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Hood Canal

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9722-BBR - Estimating Ground-Water Discharge and Nitrogen Loading in Hood Canal, WA - Completed FY2011

Introduction and Problem - In September 2002, it was observed that fish on the west side of Hood Canal near Hoodsport were under stress from low concentrations of dissolved oxygen. This observation prompted the Washington State Department of Fish and Wildlife to temporarily close parts of Hood Canal to some types of fishing during the month of October. In 2003, low dissolved-oxygen conditions worsened, some fish kills were observed as early as June, and by October it is estimated that about 30 percent of rockfish had perished. Although low concentrations of dissolved oxygen in Hood Canal have been observed during late summer and early fall as far back as the 1950ís, available data suggest that concentrations vary from year to year but have been trending lower over time, with longer durations of low concentrations. The U.S. Geological Survey (USGS) was asked by Congress to collaborate with stakeholders in studying the causes of low dissolved-oxygen concentrations in Hood Canal. In 2004, the USGS conducted field work and made initial estimates of nitrogen loading to Hood Canal from various sources. The USGS study identified the southern end of the canal, from the Great Bend to Lynch Cove, as the most susceptible to low dissolved-oxygen concentrations and estimated that 8+ 5 metric tons per year (t/yr) of nitrogen enter the canal from the more developed Lynch Cove area. In 2005, the USGS is continuing to work in partnership with the Puget Sound Action Team and the University of Washington, who are working on revising the surface-water load estimates and the nitrogen contribution from septic systems. The USGS work includes the following specific tasks: 1) develop a hydro-dynamic model of marine circulation in Hood Canal, 2) conduct a study of the effects of hypoxia on the health of Hood Canal fish populations, and 3) revise estimates of ground-water discharge and nutrient loading to the canal from the regional ground-water system.

Objective - This proposal addresses the task of estimating the amount of regional ground-water discharge into southern Hood Canal so that the total amount of nitrate-nitrogen loading from ground water can be more accurately quantified.

Relevance and Benefits - The United States Congress has deemed the oxygen depletion in Hood Canal a significant ecological problem and has appropriated funding specifically for this study, which helps to address a major concern to fisheries and other uses of Hood Canal. This study also addresses the second major mission goal of the USGS 2000-2005 Strategic Plan of better understanding the nation's environment and natural resources. The study fills an important data gap by providing improved estimates of nitrate-nitrogen inputs from the regional ground-water system. The study also will provide insight into freshwater and seawater interactions and the potential for denitrification of nitrate in aquifer soil and nearshore marine sediments. This study directly addresses the priority water-resource issue Effects of land use and population increases on water resources in the coastal zone described in the STRATEGIC DIRECTIONS FOR THE WATER RESOURCES DIVISION, 1998-2008.

Approach - Nearshore ground-water discharge will be assessed in detail at three study sites (Twanoh State Park, Sunset Beach, and Landon Road), each with unique geology, land use, and population density. Ground-water discharge at each site will be measured directly using an array of conventional seepage meters (Lee, 1977) and an automated seepage meter designed to collect continuous ground-water seepage data (Rosenberry and Morin, 2004). Vertical hydraulic gradients will be determined at each site by installing mini-piezometers next to the seepage meters and instrumented to record relative water-table altitudes during several tidal cycles. Slug tests and drawdown tests also will be conducted in order to calculate hydraulic conductivity parameters.

Once the spatial and temporal variability of ground-water discharge has been evaluated, a sampling plan will be devised to examine the concentration of nitrate and other nutrients in ground water. Approximately 15 to 20 domestic wells near the shoreline will be identified for sampling (in addition to the three detailed study sites). The wells will be evenly distributed around the southern arm of Hood Canal. Well logs will provide geologic information, water-table altitudes will be measured, and water samples will be collected and analyzed for nutrients and major ion chemistry at the National Water Quality Laboratory (NWQL). Adjacent to each well location, a piezometer will be installed in the tidal zone and a seepage meter (or array of meters) will be installed offshore in a line along the same flow path as the well. Where possible, water samples will be collected from the piezometer and seepage meter in order to better understand processes that may affect nitrate concentrations at or near the point of discharge. Microbiological processes can affect the nitrate flux in ground water that contains sufficient dissolved-organic matter. At low dissolved-oxygen concentrations, the microbes begin to use the oxygen of the nitrate to oxidize organic matter in the ground water (denitrification), which results in decreasing nitrate flux along the ground-water flow path.

Finally, ground-water-discharge data will be combined with ground-water-quality data, and the total nitrogen loading for the southern end of Hood Canal will be computed. If necessary, a two-dimensional, ground-water flow net along a vertical line of section including a well, piezometer, and seepage meter may be constructed at sites where numerical modeling may be informative. The flow nets will be constructed using geologic-well-log information, water-level data, and seepage-discharge data. Because the assumption of steady state is not likely to be valid, the flow nets will only be used as a tool to improve the conceptual understanding of a localized flow system. Ultimately, data from each site will be evaluated with regard to geology, land use, population density, or forest cover in the adjacent watershed in order to better understand relative impacts.

9722-BBR - Proposal for U.S. Geological Survey Studies Addressing Low Concentrations of Dissolved Oxygen in Hood Canal

Introduction and Problem - Low concentrations of dissolved oxygen in Hood Canal are causing fish kills. It is believed that the duration and extent of anoxic conditions have increased over time. It is thought that an increasing trend in the discharge of nitrogen compounds to Hood Canal is accelerating primary productivity resulting in anoxic conditions.

Objective - Recently, the U.S. Geological Survey (USGS) has been asked by Congress to become actively involved in the study of the causes of low dissolved-oxygen concentrations in Hood Canal. A meeting between the USGS, local groups and governments, agencies, Tribes, and the University of Washington was held on January 8, 2004, and it was agreed that the USGS could make a significant contribution to the overall efforts to determine the causes and solutions of the low dissolved-oxygen concentrations in Hood Canal by assessing the sources and quantifying the amounts of nitrogen compounds discharged by those sources to Hood Canal.

Relevance and Benefits - Congress appropriated money for this study, which helps to address a major concern to fisheries and other uses of Hood Canal. This study will address the second major mission goal of the USGS 2000-2005 Strategic Plan of better understanding the Nation's environment and natural resources.

Approach - Assessing sources and quantifying amounts of nitrogen compounds discharged to Hood Canal will be accomplished by 1) using available information and data to make initial estimates of the amounts and spatial distribution of nitrogen compounds discharged to Hood Canal from all sources, 2) using the results of initial estimates to design and conduct focused studies needed to better quantify nitrogen loading from major sources and/or to sensitive areas of Hood Canal, and 3) applying the results of the focused studies to improve the overall estimates nitrogen loading to Hood Canal.

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