Wave, tidal, and seasonal dynamics of groundwater flow, saltwater-freshwater mixing, and reactive transport in beach aquifers
Date
2017
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Nutrient, metal, and carbon fluxes in submarine groundwater discharge can
adversely impact the chemistry of nearshore marine ecosystems. These solutes can
undergo biogeochemical transformations in saltwater-freshwater mixing zones that
form when fresh groundwater flowing toward the sea meets with saline groundwater
of marine origin in sandy beaches. This dissertation focuses on the driving
mechanisms of flow, the time scales of mixing, and biogeochemical processes that are
key in altering the fate and fluxes of nutrients prior to discharge to the sea. ☐ A field and numerical modeling study investigated intertidal salinity dynamics
in a sandy tidally-influenced beach aquifer across a wide range of temporal scales and
hydrologic forcings. Seasonal variations in the terrestrial freshwater gradient were
primarily responsible for controlling the shape and size of the intertidal saltwaterfreshwater
mixing zone, followed by spring-neap variability in tidal amplitude, and
tidal stage. Intertidal salinities decreased as the seasonal freshwater hydraulic gradient
increased in spring and winter, and increased as freshwater forcing decreased in
summer. The effects of Hurricane Sandy and seasonal sea level anomalies on the
subsurface salinity distribution were minor compared to seasonal freshwater forcing. ☐ Coupled surface and subsurface measurements reveal for the first time the
motion and areal extent of infiltration, recharge, and discharge zones at swash and
tidal time scales under and across the beachface. Infiltration was controlled by the
location of wave runup and occurred across a part of the beach widely accepted in
literature to be a zone of groundwater discharge, while recharge occurred at both
swash and tidal time scales. The results demonstrate that identification of infiltration,
recharge, and discharge zones can be achieved only through coupled measurements of
the swash location and water content conditions in the beach. A more accurate
conceptual model of groundwater-surface water interactions in the intertidal zone is
developed that will require that sediment transport models be reevaluated to properly
represent zones of groundwater-surface water exchange. ☐ A variable-density groundwater flow and reactive solute transport model of a
beach aquifer was used to investigate the influence of 5 physical factors, including
tidal amplitude, freshwater flux, hydraulic conductivity, beach slope, and dispersivity
on the biogeochemical reactivity of the intertidal zone for mixing-dependent and
mixing-independent reactions, modeled as denitrification and sulfate reduction,
respectively. A sensitivity analysis of nitrate and sulfate removal efficiencies
demonstrates that tidal forcing promotes denitrification along the boundary of the
intertidal saltwater circulation cell between 1 and 10 ppt. Denitrification increases
with the size of the mixing zone, while sulfate supply is the main factor controlling
sulfate reduction. The results reveal the type of beaches that are most chemically
active and have the largest role in moderating chemical fluxes to the sea. ☐ These studies demonstrate that hydrologic and biogeochemical processes
interact across time and space, and that these interactions moderate chemical fluxes to
the sea. Thus, the findings have important implications for managing marine
ecosystems, and the recreational and economic resources they provide.