INVESTIGATING SUBMARINE GROUNDWATER DISCHARGE AND SUBTERRANEAN FLOW PATHS IN A COASTAL LAGOON USING FIELD PERMEAMETER AND LIGHT-OIL PIEZOMANOMETER OBSERVATIONS

Submarine groundwater discharge is now widely recognized as an important, yet difficult to quantify contributor of water and dissolved materials to the coastal ocean. Several methods have been proposed for determining rates of submarine groundwater discharge including seepage meters, Darcy calculations, numerical models, and natural radiogenic tracers. Each method introduces a unique set of limitations; therefore, multiple methods are preferred to capture the convoluted nature of submarine groundwater discharge. Here we investigate the application of a light-oil piezomanometer to map the potentiometric profile and determine the direction and relative magnitude of flow within the subterranean estuary of Little Lagoon, Alabama during the fall of 2017. The piezomanometer consists of a 3-m, gas vapor probe connected to a manometer equilibrated with surface water. In order to amplify the small head pressures just below the subsurface, oil was used in place of air in the manometer loop. A field permeameter was used to determine vertical hydraulic conductivity, and Darcy calculations were used to determine the vertical flux of submarine groundwater. An integrative approach was used to convert the vertical fluxes measured along a shore-line perpendicular transect to discharges. Our results reveal that a pressure gradient drives submarine groundwater flux from the subterranean estuary into Little Lagoon within 7.2 m of the shore. Groundwater flux decreases with distance from shore, and a slight negative flux (i.e. groundwater recharge) was detected 9.6 m offshore. Vertical gradients dominate the subterranean estuary; however, a lesser offshore lateral gradient was observed. Vertical submarine groundwater fluxes from the seafloor averaged 16 cm d^-1 and had a range of 41 cm d^-1 to -0.37 cm d^-1. The integrated submarine groundwater discharge was 0.15 m^-3 d^-1 per m shoreline, which is similar in magnitude to fresh submarine groundwater discharge measurements from nearby Mobile Bay. Our experience suggests that the piezomanometer method is a handy alternative to other direct methods when detailed information about the subterranean estuary is needed. In addition the method requires very little labor or infrastructure, allowing for multiple measurements during a day of fieldwork.