Submarine groundwater discharge in the Sarasota Bay system: Its assessment and implications for the nearshore coastal environment

A study was conducted from July 2002 through June 2006 in order to assess the significance of submarine groundwater discharge (SGD) to Sarasota Bay (SB), Florida. The assessment approaches used in this study included manual seepage meters, geochemical tracers (radon, 222Rn and methane, CH4), and subseafloor resistivity measurements. The estimated SGD advection rates in the SB system were found to range from 0.7 to 24.0 cm/day, except for some isolated hot spot occurrences where higher rates were observed. In general, SGD estimates were relatively higher (5.9–24.0 cm/day) in the middle and south regions of the bay compared to the north region (0.7–5.9 cm/day). Average dissolved inorganic nutrient concentrations within the SB water column ranged: 0.1–11 μM (NO2+NO3), 0.1–9.1 μM (NH4) and 0.2–1.4 μM (PO4). The average N/P ratio was higher in the north compared to the middle and south regions of the bay. On average, we conservatively estimate that about 27% of the total N in the SB system was derived via SGD. The prevalence of shallow embayed areas in the SB system and the presence of numerous septic tanks in the surrounding settlements enhanced the potential effects of nutrient rich seepages. Statistical comparison of the quantitative approaches revealed a good agreement between SGD estimates from manual seepage meters and those derived from the 222Rn model (p=0.67; α=0.05; n=18). CH4 was found to be useful for qualitative SGD assessments. CH4 and 222Rn were correlated (r2=0.31; α=0.05; n=54). Large scale resistivity surveys showed spatial variability that correlates more clearly with lithology than with SGD patterns.

► Average advection rates in Sarasota Bay system ranged from 0.7 to 24.0 cm/day. ► SGD is important in the Sarasota Bay system for dissolved inorganic nutrient fluxes. ► Seepage meters and 222Rn are principle complementary tools for SGD assessments. ► CH4 and resistivity profiling are viable for qualitative SGD assessments.