Time-lapse Electric Resistivity Imaging of Subsurface Salt Mobilization in an Impounded Mangrove Forest.

Abstract:

The study attempts to image the mobility of saline porewater beneath a black mangrove forest on a siliciclastic barrier island off the Atlantic coast of Florida. The forest is broken up into a series of impoundments bounded by earthen dikes, which prevent surface water exchange between both neighboring impoundments and a brackish, tidal intracoastal lagoon. Water management practices pump lagoon water into impoundments for six months (‘wet’ season) and leave them to dry up for another six (‘dry’ season). Our study was conducted during the end of a ‘dry’ season. The movement or storage of pumped salts is unknown. Our objective is to capture that movement-- in the absence of surface interaction between impoundments and the lagoon-- over a diurnal cycle with time-lapse electric resistivity.

The resistivity transect crosses over a saltpan with little to no vegetation and extended into black mangrove trees at either end. Throughout the diurnal period, a relatively conductive zone persists from a depth of 1 meter to as deep as 4 meters. This zone becomes less laterally continuous in the evening. There is a prominent resistive zone positioned beneath a small patch of mangrove shrubs that cuts through this conductive zone overnight. The vegetated ends of the array also appear most resistive overnight when black mangroves excrete the most salt through their leaves. These areas become more conductive during the day, which is when roots take in more water, and salt, to maintain high osmotic pressure. Direct groundwater data via CTD sensors recorded at 15-minute intervals for the survey period. Though head values from the first 1.2 meters of the subsurface do not appear to be tidally influenced, there is a rough cyclical pattern and upward (toward the surface) potentiometric flow. That gradient is largest at midday and smallest after midnight, suggesting a diurnal and/or vegetation mechanism. Direct porewater conductivity changes are small and consistent between sensor depths.

The overall balance in a given tree is maintained, but the subsurface experiences a net gain in salt concentrations. The fate of excess salt is, to our knowledge, poorly understood. Repeated electric resistivity arrays provide spatial and temporal information about these salts and contribute to an overall understanding how impounded mangrove forests behave.