Temporal and spatial fluctuations of groundwater-derived alkalinity fluxes to a semiarid coastal embayment

Highlights

• SGD rates vary spatially and seasonally.
• Fluxes of TA, DIC and DOC are driven by both SGD rates and porewater chemistry.
• Production of TA in summer, fall and winter is driven by SGD and denitrification.
• Consumption of TA, in spring, is related to CaCO3 precipitation.
• SGD and autochthonous biogeochemical process offset TA-salinity relationships.

Abstract

We conducted a comprehensive analysis of a variety of geochemical data including total alkalinity (TA), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), major ions, stable isotopes, and submarine groundwater discharge, to understand biogeochemical and hydrologic processes driving the seasonal to annual estuarine buffering capacity in Nueces Bay, Texas. These measurements, together with statistical analysis and geochemical modeling, show large variability of freshwater influence. TA consumption, common to spring seasons, was mainly driven by CaCO3 precipitation and, to some extent, by aerobic respiration. TA production occurred in some parts of the bay during summer, fall and winter, likely driven by denitrification. CaCO3 dissolution is stimulated by input of undersaturated river waters following significant flooding events. Since consumption and production of TA was not necessarily associated with different salinity zones, SGD, identified to be significant year-round, likely offsets the effects of salinity changes. Net DIC and TA fluxes exceeded dissolved organic carbon flux by an order of magnitude, except for winter 2014 when it was in the same order of magnitude. In addition to generally larger SGD rates when compared to other studies, production of TA (DIC and DOC) in the bottom sediments, as observed in this study, leads to larger fluxes, especially for the driest season (winter 2014), in the mid-bay area (6.27 · 106 μM m−2 d−1). Consistently larger inputs occur along the shoreline stations (6.14 · 106 μM m−2 d−1) following the flood recession, when compared to mid-bay (1.26 · 106 μM m−2 d−1) and are associated with lower SGD following the summer 2015 flooding. This study demonstrates that the carbonate chemistry of estuaries in semiarid areas is affected by non-conservative processes because of seasonal variability of hydroclimatic conditions.

Murgulet D., Trevino M., Douglas A., Spalt N., Hu X. & Murgulet V., 2018. Temporal and spatial fluctuations of groundwater-derived alkalinity fluxes to a semiarid coastal embayment. Science of the Total Environment 630: 1343-1359. Article (subscription required).

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