VARIATIONS IN THE FRESHWATER/SALTWATER SYSTEM ON NORTH PADRE ISLAND, TEXAS: FREE CONVECTION OR GHYBEN-HERZBERG LENS?

The University of Texas hydrogeological field methods classes have examined the saltwater-freshwater interface on North Padre Island, Texas, periodically over a ten-year period for late May/early June 1997, 2001, 2003, 2005, and 2007. Electrical resistivity surveys of the shallow subsurface, along with measurements of water levels and chemical field parameters, indicate a wide range of groundwater system conditions over time on this barrier island. Salinity levels can reach more than twice that of seawater. In some cases, such as in 1997 and 2007, data show a general Ghyben-Herzberg type freshwater-saltwater interface, although in 2007, there may be a more complicated system at depth. In 2001, 2003, and 2005, the data show a more saline water layer above less saline water, with downward fingers of saline water associated with salinity-driven free convection. This inverted salinity pattern was confirmed by Geoprobe profiles. It is inferred that the freshwater-saltwater system is highly sensitive to climatic conditions and local heterogeneities. The freshwater lens and associated flow system varied significantly over the period of study. When evaporation is high and precipitation is low, water near the surface can reach high salinities and conditions are ripe for density-driven downward flow to take place. Periods of heavy rainfall provide freshwater input and can be used to assess likely areas of recharge, ponding, and salinity buildup.