Browsing by Author "Heiss, James"
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Item Intertidal mixing zone dynamics and swash induced infiltration in a sandy beach aquifer, Cape Henlopen, Delaware(University of Delaware, 2011) Heiss, JamesSubmarine groundwater discharge has been shown to be an important source of nutrients, heavy metals, and organic compounds to the coastal ocean. Physical flow and mixing dynamics in the intertidal zone may influence these contaminant fluxes; however the mechanisms that contribute to the spatial and temporal dynamics of mixing of saltwater and through-flowing freshwater are not well understood. Flow systems in two shallow beach aquifers at Cape Henlopen, DE were characterized over multiple time scales using multilevel porewater samplers, pressure transducers, and moisture sensors. Porewater samples indicate the presence of a saline groundwater circulation cell beneath the beachface that is formed by seawater infiltrating across the upper beachface, which then circulates downward and seaward before discharging farther offshore. The circulation cell deepens and its areal extent increases when forced with a larger tidal amplitude. Longer term monitoring shows that higher inland water tables during spring and the resultant increase in freshwater forcing cause the circulation cell to freshen while lower inland water tables associated with winter result in a well-developed density inversion. A study of swash-induced infiltration was performed to quantify the effects of swash zone width and tidal elevation on the flux of seawater into the beach aquifer. High-frequency pressure and soil moisture measurements from shore-perpendicular arrays across the beachface were used to infer influx rates. Direct measurement of moisture content in the unsaturated zone and capillary fringe is needed to infer fluxes due to moisture conditions above the water table that are not measured by the pressure sensors. Infiltration occurred during the rising tide at the leading edge of the swash zone and increased in magnitude from low tide to high tide. Infiltration rates were on average 2.2 times greater near mean higher high water than near mean lower low water. Measurements revealed that swash zone width influences infiltration: influx rates associated with a beach with a wider swash zone were 2.7 times higher than those of a beach with a narrower swash zone. These insights into beach groundwater dynamics have implications for estimating fluid and chemical fluxes to coastal waters.Item Wave, tidal, and seasonal dynamics of groundwater flow, saltwater-freshwater mixing, and reactive transport in beach aquifers(University of Delaware, 2017) Heiss, JamesNutrient, 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.