A challenge to all sediment remediation technologies is the continued influx of chemicals from uncontrolled sources. However, chemicals deposited on sediments remediated with chemically active sequestering agents (such as those in active caps) may be affected by the sequestering agents resulting in reduced impacts. The objective of this project was to evaluate the effectiveness of in situ remediation technologies (active and passive caps) subjected to continued low level, metal influxes (e.g., arsenic [As], cadmium [Cd], copper [Cu], zinc [Zn], and lead [Pb]) from uncontrolled sources and to better understand relationships among remediation methods, low level influxes of chemicals, bioturbation, and effects on biological receptors.

The research objectives were addressed in three tasks:

• Task 1 - Linkages between chemical loading and recontamination of remediated sediments – flow-through mesocosms with continuous metal influx.
• Task 2 - Understanding relationships among remediation methods, low level influxes of contaminants, bioturbation, and effects on biological receptors.
• Task 3 - Development of numerical models for predicting long-term relationships between low level contaminant influxes and remediated sediments.

Task 1 examined recontamination without bioturbation, Task 2 examined recontamination coupled with bioturbation, and Task 3 developed numerical models for assessing the long–term effectiveness of remedial methods based on results from Task 1.

The main findings of Task 1 were:

1. Sequestering agents in active caps bound metals from ongoing sources thereby reducing their bioavailability and protecting underlying sediments from recontamination. In contrast, metals from ongoing sources impacted passive caps and uncapped sediment.
2. Element concentrations in Lumbriculus variegatus were significantly higher in mesocosms with untreated sediment than mesocosms with active caps and were related to element influx and chemical concentrations in sediment as measured by diffusive gradients in thin films probes.
3. The toxicity of Cu mixed with other elements was greater than the toxicity of Cu alone in treatments without active caps, but the ability of active caps to control Cu was not affected by the presence of other elements in in-flowing contamination.

In Task 2, impacted sediment was deposited over apatite caps and uncapped sediment to simulate recontamination by in-flowing particle-bound chemicals. Remedial effectiveness was assessed in the presence and absence of the bioturbating organism Corbicula fluminea. Findings were as follows:

1. The metal sequestration capacity of apatite caps was unaffected or improved by bioturbation for all elements except As.
2. The effects of bioturbation on uncapped sediment were metal specific including reductions in bioavailable Ni, Cd, and to a lesser extent, Pb, increases in bioavailable As and Cu, and little effect for Zn.
3. Bioturbation distributed metals more uniformly; differences in metal levels among sediment layers were usually significant in the absence of bioturbation but not in its presence.
4. Apatite caps reduced sediment metal levels compared with uncapped sediment in both the presence and absence of bioturbation.

Task 3 focused on review and diagnostic testing of two modeling frameworks: the Sediment Flux Model (SFM) and the Tableau Input Coupled Kinetic Equilibrium Transport (TICKET) model. The tableau approach used in the TICKET was integrated into the SFM producing a combined SFM-TICKET model capable of simulating the effects of pH on free metal activity, adsorption to representative sequestering components, and competition between metal and hardness cations for ligand binding sites.

This research showed that active caps can protect remediated sediments from recontamination by reducing the pool of bioavailable elements in ongoing sources of contamination. It contributes to a better understanding of the relationships among low level metal influxes, remediated sediments, and biological receptors, which can help set more defensible cleanup goals and priorities while ensuring the protection of human health and the environment. (Project Completion - 2020)

Knox, A.S. and M.H. Paller. 2020. Effect of Bioturbation on Contaminated Sediment Deposited over Remediated Sediment. Science of the Total Environment, 713:136537.

Knox, A.S., M.H. Paller, and J.C. Seaman. 2019. Removal of Low levels of Cu from Ongoing Sources in the Presence of Other Elements – Implications for Remediated Contaminated Sediments. Science of the Total Environment, 668:645-657.

Knox, A.S., M.H. Paller, C.E. Milliken, T.M. Redder, J.R. Wolfe, and J. Seaman. 2016. Environmental Impact of Ongoing Sources of Metal Contamination on Remediated Sediments. Science of the Total Environment, 563-564:108-117.

Knox, A.S., M.H. Paller, and K.L. Dixon. 2014. Evaluation of Active Cap Materials for Metal Retention in Sediments. Remediation, 24(3):49-69.

Paller, M.H., S.M. Harmon, A.S. Knox, W.W. Kuhne, and N.V. Halverson. 2019. Assessing Effects of Dissolved Organic Carbon and Water Hardness on Metal Toxicity to Ceriodaphnia dubia using Diffusive Gradients in Thin Films (DGT). Science of the Total Environment, 697:134107.