Spatial and temporal chemical variability in the Hillsborough River system

The Hillsborough River originates in the Green Swamp, a principal recharge area of the Floridan Aquifer system. As the river flows to Tampa Bay, its chemistry is influenced by a variety of natural and anthropogenic inputs. Spatial and temporal variations in the riverʹs major ion concentrations, CO2-system chemistry, and nutrients were examined in a two-year study between 1999 and 2001. At thirteen sampling stations along approximately 87 km of the river, water samples were collected in surroundings that ranged from pristine to urban. Concentrations of major ions were lowest in the riverʹs headwaters, showed only minor spatial variations mid-river, and sharply increased in tidally influenced waters below the dam on the lower river. The mid-river major ion composition is relatively constant in large part because two of the riverʹs most compositionally distinctive inputs, Blackwater Creek and Crystal Springs, are located well upstream. Below the confluence of Blackwater Creek and river water that is largely derived from Crystal Springs, concentrations of Na+, K+, Mg2+, Ca2+, HCO3−, F−, Cl− and PO43− showed spatial variations that were smaller than +/− 30%. Only SO42− and NO3− exhibited strong downstream variations. Sulfate concentrations increased by more than a factor of two, and NO3− decreased by more than an order of magnitude. In contrast to spatial variations in the riverʹs chemistry, temporal variations were quite large. Concentrations of major ions decreased during the rainy season (June–September) by as much as a factor of 3–5, and phosphate concentrations increased by approximately an order of magnitude. The riverʹs CO2-system also showed strong seasonal variations. River pH and CaCO3 saturation state decreased sharply during periods of high precipitation. CaCO3 supersaturation was observed during the exceptionally dry periods of the study, and undersaturation was observed during periods of high rainfall. Overall, the Hillsborough Riverʹs delivery of solutes to Tampa Bay is greatly influenced by temporal variations in river hydrology, and distinct chemical signatures from the riverʹs tributaries, groundwater sources, and anthropogenic inputs. The riverʹs output of phosphate to the bay, which is exceptionally high during periods of high river-flow, is especially notable.