Hydrologic Studies in Oregon

Project Chief: Kathleen McCarthy

Cooperator: City of Portland

The Columbia Slough watershed is a valuable resource--both ecologically and recreationally--for the City of Portland, Oregon. Over the past decade, assessments have shown that several toxic organic compounds are present in the slough, and in November 1998, the U.S. Environmental Protection Agency approved a Total Maximum Daily Load (TMDL) for the slough. The TMDL parameters include a number organochlorine compounds whose use has been severely restricted or banned altogether over the last several decades (chlordane, DDT, dieldrin, PCBs). These compounds present a unique challenge to effective slough management because ongoing sources of such compounds can be difficult to identify and control, and because many of these compounds pose environmental risk even at concentrations orders of magnitude below the detection limits associated with conventional water-sampling techniques. A new water-quality monitoring tool that is well suited to addressing this challenge is the semipermeable membrane device (SPMD).

A standard SPMD (figs. 1 and 2) consists of a 1-m length of 2.5-cm wide low-density polyethylene lay-flat tubing, and contains a thin film (1 mL) of triolein--a neutral lipid characteristic of many living organisms. When placed in a medium such as water or air, SPMDs passively accumulate lipophilic (hydrophobic)compounds over time. At the end of the deployment period, the SPMDs are analyzed for the compounds of interest and the corresponding water concentrations are then estimated from the SPMD concentrations.

Figure 1. Diagram of semipermeable-membrane device

Figure 2. Semipermeable membrane device (left) mounted on deployment
rack, which is fitted into the deployment canister (right).

Buffalo Slough, a tributary arm of the Columbia Slough (fig. 3), is approximately 1 mile long and consists of four segments connected to one another by culverts through earthen dikes. In addition to ground-water inflow, the Buffalo Slough receives discharge from a number of urban and industrial stormwater outfalls. As a consequence, this site offers an excellent opportunity for exploring potential sources and behavior of persistent compounds in the slough system, as well as fine tuning methods for investigating and monitoring toxic organic compounds prior to applying them at a larger scale.

Figure 3. Buffalo Slough and surrounding area

The overall objective of the study is to provide information and understanding to support effective resource-management decisions. Specific study objectives are to:

• Measure the spatial and seasonal distribution of target organochlorine compounds--dieldrin, DDT, DDE, and PCBs--in Buffalo Slough

• Assess the relative importance of stormwater outfalls and bed sediment as sources of these target compounds in Buffalo Slough

• Investigate the usefulness of SPMDs as a tool for investigating and monitoring hydrophobic compounds in the Columbia Slough and elsewhere

• Evaluate the utility of SPMDs as a tool for measuring the long-term effects of watershed improvement activities in Columbia Slough and elsewhere

• Train City of Portland Bureau of Environmental Services (BES) personnel in the SPMD method

RELEVANCE AND BENEFITS

SPMDs are an emerging technology that was developed at the USGS Columbia Environmental Research Center in Columbia, Missouri (CERC, formerly the Midwest Science Center, then a part of the U.S. Fish and Wildlife Service). Furthermore, the results of this study--especially those associated with the use of PRCs--will provide an important contribution to the literature on SPMD theory in general, and specific data on sampling rates and partition coefficients in particular.

Throughout this study, the project team will work closely with a Technical Advisory Committee (TAC) composed of local professionals with expertise relevant to the slough system. The TAC will include representatives from BES, Portland State University, the Oregon Department of Environmental Quality (ODEQ), the Multnomah County Drainage District, the Columbia Slough Watershed Council, and USGS. The TAC's involvement in this study will serve to increase understanding and acceptance of the SPMD method by a variety of users including the regulatory community. The USGS Oregon District office recently conducted the largest SPMD study yet to be published and thus has the expertise to be pivotal in moving this promising new technology into the mainstream.

Finally, the results of this study will provide useful and relevant information to local resource managers, assisting them in making effective resource-management decisions to comply with regulations and to meet local water-resources goals. Information gained through this study will be transferable to other urban areas facing similar water-resources issues.

To take advantage of the considerable expertise that has been developed during previous investigations of the Columbia Slough system, the TAC will be established to provide guidance during the design and implementation of this study. As mentioned above, the TAC will include local professionals with relevant expertise from BES, Portland State University, the Oregon Department of Environmental Quality (ODEQ), the Multnomah County Drainage District, the Columbia Slough Watershed Council, and USGS.

Hydrology

Ground-water/surface-water interaction.--To improve our overall understanding of water movement through the slough system, we will use a potentiomanometer to map the interaction between ground water and surface water. Winter et al. (1988) describe the use of the potentiomanometer, which consists of a small piezometer connected by a length of tubing to a manometer. The piezometer will be driven into the bed of the slough at a number of locations, and the manometer will then provide a direct reading of the difference in hydraulic head between ground water and the slough. During this exercise, we will also measure water temperature and possibly other field parameters (e.g., specific conductance, dissolved oxygen) to further elucidate ground-water/surface-water interactions. In addition to improving our understanding of the hydrology of the slough, information from this survey will provide insight into potential sources and transport pathways of target compounds. This insight will be useful for optimal selection of SPMD deployment sites and for data interpretation.

Surface-water velocity and discharge.--Water movement through the Buffalo Slough is influenced by ground-water discharge, stormwater outfalls, and flood-control pumping by the Multnomah County Drainage District. A clear understanding of the resulting complex flow regime--especially during SPMD deployments--will be important for accurate interpretation of the SPMD data. To obtain this information, we will install one side-looking acoustic doppler velocity/state sensor (SonTek/YSI, Inc., San Diego, CA) for the duration of the SPMD deployment periods. The TAC will provide guidance on selecting the optimum location for this installation.

SPMDs

Based on data from the mini-piezometer survey and input from the TAC, approximately six specific sites for SPMD deployment will be selected. These sites will be chosen to elucidate the potential sources of target analytes (e.g., bed sediments, stormwater outfalls, and ground-water dischar ge) and the movement and fate of these compounds within the slough. Working together, USGS and BES personnel will deploy SPMDs at these strategic locations during two 30-day periods. The deployment periods will be selected to capture differences between baseflow (e.g., July through September) and storm flow (e.g. December through March) conditions. In addition to assisting in site selection, the expertise of TAC members will be important for determining the most appropriate timing for deployments.

SPMDs used in this study will be spiked with a suite of permeability reference compounds (PRCs) and premounted on deployment racks by the manufacturer (Environmental Sampling Technologies; St. Joseph, Missouri) prior to shipping. The rate at which PRCs are lost from SPMDs over the course of the deployment will be measured, providing in situ calibration data to aid in interpretation of target analyte data.

Deployment.--At each selected sampling site, one canister containing five mounted SPMDs (fig. 2) will be deployed in the slough and secured with an anchor. Where needed, either a float or a support plate will be used to ensure that the canisters do not sink into bed sediments. Canisters may also be tethered to a fixed structure. During each of the two selected deployment periods, SPMDs will be left in place for approximately 30 days.

At the end of each deployment period, SPMDs will be shipped to the EST laboratory for cleanup. After exterior cleaning, the five SPMDs from each site will be composited into a single sample for dialysis and initial purification by high-performance gel-permeation chromatography. Purified dialysates from this processing will be shipped in sealed glass ampoules to Texas A & M University (or another BES-designated laboratory) for analyses.

Field Quality Control.--SPMDs will be deployed in duplicate at selected locations to help quantify the overall precision of the sampling and analytical methods. During each sampling period, fie ld-blank SPMDs will be exposed to the atmosphere during both deployment and retrieval to help quantify sample contamination resulting from handling and exposure to the atmosphere. Duplicate deployments and field blanks will each consist of the same number of SPMDs (5) as the associated primary deployment. All SPMD sets--primary deployments, duplicate deployments, and field blanks--will be processed and analyzed using the same methods.

During SPMD deployments, USGS personnel will train BES personnel in the proper handling, deployment, and retrieval of SPMDs.