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Pacific Northwest National Laboratory, Richland, WA 99354
* Corresponding author (mike.fayer{at}pnl.gov)
Received for publication October 20, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AP, ambient precipitation • DRV, deep-rooted vegetation • EP, enhanced precipitation • FLTF, Field Lysimeter Test Facility • HB, Hanford Barrier • HMS, Hanford Meteorological Station • NV, novegetation • SRV, shallow-rooted vegetation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Surface covers are applied to a variety of sites ranging from municipal landfills to hazardous and low-level radioactive waste sites. The performance requirements of a particular cover will depend on site conditions, the nature of the waste, and applicable regulatory requirements. For the majority of waste sites, the ground water pathway is the primary exposure pathway. Therefore, a key factor governing the cover design is the ability to minimize deep drainage, which can carry contaminants from the waste zone to the ground water.
At some sites, the quantities of hazardous and/or low-level radioactive waste require that surface covers remain functional for at least 500 yr. Durations of this and greater magnitude are considered "long-term" and create a unique set of issues (Matthern and Nickelson, 1997). One of those issues is lack of data to demonstrate the performance of covers for such long durations. Instead, the long-term performance of covers can only be represented with short-term tests and inferred from analogs and modeling.
Historically, surface cover designs have been tested at many locations, but the test durations have typically been limited to a few years (e.g., Warren et al., 1996; Nyhan et al., 1997; Melchior et al., 1994). Recently, the USEPA initiated the Alternative Cover Assessment Program (ACAP) to develop field-scale performance data for multiple cover systems located at 12 sites around the country (Albright et al., 2002, 2004). While the range of designs being tested is large, the intent of ACAP is to monitor the field sites for only 5 yr. Nyhan (2005) reported a testing duration of 7 yr for four 10-m2 field plots. Ward and Gee (1997) described a heavily instrumented field-scale soil cover in Richland, Washington, that was installed in the fall of 1994; the cover has now been monitored for 10 yr (United States Department of Energy, 1999; Ward et al., 2005).
For more than 20 yr, the U.S. Department of Energy has supported the evaluation of surface covers for waste isolation at the Hanford Site. As part of that effort, the Field Lysimeter Test Facility (FLTF) was built in 1987 at a site north of Richland, Washington. The FLTF contains a set of 24 lysimeters that were established to study the water balance of specific surface cover designs (Gee et al., 1989). Some of the original tests are still being monitored, thus providing up to 17 yr of performance data (Gee et al., 1993; Fayer and Szecsody, 2004). The primary objective of this paper is to present long-term water balance data that demonstrate a silt loam cover design can limit deep drainage to near-zero amounts in a semiarid setting. Secondarily, this paper will demonstrate the sensitivity of drainage rates to two deterioration scenarios.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The climate of the Hanford Site is semiarid and dominated by cool, wet winters coupled with hot, dry summers (Hoitink et al., 2003). Weather data have been collected since 1946 at the Hanford Meteorological Station (HMS), which is located about 0.5 km west of the FLTF and at the same elevation (Hoitink et al., 2003). The HMS is a complete weather station, providing hourly measurements of all variables, including air temperature, dewpoint temperature, solar radiation, wind speed, cloud cover, and precipitation.
Between 1946 and 2002, annual precipitation at the HMS averaged 172 mm and varied between 76 and 313 mm. About half of the annual precipitation occurs from November through February. A rainfall intensity of 20 mm h–1 persisting for 1 h is expected only once every 1000 yr. A day with more than 27 mm precipitation is expected to occur about once every 10 yr, while a day with 52 mm precipitation is expected only once every 1000 yr. Monthly average snowfall is greatest in December (132 mm) and January (124 mm). The seasonal snowfall record occurred during the winter of 1992–1993 and totaled 1425 mm (versus the normal 373 mm). This record amount has a return period of 500 yr.
Winter monthly average temperatures ranged from –11.1 to 6.9°C. Summer monthly average temperatures ranged from 17.2 to 27.9°C. The potential for plant activity can be represented by the number of growing days, which is the number of days between the last freezing temperature in spring and the first freezing temperature in autumn. Since 1945, the number of growing days has averaged 181 d per year and ranged from 142 to 216 d.
The vegetation of the Hanford Site is characterized as a shrub-steppe ecosystem that is adapted to the region's mid-latitude semiarid climate (Neitzel, 2003). Such ecosystems are typically dominated by a shrub overstory with a grass understory. In the early 1800s, dominant plants in the area were Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. wyomingensis Beetle & Young) and an understory consisting of perennial Sandberg bluegrass (Poa secunda J. Presl) and bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) A. Löve]. Other species included threetip sagebrush (Artemisia tripartita Rydb.), antelope bitterbrush [Purshia tridentata (Pursh) DC.], rubber rabbitbrush [Ericameria nauseosa (Pallas ex Pursh) Nesom & Baird ssp. nauseosa], spiny hopsage [Grayia spinosa (Hook.) Moq.], needle and thread [Hesperostipa comata (Trin. & Rupr.) Barkworth], Indian ricegrass [Achnatherum hymenoides (Roemer & J.A. Schultes) Barkworth], and prairie junegrass [Koeleria macrantha (Ledeb.) J.A. Schultes].
With the advent of settlement, livestock grazing and agricultural production contributed to colonization by non-native vegetation species that currently dominate portions of the landscape. Of the 727 species of vascular plants recorded for the Hanford Site, approximately 25% are non-native. The dominant non-native species, cheatgrass (Bromus tectorum L.), is an aggressive colonizer and has become well established.
Field Lysimeter Test Facility Design
Figure 1
shows that the FLTF is a subsurface facility that contains 14 drainage lysimeters, 4 weighing lysimeters, and 6 small-tube lysimeters (Gee et al., 1989). The drainage lysimeters are 2 m in diameter by 3 m deep and form the walls. The four weighing lysimeters are 1.5 m on a side, 1.7 m deep, and rest on platform scales that provide hourly water storage changes. The six small-tube lysimeters are 0.3 m in diameter and 3 m deep.
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Irrigation
Table 1 identifies the lysimeters that were irrigated to mimic an increased precipitation regime. The water was delivered through six nozzles spaced 0.41 m apart along a 2.4-m boom that was connected to the water source. The boom was 0.5 m above the ground surface and was moved automatically down the length of the facility at the rate of about 0.7 m min–1. Four rain gauges were positioned within the irrigation path and monitored manually at the end of irrigation. Untreated water from the Columbia River was applied in increments ranging from 3 to 35 mm per application. The rate was typically 4 mm h–1. During several years, up to 73 mm of water were applied in a single irrigation event to simulate a 1000-yr storm. The total quantity and frequency of application were determined by the historical monthly average. Specific application rates each month were constrained by weather, depending on the amount of rain or snowfall. Irrigation timing also depended on having frost-free days during winter months. Application rates were such that a total of 320 mm yr–1 (including natural precipitation) was applied for the first 3 yr (November 1987 to October 1990), then a total of 480 mm yr–1 for all subsequent years. These values are multiples (2x and 3x) of 160 mm yr–1, which was the average annual precipitation (through 1980) that was used to design the Hanford Barrier.
Water Storage
For the weighing lysimeters, water storage was measured directly using platform scales under the lysimeters. The resolution was ±0.02 mm. For the drainage lysimeters, water storage was calculated from water contents that were measured with a neutron probe. The measurement frequency was biweekly in the first 6 yr and sporadic thereafter. The measurement depths were every 0.15 m, starting at the 0.15-m depth; measurements were not made below the silt loam–sand interface. The resolution was ±5 mm.
Soil Matric Head
Soil matric head was measured biweekly with a pressure transducer and ceramic-cup tensiometers in those lysimeters that had matric heads above –800 cm. Depths of measurement were 1.0 and 1.5 m. The resolution was ±1 cm of water (pressure head).
Vegetation
Plant activity was monitored intermittently to identify species, estimate percent cover, and measure shrub height. Roots were not monitored or measured except as noted during excavations of some lysimeters for other purposes (e.g., to change the test conditions).
Drainage
Drainage was measured biweekly in all lysimeters by collecting free water from the outlet located at the base of each lysimeter. The collected water was weighed immediately at the facility to the nearest gram and reported on a depth basis (resolution << 0.01 mm). The collection of drainage in this manner (i.e., free drainage) implies a seepage face exists above the outlet, which suggests that water contents near the outlet may potentially be high. Field and simulation results have shown that seepage face boundaries can alter the water balance of lysimeters and affect drainage rates (Colman and Hamilton, 1947; Scanlon et al., 2002, 2005). If plant roots have access to this zone, the resulting evapotranspiration can remove water that might normally have drained in the absence of the lysimeter bottom. Excavation of several drainage lysimeters revealed no roots penetrating deeper than the sand filter layers. Additionally, the drainage lysimeters have a large gravel zone beneath the sand filter layers that does not allow high water contents to develop above the seepage face at the outlet. The weighing lysimeters do not have a large gravel zone and are thus susceptible to having plant roots affect drainage. However, no weighing lysimeter has ever been excavated to confirm or refute this assumption.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Weather
Monthly average air temperatures for the period of FLTF operation (defined as November 1987 through December 2003) were 0.8°C higher, on average, than the temperatures for the pre-FLTF period (1945 to October 1987). The individual monthly averages were warmer by amounts ranging from 0.3 to 2.6°C (which occurred in January). Average minimums during the FLTF period were 2.2°C higher; for January, the minimum was 7.1°C higher.
The average monthly precipitation amounts during the FLTF period were generally higher than amounts during the pre-FLTF period. The FLTF monthly averages ranged from 3.7 mm less to 8.25 mm greater than pre-FLTF. The maximum annual precipitation record of 313 mm was set in 1995. This record was nearly broken in the following year when annual precipitation totaled 310 mm. During the FLTF period, two records for maximum monthly snowfall were reported: 432 mm in February 1989 and 574 mm in December 1996. The annual snowfall record of 1425 mm was set during the winter of 1992–1993. During the FLTF period, the annual average precipitation was 181 mm, which was 13 mm greater than the pre-FLTF average of 168 mm.
Overall, the weather during the FLTF period could be characterized as warmer and wetter than during the pre-FLTF period. The impact on drainage rates is uncertain. Warmer weather increases potential evaporation and lengthens the season of plant activity, both of which reduce drainage. Higher precipitation can lead to higher drainage, but it could be offset by increased plant growth encouraged by the extra water.
Vegetation
Table 2 shows the list of common species observed on the FLTF lysimeters. Most are annuals and were present for only a portion of each year. Sagebrush health and survival were mixed. One explanation is that shrubs were kept trimmed at a height of 0.5 m to maintain a clear path for the irrigation boom. However, even small untrimmed shrubs had difficulty in some lysimeters, so other mechanisms must be affecting health. One possibility is that shrubs need large areas to harvest sufficient water and nutrients and the FLTF lysimeters may be insufficient in size. For example, sagebrush roots were documented at least 1.0 m from the shrub center (Fayer et al., 1999). The implication for the FLTF is that the drainage lysimeter walls could impact roots; walls of smaller lysimeters would be even more likely to do so. The issue of sagebrush health and survival is further complicated by the observation of a general decrease in sagebrush health, and some die-off, at the Hanford Site. Poston et al. (2000) reported areas totaling 1776 ha that showed evidence of sagebrush decline, and 280 ha where sagebrush death was 80% or greater. Although shrubs are effective at reducing drainage, the drainage data presented later in this report show that shrubs are not critical to the drainage-limiting ability of the barrier designs that use silt loam for the surface layer.
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Lysimeters that are kept free of vegetation have more water stored in the soil throughout the year. One of the most difficult tasks in operating the FLTF has been maintaining the no-vegetation status of those lysimeters. Because of this persistent germination, the observed drainage may not represent the maximum possible drainage that could be expected under nonvegetated conditions. Our experience has been that these lysimeters must be weeded every 2 wk in the spring and monthly during the remainder of the year. On several occasions, a 1- or 2-mo hiatus in weeding during the spring resulted in an extensive crop of plants, usually Russian thistle (Salsola kali L.) but also sagebrush seedlings, cheatgrass, and other species. The implication is that a nonvegetated condition (e.g., due to fire, disturbance) will probably not persist for silt loam surface barriers for more than a couple of months, and certainly not for more than 1 yr.
Water Contents
Figure 3
shows the water content changes in the silt loam layer of the Hanford Barrier. For the driest condition (D4: ambient precipitation, vegetation), water contents at the shallower depths increased during winter months but always returned to a low value between 0.05 and 0.07 cm3 cm–3 by summer. At the deeper depths (e.g., 1.2 m), water contents remained constant at the low values because evapotranspiration from the shallower depths was sufficient to prevent significant quantities of water from draining to the deeper depths. For the wettest condition (D10: enhanced precipitation, no vegetation), water contents at all depths increased substantially during winter months and decreased during summer months. In contrast to the results for D4, the water contents in D10 did not return to a consistent low value each summer. In addition, the water contents at the 1.2-m depth increased to as great as 0.41 cm3 cm–3 in winter compared to 0.07 cm3 cm–3 at the same depth in D4. Such high water contents in the vicinity of the capillary break (located at 1.5 m) are a precursor to the onset of drainage.
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Soil Matric Head
Only a few lysimeters (HB test: D10, D12, and W4) had matric heads high enough to measure with a tensiometer. For these lysimeters, Fig. 5
shows that heads at the 1.5-m depth were quite consistent from 1995 through 1997. During November 1997, D12 and W4 were modified for the silt loam erosion and dune sand deposition test (HB–dune sand). In the following summer, heads in D10 dropped as in previous years, but heads in D12 and W4 remained above –100 cm. The HB–dune sand lysimeters D12 and W4 were mostly unvegetated, so the contrast in heads was due primarily to the surface-soil property differences and how those properties affected the evaporation process. The dune sand was much less able than the silt loam to store water near, and/or transmit water to, the evaporation surface. These lysimeters were intended to have only shallow-rooted vegetation, but deep-rooted tall tumblemustard (Sisymbrium altissimum L.) invaded D12 in 1999 and covered 30% of the lysimeter surface before being removed in May. The effect can be seen in Fig. 5 as the rapid drop in D12 matric heads during the spring of 1999.
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The only treatment that led to significant drainage from a HB test was enhanced precipitation and no vegetation. Figure 6 shows that, for the first 3 yr (under 2x precipitation), the three lysimeters (D10, D12, and W4) containing an unvegetated Hanford Barrier had no significant drainage. In 1993, 3 yr after increasing precipitation to 3x, these lysimeters began to show significant drainage. The onset of drainage coincided with the melting of a large snow pack in February 1993. In early 1997, a similar event occurred that also resulted in significant (>0.5 mm) drainage from these lysimeters. Curiously, drainage from D10 and D12 as a result of this event was fivefold greater than that from W4. After November 1997, D12 and W4 were converted to the erosion and dune sand test (HB–dune sand). Despite having identical treatments, lysimeter D12 drained about twice as much as W4. Such a difference may indicate a lysimeter effect (drainage versus weighing) or the variability that could be expected in a full-scale cover.
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The pattern of drainage is generally predictable. Lysimeters that produced drainage typically did so in the spring and early summer in response to winter precipitation. In contrast to this pattern, some lysimeters drained very small amounts sporadically but always in late summer and fall when drainage is expected to be the smallest. Figure 7 shows the late-summer to fall drainage amounts are quite variable. This anomalous drainage was attributed to vapor flow (Campbell and Gee, 1990). As it penetrated the underlying cooler sands and gravels, the vapor condensed and eventually led to drainage. Figure 7 also shows that late summer drainage was recently detected in lysimeter D5, which contains the erosion–deposition test and only shallow-rooted plants. The matric head at the base of the silt loam layer is much higher (–300 cm) than normally occurs at that time of year in a silt loam receiving ambient precipitation and could facilitate vapor movement.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Seasonal Drainage
One aspect of the FLTF results that deserves attention is the observation of small quantities of late summer and fall drainage from some of the tests. Seasonal temperature changes are thought to be causing such drainage, leading to questions about whether the design of the lysimeter facility may be responsible. The sides of the lysimeters that face toward the facility are exposed to temperature changes whenever the facility is vented for extended periods, such as when lysimeters are modified. Using the TOUGH2 numerical model, Holford and Fayer (1990) showed that lateral temperature gradients within the lysimeters could cause late summer condensation within the basalt layer and along the outer lysimeter walls; the condensation eventually became drainage. In addition, the origin of the drainage water is not clear. The water could be draining from the silt loam layer or it could originate from residual water (i.e., from construction and leak testing) within the basalt riprap and gravel.
Independent of the FLTF tests, Ward et al. (1997) reported "small seasonal discharges" in late summer from a prototype barrier test initiated in November 1994. The four vegetated test plots were large (322 m2) and located within a field-scale surface cover, suggesting that the FLTF design is not the sole possible cause of seasonal discharge. Wittreich et al. (2003) did not report any seasonal discharge after 1997. However, the plant community had become well established after 1997, so it may be that the silt loam layer was too dry for significant downward vapor flow to occur. Such a result is consistent with the observed lack of drainage from vegetated lysimeters receiving ambient precipitation at the FLTF.
Currently, no method exists that can identify the source of the seasonal drainage water. Given sufficient years, residual water from construction or leak testing ought to diminish. If so, continued seasonal discharges would imply the drainage water was coming from the silt loam. Fortunately, the average drainage observed under these seasonal discharge conditions after the initial 3 yr is less than 0.5 mm yr–1. Therefore, seasonal temperature-induced discharge is not considered to be a performance issue for the HB design at this time.
Long-Term Considerations
The long-term performance of surface covers can be influenced by climate change, ecological change, and soil development. Perhaps the most dramatic impact would be from climate changes involving precipitation and temperature. Fayer et al. (1999) examined these variables using a water balance simulation approach and paleoclimate observations derived from pollen data from Carp Lake, which is located about 175 km southwest of the Hanford Site, at an elevation of 714 m (versus 220 m at Hanford). Carp Lake was chosen because pollen records at the Hanford Site were eliminated during the glacial flooding 13c000 yr ago. Whitlock and Bartlein (1997) described a 125c000-yr paleoclimate record constructed from the pollen record in cores taken from Carp Lake. Wing et al. (1995) described the Carp Lake pollen interpretation relative to precipitation and temperature. For the entire Holocene (i.e., the last 10 000 yr), the data suggested that annual temperatures and precipitation ranged from 0 to 2.8°C warmer and 0 to 50% drier compared to modern climate. During the glacial period before the Holocene, annual temperatures ranged from 0.2°C warmer to 2.5°C cooler and precipitation ranged from 75 to 128% of modern levels. In summary, for the last 100c000 yr, annual precipitation ranged from 50 to 128% of modern levels and annual temperatures ranged from –2.5 to 2.8°C of modern levels. Fayer et al. (1999) used these limits to represent future climate conditions in their simulations by scaling the current temperature and precipitation data. For the climate change conditions most likely to promote recharge (i.e., higher precipitation and lower temperature), their results suggested that similar covers with just 1 m of silt layer (equivalent to the eroded Hanford Barrier test in this paper) would prevent drainage if future climate is not more extreme than the paleoclimate reconstruction.
Vegetation changes are difficult to predict. Conversion from shrub-steppe to grass-dominated surfaces has occurred extensively at Hanford and may continue in the future. Just as important are the recent increases in non-native plant species and the possibility that such increases may alter the composition of the shrub-steppe (Neitzel, 2003). Regardless, the data from the lysimeter tests show that if silt loam surfaces persist, the type of vegetation (shallow-rooted grasses vs. deep-rooted shrubs) should not affect the ability of the surface cover to prevent drainage. The silt loam soil has such a large storage capacity and ability to evaporate the excess stored water even under the envisioned climate change scenarios. This result is consistent with other studies. For example, Kurc and Small (2004) showed that semiarid shrubland and grassland in New Mexico had similar overall evapotranspiration fluxes (essentially equivalent to precipitation). They found that the differences in transpiration from the two plant communities were offset by differences in the evaporation fluxes.
Soil development, specifically bioturbation, can impact drainage by changing the physical and hydraulic properties of the silt loam. Shafer et al. (2004) studied the issue of bioturbation as it relates to surface covers at the Nevada Test Site (NTS). They examined four locations on the NTS that were analogs for various cover "ages" ranging from 30 to 125c000 yr old. On all four analog sites, small mammal bioturbation was largely limited to the upper 0.7 m. At Hanford, short-term animal tests suggested that bioturbation by mammals created temporary macropores that increased soil infiltrability (which could increase deep drainage) and deep soil drying (which decreases deep drainage). Overall, bioturbation did not appear to increase water storage and, by inference, drainage (Landeen, 1994). Assuming bioturbation is confined to the upper meter, turnover and mixing of the silt loam is expected and not considered detrimental to performance. With respect to hydraulic properties, insufficient time has elapsed to draw any conclusions. However, observations at the FLTF did not reveal any detectable macropore flow in vegetated lysimeters where roots have been present for the past 17 yr.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Reducing the silt loam thickness to 1.0 m (as might occur via erosion), with or without plants, did not lead to drainage under ambient conditions. Under 3x precipitation conditions, however, drainage was significant even though plants were present.
In a separate test, replacing the top 0.2 m of silt loam with dune sand and reducing plant cover did not lead immediately to the onset of drainage, but soil matric heads within the silt loam noticeably increased and what is believed to be small quantities of thermally induced drainage were sometimes observed. This observation suggests that dune sand migration onto a surface cover has the potential to reduce a cover's ability to minimize deep drainage. The presence of vegetation, especially deep-rooted shrub-steppe plants, mitigates this potential problem, but makes the barrier's success susceptible to plant disturbances and fire. Thus, deposition of wind-blown sand could affect long-term barrier performance.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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