Sediments will be sampled in two areas: one set from the Buffalo Slough (representative of the upper slough) and one set from the Wapato Wetlands near the City of Portland outfall 55A (representative of the tidally influenced lower slough). Replicate sediment samples will be collected in an appropriately cleaned Ekman-type (box dredge) sampler. A pair of locations will be sampled in each water body at positions at or near to centrally located stations among those sampled by Parametrix in the focused RI sampling. We will not attempt the extensive spatial transects collected by Parametrix because our studies focus on the detailed character- istics of a few samples rather than the more general characteristics of a wide range of locations. In this way, the two studies should complement one another well.
Standard procedures will be followed to ensure intact, representative, and minimally disturbed samples. These include steady dropping of the sampler, slow retrieval, and visual verification that sediments have not been winnowed or extensively disturbed. Once a suitable sample has been obtained, the location, water depth, and visual observations of sample appearance will be recorded. Overlying water will be carefully siphoned off and the sediment pH and redox potential measured. Samples of the top 2 cm will be removed witha Teflon coated scoop, designed to minimally mix or disrupt the solids and pore water. These samples represent the shallow sediments most in contact with benthic organisms and overlying water; they will be comparable to the grab samples taken by Parametrix via an almost identical protocol. Samples will be transferred without mixing to glass jars, purged on site with N2 and sealed. Capped samples will be stored immediately on ice for transport to the lab.
In the lab, samples will be handled in an anaerobic chamber. Full anaero- bic handling minimizes the potential loss of acid volatile sulfides (AVS) and the alteration of iron and manganese phases that will occur if a sample is exposed to the atmosphere. Sample splits will be taken for QA/QC procedures. One set of subsamples will be "squeezed" in a piston press to remove pore water for sampling, then resuspended in a wash solution with an ionic composition similar to slough water, re-squeezed and either wet-frozen or dried for future use. Another set of subsamples will be kept wet and unhomogenized and used immediately or stored frozen for comparison experiments to determined any artifacts introduced by sample homogenization. Dried samples and selected wet samples will be analyzed for organic carbon, Fe, Mn, Al, and acid-volatile sulfides (AVS) content, approximate grain-size distribution, and solid-phase concentrations of the target pollutants (Ni, PCB, etc.)
A selection of both dry and wet samples will be used in experiments to determine the equilibrium adsorption or partitioning parameters for Pb, Ni, PCB, and PAH. Partitioning or adsorption behavior will be measured in batch tubes, gently rotated for a time sufficient to effect an essentially complete equilibrium. All sample handling and incubation will be done under anaerobic conditions. Preliminary experiments will resolve the necessary experimental conditions such as appropriate adsorbate concen- tration ranges. Other preliminary experiments will compare the results obtained with wet versus dried samples. For compounds for which little or no difference can be found between wet and dry samples, dry samples will be used subsequently as the primary experimental materials, due to the greater ease of handling.
Standard isotherms will be constructed for both metals and organics over as wide a range of concentrations as practicable. Because the slough pH varies seasonally and diurnally, metal adsorption also will be measured as a function of pH (yielding so called pH "edges"). Data will be reduced to linear Kp values for organics and both Langmuir and diffuse-layer model (DLM) K values for metals. Kp values will be normalized by organic carbon content to yield site-specific estimates of Koc, a parameter for which non-site-specific values had to be used in the SLRA. The directly-measured values of metal and organic sorption parameters obtained can be used to refine the risk assessments.
An optional task is to perform a preliminary measurements of the partitioning of selected pollutants to algal samples taken from slough waters. Algae are known to have a high affinity for both metals and organic contaminants and may be an important pathway for conveying soluble pollutants to the sediments.
Cr has been detected at relatively high levels in a number of slough sediment samples, particularly near COP Outfall 55A in Wapato Wetlands. Preliminary analyses measured total Cr and did not distinguish between the two common forms of Cr, Cr(III) and Cr(VI). Because the sediments are thought to be anoxic, organic rich, and strongly reducing, it is unlikely that any of the toxic Cr(VI) can exist there. If indeed all of the observed Cr is in the relatively harmless Cr(III) form, then risk assessments for this metal can be revised downward to reflect the low risk presented by this metal to humans and other organisms.
The diphenylcarbazide (DPC) colorimetric method is a sensitive and accurate measure of Cr(VI); the method does not respond to Cr(III). We will test expressed pore water samples for Cr(VI) with DPC analysis. We also will extract sediment samples with a NaHPO4 solution that will desorb any adsorbed Cr(VI) (chromate or bichromate). The extracts will then be analyzed for Cr(VI) with DPC. Appropriate controls and other precautions will be taken to ensure that analytical interferences are eliminated. The samples also will be acid-extracted according to standard procedures and total Cr determined for reference. The fraction of extractable Cr that is the toxic Cr(VI) form can then be deduced.
Use the results of the equilibrium experiments described above to define the details of experiments to measure adsorption and desorption rates of the target contaminants. For example, equilibrium partitioning experi- ments will indicate the magnitude of changes in speciation due to adsorp- tion or partitioning under various conditions, and preliminary tests will have revealed the approximate time scales of uptake. These observations will greatly streamline the rate experiments because we will already know the general behavior of the sediments and will be able to move more efficiently to the key rate experiments.
Contaminant uptake or release by the sediment is expected to be heavily influenced by diffusional limitations in the porous structure of the natural sediment particles. This diffusion typically depends in part on the flow conditions surrounding the particles: diffusive fluxes are fastest under well mixed conditions and very slow in stagnant water. We will conduct selected uptake- and release-rate experiments under a range of mixing conditions to replicate various sediment mixing scenarios in the slough.
In a typical experiment, subsamples of sediment will be either amended with additional doses of target contaminants or left unamended (i.e., will contain only the in situ contaminant levels). Samples will then be resuspended in a series of wash solutions and the release of contaminants monitored as a function of time. The converse (uptake) experiments will be conducted by spiking samples with target contaminants and tracking the rate of removal of the contaminants from the water onto the sediments.
Because the pH and ionic composition of the water may affect uptake and release rates, several tests will examine the role of these parameters on rates for a range of conditions typical of the slough. For example, we will look at the range of pH observed during the daily or annual pH cycle typical in the slough due to algal growth.
During the conduct of equilibrium and rate experiments described in (1) and (2) above, parallel studies will be made on the presence and behavior of fine colloids that easily elute from sediments when they are mixed and which appear to bear a disproportionate amount of metal and organic contamination. Typical studies will involve separating the sediments from the aqueous phase via carefully controlled centrifugation and filtration methods that will distinguish settleable particles, "nonsettleable" colloids, and contaminants in true solution.
The importance of colloids will be quantified by measuring the fraction of total contaminant in the experimentally distinguished colloidal fraction. Colloids are expected to reduce the bioavailability of contaminants for most organisms, so their role should be properly factored into risk assessments. However, colloids may also be a means by which contaminants spread through the slough or are exported to the Willamette and Columbia Rivers, especially during dredging or other disruption of the sediments.
An optional task is to conduct preliminary experiments to assess the stability of the colloids under various chemical conditions. For example, colloids are less stable in the presence of elevated levels of calcium, iron, and aluminum ions. Such conditions may occur naturally in some parts of the slough. In addition, the studies may suggest chemical treatments that could be employed during dredging to minimize the dispersal of contaminant-bearing colloids from disturbed sediments.
The chemical potential of a compound, expressed as the "fugacity" of that compound, can be used to quantitatively assess its tendency to transfer from one phase to another. In simple terms, fugacity is a measure of a molecule's "urge to flee" or escape from one component (such as the water) into another component (such as the air or fish tissues). Each of these four components is important for risk and transport assessments. Sediments are the main repository of contaminants, water is the phase in which contaminants move around in the system, fish tissues are a key exposure pathway for humans and wildlife, and air (in this situation) is the phase into which some contaminants are slowly "lost".
Various practical methodologies based on the fugacity concept have been developed to represent and forecast the distribution and availability of hydrophobic organic compounds such as PCB. We will organize litera- ture data (Stage 1) and our own experimental data (Stage 2) on hydropho- bic compounds to formulate a fugacity based model of contaminant distribution within the slough system. This work will be coupled into the existing framework of the risk assessment program to improve and refine the estimates of risk to human and ecosystem health.