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CIVIL I,N(;INFERING/O(:'rOBI.1R 1994 Transpiration Precipitation The overlying fine soils must become nearly saturated for the water pressure to approach atmospheric pressure and allow water to flow into the sublayers. This re- sistance to drainage explains the large storage capacity of the overlying fine soil. mix Gravel NEx Keeping the water in the fine -textured layer provides time for evaporation and transpi- ration to remove it. Results from tests conducted at the FLTF indicate the capillary barrier func- tions will work as designed. During the first three years of testing, twice the annual average precipitation (320 mm) was added to eight lysimeters to simulate a wetter cli- mate. During the next two years, three times the annual average precipitation (480 mm) was added to the same lysime- ters. During this entire five-year testing period, water losses by evaporation and transpiration exceeded water gains by precipitation and irrigation, even for the lysimeters receiving treatments repre- sentative of wetter climatic conditions. Although the vegetated lysimeters were most effective at removing soil moisture, even the soil water stored in the unvegetated lysimeters decreased during the five-year test period. No drainage was collected from any of these lysimeters. The capillary barrier concept does have its limits. At the be- ginning of the sixth year of testing, during the unusually wet winter of 1992-93 when record snowfalls were recorded, scien- tists and engineers observed drainage from several unvegetat- ed lysimeters already receiving supplemental precipitation. The routine supplemental irrigation treatments, when Com- bined with the unusually large amount of precipitation received during that winter, resulted in greater than three times (480 mm) the annual average precipitation added to the lysimeters. The net result was that the storage capacity of the fine -soil reservoir was exceeded and the unvegetated lysimeters began draining. The lysimeters with vegetation did not drain even though they received the same amount of moisture. Because of earlier tests conducted on two of our field lysimeters, we already had some understanding of the limits of the capillary barrier's performance. In two drainage lysimeters, we added enough precipitation to force water to break through the capillary barrier. As expected, water does not pass through the capillary barrier in the liquid phase until the soil approach- es saturation and capillary pressure approaches zero. Once breached, the capillary barriers in the lysimeters drained slow- ly until they reached a stable water content, resulting in a stor- age of more than 500 mm of water, twice as much as that nor- mally held by that soil against gravity. The observations at the FLTF indicate that both vegetated and unvegetated barrier systems are able to store and evapo- transpire at least three times the annual average precipitation, simulating the upper bound of projected climate changes at the Hanford site during the next 1,000 years or more. Vegetated barrier systems are able to accommodate even greater amounts of precipitation because of the water extraction capa- bilities of plants, thereby providing increased storage capacity. 1.5 m Fractured Basalt Riprap 0.30 m Drainage Gravel/CwWon 0.15 m Asphaltic Concrete Coated with Fluid -Applied Asphalt 0.10 m Top Course Compacted Sol Foundation In Situ Sol (Variable Thidmess) The fine -soil layer stores moisture until the processes of evaporation and transpiration can recycle any excess water back to the atmosphere. This layer also supports plants nec- essary for transpiration. The coarser materials placed directly below the layer create a capillary break that inhibits the downward percolation of water through the barrier until the layer becomes nearly saturated. The placement of the fine - soil layer directly over the coarser materials creates a favor- able environment for containing the biological cycles in the upper portion of the barrier, thereby reducing biointrusion in- to the lower layers. The coarser materials also may help deter inadvertent human intruders from digging deeper into the barrier profile. Low -permeability asphalt layers placed in the barrier profile below the capillary break are used in the protective barriers. These layers should divert any percolating water that gets through the capillary break away from the waste zone, and lim- it the upward movement of noxious gases from the waste zone. The coarse materials located above the low -permeability as- phalt layers also serve as a drainage medium to channel any percolating water to the edges of the barrier. CONFIRMING THE CAPILLARY BARRIER In developing the prototype design, we had to provide defensi- ble evidence through laboratory experiments, field tests, com- puter modeling and other studies that the final barrier design would meet the performance objectives. We began by testing capillary barrier functions at the Field Lysimeter Test Facility (FLTF). The capillary barrier is effective in controlling the down- ward movement of moisture through the barrier. It is con- structed by placing a fine -soil layer directly over a layer of coarser materials (such as silt over sands and/or gravels). The differences in textures between the barrier materials provide a capillary barrier for percolating water. In an unsaturated system, the capillary pressures are much less than atmospheric pressure. For significant quantities of water to flow into and through the coarser sublayers, the water pressure must be raised to nearly equal atmospheric pressure. 40 at n E� io 1 1.0 m Sift LoamlAd u r - '* 1.0 m SM Loam 0.15 m Sand Ffter 0.30 m Gravel Filter The overlying fine soils must become nearly saturated for the water pressure to approach atmospheric pressure and allow water to flow into the sublayers. This re- sistance to drainage explains the large storage capacity of the overlying fine soil. mix Gravel NEx Keeping the water in the fine -textured layer provides time for evaporation and transpi- ration to remove it. Results from tests conducted at the FLTF indicate the capillary barrier func- tions will work as designed. During the first three years of testing, twice the annual average precipitation (320 mm) was added to eight lysimeters to simulate a wetter cli- mate. During the next two years, three times the annual average precipitation (480 mm) was added to the same lysime- ters. During this entire five-year testing period, water losses by evaporation and transpiration exceeded water gains by precipitation and irrigation, even for the lysimeters receiving treatments repre- sentative of wetter climatic conditions. Although the vegetated lysimeters were most effective at removing soil moisture, even the soil water stored in the unvegetated lysimeters decreased during the five-year test period. No drainage was collected from any of these lysimeters. The capillary barrier concept does have its limits. At the be- ginning of the sixth year of testing, during the unusually wet winter of 1992-93 when record snowfalls were recorded, scien- tists and engineers observed drainage from several unvegetat- ed lysimeters already receiving supplemental precipitation. The routine supplemental irrigation treatments, when Com- bined with the unusually large amount of precipitation received during that winter, resulted in greater than three times (480 mm) the annual average precipitation added to the lysimeters. The net result was that the storage capacity of the fine -soil reservoir was exceeded and the unvegetated lysimeters began draining. The lysimeters with vegetation did not drain even though they received the same amount of moisture. Because of earlier tests conducted on two of our field lysimeters, we already had some understanding of the limits of the capillary barrier's performance. In two drainage lysimeters, we added enough precipitation to force water to break through the capillary barrier. As expected, water does not pass through the capillary barrier in the liquid phase until the soil approach- es saturation and capillary pressure approaches zero. Once breached, the capillary barriers in the lysimeters drained slow- ly until they reached a stable water content, resulting in a stor- age of more than 500 mm of water, twice as much as that nor- mally held by that soil against gravity. The observations at the FLTF indicate that both vegetated and unvegetated barrier systems are able to store and evapo- transpire at least three times the annual average precipitation, simulating the upper bound of projected climate changes at the Hanford site during the next 1,000 years or more. Vegetated barrier systems are able to accommodate even greater amounts of precipitation because of the water extraction capa- bilities of plants, thereby providing increased storage capacity. 1.5 m Fractured Basalt Riprap 0.30 m Drainage Gravel/CwWon 0.15 m Asphaltic Concrete Coated with Fluid -Applied Asphalt 0.10 m Top Course Compacted Sol Foundation In Situ Sol (Variable Thidmess) The fine -soil layer stores moisture until the processes of evaporation and transpiration can recycle any excess water back to the atmosphere. This layer also supports plants nec- essary for transpiration. The coarser materials placed directly below the layer create a capillary break that inhibits the downward percolation of water through the barrier until the layer becomes nearly saturated. The placement of the fine - soil layer directly over the coarser materials creates a favor- able environment for containing the biological cycles in the upper portion of the barrier, thereby reducing biointrusion in- to the lower layers. The coarser materials also may help deter inadvertent human intruders from digging deeper into the barrier profile. Low -permeability asphalt layers placed in the barrier profile below the capillary break are used in the protective barriers. These layers should divert any percolating water that gets through the capillary break away from the waste zone, and lim- it the upward movement of noxious gases from the waste zone. The coarse materials located above the low -permeability as- phalt layers also serve as a drainage medium to channel any percolating water to the edges of the barrier. CONFIRMING THE CAPILLARY BARRIER In developing the prototype design, we had to provide defensi- ble evidence through laboratory experiments, field tests, com- puter modeling and other studies that the final barrier design would meet the performance objectives. We began by testing capillary barrier functions at the Field Lysimeter Test Facility (FLTF). The capillary barrier is effective in controlling the down- ward movement of moisture through the barrier. It is con- structed by placing a fine -soil layer directly over a layer of coarser materials (such as silt over sands and/or gravels). The differences in textures between the barrier materials provide a capillary barrier for percolating water. In an unsaturated system, the capillary pressures are much less than atmospheric pressure. For significant quantities of water to flow into and through the coarser sublayers, the water pressure must be raised to nearly equal atmospheric pressure. 40