EVALUATION AND MODIFICATION OF THE DREDGING ELUTRIATE TEST

Transcription

1 EVALUATION AND MODIFICATION OF THE DREDGING ELUTRIATE TEST K. K. Salkuti 1, G. B. Gummadi 2, D. F. Hayes 3 ABSTRACT The Dredging Elutriate Test (DRET) was developed by DiGiano et.al (1995) to estimate water column contaminant concentrations resulting from sediment resuspension during dredging operations. Even though the DRET is widely used, it has not been thoroughly verified. Additionally, several studies have shown that the results obtained from the DRET can vary from field results by an order of magnitude. This paper will describe recent efforts to evaluate the DRET results and consider potential modifications to the testing procedure to improve its correlation with field results. DRET tests were conducted on sediments obtained from a fresh water Lake. These sediments contained significant concentrations of mercury and VOCs, both of which have significantly different partitioning behavior than PCBs, which were used to develop the DRET procedure. The DRET results showed contaminant concentrations one to two orders of magnitudes higher than observed during sediment removal operations. Several modifications to the DRET test were evaluated to more closely approximate dredging conditions. DRET test results are presented and discussed in detail. Water quality monitoring results are compared with DRET concentration with and without modifications to the protocol. Conclusions are drawn about the value of these test modifications. Finally, recommendations for future DRET testing are presented. Keywords: Partitioning coefficient, mechanical mixing, sediment resuspension, mixing intensity, water quality INTRODUCTION Before the 1970 s, industrial and urban wastewaters often contained toxic pollutants such as polychlorinated biphenyls (PCBs), pesticides, and metals (USEPA, 2008). These pollutants were released into lakes, rivers, and oceans which led to sediment contamination that directly or indirectly affects human and aquatic life. According to USEPA (2008), 5.5 million cubic yards of contaminated sediment have been remediated by dredging from the U.S Great Lakes Basin from 1997 to 2007 (EPA, 2008). Different operations such as dredging, adding amendments such as carbon to contaminated sediments, and in-situ capping have been developed to remediate the contaminated sediment (USEPA, 2008, Palermo et. al, 2008). Operations such as adding amendments to contaminated sediments and in-situ capping are often site-specific; amendments may be effective for only specific contaminants. Dredging operations can be used to remove sediments with any type of contaminants, but there are concerns about increased sediment resuspension and contaminant release. A wide range of sediment resuspension rates have been hypothesized. Some suggest that as much as 2 to 5 percent of the in situ sediment volume could potentially be lost due to resuspension (Chapman, 1997). The DRET was developed by the US Army Corps of Engineers to estimate water column contaminant concentrations during dredging operations. DiGiano et.al, (1995) developed the test using New Bedford Harbor sediments which are contaminated primarily with PCBs. Although DRET results have not been validated for other constituents, the test has often been used to estimate losses for a wide range of contaminants such as metals, volatile organic compounds, and semi-volatile organic compounds. This study compares water column concentrations for mercury collected during a backhoe dredging operation for sediment sampling purposes with DRET results. Mercury concentrations in the field samples were significantly lower than those measured in the DRET; DiGiano et. al (1995) found similar results for PCBs in New Bedford Harbor. Modifications to the DRET protocol were evaluated to determine if test conditions closer in line with actual dredging operations would produce results closer to the field observations. 1 Graduate Student, Department of Chemical Engineering, University of Louisiana at Lafayette, P.O. Box 42291, Lafayette, LA , kirankumarche@gmail.com 2 Graduate Student, Department of Chemical Engineering, University of Louisiana at Lafayette, P.O. Box 42291, Lafayette, LA , godsonbabu@gmail.com 3 Director, Institute for Coastal Ecology and Engineering and Professor, Department of Civil Engineering, University of Louisiana at Lafayette, P.O. Box 42291, Lafayette, LA ; Phone: 337/ , FAX: 337/ , hayes@louisiana.edu. 22

2 RELATED RESEARCH Empirical models such as the Turbidity Generation Unit method (Nakai, 1978) and computer models such as DREDGE (Hayes and Je, 2000) were developed to estimate sediment resuspension during dredging operations. The release of constituents from sediment suspensions has been studied and a series of elutriate tests and models have been developed to estimate site-specific partitioning behavior. Equilibrium and kinetic models have also been developed to estimate contaminant partitioning, fate, transport, adsorption, and desorption processes of contaminated sediments (Lick and Rapaka, 1996; Birdwell and Thibodeaux, 2007). The first identified approach for estimating water quality impacts during dredging was by Nakai (1978). This model uses a Turbidity Generation Unit to quantify the resuspension which corresponds to the quantity of turbidity generated when a unit quantity of bed material is dredged (Hayes et.al, 2000). A model was developed by Collins (1995) to measure sediment resuspension rates for open clamshell buckets and cutterhead dredges at the dredging site. Collins defined a factor R called resuspension factor or the source strength of resuspended sediment, which is defined as the temporal rate at which the mass of sediment is introduced in to the near field waters due to dredging operations (Collins, 1995). Hayes et.al (2000) developed a dimensional model (DM) and non-dimensional models (NDM) which were based on stepwise regression analysis and estimates the sediment resuspension due to cutterhead dredges (Palermo et.al, 2008). Different equilibrium and kinetic approaches have been used to estimate the contaminant concentrations during dredging. DiGiano et.al (1995) presented an equilibrium partitioning model to predict soluble polychlorinated biphenyls (PCB) and other constituent concentrations. This model uses simple mass balance approach between the soluble and sorbed PCBs at equilibrium in the water column based upon the DRET test. Borglin et. al (1996) conducted long term batch experiments and stated that the equilibrium assumption is not a good approximation, since the Adsorption and Desorption processes from Hydrophobic Organic Chemicals to sediment can be slow. Cheng et. al (1995) studied a hypothetical test case for the Buffalo River and concluded that the equilibrium assumption over predicts contaminant release. Cheng et. al (1995) concluded that an equilibrium model could overpredict 50% to 90% of the total mass desorbed for higher partitioning coefficient values (K p = 10 5 ml/g). However, an equilibrium model could produce reasonable results for smaller partitioning coefficient values depending on the particle aggregate sizes (Cheng et. al, 1995). Birdwell and Thibodeaux (2007) stated that the desorption of hydrophobic organic compounds (HOCs) due to resuspension events depends on their biphasic behavior and due to the presence of more than one compartment in the sediment. The biphasic behavior consists of two fractions, where the first fraction desorbs rapidly within 24 hours and the second fraction desorbs slowly (months to years). MATERIALS AND METHODS 18.9-liter (5-gallon) containers of the homogenized sediment and site water obtained from the fresh water Lake were provided to the University of Louisiana at Lafayette for this research. Once received, sediment in the 18.9-liter (5- gallon) container was again homogenized. Standard DRET tests using the procedure presented in DiGiano et.al (1995) were conducted in triplicates. All samples from the DRET tests were sent to the same EPA-certified laboratory for analysis. Site water was used to dilute the sediment to the proper suspended sediment concentration for all tests. The potential for stripping of volatile contaminants during the mixing portion of the DRET test is a concern since the protocol specifies pneumatic mixing. Mechanical mixing was used as an alternative in all further testing; mechanical mixing also provides better uniformity and control during mixing. The DRET protocol specifies pneumatic mixing at an air flow rate of m 3 /hr (0.5 ft 3 /hr) (DiGiano et.al, 1995). A range of mixing intensities and total applied mixing were evaluated to assess their effect on the release of constituents from the particulate to the dissolved phase. Assuming a discharge point air pressure of 30 psi and water viscosity of N-s/m 2, the DRET protocol results in a mixing intensity (G) of about 335 s -1 in the 4-L graduated cylinder (using the Camp-Stein equation as presented by Tchobanoglous and Burton, 1991). Figure 1 compares mixing intensity associated horizontal auger and cutterhead dredge heads to the DRET test. Cutters on these dredge heads act as impellers and induce mixing through their rotation. It should be noted that mixing intensity in these graphs does not include mixing due to dredge movement or hydraulic currents. The actual mixing intensity 23

3 experienced by a sediment suspension also varies significantly with proximity to the dredge head and dredge movement. Mixing intensity values of 200s -1, 335s -1 and 670s -1 were selected for testing. Figure 1. Mixing intensity associated with horizontal auger dredges (left) and mixing intensity associated with hydraulic cutterhead dredges (right). Mixing time represents the time during which mechanical or hydraulic dredging actions result in an agitated environment in the vicinity of the dredge head and dredge operation. The current DRET requirement of 1 hour is thought to be considerably greater than the time sediment suspensions experience mixing due to dredging operations. Experiments were conducted with shorter mixing times of 5 min, 15 min, and 30 min to evaluate the effects of mixing time. The combination of mixing intensity and duration also result in a range of total mixing. Total mixing (GT) experienced during the 1-hr mixing portion of the DRET test is about 1.2 x Cheng et.al (1995) studied a hypothetical test case for the Buffalo River and concluded that the equilibrium assumption over-predicts contaminant release; Borglin et.al (1996) also concluded Adsorption and desorption processes from hydrophobic organic chemicals to sediment can be slow from a series of long-term batch experiments. Settling time represents the time during which resuspended sediment concentrations change while contaminant partitioning moves toward equilibrium. The DRET protocol calls for one-hour of settling after the mixing period. This research included experiments at shorter settling times of 5 min, 10 min, 20 min, and 30 min to study settling time effects. RESULTS AND DISCUSSIONS Sediment and water column samples collected during the sampling effort were analyzed by the same commercial laboratory for the constituents of concern. This study focused on solids, mercury, and naphthalene concentrations; these are shown in Table 1 along with the DRET results. The results suggest that the DRET test is a very conservative estimator of water column concentrations of total suspended solids (TSS), naphthalene, and mercury. TSS concentrations at the end of the DRET test were still 68 mg/l while water column TSS concentrations averaged 13 mg/l inside the silt curtain and 4 mg/l outside of the silt curtain. 18% of the DRET results and all other field constituent concentrations were less than 10% of the DRET concentrations. Soluble and total mercury concentrations from the DRET were about 2 orders of magnitude higher than those seen inside the silt curtain and 3 orders of magnitude higher than those outside the silt curtain. Naphthalene concentrations inside and outside the silt curtain were closer together, but still 2 orders of magnitude lower than see in the DRET (Figure 2). 24

4 Table 1. Sediment, water column, and DRET results; data ranges in parentheses. Naphthalene ( g/l) Mercury ( g/l) Solids Conc. Total Soluble Total Soluble Sediment 49.2 % 100 mg/kg mg/kg - DRET 68 mg/l 397 g/l 237 g/l 1.2 g/l 0.70 g/l ( ) ( ) ( ) ( ) ( ) Inside Silt Curtain 12.6 mg/l ( ) 6.3 g/l (1.9-11) 6.7 g/l (1.9-14) g/l ( ) g/l ( ) Outside Silt Curtain 3.9 mg/l ( ) 1.9 g/l ( ) 3.1 g/l ( ) g/l ( ) g/l ( ) 1.0 Field Concentration/DRET Concentration TSS (mg/l) Total Hg (ug/l) Dissolved Hg (ug/l) A MAXIMUM AVERAGE MINIMUM Total Napthalene (ug/l) Dissolved Napthalene (ug/l) Figure 2. Field water column constituent concentrations relative to DRET results. The disparity in the DRET results for TSS, naphthalene, and mercury seems to be much greater for the fresh water Lake samples than that reported for PCBs by DiGiano, et.al (1995) (Table 2). Some of the difference may result from the significantly different behavior of the constituents and their environments. Some of the difference may also be associated with the different dredging equipment used and the rate of production achieved; However, much of the difference may result from the location of the field data reported. The New Bedford data is from samples collected within just a few feet of the dredge head operation while the fresh water Lake samples were collected from the water column about 30.5 meters (100 feet) or more (estimated) from the backhoe operation. The significantly higher TSS concentrations in the New Bedford field samples are indicative of the stronger signal observed at these closer locations. Table 2. Summary of New Bedford DRET and field data (DiGiano, et.al 1995). TSS (mg/l) Total PCBs ( g/l) Soluble PCBs ( g/l) DRET Cutterhead 133 (46-388) 7 ( ) 0.6 ( ) Horizontal Auger 1931 ( ) 54.9 ( ) 10.1 ( ) Matchbox 179 (62-582) 2.6 ( ) 0.5 ( ) 25

5 Modifications to the DRET tests were evaluated as described to determine if test conditions closer in line with field data for mercury. These experiments used distilled water for diluting sediments because of a lack of site water; although the use of distilled water may affect the quantitative results, it is believed that they should not significantly affect the comparative results. The first set of experiments used a constant mixing time of 5 minutes rather than the 60 minutes called for in the DRET protocol. During dredging, suspensions are thought to be subject to significant mixing for only a short time, possibly even less than one minute. Settling time was varied from 5 minutes to 30 minutes. Tests were conducted for three mixing intensities of 200/s, 335/s, and 670/s. These results (presented in Table 3) show a decrease in observed total and soluble mercury concentrations as settling time increases for all mixing intensities. Some of the decrease in total mercury concentration (C T ) is likely due to the decrease in total suspended solids due to settling. All soluble constituent concentrations measured in this study also include any constituent associated with particulate matter smaller than the 0.47µm filter used to filter suspended solids from the samples. No attempt was made in this research to quantify the colloidal contribution. Table 3. Mercury concentrations observed in modified DRET experiments using distilled water; all tests were conducted with a mixing time of 5 minutes. Settling Time (min) TSS (mg/l) C d (µg/l) C T (µg/l) Log K d (L/kg) Mixing Intensity, G = 200/sec Mixing Intensity, G = 335/sec ND ND Mixing Intensity, G = 670/sec ND = no data for this entry Table 4 shows the results observed in a second set of experiments conducted by maintaining a constant settling time of 30 min and varying the mixing time from 5 min to 30 min with mixing intensities of 200/s, 335/s, and 670/s. The results show an increase in the total and dissolved mercury concentrations as mixing times increase. This increase generally corresponds to an increase in TSS, although the 670/s mixing intensity does not follow this trend. The results also show that the rate of contaminant being released into the dissolved phase generally decreases with mixing time and soluble and total mercury concentrations increase with increasing mixing intensity (G) over the range tested. 26

6 Table 4. Mercury concentrations observed in modified DRET experiments using distilled water; all tests were conducted with a settling time of 30 minutes. Mixing Time (min) TSS (mg/l) C d (µg/l) C T (µg/l) Log K d (L/kg) Mixing Intensity, G = 200/sec Mixing Intensity, G = 335/sec 5 ND ND 15 ND ND 30 ND ND Mixing Intensity, G = 670/sec ND = no data for this entry CONCLUSIONS The standard DRET tests significantly over-predicted naphthalene and mercury concentrations in comparison to the water column samples collected during the backhoe dredging operation. Variations to the DRET protocol show that mercury release to the soluble phase increases with mixing intensity and duration. Settling of suspended particulates removes contaminants from the water column faster during the tests than they are released from a particulate into a soluble phase. The results suggest that modifications to the DRET test may help bring the results closer to the field observations, but even under the most favorable conditions the mercury concentrations were still substantially higher in the DRET test. Additional work is necessary to prove the veracity of the DRET test for predicting contaminant release during dredging operations. REFERENCES Birdwell, J. E., Thibodeaux L.J. (2007). A Kinetic Model of Short-Term Dissolved Contaminant Release during Dredge-Generated Bed Sediment Resuspension, Environmental Engineering Science, 24(10): doi: /ees Borglin, S., Wilke, A., Jespen, R., and Lick, W. (1996). Parameters Affecting the Desorption of Hydrophobic Organic Chemicals from Suspended Sediments, Department of Mechanical and Environmental Engineering, University of California, Santa Barbara, California 93106, USA. Chapman, P. M., (1997). Contaminated Sediments in Ports and Waterways, Cleanup Strategies and Technologies, US National Research Council, National Academy Press, Washington, DC. pp Cheng, C. Y., Atkinson, J. F., and DePinto, J. V., (1995). Desorption during Resuspension Events: Kinetic v. Equilibrium Model, Great Lakes Program, Department of Civil Engineering, State University of New York at Buffalo, Buffalo, New York 14260, USA. Collins, M. A. (1995). Dredging Induced near Field Resuspended Sediment Concentrations and Source Strengths, Miscellaneous Paper D-95-2, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS. DiGiano, F. A., Miller, C. T., Yoon, J. (1995). Dredging Elutriate Test (DRET) Development, Contract Report D- 95-1, U.S. Army Engineers Waterways Experiment Station, Vicksburg, MS. EPA (2008) Contaminated Sediment Program-Sediment Remediation, Internet: (14 April 2009) Hayes, D. F., Crockett, T. R., Ward, T. J., and Averett, D. (2000). Sediment Resuspension During Cutterhead Dredging Operations, Journal of Waterway, Port, Coastal, and Ocean Engineering 126(3): American Society of Civil Engineers. Hayes, D. F., and C. H. Je. (2000). DREDGE Module User s Guide, Draft. Vicksburg, MS: U.S. Army Engineer Research and Development Center. Lick, W., Chroneer, Z., and Rapaka, V. (1997). Modeling the Dynamics of the Sorption of Hydrophobic Organic 27

7 Chemicals to Suspended Sediments, Department of Mechanical and Environmental Engineering, University of California, Santa Barbara, CA Nakai, O. (1978). Turbidity Generated by Dredging Projects, Management of Bottom Sediments Containing Toxic Substances, Proceedings of the Third U.S.-Japan Experts Meetings. EPA-600/ , Palermo, M. R., Schroeder, P. R., Estes, T. J., Francingues, N. R. (2008). Technical Guidelines for Environmental Dredging of Contaminated Sediments, Technical Resource Document ERDC/EL TR-08-29, Engineer Research and Development Center, Environmental Laboratory, Vicksburg, MS. Tchobanoglous, G., Burton, F.L. (1991). Wastewater Engineering Treatment, Disposal and Reuse, McGraw Hill, Inc. CITATION Salkuti, K.K., Gummadi, G.B., and Hayes, D.F. Evaluation and modification of the dredging elutriate test, Proceedings of the Western Dredging Association (WEDA XXXI) Technical Conference and Texas A&M University (TAMU 42) Dredging Seminar, Nashville, Tennessee, June 5-8,