METHODS

The objective of the monitoring program is to provide information on various components of the water balance equation:

where ET is evapotranspiration, P is precipitation, RO is surface runoff, S is water storage, L is lateral drainage, and D is deep drainage or percolation (Table 1). All parameters on the right of the equation are being monitored, and evapotranspiration will be calculated.

 

Table 1. Water budget equation with various parameters and their methods of measurement described. Supplemental data for temperature and potential energy are also being collected.

 

Installation of vertical instruments was restricted to the area above the GCL/asphalt- and capillary-barrier layers because the performance of the covers relies on the integrity of the barriers. We installed horizontal neutron probe access tubes above and below the barrier horizons to provide monitoring data without compromising the barrier layers. We installed all instrumentation during construction except the vertical neutron probe access tubes, which were installed from the surface after construction was completed. The plots will be allowed to equilibrate with ambient conditions before we add an irrigation system to simulate rainfall. An instrument silo is located in the center of the installation to house data loggers and access ports for retrievable instrumentation. Two instrument signal cable trees constructed of 0.3-m-diameter PVC were installed in each plot during construction (Figure 5 and Figure 6). The signal cable trees in the GCL/asphalt design extend from the surface to the top of the GCL/asphalt (1.3 m in depth), and those for the capillary-barrier design extend from the surface to the capillary barrier (2.0 m in depth).

Figure 5. Typical instrument signal cable tree. The trees were constructed of 0.3-m ID schedule 80 PVC and are placed on a concrete pedestal. There are two trees per subplot, resulting in a total of eight trees (Figure 3). Instrument cables are routed through water-tight fittings and either terminate at one of two multiplexers located inside each tree or are routed directly to a multiplexer located in the central instrument silo. There are a total of 128 time-domain reflectometry (TDR) probes, 56 heat-dissipation sensors, and 24 thermistors installed in this manner at various depths and orientations throughout the soil profile above each of the barrier horizons.

 

Figure 6. One of the instrument trees on a concrete pedestal during construction. None of the instruments had been installed at that time. Note the large bundle of instrument cables to be later routed through conduits.

 

 

Rainfall, Runoff, and Drainage Measurements

We are monitoring drainage from the lysimeters (Figure 7), lateral drainage from the GCL/asphalt layer (Figure 8), and precipitation with rain gauges. Runoff from each 17-m2 test plot drains to one of four tanks designed to hold the runoff depth from a 25-yr probability storm event of 6.5 cm. Each storage tank has a 0.9-m-diameter sump well built into its base, which is used to measure runoff of <3.5 mm in depth. The volume of surface runoff is measured with a pressure transducer located at the bottom of each sump well. Lateral drainage is measured for two 15-m2 areas of the asphalt layer. Deep drainage is monitored from the base of each plot with pan lysimeters constructed of flexible membrane liners (60 mil very flexible polyethylene).One of three methods is used to monitor these flows, each designed to obtain a minimum accuracy of 1 percent over the anticipated range of flow rates. Each measurement mechanism will be placed sequentially in line. At the lowest anticipated flow rates of 0 to 5 L d-1, an infrared drop-counting mechanism will be used. Intermediate flow rates from 0.5 to 25 L d-1 will be measured by a tipping-bucket rain gauge that has a 5-mL tipping capacity. We will measure flow rates of 25 L d-1 by determining the water depth in the storage drum with a wireline probe (0.1-m3 capacity). Each lysimeter is 12 m2 and is located directly beneath the center of each cell. The successive insets from each edge with depth are designed to prevent the collection of water that did not percolate through the 17-m2 caps.

Figure 7. Pan lysimeters located at the base of each of the four experimental subplots. The instrument silo can be seen partially buried in the center of the installation. The lysimeters are constructed of 60-mil very flexible polyethylene with a 2 percent slope toward a subsurface drain-collection system. Drainage is then routed through pipes back to the central instrument silo where volumes are measured with rain gauges.

 

Figure 8. The asphalt-barrier layer during construction. Drains located at the right route any intercepted lateral drainage and return it to the silo for measurement using rain gauges, similar to the lysimeter drainage measurement system. Also noted is one of the aluminum horizontal neutron probe access tubes, here temporarily capped.

 

 

Water Content Monitoring

Two instruments are used to monitor temporal variations in water content: neutron probe and time domain reflectometry. In addition, we are testing the ability of electromagnetic induction as a noninvasive technique to monitor water content. A neutron probe (Model 503 DR, Boart Longyear Campbell Pacific Nuclear Corp., Martinez, CA) will be used to monitor volumetric water content in both horizontal and vertical access tubes in each plot. We installed 20 vertical access tubes to the depth of the asphalt (1.3 m) in the GCL/asphalt design and to the top of the capillary barrier (2 m). The access tubes consist of PVC pipe sealed at the base with a machined drive point and a threaded cap at the top. We installed the access tubes after construction was completed. Horizontal neutron probe access tubes are open at both ends (Figure 8 and Figure 9). The 1.3-m depth targets the top of the GCL/asphalt and the 2-m depth targets the top of the capillary barrier. We installed two access tubes at 0.3 and 3 m in each design, one aluminum (10-cm-diameter) and the other bisque-fired clay (15-cm-diameter). All other monitored depths consist of aluminum access tubes. The SEAMIST system is used to position the neutron probe inside the horizontal access tubes. Advantages from using the SEAMIST system include the possibility of using logging tools that may be developed in the future. Bisque-fired clay has not been used in access tubes previously and is being tested in this program. We will also evaluate the use of absorbent pads for collecting pore fluids in the event of leakage.

Figure 9. Installation of an aluminum horizontal neutron probe access tube.

 

Time-domain reflectometry (TDR) is used to automatically monitor temporal variations in water content in the shallow subsurface (Figure 10). Two different TDR systems (Campbell Scientific, Logan, Utah, and Soilmoisture Equipment, Santa Barbara, California) and three different probe designs (all three-wire probes; 30-cm-long uncoated probes [40, CS610, Campbell Scientific]; 20-cm uncoated [40] and 20-cm coated probes [48] [Soilmoisture Equipment] were installed to address problems of signal attenuation in high-conductivity soils. The basic theory of TDR is described in Topp et al. (1980). Unlike neutron-probe measurements of water content, which must be conducted manually, TDR measurements can be fully automated. Coated TDR probes were installed horizontally to monitor downward movement of steep wetting fronts. Uncoated probes were installed vertically to obtain an integrated measure of water-content change for water-balance modeling (Table 1). A total of 128 TDR probes were installed about 1 m laterally from each of two instrument trees in each plot. Water content is monitored daily at all depths.

Figure 10. A (coated) time-domain reflectometry (TDR) probe and a heat-dissipation sensor (HDS) just prior to installation. Coated TDR probes were installed horizontally at various depths to monitor the arrival of steep wetting fronts, and uncoated TDR probes were installed vertically to obtain integrated average water contents for water budget calculations. Heat-dissipation sensors measure soil water matric potential at near-saturation values.

 

We are testing electromagnetic (EM) induction as a technique to noninvasively monitor water content. We monitored apparent electromagnetic conductivity adjacent to the vertical neutron probe access tubes to evaluate the relationship between EM induction and water content of the prototype barriers. Electromagnetic transects measured across fissured sediments in the natural system indicate that the sediments in the natural system may be too dry to provide a useful signal (Scanlon et al., 1999b). Sheets and Hendrickx (1995) found a good correlation between neutron probe data and apparent EM conductivity while monitoring temporal variations in water content near Socorro, New Mexico.

 

 

Potential Energy

Water potential is monitored throughout the profile by thermocouple psychrometers. These instruments were calibrated in the laboratory for the expected range of temperatures and water potentials. Probes were installed horizontally in PVC pipe (2.5 cm OD) and are retrievable (Figure 11). Two vertical profiles of PVC pipe (~0.3-m vertical spacing) were installed in each plot to provide vertical profiles in water potential. Thermocouple psychrometers were installed in duplicate at each depth and are connected to CR7 data loggers (Campbell Scientific Inc., Logan, Utah) in the instrument silo.

We are using 56 heat-dissipation sensors (Figure 10) to monitor matric potential in the field. These were calibrated in the laboratory by means of pressure-plate extractors and were then installed horizontally adjacent to the TDR probes. Thermistors are used to monitor temperature variations with depth at the same locations as the heat-dissipation sensors in one instrument tree per plot, resulting in a total of 24 thermistors. The heat-dissipation sensors and thermistors are attached to AM416 multiplexers (Campbell Scientific Inc., Logan, Utah) at the instrument trees, which are connected to CR10X data loggers at the silo. Installation of the instruments was completed in September 1997 (Figure 12).

 

Figure 11. Two thermocouple psychrometer (TCP) conduits during installation. TCPs are retrievable and are installed into a small chamber located at the outer end of each conduit. The chambers are open to the soil and sealed off from the conduit and thus equilibrate with the surrounding soil. There are 80 such conduits buried at nominal 0.3-m increments from the surface to a depth of 3.0 m. One half of the conduits contain duplicate instruments, resulting in a total of 120 installed TCPs.

 

Figure 12. The engineered cover installation upon completion of the primary construction phase in September 1997.

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