MULTISCALE GROUNDWATER-SURFACE WATER FLOW PATHS AND RESIDENCE TIMES IN COMPLEX COASTAL WETLANDS’ CHANNEL-ISLAND-LAGOON TOPOGRAPHY

In coastal wetlands, tides and river discharge often force surface water through complex channel, island, and lagoon topography. Resultantly complex flow paths, groundwater-surface water mixing, and residence times likely substantially affect the ecology and biogeochemical functions of the wetlands but are seldom characterized. We developed a transient, three dimensional, coupled groundwater-surface water model for a channel-island-lagoon system in the Wax Lake Delta (LA, USA), to quantify the variability of these dynamics on a tidal timescale. Tidally forced surface water interacting with the complex topography of the island set up head gradients that caused surface water and groundwater flow path, and residence times to span a wider spatial and temporal scale. The lag in the water level between the tidal channel flow and the slower lagoon filling/draining caused surface water to recharge into the lagoon bed and discharge along the outer shoreline of the island. Rapid tidal changes in the channel water level also induced water to recirculate in the outer island banks, in a narrow zone exhibiting high flow-direction variability and high mean velocity. The rest of the island exhibited generally low-velocity, and almost unidirectional lagoon-to-channel flow on the tidally-averaged timescale. The tidally-averaged exchange flux from groundwater to surface water varied from -6 to +6 cm d^-1 and was highest along the outer shoreline of the island during low tide, when the water level difference between the higher lagoon and lower channel was largest. The average volume of water exchanged over half a tidal cycle is 1.2×10⁴ m³ which is about 1% of the volume of filling/draining tide water over the surface domain. For surface water around the island, residence time estimates were on the order of hours-to-days in the channel and near the lagoon mouth but surprisingly long in the inner lagoon, at several months to more than a year. A groundwater transit time in the order of 5 years was estimated based on a lagoon to channel mean horizontal groundwater velocity of 5 cm d^-1. This work demonstrates the complex dynamics and interactions of groundwater and surface water in channel-island-lagoon systems, which are ubiquitous features in coastal areas, but little-characterized in terms of coupled surface water-groundwater dynamics.