Journal of Environmental Quality 31:1453-1461 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
Spectral Properties of Salt Crusts Formed on Saline Soils
F. M. Howari*,a,
P. C. Goodellb and
S. Miyamotoc
a University of Texas at El Paso, Center for Environmental Resource Management, PMB 113, El Paso, TX 79968
b University of Texas at El Paso, Dep. of Geological Sci., El Paso, TX 79968
c Texas A&M Agricultural Research Center at El Paso, 1380 A&M Circle, El Paso, TX 79907
* Corresponding author (f-howari{at}tamu.edu)
Received for publication January 25, 2001.
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ABSTRACT
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Rapid identification and large-scale mapping of salt-affected lands will help improve salinity management in watersheds and ecosystems. This study was conducted to examine spectral reflectance of soils treated with saline solutions containing NaCl, NaHCO3, Na2SO4, and CaSO4·2H2O. Spectral reflectance was measured upon salt crusts formed on two soils (Torrifluvents) subirrigated with saline solutions of 500, 1000, and 1500 mmolc L-1 with a spectroradiometer in the visible and near-infrared region (400–2500 nm). Spectral analyses revealed that samples of gypsum crusts have diagnostic absorption features near 1023, 1225, 1457, 1757, 1800, and 2336 nm, whereas halite crusts have diagnostic absorption features near 1442, 1851, 1958, and 2226 nm. Several broad absorption features were seen in the spectra of the crusts of sodium bicarbonate at 1243, 1498, 1790, 1988, and 2356 nm. The spectrum of soils treated with sodium sulfate exhibited absorption features at 1243, 1472, 1677, 1774, 1851, 1968, and 2245 nm. Crystal size or salt concentrations did not affect the positions of the absorption bands of the salt crusts. However, reflectance increased as particle sizes decreased or with increasing presence of salt crusts. Spectroscopy can be used under certain conditions to identify the presence of primary diagnostic spectral features of gypsum, nahcolite, thenardite, and halite crusts.
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INTRODUCTION
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HIGH SALINITY ADVERSELY affects biological functions in ecosystems and causes degradation of soil and water resources (Tanji, 1990, 1996; Sumner and Naidu, 1998; Sumner, 2000). Estimates indicate that salt-affected soils occupy 1 billion ha, or 7.7% of the earth's surface (Massoud, 1990). Irrigated lands are subject to salinization, and according to the International Soil Reference and Information Center (1990), as much as 1.7 million ha per year worldwide are affected. Degradation of riparian zones due to salinization is also emerging as an environmental issue (Busch and Smith, 1993; Briggs, 1996; Glenn et al., 1998). To manage salts in these environments, it is desirable to have a rapid and cost-effective method of identifying and mapping salt-affected lands on a large field scale.
Saline soils usually have high concentrations of Na+, Mg+2, Ca+2, Cl-, and/or SO-24 in soil solutions. These ions, upon evaporation, form salt crusts and efflorescences on the surface of the soil. Soil scientists identify these soils by several methods, such as aerial photography, electrical resistivity, and electromagnetic methods (Everitt et al., 1988; Rhoades and Miyamoto, 1990; Mougenot et al., 1993). Reflectance spectroscopy of salt-affected lands uses the physics of the atomic and molecular vibration processes, which are important in creating the absorption features in mineral and soil spectra (Beck et al., 1976; Condit, 1970; Obkuhov and Orlov, 1964). It is the vibrational processes that are important for investigating salts and evaporate minerals (Everitt et al., 1988; Mougenot et al., 1993; Csillag et al., 1993; Metternicht and Zink, 1997; Ben-Dor et al., 1999). Vibrational absorption features are caused by stretching and bending of bonds between anion groups of molecular species such as sulfate, carbonate, and other anion groups (Hunt and Salisbury, 1970; Hunt, 1982; Crowley, 1991; Csillag et al., 1993; Drake, 1995; Clark, 1999). Many of the spectral features of evaporate minerals can also be explained by vibrational absorption due to water molecules chemically bound as part of the crystal structure, as fluid inclusions, or adsorbed on these minerals. Middle infrared bands with water and OH- absorption features allow distinction of soil surfaces affected by Cl- and SO-24 salts (Mulders, 1987; Everitt et al., 1988; Howari et al., 2000a,b,c).
Everitt et al. (1988) found that well-developed saline efflorescences and crusts are always associated with high reflectance in the visible and near-infrared spectra. Their study indicated that crusted saline soil surfaces are generally smoother than nonsaline surfaces, giving higher reflectivity. Sulfate and chloride salts in dry soils can be distinguished with middle infrared wavelengths (Metternicht and Zink, 1997; Mougenot et al., 1993; Howari et al., 2000a, b,c). Metternicht and Zink (1997) found that absorption bands of soil surfaces containing CaSO4·2H2O were distinguishable from a halite absorption band. In their study, nonsaline and slightly salt-affected areas were not recognizable from each other. Howari et al. (2000a)(b,c) used a portable spectroradiometer to examine spectral properties of evaporites formed in Petri dishes, and found that the prominent peaks of the examined salts did not overlap (Fig. 1)
. However, it is yet to be proved if laboratory-determined spectral properties of salt crystals and evaporites apply directly to salt crusts formed on soil surfaces in the natural world. This work was conducted to determine spectral characteristics of salt crusts formed on soils. Spectral properties of crusts having mixtures of multiple salt species were also examined.
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MATERIALS AND METHODS
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Two samples, representing Harkey silty clay loam (coarse-silty, mixed, superactive, calcareous, thermic Typic Torrifluvent) and Vinton sandy loam (sandy, mixed, thermic Typic Torrifluvent), were collected from the flood plain of the middle Rio Grande, 3.2 km south of Canutillo, Texas. Harkey silty clay loam has average grain sizes that range from 7.8 x 10-3 to 1.95 x 10-3 mm, whereas Vinton sandy loam has predominant grain sizes that range from 0.25 to 0.125 mm. Additional properties are given in USDA (1971) and Miyamoto (2000). Soil samples were crushed with a rubber hammer and air-dried. Harkey soil samples were passed through a 1-mm screen; Vinton samples through a 2-mm screen.
Saline solutions were prepared by adding analytical-grade salts (CaSO4·2H2O, NaCl, NaHCO3, CaCO3, and Na2SO4) to distilled water at 500, 1000, and 1500 mmolc L-1. In addition, nine different saline solutions were prepared by mixing the 500 mmolc L-1 solution of NaCl and CaSO4·2H2O; NaHCO3 and CaCO3; and CaSO4·2H2O and Na2SO4 at molar ratios of 3:1, 1:1, and 1:3. The solutions consisting of CaCO3 and CaSO4·2H2O at those concentrations are actually suspensions, as the target concentrations exceeded the respective solubility limits.
Soil Conditioning
The background level of the salt content of the soil sample was standardized by leaching the soil samples with a 500 mmolc L-1 saline solution. The soil samples (100 g each) were placed in a funnel lined with filter paper and leached with 1000 mL of the same solutions. The samples were air-dried, crushed again, and sieved through a 1-mm screen for the Harkey soil and a 2-mm screen for the Vinton soil. The soil samples were then placed in Petri dishes (10 cm in diameter) for additional treatment. Original fractions of the soil samples (100 g) without conditioning were used as reference or background samples.
Salt Crust Preparation and Observation
Subirrigation was performed by placing the soil samples on a coil of packed microtubes (2 mm in diameter with small pores of 65 µm in diameter) placed on Petri dishes (10 cm in diameter). A saline solution was fed through the microtubes. The soil samples were approximately 10 mm in thickness, and were irrigated by 150 mL of the prepared pure and mixed saline solutions, individually. The saline water was applied from the bottom to the top of the soil surface. The water was evaporated from the top of the soil at 40°C to form salt crusts with average thicknesses ranging from 0.3 to 1.7 mm on top of the soils. Duplicate samples were prepared for all cases. The prepared samples were studied by optical microscope for crystal identification and for optical properties (Ehlers, 1987; Nesse, 1991; Shelley, 1985).
Reflectance Measurements
Before initiating spectral tests, a GER (Millbroke, NY) 3700 spectroradiometer was calibrated by mounting it on a stand to measure the reflectance of its special calibration board. The results were compared with standard curves. The reference and target samples were placed to completely fill the field of view of the instrument. The optical sight of the instrument was used to align targets to the field of view at extended distances. Before taking measurements, the alignment of this sight was verified by two individuals. At the time of measurement, the operator wore nonreflective clothing, and the measurements were taken a distance away from any reflective objects. Replicate measurements of each sample were taken. Two different people collected and entered the data, including the reference data, into JMP/SAS (SAS Institute, 1999) and Excel (Microsoft, 2000) software programs. The spectral data from the experiments were studied by computerized peak picking and analysis techniques with Spec-view (Jet Propulsion Laboratory and United States Geological Survey, 1992) and Excel software packages, and based on visual spectral analogy with the salt and soil reflectance curve classifications of Condit (1970), Stoner and Baumgardner (1981), Crowley (1991), Csillag et al. (1993), Drake (1995), and Clark (1999).
The average recorded standard deviation and standard error of the spectral readings of the same sample were 8.4 x 10-3 and 1.84 x 10-3 nm, respectively. For the replicate samples, the average recorded standard deviation and standard error were 0.319 and 0.0438 nm, respectively.
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RESULTS
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Mineralogical Observations
After treating Harkey and Vinton soils with the NaCl solutions, a crust with a glassy transparent appearance developed on the soil surfaces. The salt crusts developed on Harkey silty clay loam had prevailing grain sizes of 0.0098 to 0.00495 mm, whereas salt crystals on Vinton soils had grain sizes of 0.55 to 0.325 mm. The crusts formed on soils treated with 500 mmolc L-1 NaCl solution showed under the microscope to be transparent to translucent halite crystals with vitreous luster. The crystals were white to colorless, cubic crystals (Fig. 2)
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The soils treated with CaSO4·2H2O solutions formed white, transparent to translucent, gypsum crusts. The crystals examined had prismatic, curved, or twisting monoclinic shapes, with vitreous luster, and also appeared as earthy, foliated, or granular masses. The crystals had good cleavage in one direction. The samples were white to colorless with some elongated fibrous crystals (Fig. 2).
The soils treated with sodium bicarbonate formed brown to gray monoclinic crystals of nahcolite that had vitreous, transparent to translucent crystals with perfect cleavage. The soils treated with sodium sulfate typically formed gray to brownish white, orthorhombic crystals of thenardite. The soils subirrigated with CaCO3 solution formed white rhombohedral calcite crystals, with light colors. The crusts were very fine aggregates of crystals (Fig. 2), and exhibited significant double refraction.
Spectral Reflectance of Crusts with Single Salts
The reflectivity of salt crusts formed on the soils increased with increasing concentrations of saline solutions used. Examples are shown in Fig. 3
for the crust consisting of NaHCO3. However, the concentrations of the solutions did not affect the absorption features, thus the data from the solution concentrations of 500 mmolc L-1 are reported here. The absorption features observed are summarized in Table 1.
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Table 1. The location of the absorption features for Vinton and Harkey soils treated with gypsum (CaSo4·2H2O), halite (NaCl), calcite (CaCO3), nahcolite (NaHCO3), thenardite (Na2SO4), and their mixtures.
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Halite (NaCl)
The spectra of Harkey and Vinton soils treated with the NaCl solution and the spectra of the untreated soils are shown in Fig. 4
. The halite spectrum is similar to the spectral pattern of the untreated soils, as they are parallel to each other. However, halite-affected soils imparted higher reflectivity than the untreated soils. The spectral pattern of the soil treated with NaCl solution is similar to the spectral pattern of pure halite (Fig. 1 and 4). Pure halite itself cannot induce absorption bands in the visible and near thermal infrared (Hunt et al., 1971a, b; Mougenot et al., 1993). Water bands (1400, 1900, and 2250 nm) were observed in the spectra of halite crusts (Fig. 4) due to moisture and fluid inclusions, and those are the dominant absorption features of halite. Similar results were found in Hunt et al. (1971a), Mulders (1987), and Mougenot et al. (1993).
Gypsum (CaSO4·2H2O)
The spectrum of the Harkey soils treated with the CaSO4·2H2O solution had strong absorption features at 1023, 1225, 1779, and 2336 nm, and even stronger ones at 1457, 1865, and 1988 nm. The Vinton soil samples with gypsum crusts had several portions of the spectrum that exhibit distinct absorption features. These features were at 1204, 1234, 1457, 1768, 1865, 1978, 2103, and 2236 nm (Fig. 4). Those absorption features were consistent with those reported by Hunt et al. (1971a), Mulders (1987), and Mougenot et al. (1993). Gypsum-affected soils imparted higher reflectivity than the nonsaline soils (the background soils), and this is consistent with earlier findings by Stoner and Baumgardner (1981). The spectra of the soils treated with CaSO4·2H2O solutions were similar to the spectra of gypsum (Fig. 1 and 4). In gypsum the overtones or combination tones from fundamental vibrations of the water molecules produced a series of bands affecting the infrared spectrum between 1000 and 2500 nm (e.g., 1464, 1550, 1750, 1978, and 2300 nm), and can be detected in the spectra of gypsum crusts (Fig. 4). However, the most dominant absorption features occurred at 1464, 1750, and 1978 nm; in addition, two small peaks maintain consistency in appearance at 1755 and 1865 nm. These findings are in agreement with the earlier findings of Hunt et al. (1971a)(b), Hunt (1982), Clark (1999), Mulders (1987), and Mougenot et al. (1993).
Nahcolite (NaHCO3)
The spectra of soils treated with NaHCO3 solutions and the spectra of the untreated soils are shown in Fig. 4. The spectra of the Vinton soils treated with NaHCO3 solution had absorption features at 1199, 1334, 1472, 1997, and 2170 nm (Fig. 4; Table 1). The spectra of the Harkey soils treated with NaHCO3 solution had absorption features at 1023, 1334, 1472, and 1997 nm (Table 1). However, the most dominant absorption features of nahcolite crusts occurred at 1334, 1472, and 1997 nm. The reflectance of nahcolite sharply decreased in the wavelength range of 1199 to 2170 nm. All of these absorption features are presumably related to vibrational overtones or combination tones of the water molecules and of the HCO-23 ion. However, the broad nature of the absorption features of the soils treated with sodium bicarbonate could be due to the network of symmetrical H-bonds between carbonate ions. The maximum reflectance of the Harkey soils treated with sodium bicarbonate ranged from 35 to 42%, whereas the maximum reflectance of the Vinton soils treated with sodium bicarbonate ranged from 32 to 46% (Fig. 4a,b). These values were higher than reflectance values of the untreated soils.
Thenardite (Na2SO4)
The Harkey soils treated with Na2SO4 solutions had absorption features at 1225, 1450, 1988, and 2075 nm (Fig. 4; Table 1), whereas the Vinton soils treated with Na2SO4 solution had absorption features at 1234, 1486, 1800, 1988, and 2103 nm (Fig. 4). The absorption features in the soils treated with Na2SO4 are due to water molecules of fluid inclusions. The spectral pattern of thenardite crusts is similar to the spectra of pure thenardite (Fig. 1 and 4). As the concentration of Na2SO4 increases in the treatment solution, so did the reflectivity of the thenardite crust evolved from that solution (Fig. 3).
Spectra of Crusts with Multiple Salts
Halite and Gypsum (NaCl and CaSO4·2H2O)
The spectra of salt crusts consisting of halite and gypsum are shown in Fig. 5
. The most noticeable features in this system are located around 1450, 1750, and 1950 nm. The absorption feature at 1865 nm was useful to differentiate between Vinton soils treated with solutions of CaSO4·2H2O (100%) and solutions of NaCl (100%), and Harkey soils treated with solutions of CaSO4·2H2O (100%) and CaSO4·2H2O (75%) and NaCl (25%) (Table 1). Between 400 and 700 nm it is difficult to differentiate between the spectral patterns of the treated soils with CaSO4·2H2O–NaCl solutions. The differences in the reflectance values of the spectra of the Harkey and Vinton soils treated with mixed solutions of NaCl and CaSO4·2H2O were most pronounced between 700 and 1700 nm and 750 and 2000 nm. In this latter range the most frequent absorption features occurred at 1464, 1750, and 1890 nm (Fig. 5; Table 1). In the 500- to 2500-nm range, most of the reflectance values of Vinton soils treated with the solutions of 25% CaSO4·2H2O and 75% NaCl lie outside those of the end members (Fig. 5).
Thenardite and Gypsum (Na2SO4 and CaSO4·2H2O)
The spectra of the crusts consisting of thenardite and gypsum were dominated by the spectra of thenardite (Fig. 6)
. Reflectance from thenardite increased with gypsum additions up to 25%, and it decreased with additional applications. The two characteristic peaks of gypsum did not appear even at gypsum molar fractions as high as 75%. The dominant absorption features of salt crusts consisting of Na2SO4 and CaSO4·2H2O occurred at 1236, 1457 ± 37, 1997, and 2075 nm for Harkey soil, and at 1476 ± 10, 1800, 1988, and 2017 nm for Vinton soil (Table 1).
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Fig. 6. Spectra of (a) Harkey and (b) Vinton soils treated with Na2SO4, CaSO4·2H2O, and their mixtures.
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Nahcolite and Calcite (NaHCO3 and CaCO3)
The spectra of crusts consisting of nahcolite and calcite were dominated by the spectral contribution of nahcolite. The primary differences of the absorption features of this system were observed at 1033 and 1997 nm, but were minimal at 1472 nm (Fig. 7
; Table 1). The most frequent absorption features were recorded at 1033, 1334, 1472, 1677, 1977, and 2170 nm for Harkey soil, and at 1199, 1334 ± 2, 1479, 1677, 1988, and 2170 nm for Vinton soil. A consistent, sharp decrease in the reflectance value was observed from 1199 to 2170 nm.
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DISCUSSION
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Reflectivity of the treated Harkey soil was higher than the reflectivity of treated Vinton soil (Fig. 4). Harkey soil has a finer texture than Vinton soil; therefore, Harkey soils tend to have finer salt crystals and brighter surfaces than the relatively coarse-textured Vinton soil. Smoother surfaces of Harkey soil may cause less shadowing by particles on the surface. Reports state that fine-grained crystals reflect more light than coarser grained (Vincent, 1997; Clark, 1999; Howari, et al., 2000a,b,c).
Spectral identification of different types of salts present in the crust is easiest if the crust consists of single salts. Crusts consisting of nahcolite (NaHCO3), for example, can be identified by the sharp decline in reflectance as the wavelength increases from 1199 to 2170 mm (Fig. 4 and 7). The slope of the spectrum of the nahcolite crust can be considered diagnostic, because no other spectra investigated here exhibit such a pattern. Crusts consisting of gypsum (CaSO4·2H2O) can be identified by the three dominant spectral features, which appear at 1464, 1750, and 1978 mm. However, some of these absorption features also appear in thenardite (Na2SO4), thus the distinction between gypsum and thenardite may be difficult unless other dominant spectral features can be identified. It would not be easy to identify a crust consisting of halite, as its reflectance had no distinguishable features. Calcite (CaCO3) can be identified by the presence of absorption features at 1052, 1479, and 2100 nm. The absorption features around 900 nm could be due to iron oxide (Clark, 1999). The following order of ability to distinguish the soils treated with salts is suggested based on the shapes of the spectra patterns and dominant absorption features (Fig. 1, 3, 4, 5, 6, and 7; Table 1): nahcolite > gypsum 
thenardite > calcite > halite. The shape and depth of the absorption features of the soils treated with salts are different from those soils without treatment. Features increase as the soil is treated with salts, as can be observed in soils treated with gypsum solutions (Fig. 4 and 5). In some cases the spectral pattern of single pure salts and crusts of the same chemical compounds might be slightly different due to the grain size effects (Fig. 4). Although any two salts may well have one or more absorption features in similar positions, the slight shift in wavelengths, spacing size, and shape of these features mean that their spectra can be diagnostic at least in terms of other minerals found in soil and evaporitic environments, and can be an advantage for the identification of salt spectra.
Spectral identification of salt crusts consisting of multiple salts cannot be made with an assumption that salt crusts are a homogenous substance. In the case of the saline solutions consisting of NaHCO3 and CaCO3 it is most likely that CaCO3 has never dissolved completely. The spectra of the multiple salt crusts in this case are similar to nahcolite, suggesting that CaCO3 precipitated first and then NaHCO3. In the case of saline solutions consisting of Na2SO4 and CaSO4·2H2O, evaporation of the solution first causes preferential precipitation of CaSO4·2H2O. The crusts are thenardite formed on the precipitated gypsum, thus masking out the gypsum spectra. The crusts formed from the saline solutions consisting of NaCl and CaSO4·2H2O contained gypsum crystals, thus presenting the spectral pattern of gypsum. When the CaSO4·2H2O content is low (e.g., lower than 25%), the surface of the crusts consisted of halite developed on gypsum. In this case, spectral contribution of halite eclipses the spectral features of gypsum.
In Harkey and Vinton soils treated with multiple salt solutions of nahcolite and calcite (NaHCO3–CaCO3), the diagnostic spectral slope of nahcolite (between 1119 and 2170 nm) and the dominant spectral features of this mineral (e.g., absorption features at 1334, 1472, and 1997 nm) appear as dominant diagnostic features (Fig. 7). Evaporation of saline solution of multiple salts of NaHCO3 and CaCO3 first causes calcite precipitation (solubility of 0.5 meq L-1). This early phase will precipitate, and then nahcolite (solubility of 1642 meq L-1) will precipitate on its surface. This explains the dominant spectral contribution (slope) of nahcolite in this system.
These observations point out that the spectra of salt crusts are likely to reflect the salt species that formed last on their surface. If these crusts were formed upon evaporation, the surface of the crusts is most highly influenced by the evaporite minerals that had the highest solubility product and appropriate concentrations. There is a need to further evaluate spectral characteristics of salt crusts consisting of highly soluble evaporate minerals (e.g., salt crusts evolved from mixed solutions of halite and thenardite). These highly soluble evaporites can easily be leached with rain or sprinkler irrigation, thus exposing less soluble evaporites such as gypsum to the surface. It is highly probable that spectra of salt crusts change with seasons and areas, and this question has to be resolved with salt crusts existing in saline soil environments. The present research needs to be expanded to study the relationship between spectra reported here and satellite images with high spectral and spatial resolution. This trend of research could lead to better understanding of salt crust formation in agricultural and riparian areas of river systems. Such knowledge can be used for agricultural land management purposes, and would be based on cost-effective ground and airborne spectroscopic data.
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CONCLUSIONS
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Any two pure or mixed salts may well have one or more of their absorption features in a similar position, but the observed variations in the spacing, size, and shape of these features means that their spectra can be diagnostic. Salt grains are smaller on finer-grained soil, giving higher reflectivity. Soils treated with increasingly higher salinity solutions arrive at a point where the soil particles are covered with salt, and the spectra of the soil disappears. It can be concluded from the results reported in this study that the spectral differences of evaporites consisting of single salt compounds appear to be significant enough for determining the type and mineralogy of the salt crusts, at least under laboratory conditions. In multiple salt crusts, spectral features of halite increase by increasing its weight fraction; it becomes dominant when its quantity exceeds 75%. When gypsum, nahcolite, and thenardite are found as an end member in treatment solutions consisting of multiple salts, their primary diagnostic features appear in the spectra of salt crusts evolved from that solution.
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ACKNOWLEDGMENTS
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We would like to extend our thanks and appreciation to the Center for Environmental Resource Management of the University of Texas at El Paso (UTEP), the U.S. Environmental Protection Agency (Grant no. ERC-2K-R), the Pan American Center for Earth and Environmental Studies at UTEP, and Texas A&M Agricultural Research and Extension Center at El Paso for funds and technical assistance.
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ABSTRACT
INTRODUCTION
