Phototransformation of Contaminants at the Sediment-Water InterfaceEPA Grant Number: R830394
Title: Phototransformation of Contaminants at the Sediment-Water Interface
Investigators: Tratnyek, Paul G.
Institution: Oregon Health & Sciences University
EPA Project Officer: Carleton, James N
Project Period: March 1, 2002 through February 28, 2004 (Extended to September 30, 2006)
Project Amount: $198,226
RFA: Futures Research in Natural Sciences (2001) RFA Text | Recipients Lists
Research Category: Futures , Ecological Indicators/Assessment/Restoration , Land and Waste Management , Hazardous Waste/Remediation
Solids that have settled at the bottom of shallow surface waters form a macroscopic sediment-water interface (SWI) that can be exposed to significant amounts of sunlight. Rapid and unusual photoreactions at the sunlit SWI are likely because the interface represents a narrow mixing zone for reactive species from above and below. Enhanced phototoxicity has been attributed to PCBs and other contaminants at the SWI, and more UV light is reaching the SWI due to thinning of the ozone layer. Yet, the photochemistry of the SWI had never been subject to a systematic investigation.
The major hypothesis underlying the proposed study is that the lack of information on photochemistry at the SWI is a significant contributor to uncertainty in risk assessments involving contaminated sediments. To reduce this uncertainty, we propose to:
(1) Survey a range of contaminant-sediment combinations using a variety of protocols, to identify the types of situations where photoeffects at the SWI are most significant.
(2) Develop improved and alternative methods for studying SWI photoeffects, to validate the conclusions from (1) and allow more detailed process-level investigations as in (3).
(3) Apply a range of chemical and physical probes to obtain a detailed understanding of processes such as (i) indirect photolysis sensitized by sediment organic matter, (ii) photocatalysis by metastable precipitates at the SWI, and (iii) catalysis by pore water constituents that diffuse up into the photic zone.
(4) Perform a preliminary assessment of the overall importance of these effects relative to other environmental fate processes, and use this understanding to anticipate future problems and/or opportunities for improved management practice.
A major impediment to assessing the effects of sunlight at the SWI is the difficulty of sampling in the presence of such steep vertical gradients. However, preliminary experiments we have performed using relatively simple laboratory microcosms have given remarkably robust results. The proposed project will involve development of more sophisticated sampling protocols for use with laboratory and field microcosms, as well as adaptation of existing exposure models to describe the complex geochemical dynamics of the interfacial region.
Currently, assessments of the risk from contaminated sediments do not take into account the effect of sunlight at the SWI even though there is a strong possibility that phototoxicity involving adsorbed contaminants is an important stressor of benthic organisms. The proposed project will provide the first comprehensive evaluation of what photochemical processes need to be considered, what types of contaminants provide the greatest risk, and what types of sedimentary environments are most likely to be impacted.