Submarine groundwater discharge and nutrient input to a semiarid and hypersaline estuary: Baffin Bay, Texas

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Date Issued
2018-122018-12
Author
Lopez, Cody Vaughn
Lopez, Cody Vaughn
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This study evaluates the role of submarine groundwater discharge (SGD)-derived
nutrients to Baffin Bay, a semi-arid, hypersaline bay in south Texas. SGD can be equivalent to
riverine input in some places, making SGD’s role in nutrient input important. Dissolved
inorganic nitrogen (DIN), and total alkalinity combined with SGD measurements, using two
different geochemical tracers (radium-226 [226Ra], radium 224 [224Ra] and radon [222Rn]) and
geophysical investigations were conducted. SGD rates were found to have slight spatial variation
with higher rates near the shoreline around areas characterized by course-grained sediments and
relic serpulid reefs. The222Rn and 226Ra-based SGD estimates produced agreeable results, within
the range of uncertainties, and no significant changes in SGD from July to November, within the
same year, were observed. However, 226Ra and 224Ra activities decreased from July to November
and are associated with large decreases in porewater DIN concentrations. July and November
222Rn-derived SGD rates were 31.4±32.7 and 30.0±30.9 cm∙d-1, respectively while those derived
from 226Ra were 16.6±1.7 and 13.2±1.3 cm∙d-1, respectively. Given the lack of change in SGD
between the two seasons, organic matter (OM) decay may be the driving force for changes in
radium activities as it can lead to reducing conditions that enhance radium solubility from
sediments. In addition to OM remineralization from phytoplankton, a shift from a seawater to a
terrestrial groundwater source in the subterranean estuary is also likely to be responsible for the
larger porewater radium activities and nutrient concentrations in July. A comparison of bay-wide
solute fluxes indicates that DIN inputs, mainly in the form of ammonium (NH4+), are almost five
orders of magnitude higher in the SGD component than the surface runoff. Therefore, regardless
of the magnitude of SGD and its nature (i.e. fresh or saline; groundwater or recirculated saline),
the associated nutrient input is likely significant in this shallow bay system in warmer months.
This study helps provide an understanding of the possible effects of OM decay on radium and
DIN fluctuations and inputs in a hypersaline estuary. Studying these relationships is important as
hypersalinity is a developing problem in freshwater-limited environments.
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