Geochemical and Geophysical Examination of Submarine Groundwater Discharge and Associated Nutrient Loading Estimates into Lynch Cove, Hood Canal, WA

Abstract

Geochemical tracer data (i.e., 222Rn and four naturally occurring Ra isotopes), electromagnetic (EM) seepage meter results, and high-resolution, stationary electrical resistivity images were used to examine the bi-directional (i.e., submarine groundwater discharge and recharge) exchange of a coastal aquifer with seawater. 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 summer-time 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 activities (0.5-29 dpm L(-1)) and water level fluctuations, and provides compelling evidence for tidally modulated exchange of groundwater across the sediment/water interface. 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 during low tide and recharge during high tide events. To evaluate the importance of fresh versus saline SGD, Rn-derived SGD rates (as a proxy of total SGD) were compared to excess 226Ra-derived SGD rates (as a proxy for the saline contribution of SGD). The calculated SGD rates, which include a significant (>80%) component of recycled seawater, 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 10(4) mol d(-1) is 1-2 orders of magnitude larger than similar estimates derived from atmospheric deposition and surface water runoff, respectively.

Atoll Groundwater Movement and Its Response to Climatic and Sea-Level Fluctuations

“…The groundwater samples, that were pumped from a depth of up to 8 m, were analyzed for ammonium (NH +4 ), dissolved silicate (DSi), total dissolved phosphorus (TDP), molybdenum (Mo), barium (Ba), uranium (U), and a suite of hydrological parameters, including pH and salinity. As per methods summarized in Swarzenski et al [30] nutrients were determined on a Lachat Instruments QuickChem 8000 at Woods Hole Oceanographic Institute (WHOI), while the suite of trace elements was analyzed on a High Resolution Inductively Coupled Plasma Mass Spectrometer at the University of Southern Mississippi. From these measurements, tidal lag and efficiency could also be determined, and is useful for estimating aquifer permeability and storage properties [15].…”

Examining lake-bottom structures with the resistivity imaging method in Ilan's Da-Hu Lake in Northeastern Taiwan

“…Loke et al (2013) gave a good review of the developments of a variety of resistivity surveys. Among these studies, a few researchers used waterborne resistivity surveys for geological mapping (Rinaldi et al, 2006;Rucker et al, 2011) and submarine groundwater discharge (Day-Lewis et al, 2006;Henderson et al, 2010;Swarzenski et al, 2007). In the current study, we conducted resistivity imaging surveys with the Lippmann 4-point Light system and passive electrodes.…”

Determining Distribution and Size of Larval Pacific Geoduck Clams (Panopea generosaGould 1850) in Quartermaster Harbor (Washington, USA) Using a Novel Sampling Approach

“…Highly detailed groundwater level measurements and field investigations along the marine shoreline, including the use of electrical resistivity profiles and geochemical tracers, were used to estimate groundwater inputs to Lynch Cove in Hood Canal as part of the Hood Canal Dissolved Oxygen Program (Swarzenski et al 2007, Simmonds et al 2008). However, these types of intensive field studies are beyond the scope of the Quartermaster Harbor Nitrogen Management study.…”