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 (6.27·106µMm-2d-1). Consistently larger inputs occur along the shoreline stations (6.14·106µMm-2d-1) following the flood recession, when compared to mid-bay (1.26·106µMm-2d-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.
ABSTRACT
There is a lack of understanding and methods for assessing the effects of anthropogenic disruptions, (i.e. river fragmentation due to dam construction) on the extent and degree of groundwater-surface water interaction and geochemical processes affecting the quality of water in semi-arid, coastal catchments. This study applied a novel combination of electrical resistivity tomography (ERT) and elemental and isotope geochemistry in a coastal river disturbed by extended drought and periodic flooding due to the operation of multiple dams. Geochemical analyses show that the saltwater barrier causes an increase in salinity in surface water in the downstream river as a result of limited freshwater inflows, strong evaporation effects on shallow groundwater and mostly stagnant river water, and is not due to saltwater intrusion by tidal flooding. Discharge from bank storage is dominant (~84%) in the downstream fragment and its contribution could increase salinity levels within the hyporheic zone and surface water. When surface water levels go up due to upstream freshwater releases the river temporarily displaces high salinity water trapped in the hyporheic zone to the underlying aquifer. Geochemical modeling shows a higher contribution of distant and deeper groundwater (~40%) in the upstream river and lower discharge from bank storage (~13%) through the hyporheic zone. Recharge from bank storage is a source of high salt to both upstream and downstream portions of the river but its contribution is higher below the dam. Continuous ERT imaging of the river bed complements geochemistry findings and indicate that while lithologically similar, downstream of the dam, the shallow aquifer is affected by salinization while fresher water saturates the aquifer in the upstream fragment. The relative contribution of flows (i.e. surface water releases or groundwater discharge) as related to the river fragmentation control changes of streamwater chemistry and likely impact the interpretation of seasonal trends.
