Projects & Programs
Natural Sediment Recovery Modeling in Ward Cove, Alaska
A pulp mill discharged an average of 30 to 40 million gallons per day of wastewater to an adjacent cove during its operations, contributing to low-dissolved oxygen and high organic content in the sediments. As part of a consent decree with the Environmental Protection Agency, the mill agreed to address the contaminated sediment.
Disturbance of sediments can sometimes be more harmful to the environment than leaving the sediments to recover in place naturally. This is recognized by the regulatory agencies and is also the reason that evaluation of source control and natural recovery are a requirement before any cleanup action. The most common and effective mechanism for natural recovery calculations, widely used by regulatory agencies, is the dilution and cover provided by fresh, clean sediments that are delivered to the contaminated site by natural means. In locations without significant sources of fresh sediments, natural recovery still occurs through other mechanisms such as biodegradation and diffusion and flushing by overlying waters.
A powerful combination of hydrodynamic and toxic models was used to assist with sediment remediation analysis. PNNL staff, , used EFDC, a hydrodynamic model, and WASP/ TOXI5, a toxics fate and transport model, to better characterize impacted regions, "hot spots" to be targeted for cleanup and regions showing potential for quick, natural recovery. The toxics fate and transport model was calibrated and applied to simulate the 43-year discharge of pulp mill effluents. Natural recovery of sediments was simulated with the help of processes such as: 1) burial by new clean sediments 2) chemical biodegradation and 3) diffusion and tidal flushing and the implementation of zero discharge. Model results showed sediment recovery of total organic carbon in the top 10 cm of sediment within 15 years. Results for 4-methylphenol and ammonia also showed recovery with the exception of some hot spots.
Project Highlights:
- The study showed natural recovery was feasible through bio-degradation in the absence of fresh sediment loads
- The study helped identify hot spots where natural recovery would exceed 10 years
- Model results were used to optimize dredging requirements resulting in a 50% reduction in dredging area and a cost savings of about $15 million.