Submarine groundwater discharge to Tampa Bay: Nutrient fluxes and biogeochemistry of the coastal aquifer

Abstract

To separately quantify the roles of fresh and saline submarine groundwater discharge (SGD), relative to that of rivers, in transporting nutrients to Tampa Bay, Florida, we used three approaches (Darcy's Law calculations, a watershed water budget, and a 222 Rn mass-balance) to estimate rate of SGD from the Pinellas peninsula. Groundwater samples were collected in 69 locations in the coastal aquifer to examine biogeochemical conditions, nutrient concentrations and stoichiometry, and salinity structure. Salinity structure was also examined using stationary electrical resistivity measurements. The coastal aquifer along the Pinellas peninsula was chemically reducing in all locations sampled, and that condition influences nitrogen (N) form and mobility of N and PO 4 3− . Concentrations of NH 4 + , PO 4 3− and ratio of dissolved inorganic N (DIN) to PO 4 3− were all related to measured oxidation/reduction potential (pε) of the groundwater. Ratio of DIN: PO 4 3− was below Redfield ratio in both fresh and saline groundwater. Nitrogen occurred almost exclusively in reduced forms, NH 4 + and dissolved organic nitrogen (DON), suggesting that anthropogenic N is exported from the watershed in those forms. In comparison to other SGD studies, rate of PO 4 3− flux in the seepage zone (μM min Tampa Bay was higher than previous estimates, likely due to 1) high watershed population density, 2) chemically reducing conditions, and 3) high ion concentrations in fresh groundwater. Estimates of freshwater groundwater flux indicate that the ratio of groundwater discharge to stream flow is ∼ 20 to 50%, and that the magnitudes of both the total dissolved nitrogen and PO 4 3− loads due to fresh SGD are ∼ 40 to 100% of loads carried by streams. Estimates of SGD based on radon inventories in near-shore waters were 2 to 5 times greater than the estimates of freshwater groundwater discharge, suggesting that brackish and saline SGD is also an important process in Tampa Bay and results in flux of regenerated N and P from sediment to surface water. Published by Elsevier B.V.

Dynamics of groundwater‐derived nitrate and nitrous oxide in a tidal estuary from radon mass balance modeling

“…When surface discharge returned to baseflow conditions (on the days prior to survey), reinvasion of the salt wedge at the sampling station mobilized the 222 Rn-rich pore water from the surface sediment as a result of buoyancy-driven mixing between the fresh pore water and the salt wedge (Santos et al 2012). (Garrison et al 2003;Kroeger et al 2007), on coral reefs (Blanco et al 2011), and on the continental shelf (Burnett et al 2007). In this study, the NO { 3 fluxes were in the upper range (river : SGD ratio , 3.3 3 10 22 ) in comparison with the nitrogen fluxes from other similar studies on coastal environments (river : SGD ratio of 0.5 : 11; see Table 4).…”

Nitrogen biogeochemistry of submarine groundwater discharge

“…However, in groundwater from individual watersheds, NH z 4 contributed as much as 24% of TDN. In watersheds with widespread reducing conditions in the fresh aquifer, NH z 4 can compose ,100% of DIN concentration (Kroeger et al 2007). Regardless of their source, the nitrate and ammonium plumes allow us to observe the behavior of nitrogen in the nearshore aquifer, as we will discuss in the section Biogeochemical transformations.…”

“…Naturally vegetated areas range from grass and shrublands to pitch pine and mixed oak forests. The head of Waquoit Bay watershed has a human population density of 190 km 22 , and wastewater disposal is through on-site septic systems (Kroeger et al 2006a). At our study site in the nearshore aquifer, solid phase organic carbon is low (nondetectable to 0.075%; Charette et al 2005), and dissolved organic carbon concentrations range from 7 to 700 mmol L 21 (Charette and Sholkovitz 2006; Web Appendix www.aslo.org/lo/toc/vol_53/issue_3/1025al.…”

“…Hence, SGD often consists of a substantial amount of recirculating seawater, and thus results in fluxes of sediment-regenerated nutrients and other pore-water materials to coastal waters. Comparisons at a range of scales indicate that chemical loads to coastal waters from SGD commonly rival loads from riverine transport (Taniguchi et al 2002;Slomp and Van Cappellin 2004;Kroeger et al 2007). …”

Nitrates in Groundwater Discharges from the Azores Archipelago: Occurrence and Fluxes to Coastal Waters

“…The concentration of nutrients from discharging groundwater is often higher than in coastal receiving waters [4], as shown for nitrates which present levels that are two to three orders of magnitude higher than typical coastal waters [5]. Therefore, transport of nutrients toward coastal waters through groundwater discharge can be a driver to the deterioration of water quality in these receiving environments, leading to ecosystem changes [6][7][8][9].…”