A HYDROGEOLOGIC TRANSECT OF NORTH PADRE ISLAND: LAGUNA MADRE TO THE GULF OF MEXICO

Since 1985, The University of Texas has taught a summer hydrogeology field methods course for seniors and graduate students in which they learn a suite of field tests for analyzing hydrogeological systems. The course concludes with a project where the students apply selected methods at a site or area and write a formal hydrogeologic report. The 2003 class analyzed a cross section of North Padre Island, Texas, between the Gulf of Mexico and Laguna Madre. They hypothesized that the island’s fresh-water system is a locally isolated system, the shape of the lens is skewed by fluid density differences, and that natural (free) convection occurs in the sediments underneath and adjacent to Laguna Madre. Water chemistry, grain size analyses, field reconnaissance, surveying, piezometer installation, analysis of tidally-induced water level fluctuations, seepage meters, earth resistivity, and Geoprobe techniques were applied. Conductivity in the Laguna Madre, which experiences evaporative concentration, was significantly higher than in the Gulf of Mexico. Seepage meter measurements from ponds distributed across the island demonstrate freshwater discharge. Water chemistry and piezometers characterized the shallow groundwater and surface water systems. Electrical resistivity determined depth of fresh and saline water interfaces and extent of mixing zones; the data show a fresh-saline water interface at a depth of ~14 meters at the island center that shallows to the east and west. Three electrical conductivity (EC) logs were taken using an EC probe driven by a Geoprobe at the west, central, and eastern areas The west EC log displayed a high conductivity peak of 27311 mSm-1 at a depth of 7.13 m. On the east, the EC log peaked at 4064.24 mSm-1 at a depth of 9.01 m. The center EC probe was pushed to a depth of 21.17 m where the highest recorded conductivity was 549.36 mSm-1. The EC log and resistivity data show salinity peaks at varying depths across the transect and demonstrate that the freshwater flow system is a localized system isolated from the underlying Chicot aquifer. Applying the Ghyben-Herzberg equation, the fresh-water lens is estimated to be thinner on the Laguna Madre than on the Gulf side of the island. Thermohaline free convection may also thin the lens on the Laguna Madre side and push the interface inland.