm long coated probes (Soilmoisture Equipment Corp. model 6005C). The coated probes were used to reduce signal attenuation in high conductivity materials. TDR also measures bulk soil electrical conductivity, which may be useful for monitoring infiltration. TDR probes were installed in pits dug to a maximum depth of 1.8 m. The probes were inserted into the walls of the pit to minimize disturbance of soils above the probes. Water content varies with soil texture; therefore, spatial variations in water content cannot be used to determine the direction of water movement.

The direction of water movement can be determined from the gradient in potential energy because water moves from regions of high energy to low energy. Potential energy is the energy that results from position in a force field including gravity, capillarity (under wet conditions), adsorption (under dry conditions), and osmotic force fields in the unsaturated zone. Matric potential describes the forces related to the soil matrix and includes capillary and adsorptive forces. Osmotic potential results from the addition of solutes to the pore water. Water potential includes matric and osmotic potential. The Bureau has used a wide variety of instruments to measure or monitor these pressure potentials in the unsaturated zone. Water potential can be measured in the laboratory on soil cores collected in the field using a thermocouple psychrometer sample changer (Decagon SC10A model) or a water activity meter (Decagon CX2 model). These instruments measure the relative humidity of the air around the soil, which is related to the water potential using the Kelvin equation. The Bureau has installed tensiometers and advanced tensiometers to monitor potentials in the wet range in the field. The Bureau has also installed thermocouple psychrometers and heat dissipation sensors to monitor potentials in the dry range (Figure 1). Heat dissipation sensors measure the rate at which a heat pulse is dissipated in the instrument matrix material, which increases with increasing saturation and is related to pressure through laboratory calibration. Monitoring data provide information on infiltration and deep drainage in the vadose zone. Field installations include data loggers (CR7x, CR10x, CR21x, or CR23X) and are transmitted to the office using cellular data telemetry. Figure 2 shows the results of long term monitoring of water potentials at a sandy site in the Hueco Bolson in west Texas. The data show infiltration of water to 0.8 m depth. The lack of infiltration to greater depths reflects the ability of vegetation to remove water through evapotranspiration and minimize subsurface flow. Typical upward water potential gradients result from long-term drying of unsaturated sediments for the past 10,000 yr and indicate that there is no recharge in these interdrainage settings in arid regions.

Permeability in Unsaturated Media

The Bureau has estimated permeability in unsaturated media by monitoring atmospheric pumping as a result of barometric pressure fluctuations and performed pneumatic pressure tests as part of a research program conducted at the Maricopa Environmental Monitoring site to evaluate unsaturated zone monitoring techniques and strategies. This study was funded by the Nuclear Regulatory Commission.

Saturated Hydraulic Conductivity

The Bureau routinely measures saturated hydraulic conductivity in the laboratory using constant head or falling head approaches. The Bureau has a flexible wall permeameter to measure hydraulic conductivity in sediments of low conductivity in the laboratory

Guelph Permeameter

The Bureau has used a Guelph permeameter (model 2800K, Soilmoisture Equipment Corp, Santa Barbara, CA) and borehole permeameter to measure saturated hydraulic conductivity (Ks) in unsaturated sediments. The Guelph permeameter is generally used to measure Ks in the shallow subsurface. Constant head borehole permeameter tests were used to estimate Ks at much greater depths in the unsaturated zone. We have also evaluated techniques for analyzing the borehole permeameter tests.

References:
Xiang, J., Scanlon, B. R., Mullican, W. F. I., Chen, L., and Goldsmith, R. S., 1997, A multistep constant-head borehole test to determine field saturated hydraulic conductivity of layered soils: Advances in Water Resources, v. 20, pp. 45-57. [PDF]

The Bureau also develops water retention functions for different soils, which describes the relationship between water content and water potential. Hanging water columns are used to measure water retention in the wet range (0-0.25 bar) and pressure plate extractors are used in the dry range (1, 5, and 15 bar chambers).

March 2006