Vadose Zone Journal 1:150-157 (2002)
© 2002 Soil Science Society of America
Chloride Distributions as Indicators of Vadose Zone Stratigraphy in Palouse Loess Deposits
A. T. O'Geen*,a,
P. A. McDaniela and
J. Bollb
a Soil Science Division, Dep. of Plant, Soil, and Entomological Sciences, University of Idaho, Moscow, ID 83844-2339
b Dep. of Biological & Agricultural Engineering, University of Idaho, Moscow, ID 83844-0904
* Corresponding author (ogee2191{at}uidaho.edu)
Received 21 November 2001.
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ABSTRACT
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Chloride is often used as a conservative tracer to estimate groundwater recharge rates in arid and semiarid regions. Relationships between Cl- depth profiles and vadose zone stratigraphy have revealed new information on the behavior of this dissolved constituent in pore waters of heterogeneous materials. We measured pore-water Cl- in loess deposits of the eastern Palouse region in northern Idaho, where multiple sequences of buried soils extend to 
20 m depth. Three cores were collected to bedrock at summit, side slope, and valley positions. Pore-water Cl- distribution, clay content, soil strength, and secondary Mnd/Fed ratios were measured to identify relationships between natural tracer migration and vadose zone stratigraphy. Characterization of deep strata revealed complex sequences of extremely dense paleosol fragipans interstratified with less dense leached horizons. Abrupt changes in Cl- concentration reflect boundaries between these stratigraphic units that display contrasting physical and morphological properties. Results illustrate that loess stratigraphy influences vadose zone water movement in the Palouse. In addition, Cl- depth profiles can be used as indicators of deep stratigraphy across various landscape positions.
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INTRODUCTION
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LOESS-DERIVED SOILS in the eastern Palouse region of Washington and northern Idaho contain fragipans and horizons with fragic character that restrict vertical percolation of water (Fig. 1) (McDaniel and Falen, 1994; McDaniel et al., 2001). Fragipans are brittle subsurface soil horizons that exhibit high bulk density to the extent that movement of water and roots is restricted (Soil Survey Staff, 1999). Soilscapes that contain hydraulically restrictive horizons occupy areas of greatest mean annual precipitation in the Palouse Basin, which is a drainage basin that occupies approximately 100 000 ha of the eastern Palouse region (Fig. 1). Since water-restrictive horizons are believed to play a critical role in groundwater recharge, understanding the ability of soils to accommodate deep percolation is an important step toward characterizing the regional hydrology of the Palouse Basin.
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Fig. 1. Location of the study site showing distribution of soils with hydraulically restrictive horizons in eastern Washington and northern Idaho. Topographic attributes are displayed using a 2-m grid elevation model and location of core samples are indicated with X.
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Investigations of near-surface hydrological processes in soils with fragipans are common, yet no studies have directly measured deep percolation below a fragipan, particularly in xeric moisture regimes such as those found in the Pacific Northwest. Observations of perched water table dynamics in the eastern Palouse region indicate that E horizons overlying restrictive horizons remain saturated for long periods and water moves laterally as throughflow in more permeable Ap and Bw horizons (Reuter et al., 1998; McDaniel et al., 2001). Measured saturated hydraulic conductivity values range from 0.06 to 0.9 cm d-1 in restrictive horizons and 14 to 129 cm d-1 in Ap and Bw horizons (McDaniel et al., 2001; Reuter et al., 1998). As a result, up to 90% of the perched water volume can be shed from uplands as lateral throughflow in Ap and Bw horizons (Brooks et al., 2000). Moreover, multilevel tensiometers indicate that saturation does not occur below the uppermost fragipan in uplands of the eastern Palouse region (O'Geen et al., 2001).
Water mass balance techniques are commonly used to assess water movement in soils with fragipans under udic soil moisture regimes. In New York, Parlange et al. (1989) discovered that a majority of irrigation water moved laterally through a network of cracks present within a fragipan. A similar study using in situ soil block experiments identified 23% of water flow as vertical percolation below the upper boundary of a fragipan (Day et al., 1998). A unifying concept among all studies is that permeability of fragipans is governed by frequency and distribution of cracks, which, in turn, is influenced by the degree of soil development (Van Vliet and Langohr, 1981; Parlange et al., 1989; Ciolkosz et al., 1995; Day et al., 1998).
Pore-water Cl- is commonly used as a conservative natural tracer to assess deep percolation in vadose zones (Scanlon, 1992; Cook et al., 1992; Allison et al., 1994; Phillips, 1994; Murphy et al., 1996). In most cases, Cl- is introduced into soils through atmospheric deposition and enriched in soil solution through evapotranspiration because plants do not assimilate significant amounts of Cl-. Using Cl- depth profiles, a mass balance approach can be applied to calculate water flux under steady-state conditions (Cook et al., 1992; Allison et al., 1994, Tyler and Walker, 1994; O'Brien et al., 1996; Murphy et al., 1996). If sources and sinks of Cl- are defined and the deposition rate has remained constant through time, the flux of Cl- below the root zone can be estimated by:
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where R is recharge, P is mean annual precipitation, Cs is average Cl- concentration, and Cp is Cl- concentration in precipitation.
Although several studies have documented sedimentologic and stratigraphic characteristics to accompany natural tracer studies, few have directly quantified the influences of stratigraphy on Cl- depth profiles. Eriksson and Khunakasem (1969) discovered that spatial variability of recharge rates calculated from Cl- mass balance in groundwater was related to general trends in distribution of soils. Complex hydrological processes such as fluctuating water tables and bypass flow have been suggested as factors influencing the Cl- profiles (Allison et al., 1994; Phillips, 1994). Steep concentration gradients of Cl- depth profiles reportedly reflecting boundaries between different stratigraphic units were recognized, but not directly quantified (Johnston, 1987). In addition, Hendry et al. (2000) identified distinct Cl- signatures in different geologic units within a saturated aquitard, but no studies have focused on relating the form of Cl- depth profiles to stratigraphic sequences in unsaturated material.
Regional concerns regarding sustainability of groundwater in the Palouse Basin have fueled a need to identify hydrologic processes in soils and deep vadose zones. The overall objective of this study was to use Cl- depth profiles to interpret relationships between soil stratigraphic conditions and hillslope hydrologic processes in a Fragixeralf landscape of northern Idaho. Specifically, we wanted to compare more traditional techniques for characterization of deep sediment stratigraphy, such as morphology, physical properties, and mineralogy, with Cl- depth profiles.
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MATERIALS AND METHODS
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Environmental Setting
The Palouse region is located on the eastern part of the Columbia Plateau and is characterized as a thick sequence of loess deposits that occupy 20 000 km2 in northwestern USA (Fig. 1) (Busacca, 1989; Baker et al., 1991). The study area consists of a 1.7-ha catchment located near the eastern boundary of the Palouse region in northern Idaho (Fig. 1). The landscape of rolling loessial hills represents eolian deposition throughout the Quaternary Period (Busacca, 1989). Loess thickness ranges from 20 m in uplands to <5 m in valleys. Soils are classified as Fragixeralfs (Soil Survey Staff, 1999).
In the western Palouse region, multiple episodes of soil development are interrupted by periods of rapid loess deposition resulting in a vertical sequence of buried ancient soils (paleosols) interstratified with relatively unaltered loess (Baker et al., 1991). Because the eastern Palouse receives greater mean annual precipitation and is more distal to the loess source, superposition of paleosols is common, and overprinting of soil-forming intervals results in a continuous sequence of hydraulically restrictive paleofragipans and highly leached (E) horizons (Busacca, 1989; King, 2000).
Mean annual precipitation is 800 mm at the research site and falls predominantly during winter months. As a result, perched water tables form in November or December and persist for up to seven months, disappearing in May or June in response to increased evapotranspiration (McDaniel et al., 2001). Observations of perched water table dynamics indicate that the uppermost fragipan extends across the catchment (McDaniel et al., 2001).
Experimental Methods
Using a truck-mounted hollow-stem auger, we sampled three hillslope positions within the study area. Three, 4-cm-diam. cores extending into weathered bedrock were saved for tracer and detailed stratigraphic analysis. These three sampling sites include summit, side slope, and valley positions. Cores ranged from 6 to 13 m in length and were sectioned into 20-cm sampling increments, sealed in plastic bags, and frozen until analyzed.
In the laboratory, cores were split longitudinally, providing samples for both stratigraphic description and for extraction of pore water Cl-. Gravimetric water content was determined for all samples prior to Cl- extraction. After samples were oven-dried, 20 g of sediment were mixed with 20 mL of triple-distilled water and shaken overnight. Samples were centrifuged for 30 min, and supernatant solution was filtered through a 0.22-µm Millipore filter (Millipore Corp., Bedford, MA). Chloride concentration was measured in triplicate using a Dionex ion chromatograph (Dionex Corp., Sunnyvale, CA), and pore water Cl- concentration was calculated using gravimetric water content (Murphy et al., 1996).
Since the degree of soil development appears to control vadose zone hydrology in the eastern Palouse, we attempted to identify pedostratigraphic units across the catchment with measurements of clay, secondary Mn and Fe oxides, and soil strength. These measurements were used with morphologic descriptions of strata to further differentiate paleosol boundaries. Clay content was determined using the pipette method (Gee and Bauder, 1986). The distribution of secondary Mn and Fe oxides reflects changes between reducing and oxidizing environments, and Mnd/Fed ratios are therefore a good indicator of E-Btxb horizon boundaries in the Palouse region (McDaniel et al., 1992, 2001). The Mnd/Fed ratios were measured using a citrate-bicarbonate-dithionite extracting solution (CBD) (Soil Conservation Service, 1972). Soil strength was used as a proxy for brittleness, a characteristic of fragipans, and assessed on peds using a pocket penetrometer, where deformation of the piston spring was recorded and expressed as unconfined compressive strength in kilograms per square centimeter.
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RESULTS AND DISCUSSION
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Morphology and Physical Properties of Soils
Detailed soil descriptions were made for each sampling location according to techniques described by the Soil Survey Staff (1993). Each profile is dominated by silt loam or silty clay loam textures and contains E horizons that overlie an extremely dense paleosol fragipan (Btxb) (Table 1). The lower boundary of the uppermost fragipan at the summit position contains filaments of calcium carbonate that coat relict Mn concentrations. The dendritic Mn concentrations suggest changes in redox status associated with seasonally saturated conditions, probably when the zone was nearer to land surface. The calcium carbonate coatings illustrate how superposition of paleosols in an aggrading landscape changes the hydrology of a horizon by effectively decreasing percolation (Table 1).
Side slopes, which receive more effective precipitation from redistribution of snow, display evidence of greater leaching with a thicker E horizon and a lack of calcium carbonate. Since valley soils receive the greatest amount of effective precipitation profiles display clay-rich horizons overlying the fragipan. Depth to redoximorphic features is <30 cm. Moreover, horizons display reduced matrices, indicating prolonged saturation.
Buried fragipans are dense, brittle, and continuous across the hillslope. In uplands, fragipans have extremely hard dry consistence with very coarse prismatic structure that changes to very thick platy or a massive condition with depth (Table 1). Redox concentrations and Mn coatings are common in ped interiors. Redox depletions are present along large cracks separating structural units and in overlying E horizons (Table 1). In valley positions, buried fragipans grade into slightly less dense, brittle horizons with very hard dry consistence and medium to coarse prismatic or thick platy structure. Redox concentrations and Mn coatings are common in ped interiors and redox depletions occupy more than 50% of the horizon matrix (Table 1).
Morphology and Physical Properties of Deep Loess
We recognized several major zones of paleosol fragipan development (Btxb horizons) in the upper 6 m of loess (Fig. 2–4). Each zone of fragipan development represents multiple episodes of soil formation spanning thousands of years in which profiles are superimposed into single units (Busacca, 1989). In many cases, fragipan horizons are interstratified with subtle leached horizons (Eb horizons) that reflect periods of saturation when the horizon was near the land surface. Because superimposed profiles are sometimes difficult to recognize in core samples, we compared morphologic observations with measurements of soil strength, clay content, and secondary Mn and Fe oxides to distinguish between buried Eb horizons and fragipans (Fig. 2–4).
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Fig. 2. Pedostratigraphic descriptions and depth profiles of chloride,  m, clay, and Mnd/Fed at the summit position. Shading represents zones of soil strength greater than 3.0 kg cm-2.
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Fig. 4. Pedostratigraphic descriptions and depth profiles of chloride, m, clay, and Mnd/Fed at the valley position. Shading represents zones of soil strength greater than 3.0 kg cm-2.
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Fig. 3. Pedostratigraphic descriptions and depth profiles of chloride, m, clay, and Mnd/Fed at the side slope position. Shading represents zones of soil strength greater than 3.0 kg cm-2.
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Physical and chemical properties differ sharply between Eb and paleofragipan horizons. In each profile, Eb horizons have lower clay content, lower Mnd/Fed ratios, and decreased soil strength. For example, all fragipans have 6 to 10% more clay (absolute) than Eb horizons and have silty clay loam textures (Table 1; Fig. 2–4). Striking differences in soil strength exist where average strength increases from 1.1 kg cm-2 in Eb horizons to 3.1 kg cm-2 in fragipans (Fig. 2–4). Horizon boundaries were identified by CBD-extractable Fe and Mn oxides, where Mnd/Fed ratios at boundaries of overprinted fragipans display a twofold change (Fig. 2–4). Superposition and overprinting of paleosols complicates interpretation of clay and Mnd/Fed depth profiles, and although peaks in clay and Mnd/Fed ratios occur at slightly different depths, they occur within the same horizon (Fig. 2–4). These data illustrate that physical and chemical properties can be used to distinguish pedostratigraphic units in loess sequences. Moreover, properties of paleosol fragipans and Eb horizons appear to contrast to the extent that hydrologic barriers are identifiable.
Chloride Depth Profiles and Stratigraphic Relationships
The Cl- depth profile at the summit position is influenced by soil stratigraphy, where boundaries between Eb and paleofragipan horizons (Btxb) are associated with relatively large changes in Cl- concentration. Peaks in Cl- concentration correspond with three buried fragipans that display changes in strength and clay content. Furthermore, their boundaries are identified by increases in Mnd/Fed. Upper boundaries of each fragipan located at 1.00, 3.00, and 5.75 m reflect increases in Cl- concentration of at least 20 mg L-1 compared with interstratified Eb horizons (Fig. 2).
The side slope position illustrates similar trends, where peaks in Cl- concentration reflect boundaries between Eb horizons and four buried fragipans located at 1.00, 2.00, 3.00, and 6.50 m (Fig. 3). In the upper two fragipans, Cl- concentration is 5 to 10 mg L-1 greater than in the interstratified Eb horizons, while the upper boundaries of the third and fourth fragipans display greater than a 20 mg L-1 increase. Maximum Cl- concentration is associated with the buried fragipans, and the Cl- depth profile illustrates gradual changes in Cl- concentration. A greater degree of profile overprinting is observed at this landscape position. Hence, boundaries between Eb and fragipan horizons are more diffuse, resulting in depth profiles with smoother lines compared with the summit (Fig. 2 and 3).
In the valley, morphology of the Cl- depth profile reflects changes in strata, but is also influenced by multiple perched water tables. An abrupt increase in Cl- concentration from 5 to 90 mg L-1 is associated with the restrictive layer 1.00 m below the soil surface (Fig. 4). A similar Cl- concentration peak reflects a stratigraphic boundary at around 3 m. In addition to the near-surface perched water table at 0.5 m, as many as three zones of perched water are present in lowland positions located at 1.25, 2.50, and 8.00 m (Boll et al., 2001).
Chloride depth profiles can be used to identify hydrostratigraphic units within the hillslope. At each sampling location, E horizons that overlie the uppermost fragipan display low Cl- concentrations because this is where water contents are highest (Fig. 2–4). Water contents decrease below the uppermost fragipan in the upland positions, and high Cl- concentrations are found in multiple paleosols (Fig. 2 and 3). Chloride concentrations appear to be diluted or leached by perched water in the valley (Fig. 4). Additionally, a thick Eb horizon characterized by low Cl- concentration, is present between 2 and 3 m at the summit and valley, and 5 to 6 m at the side slope position (Fig. 2–4).
To assess relationships between individual stratigraphic measurements and Cl- depth profiles, we correlated the vertical change in Cl- concentration with the vertical change in clay, strength, and Mnd/Fed (Table 2). In the upland positions, correlations of change in Cl- concentration with clay and strength are significant at the 0.01 and 0.05 levels. Change in Cl- concentration was not significantly correlated with changes in Mnd/Fed ratios (Table 2), probably because these ratios tend to change across horizon boundaries and do not necessarily remain constant within a horizon. The vertical distribution of secondary CBD-extractable Mn and Fe has been affected by saturated flow that may have only occurred when the unit was near the land surface (O'Geen et al., 2001).
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Table 2. Correlation of the vertical change in measured stratigraphic variables with pore-water Cl- concentrations in a Palouse hillslope.
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Changes in Cl- concentration were not significantly related to stratigraphic measurements in the valley position (Table 2). This lack of correlation is due to the dynamic hydrology of this position where groundwater mounds extend into the lowermost fragipan and multiple perched water tables occur (O'Geen et al., 2001). In fact, peaks in Cl- concentration within the valley core may reflect the transport of near-surface perched water from surrounding uplands, which perhaps explains why Cl- concentration is significantly correlated to water content in the upper 4 m of the valley position (Table 2).
Mechanisms of Recharge
Multiple buried soils with contrasting hydraulic properties within deep strata control the path of percolating water and therefore influence morphology of Cl- depth profiles (Fig. 2–4). In the eastern Palouse region, rooting depth is limited to horizons that overlie the fragipan. Cracks in the fragipan extend only into the upper portions of the pan. On the basis of our understanding of the pedologic and depositional environment, we interpret zones of Cl--rich pore water that are below the active rooting depth to represent "old" water that is trapped in deep sediment. Evidence from field observations and deep-well monitoring indicates that perched water tables do not exist below the uppermost fragipan in uplands (Boll et al., 2001). Moreover, secondary calcium carbonate coatings present below the uppermost fragipan indicate that little vertical flushing of water through the soil matrix has occurred at the summit position. Since a majority of saturated flow is redistributed downslope (Brooks et al., 2000), vertical movement of Cl-rich water must occur slowly as unsaturated flow, and this water may reside in buried paleosol fragipans for extended periods of time.
Several studies have associated large increases in Cl- concentration with long pore water residence times at the lower boundary of the root zone (Stone, 1992; Allison et al., 1994; Tyler and Walker, 1994). In our case, we postulate that peaks in Cl- concentration are relicts from times when these units were near the land surface. Chloride deposition has remained constant over the last 15 000 yr in the Palouse region, and weathering of primary minerals in Palouse loess does not release significant Cl- (Murphy et al., 1996). Therefore, duration of landscape stability and degree of evapotranspiration govern the extent of Cl- accumulation within a horizon when the unit represented the land surface. In fact, buried paleosols in the Palouse are believed to have formed under conditions drier than present during periods of extended landscape stability (Kemp et al., 1998).
Similar processes probably influence Cl- accumulation in valley positions, but multiple perched water tables complicate interpretation of Cl- depth profiles. Monitoring of deep wells indicates that the groundwater table extends to the basal fragipan around 4 m below the soil surface. Piezometers installed below the uppermost fragipan indicate confined perched water tables around 1.5 and 2.5 m below the surface. Abrupt decreases in Cl- concentration at 1.2 and 3.0 m may reflect boundaries of the groundwater table and confined perched water tables (Fig. 4). There is insufficient evidence, however, to explain the complex hydraulic processes responsible for these observations.
Recharge Estimates
Because we believe old water resides in buried paleosols, changes in recharge rates associated with climatic swings of the Holocene or late Pleistocene may be preserved. In order to measure recharge rates through time we plotted cumulative water as a function of Cl inventory (Fig. 5). Slopes of straight-line segments represent reciprocals of Cl- concentration for a particular depth interval (Selker et al., 1999). Straight-line segments reflect episodes of relatively constant precipitation and Cl- deposition, thus recharge rates can be calculated for these particular depth intervals (Murphy et al., 1996; Selker et al., 1999).
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Fig. 5. Recharge rates calculated from water inventory vs. chloride inventory. Linear-line segments represent constant chloride accumulation.
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Plots of cumulative Cl- vs. cumulative water at the summit and side slope positions illustrate a characteristic stair-step appearance that corresponds to stratigraphic changes (Fig. 5). Recharge estimates are highest (3 and 6 mm yr-1) in a well-developed buried E horizon located 2 m below the surface at the summit and 4.75 m at the side slope. In addition, the lowest recharge rate (1 mm yr-1) is preserved in buried fragipans (Fig. 5).
Estimates of recharge in uplands probably reflect maximum rates because we cannot assess the dilution effects associated with the formation and lateral redistribution of perched water. In upland positions, Cl- is concentrated in the root zone during the growing season, but is probably redistributed as lateral flow in perched water tables to valley positions in the winter months. Furthermore, the presence of pedogenic CaCO3 in the summit position located at 2 m having a radiocarbon age of 9220 yr BP indicates even longer pore water residence times. Discrepancies in pore water residence time and pedogenic carbonate dates further suggest that Cl--bearing strata represent Cl- accumulation at discrete intervals through time, and the profile as whole does not reflect a continuous chronological sequence of Cl- accumulation.
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CONCLUSIONS
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Results of this study illustrate how Cl- concentration varies with depth and pedostratigraphic boundaries in the vadose zone. Abrupt changes in Cl- concentration occur across boundaries between buried Eb and paleofragipan horizons because these boundaries impose hydrostratigraphic barriers that restrict vertical flow. Moreover, the abrupt changes in Cl- concentration at upland sites indicate that water is trapped in buried, hydraulically restrictive horizons. The partially diluted signature of the valley site illustrates hydraulically active conditions. Understanding variability of the regolith is important to assess hydrologic processes in the vadose zone. Chloride depth profiles can be used as a relatively simple technique to characterize hydrostratigraphic units in deep regolith. We know of no other single technique that can be so easily used to characterize hydrostratigraphy within the vadose zone.
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ACKNOWLEDGMENTS
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The authors gratefully acknowledge the financial support of the Idaho Water Resources Research Institute, University of Idaho, United States Geological Survey-Water Resources Research Program and Idaho National Environmental Engineering Laboratory.
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REFERENCES
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ABSTRACT
INTRODUCTION
