Groundwater and surface water phosphorus concentrations, Everglades National Park (FCE), South Florida for June, July, August and November 2003

Seawater intrusion into a coastal aquifer mixes with the discharging fresh water to form a zone of mixed composition. This mixing zone is considered to be geochemical important in a carbonate aquifer as an area of enhanced carbonate mineral dissolution and,or precipitation. Ion exchange reactions are also common within the mixing zone as the dominant cation in seawater, sodium, replaces other ions adsorbed to the aquifer matrix. Phosphorus has a strong affinity for adsorption to calcium carbonate minerals. Both the dissolution of calcium carbonate minerals as well as ion exchange reactions have the potential to release phosphorus from the aquifer matrix to the groundwater. Discharge of this phosphorus-laden groundwater, as induced by the fresh/saline water interface, may then be an additional source of phosphorus to the overlying coastal environments. Both surface water and groundwater were collected across the seawater intrusion zone of the coastal Everglades in south Florida during the summer of 2003. Hydrogen sulfide was released from the groundwater wells during sampling, indicating the groundwater was most likely anoxic. In order to reduce the potential exposure of the groundwater to oxygen during sampling, groundwater samples were collected in the following manner. Groundwater wells were first purged of at least three well volumes prior to sampling. Water samples were then collected using a submersible pump with a 16-gauge needle fitted at the end of the discharge hose. The needle was pushed through a rubber stopper covering an acid-washed vacutainer that was first flushed with nitrogen gas to remove and oxygen, then evacuated with a vacuum pump to establish a negative pressure within the vacutainer. Water samples were stored on ice and transported to the laboratory. Samples were processed for total phosphorus using colorimetery following dry-oxidation/acid hydroloysis methods within 1 to 5 days following sample collection. Salinity of the groundwater and surface water was determined at the time of sampling using an Orion S-C-T meter. Total phosphorus concentrations in 11 groundwater wells ranged from 0.5 to 2.3 uM (16 to 73.6 ug/L-P). Concentrations of total phosphorus in the overlying surface water were consistently lower and ranged from 0.16 to 0.45 uM (5.1 to 14.4 ug/L-P). There was a strong positive correlation between total phosphorus concentrations in the groundwater with salinity (R2=0.6). The results of this investigation indicate enhanced phosphorus concentrations within mixing zone groundwaters underlying the southern Everglades. These concentrations exceed the expected values predicted by conservative mixing of local fresh groundwater and intruding seawater, which both have TP less than 1 uM.