ASCE Effectiveness of Moisture and Fixatives in Controlling Mobility of Contaminated Soil (TPD1007023)

Introduction

Mobilization of contaminated soil and dust particles due to wind forces during excavation, removal, and disposal of contaminated soils is a concern during site remediation activities. Airborne emissions and escape of contaminated particulates have been re- ported during remediation activities (U.S. DOE 2005). Sackschewsky (1993) reported that large amounts of radioactive and hazardous waste at U.S. DOE facilities have been either stored or disposed of within soil medium. This includes solid-waste burial grounds, cribs for the disposal of liquid effluents, and unintentional release of contaminants to soil media. Even waste material in underground storage tanks can be considered to be in the soil medium. During activities that require extensive earth moving, such as the retrieval of buried solid waste and debris or excavation and treatment of contaminated soil, may generate dust and wind-borne contaminant spread. The phenomena of soil erosion and dust generation have been studied by many researchers. The effect of wind forces on mobility of soil particles can be in different forms depending on the size of the particles and wind velocity. Experimental studies of soil mobility using wind tunnel have been conducted by Cornelis et al. (2004), McKenna- Neuman and Nickling (1989), and Hagen (2004) among others, but there are very few studies conducted on the behavior and movement of contaminated soils due to wind forces. Sackschewsky (1993) conducted wind tunnel experiments using contaminated soil to compare the ability of natural polysaccharides to suppress contaminated soil.

The purpose of this study was to evaluate the effectiveness of moisture addition and use of commercially available fixatives in controlling the mobility of soil particles on soil mounds when exposed to wind forces. Soil samples were mixed with plutonium powder simulant, cerium oxide powder, which has similar characteristics as of plutonium and is a nonradioactive material. Wind tunnel experiments were conducted to evaluate the effectiveness of two commercially available fixatives in controlling soil erosion. The fixatives tested include calcium chloride solution [RoadMaster (Tetra Technologies, Inc., The Woodlands, Tex.)], and a mixture of alkanes and alkylated saturated compounds solution [Durasoil (Soilworks, LLC, Chandler, Ariz.)]. An open-loop low-speed wind tunnel was used to test the effect of sustained wind speeds ranging from 10 to 30 mi per hour (mi/h) on mobility and displacement of soil and PM10 particulates.

Materials and Methods

Fixative Selection

The two commercially available fixatives tested included the 38% calcium chloride solution ( RoadMaster), and mixture of alkanes and alkylated saturated compounds solution ( Durasoil) listed in Table 1. These two fixatives were selected after an extensive literature search on commercially available fixatives that can be used for stabilization of soils and dust suppression.

Table 1. Chemical Composition of Durasoil

Particle-Size Distribution Analysis

As an initial step, about 500 lb of uncontaminated soil was obtained from the Hanford Reservation in Washington. Hanford Reservation is one of the Department of Energy sites with active cleanup projects. After the soil was mixed to prepare a uniform mixture, the particle-size distribution analysis was performed by a Bouyoucos hydrometer (Model ASTM 152 H, Spectrum Inc., New Brunswick, N.J.). The soil sample used for the experiments contained   an   average   of   96.2 ± 1.2%   sand   (2.0–0.05 mm), 3.7 ± 1.3% silt (0.05–0.002 mm), and 0% clay (<0.002 mm). It was  concluded  that  the  soil  could  be  characterized  as  mostly sandy soil (Lagos et al. 2006).

Moisture Analysis

The moisture content of the soil was determined by modifying the ASTM Standard D2216 (ASTM 2005) and was found to be 2.7 ± 0.1%. Approximately 2 g of soil samples were weighted and placed in an oven at 205 ° C for a period of 4–6 min. This was considered as the “baseline” (dry Hanford soil) case.

Particulate Matter „PM10… Measurements

PM10 concentrations were measured during the wind tunnel tests, downstream of the air flow, 1 in. from the soil mound in the wind tunnel’s test section. A real-time dust monitor (TSI 8520 Dust Track, TSI Inc., Shoreview, Minn.) was used to record the PM10 concentrations. The PM10 measurements were recorded for an average of 10 min during each test run.

Experimental Setup

The experiments were conducted in an open-loop low-speed wind tunnel (Engineering Lab Design, Model #304, Lake City, Minn.). The wind tunnel had a 1 ft X 1 ft X 2 ft test section with overall dimensions of 19.2 ft X 6.3 ft X 6.3 ft. The wind tunnel allowed for the samples to be exposed to sustained wind speeds ranging from 10 to 30 mi/h. The wind tunnel’s test section was modified to allow for placement of soil samples in a tray (3 in. X 3 in. X 1 in.). The test section of the wind tunnel had a cross-sectional area of 1 ft X 1 ft and was equipped with a pitot tube for dynamic pressure measurements. Velocity measurements were recorded in the section corresponding to the middle of the soil samples at five vertical distances from the surface of the sample (0.25, 3, 6, 8, and 10 in.). A Phantom V5.1 high speed camera (Vision Research, Stuart, Fla.) was installed in the test section for real-time flow visualization and observation of soil movement (creeping and saltation). An aerosol analyzer instrument (TSI 8520 Dust Track) was placed in the test section immediately downstream from thesoil tray to measure the airborne soil particles. Furthermore, downstream from the wind tunnel’s test section, a collection box was designed and installed to allow for the collection and measurement of amount of soil displaced by the wind forces. The tests were conducted by exposing the soil mounds (with and without fixatives) to wind speeds ranging from 10 to 30 mi/h.

Experimental Design

The experiments were designed to study the effects of varying environmental conditions on the fixative performance and the movement of soil particles. Two distinct soil matrices were de- signed and used for the wind tunnel experiments:

1. Soil samples with varying initial soil moisture content [2.7, 5, 10, 15, and 20% weight-to-weight ratio (w/w)]; and
2. Soil samples with an initial 2.7% moisture content sprayed with fixatives (i.e., calcium chloride solution and Durasoil). Each soil sample was placed in the test section of the open-loop low- speed wind tunnel and exposed to continuous free stream wind speeds from 10 to 30 mi/h. For Sample Matrix 1, soil samples with different moisture content were tested at velocities ranging from 10 to 25 mi/h. For Sample Matrix 2, soil samples at 2.7% moisture were sprayed with fixatives, and tested at wind velocities ranging from 10 to 30 mi/h. A total of five velocity measurements per sample were recorded at five different heights (0.25, 3, 6, 8, and 10 in.) from the soil surface. The soil was exposed to each velocity regime for 10 min. Replicate test runs were conducted for each test condition. For soil samples with different moisture content, a soil sample of 224 g was used and for soil samples with fixatives, 224 g of soil (soil sample at 2.7% moisture) was used before addition of fixatives. A mass balance was conducted after each test run by measuring the mass of the soil leaving the test section of the wind tunnel, the mass of soil displaced downstream of the test section, and the amount of soil remaining on the tray at the end of each run. Some mass was lost due to water evaporation during wind tunnel experiments.

Sample Preparation and Fixative Application

For the test runs, to study the effect of moisture on soil mobility, soil samples were prepared with moisture content of 2.7, 5, 10, 15, and 20 w/w. For experiments with fixative applications, pre- liminary wind tunnel experiments were conducted using vendor recommended application rates. However, no significant soil dis- placement was observed for the two fixatives at the recommended fixative application rates. Therefore, dilution ratios and the application rates for the selected fixatives were modified as presented in Table 2.

The calcium chloride solution was sprayed onto soil samples with 2.7% moisture content and the solution covered 100% of the soil surface for all application rates. For Durasoil application, the fixative solution could not be sprayed due to its high viscosity. Therefore, it was applied by pouring the fixative using a dropper for all three application rates ( 25, 50, and 100%) used in the experiment. When applied, the Durasoil covered approximately between 60 and 80% surface of the soil at all application rates.

Results and Discussion

Wind tunnel experiments were conducted using soil samples with moisture content of 2.7, 5, 10, 15, and 20% w/w and amount of soil displaced (in grams) in relation to wind velocity (mi/h) was measured. At wind speed of 10 mi/h, no soil displacement was observed regardless of the moisture content. At wind speeds  over 10 mi/h, some soil displacement was observed only for the samples with low moisture content (less than 10%). For samples with moisture content over 10%, no soil displacement was observed at wind velocities over 20 mi/h. During the test runs, PM10   levels   were   monitored.  The   highest   PM10   levels of 240.22 mg / m3 were observed for soil samples with 2.7% moisture content w/w at wind speeds of 30 mi/h (the highest wind velocity tested). The PM10 levels increased with increasing wind velocity. For the soil samples with 2.7% moisture content w/w, the average PM10 concentration was 8.7 mg / m3 at wind velocity of 15 mi/h. When the wind speed was increased from 15 to 30 mi/h, the PM10 levels increased by 96%. The increase in soil moisture was effective in controlling the PM10 levels. For ex- ample, as the soil moisture content was increased from 2.7 to 20% w/w, the PM10 levels decreased from 233.4 to 0.142 mg / m3 at the wind speed of 20 mi/h.

Table 2. Summary of Fixatives Application and Dilution Rates

aApplication volume (undiluted).

bSame application rate, varied dilution.

Experiments were conducted with samples containing 2.7% moisture w/w sprayed with predetermined amounts of calcium chloride solution. For 10% calcium chloride solution, no soil displacement was observed at all wind velocities tested. As the amount of calcium chloride decreased in the applied solution, some soil displacement was observed at lower wind speeds. The maximum soil displacement, 4 g, was measured after application of 2.5% solution of calcium chloride and at a sustained wind velocity of 30 mi/h.

For the test runs with Durasoil due to the high viscosity of the mixture, the application of the solution was difficult. The Durasoil solution covered approximately 80% of the surface of the soil sample, leaving 20% of the surface of the soil uncovered. Based on the results, Durasoil was effective in controlling soil erosion when in the wind velocity range of 10–30 mi/h. Soil displacement observations were also conducted for the Durasoil applications. The Durasoil results echo with the results of calcium chloride solution; however, the maximum amount of soil displaced at the higher velocities is restricted to only 2 g.

A significant decrease in the PM10 concentrations was observed with the increasing fixative application rates. For example, for the test samples with calcium chloride solution application,  the PM10 concentration at 30 mi/h was 3.097 mg / m3 at a fixative concentration of 2.5% and at the same velocity the PM10 concentration was reduced to 0.615 mg / m3 at a fixative concentration of 10%. The PM10 levels were reduced by 80% when the fixative concentration was increased by a factor of 4. A similar trend was observed for the samples with Durasoil application.

Conclusions

Wind tunnel experiments conducted with soil samples collected from Hanford Reservation in Washington state showed that soil mobility can be controlled by moisture and use of fixatives effectively. For the natural soil samples collected from the Hanford site with 2.7% moisture content, there was a 99.7% increase in the amount of soil displaced as the wind velocity increased from 15 to 25 mi/h. Soil moisture content played a significant role on soil mobility when exposed to varying wind speeds. There was no detectable soil loss when the moisture content of the soil was at 20% w/w and above at wind velocities less than 25 mi/h.

The two commercially available soil fixatives selected for this study showed significant control on soil mobility. For the calcium chloride solution, an average of 4.3% of the soil was lost at a  2.5% dilution rate (0.69 mL of calcium chloride) and 0.8% of the soil at 7.5% dilution rate (2.07 mL of calcium chloride) during the experiments. There may be significant cost saving by reducing the dilution ratio of the calcium chloride to 2.5% and/or 5% since there is no significant increase in extent of soil suppression by increasing the concentrations of calcium chloride from 2.5 to 38% (manufacturer’s recommended application). The application rate for the Durasoil fixative was also modified from those that were recommended by the manufacturer. For these experiments the fixative application dosages were tested at 25, 50, and 100% of vendor recommended application rate. At the 25% application rate, only an average of 2.1% of soil was lost, compared to a total average of 0.4% of soil loss at an application rate of 100%. Again, significant cost savings can be achieved by reducing the application rate of the Durasoil.

The PM10 measurements showed direct correlation with wind speed. The largest concentration (240.22 mg / m3) was measured for  the  soil  sample  with  2.7%  moisture  and  at  a  velocity  of 30 mi/h. For the same soil moisture (2.7%) at 15 mi/h wind velocity, the average PM10 concentration was only 8.72 mg / m3. This represents a 96% reduction in PM10 concentration when the velocity was decreased by 50%. The PM10 levels were inversely correlated with the soil moisture content. At soil moisture of 2.7% and  wind  velocity  of  20  mi/h,  the  PM10  concentration  was 233.39 mg / m3. When the soil moisture was increased to 20%, the   average   airborne   particulate   concentration   decreased  to 0.142 mg / m3. PM10 measurements with the two fixatives studies showed that fixatives were effective in suppressing the soil mobility and levels of airborne particles with wind forces.

References

ASTM. (2005). “Standard test method for laboratory determination of water (moisture) content of soil and rock by mass.” D2216, West Conshohocken, Pa.

Cornelis, W. M., Gabriels, D., and Hartmann, R. (2004). “Parameterization for the threshold shear velocity to initiate deflation of dry and wet sediment.” Geomorphology, 59, 43–51.

Hagen, L. J. (2004). Assessment of wind erosion parameters using wind tunnels, Throckmorton Hall, Kansas State Univ., Manhattan, Kan.

Lagos, L. E., Varona, J., Zidan, A., Gudavalli, R., and Wu, K.-S. (2006). “Preliminary experimental analysis of soil stabilizers for contamination control.” Proc., ICONE14, 14th Int. Conf. on Nuclear Engineering, Miami.

McKenna-Neuman, C., and Nickling, W. G. (1989). “A theoretical and wind tunnel investigation of the effect of capillary water on the entrainment of sediment by wind.” Can. J. Soil Sci., 69, 79–96.

Sackschewsky, M. R. (1993). Fixation of soil surfaces contamination using natural polysaccharides, Westinghouse Hanford Company, Richland, Wash.

U.S. DOE. (2005). Hanford site risk-based end state vision, Washington, D.C.