56 REPORT OF INVESTIGATIONS No. 50
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Figure 23. Chloride concentration in St. Johns River, northeastern Florida.
shows that the quality of the water in the St. Johns River is highly variable, especially from the headwaters to State Highway 46 and in the lower reaches where salt-water encroachment occurs. At low flow the most abundant constituent in the water is chloride, but sodium, sulfate, calcium, and magnesium are also present in high concentrations. Table 9 presents a comparison of the chemical analyses of water in the St. Johns River on June 7, 1962 at State Highways 520 (station 36), 50 (station 35), and 16.
The increase in concentration from State Highway 520 to State Highway 50 was caused by more ground-water inflow. The high concentrations of dissolved minerals at 'State Highway 16 are caused by sea-water encroachment. The lower concentrations at low flow between the Wekiva River and U. S. Highway 17 are caused by dilution from relatively fresh spring flow.
As the rainy season began in late June 1962, the flow in the St. Johns River increased and the quality of the water improved.