Seasonal dynamics in costal aquifers : investigation of submarine groundwater discharge through field measurements and numerical models

Abstract

The fresh and saline groundwater flowing from coastal aquifers into the ocean comprise submarine groundwater discharge (SGD). This outflow is an important pathway for the transport of nutrients and contaminants, and has been shown to adversely affect coastal ecosystems in many areas of the world. The focus of this work is the characterization of SGD and the mechanisms that drive it, with a specific emphasis on seasonal forcing. Field measurements during five summers in Waquoit Bay, Massachusetts reveal the pattern and composition of submarine groundwater discharge. Flow is highly variable over small spatial and temporal scales, and the salinity and radium content of the discharge demonstrates heterogeneity in groundwater origin. Maximum discharge occurred in two alongshore bands: brackish outflow nearshore and saline discharge offshore. Most of the total flow was saline, yet net seawater inflow over a tidal cycle was negligible. Circulation mechanisms such as tides, waves, and hydrodynamic dispersion cause significant saline groundwater discharge, and are potentially important for chemical loading to estuaries. However, these mechanisms can explain only 12-30% of the observed saline outflow in Waquoit Bay. A seasonal forcing mechanism is proposed to explain the source of the remaining observed saline outflow. During periods of high inland recharge, the water table rises, forcing seaward movement of the freshwater-saltwater interface and outflow of saline groundwater; the opposite is true during times of low recharge. A series of idealized simulated systems demonstrates this process for a range of realistic aquifer parameters, and a time lag between maximum recharge and simulated peak discharge may explain the observed net discharge during times of low recharge.

(cont.) Winter hydraulic gradient measurements in Waquoit Bay reveal inflow in the zone of peak summer saline discharge, confirming seasonal variation in SGD. Investigation of the subsurface salinity profile and local hydrogeology forms the basis for a hypothesized groundwater flow pattern that explains the observed discharge. A numerical model of the system supports the profile and exhibits temporally-lagged inflow and outflow of saltwater at the sea floor in response to seasonal recharge that may explain the net saline outflow observed in Waquoit Bay during the summer.