Year of Publication

2015

Degree Name

Master of Science (MS)

Document Type

Master's Thesis

College

Arts and Sciences

Department

Earth and Environmental Sciences (Geology)

First Advisor

Dr. Audrey Hucks Sawyer

Abstract

In coastal rivers, tides can propagate for tens to hundreds of kilometers inland beyond the saltwater line. Yet the influence of tides on river-aquifer connectivity and solute transport in tidal freshwater zones (TFZs) is largely unknown. We estimate that along the TFZ of White Clay Creek (Delaware, USA), more than 17% of river water exchanges through hyporheic and riparian storage zones due to tidal pumping alone. Additional hyporheic processes such as flow through bedforms likely contribute even more exchange. The turnover length associated with the tidal pumping process is 39 km, similar to turnover lengths for all hyporheic exchange processes in non-tidal rivers of similar size. Based on measurements at a transect of piezometers located 17 km from the coast, tidal pumping exchanges 0.44 m3 of water across the bank and 0.49 m3 across the bed per unit river length. Exchange fluxes range from -0.81 to 1.68 m d-1 across the bank and -0.84 to 1.88 m d-1 across the bed. During rising tide, river water infiltrates into the riparian aquifer, and the downstream transport rate in the channel is low (1.45 m3 s-1). During falling tide, stored groundwater is released to the river, and the downstream transport rate in the channel increases by 380%. Tidal bank storage zones may remove nutrients or other contaminants from river water and attenuate nutrient loads to coasts. Alternating expansion and contraction of aerobic zones in the riparian aquifer likely influence contaminant removal along flow paths. A clear need exists to understand contaminant removal and other ecosystem services in TFZs and adopt best management practices to promote these ecosystem services.

Water Table Fluctuation Logs.pdf (300 kB)
Water Table and Stage Fluctuation Data