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Pacific Coastal & Marine Science Center

Submarine Ground-water Discharge

Submarine Ground-water Discharge Diagram
 

Puget Sound Project

Hood Canal Background

The response of SGD to the large tidal range observed in Hood Canal: Puget Sound, Washington can be studied by applying our suite of geophysical and geochemical techniques. This site, where tides can be in excess of 3 meters and there is notable SGD, provides an ideal opportunity to study SGD rates as a function of tidally driven water level fluctuations.

Submarine groundwater discharge (SGD) has been shown to be important in water and chemical budgets of our coastal systems. The impact of SGD to a particular coastal water body depends on a variety of forcing functions that are both marine and terrestrial. For example:

  1. Tides, waves, storms or density/current driven gradients
  2. Density-related convection cells, induced by the instability of freshened water masses residing below more saline water
  3. The dynamic alignment of the fresh water/saltwater interface in response to climatic and anthropogenic forcing
  4. Water level variations across permeable barriers (for example, barrier islands) all may affect rates of SGD.
In this task, we will continue to assess the ecological impact of SGD within Hood Canal, WA (part of Puget Sound), where preliminary results clearly show finely defined 222Rn peaks that correspond closely to low tides events. We will investigate marine/terrestrial forcing on SGD rates in this system, which represent an endmember in terms of tidal amplitude and coastal energy. This effort is a more formal continuation of a field effort just commenced (June 2006), that was sponsored by WRD -Tacoma. While most of this task's emphasis will be devoted to examining SGD-derived nutrient loading to this estuary, the unique effects of tides and dramatic water level change on SGD are also being addressed.

Objectives

  • Examine the hydrogeologic and seasonal controls on SGD and SGD-derived nutrient loading estimates in this system.
  • Develop a Ra mass balance to estimate system-wide SGD rates
  • Continue application of resistivity techniques to study the freshwater/saltwater interface and its response to such tidal excursions.
Measuring water levels using a shallow piezometer
Measuring water levels using a shallow piezometer. Photos courtesy of Bill Simmonds and Don Rosenberry, USGS.

Methods

Current methods will include our existing suite of geochemical and geophysical techniques that include:

  1. Stationary and continuous 222Rn
  2. Amospheric 222Rn
  3. The four naturally occurring Ra isotopes, 223,224,226,228Ra
  4. stationary and streaming DC resistivity
  5. electromagnetic (EM) seepage meters

EM seepage meter logging device
The logging device for an electromagnetic (EM) seepage meter.

EM seepage meter
An electromagnetic (EM) seepage meter can assess seepage by measuring flow through an electromagnetic coil. It can continuously record the data.

Communications Plan

The task will produce both technical reports published by the USGS and external peer-reviewed journals that communicate observations and interpretations of task research. In addition task members will present results at formal scientific meetings and make informal presentations at internal meetings, meetings with collaborators and at Universities. Basic data and observations will be available on this Webpage in the near future.

Read more:
A Geochemical and Geophysical Examination of Submarine Groundwater Discharge and Associated Nutrient Loading Estimates into Lynch Cove, Hood Canal, WA (532 KB PDF)

ABSTRACT
Geochemical tracer data (for example, 222Rn and four naturally occurring Ra isotopes), electromagnetic (EM) seepage meter results, and high-resolution, stationary electrical resistivity images are used to examine the bi-directional (for example, submarine groundwater discharge and recharge) exchange of a coastal aquifer with sea water. Our study site for these experiments was Lynch Cove, the terminus of Hood Canal, WA, where fjord-like conditions dramatically limit water column circulation that can lead to recurring summertime hypoxic events. In such a system a precise nutrient budget may be particularly sensitive to groundwater-derived nutrient loading. Shore-perpendicular time-series subsurface resistivity profiles show clear, decimeter-scale tidal modulation of the coastal aquifer in response to large, regional hydraulic gradients, hydrologically-transmissive glacial terrain, and large (4-5 m) tidal amplitudes. A 5-day 222Rn time-series shows a strong inverse covariance between 222Rn concentrations (5 - 30 dpm L-1) and water level fluctuations, and provides compelling evidence for tidally-modulated exchange of groundwater across the sediment / water interface in this system. Mean Rn-derived submarine groundwater discharge (SGD) rates of 8584 cm d-1 agree closely in the timing and magnitude with EM seepage meter results that showed discharge (up to 80 cm d-1) during low tide and recharge during the high tide events. To evaluate the importance of fresh versus saline SGD, Rn-derived SGD rates (as a proxy of total SGD) are compared to excess 226Ra-derived SGD rates (as a proxy for the saline contribution of SGD). The calculated SGD rates are used to estimate associated nutrient (NH4, Si, PO4 -3, NO3+NO2, TDN) loads to Lynch Cove. The dissolved inorganic nitrogen (DIN = NH4+NO2+NO3) SGD loading estimate of 5.9 x 104 mol d-1 is one to two orders of magnitude larger than similar estimates derived from atmospheric deposition and surface water runoff, respectively.

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