Research

Material availability and low costs for reactive barrier caps are central to their consideration for any capping project. The current sources for phosphorite rock materials on the Eastern United States include PCS Phosphates in North Carolina and IMC-Agrico in Florida. Both the Phosfil product available from PCS Phosphates and the Florida phosphorite rock can be purchased for approximately $135/ton, which includes delivery by truck within 400 miles of the source. Work on the Anacostia River required 235 tons of phosphorite rock to cap the 743 m² area. Based on the above prices the cost of the Phosfil barrier material delivered was $31,725. In the future, there are several factors that may work to reduce this price including shipping materials by ocean barge, larger volume production, and improved deployment techniques to minimize material waste.

In 1997 the District of Columbia commissioned a study to determine the extent of contamination, remediation options, and cost associated with the Anacostia River (Velinsky, et al., 1997). This study determined the area and volume of contaminated sediments present in the Anacostia River, and the adjacent Washington Channel. Additionally the study estimated the costs required for four conventional remediation scenarios. Our research has expanded these four scenarios to include cost estimates and potential benefits for four additional phosphate-based reactive barrier capping systems (Figures 1 and 2).

To generate these cost estimates, the following facts and assumptions were used.

1. The availability of dredge sediment disposal space is highly limited. The first 250,000 yd³ would be used by the US ACE on the Kingman Lake project, and would cost only $5/yd³. The location of the site would require that additional dredged materials would need to be transported up to 50 miles away to the next available facility (although no facility large enough existed in the area at the time).
   
   
2. Dredging of the area assumes two scenarios. First clean dredging using a hydraulic dredge with high precision and low unwanted sediment transport would cost about $15/yd³. Lower cost hydraulic dredging could be implemented for $5/ yd³ but with high offsite sediment transport.
   
   
3. A transport system is assumed for suctioning the dredged materials via a pipeline to a disposal/transfer facility within 3 km of the site.
   
   
4. The costs for capping and filling in the dredged areas were calculated as ranging from $5-$7/yd² for the clean sand caps. This cost includes the purchase, delivery and placement of the clean sand materials. Clean sand materials being placed over the reactive barriers require greater precision, and were therefore calculated at a range of $7-$10/yd².
   
   
5. The cost for the reactive barrier cap material was estimated to be $15/yd². Assuming that the barrier was incorporated into a geofabric mesh to ensure quick and accurate deployment, the geofabric construction is estimated at $12/yd². Crane based deployment of this system is estimated to cost an additional $25/yd², resulting in a total deployment cost of $52/yd². Large uncertainties in these numbers means that a conservative pricing factor of 2x (or $104/yd²) was used as the high estimate for reactive barrier deployment.

Scenarios 1 through 4 in Figure 1 (please click to see the full-size figure) address contamination along the entire length of the lower Anacostia River and the WashingtonChannel (see Appendix A). Scenarios 5 through 8 in Figure 2 addresses contamination in only the most polluted sections of the river. Comparing capping costs between scenarios 2 and 3 (both of which deepen the channel by 1.25 ft.) it can be seen that the mid price for reactive barriers are 33% lower. This cost savings comes from two operational areas. First, less contaminated material needs to be dredged in order to deepen the channel to the same depth. This is a great benefit, as disposal options in the Washington D.C. area are extremely limited. The second factor lowering costs is that less clean sand is required on top of the reactive barrier material. This is because the clean sand in a reactive barrier only functions as a erosion and bioturbation layer, but not a chemical migration barrier.

Comparing the costs of remediation between scenarios 6 and 7 demonstrates that even when only the "hot spots" are treated, the reactive barrier system costs 45% less than the conventional barrier system. Again, the costs of sediment disposal dominate the overall operations costs. Since the only 300,000 yd3 of disposal space could be identified in the Washington D.C. area for these sediments during the original investigation. As a result, it was concluded that no remediation could be performed for a reasonable price. Table 1 summarizes the quantities of sediments dredged and the number of times above the disposal capacity. Scenario 8 uses a reactive barrier to treat the hot spots in the Anacostia River, does not change the river depth, and allows for a remediation option that fits within the current limited disposal capacities of the Washington D.C. area. This is not an option with any of the conventional capping scenarios.

Table 1: Dredged Sediment Volumes and Capacity Estimates

Scenario	Volume Dredged (yd3)	Estimate Above Disposal Capacity
1	10,935,775	36.5
2	3,645,258	12.2
3	2,733,944	9.1
4	1,093,577	3.6
5	2,530,884	8.4
6	843,628	2.8
7	632,721	2.1
8	253,088	0.8

Deployment of multiple layer reactive barriers currently requires more handling time, and higher labor costs than conventional barriers. However, the relatively similar total material costs (tons required*price per ton) for phosphorite materials compared to conventional barrier materials demonstrates that further development of efficient deployment technologies could result in an even more competitive capping alternative.

Often, reactive barrier sediment caps are more expensive than conventional capping systems. However, from the above discussion it can be seen that the use of phosphorites as a reactive barrier offers several potential economic benefits under different capping scenarios. As disposal of contaminated sediments increases, and viable disposal locations become scarce, the thinner reactive barrier system becomes more economically competitive against conventional barrier systems.

References
Velinsky, D. J., Gruessner, B., Haywood, H. C., Cornwell, J., Gammisch, R. and Wade, T. L. (1997). Determination of the Volume of Contaminated Sediments in the Anacostia River; District of Columbia. Rockville, MD, Interstate Commision on the Potomac River Basin: 99.