2-14 A Pilot Monitoring Program to Improve Understanding of the Inland Migration of Saltwater Through Coastal Canals in Miami-Dade County Florida

Session: Coastal Hydrogeology in an Age of Rising Seas

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Surface-water drainage and groundwater withdrawals in southeast Florida have lowered the inland water table and led to gradual saltwater intrusion along the base of the Biscayne aquifer.  The US Geological Survey (USGS), in cooperation with Miami-Dade County, operates an extensive monitoring well network to track inland migration of saltwater near the base of the Biscayne aquifer, an EPA designated sole source aquifer.  As early as 1942, researchers recognized that leakage of saline water from coastal canals likely also contributes to saltwater contamination in the Biscayne aquifer. Salinity control structures are operated to maintain inland freshwater stages above sea level and provide a barrier to prevent saltwater from flowing inland through canals. Saltwater can flow inland when gates at the structures are opened to allow seaward discharge or if the gates are leaky.  Saltwater can also circumvent these structures by flowing through the adjacent highly porous aquifer.  Existing canal salinity data for southeast Florida are sparse, but in 2024, the USGS began a pilot monitoring program to characterize the inland migration of saltwater through coastal canals in Miami-Dade County. This includes continuous specific conductance monitoring in canals at the headwater side of the control structure and an upstream location. Additionally, when saltwater reaches the upstream site, measurements are taken by the USGS to obtain cross-sectional profiles of specific conductance.  Data from the Biscayne Canal show that specific conductance greater than 40,000 uS/cm can persist for long periods (over a month) on the headwater side of the structure when the gates are closed. During these periods, specific conductance at the upstream site (approximately 1.5 miles upstream from the structure) increases to as high as 10,000 uS/cm.  The cross-sectional profiles of specific conductance show that the canal is not well mixed vertically or horizontally.  These observations indicate that saltwater is getting trapped upstream of the structure when the gates are closed and is likely migrating upstream through the deepest portion of the canal.  Specific conductance changes rapidly, at times fluctuating nearly two orders of magnitude in less than half an hour.  This, combined with large spatial variability due to poor mixing, make continued monitoring at high temporal and spatial resolution important for further characterization of the inland migration of saltwater through southeast Florida’s canals.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10006

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