Measuring water levels using a shallow piezometer. Photos courtesy of Bill Simmonds and Don Rosenberry, USGS.
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:
An electromagnetic (EM) seepage meter can assess seepage by measuring flow through an electromagnetic coil. It can continuously record the data.
Current methods will include our existing suite of geochemical and geophysical techniques that include:
The logging device for an electromagnetic (EM) seepage meter.
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.
A Geochemical and Geophysical Examination of Submarine Groundwater Discharge and Associated Nutrient Loading Estimates into Lynch Cove, Hood Canal, WA (532 KB PDF)
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 85±84 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.