GSA Connects 2021 in Portland, Oregon

Paper No. 235-10
Presentation Time: 4:10 PM


FLINT, Madison, MARTIN, Jonathan and OBERHELMAN, Andrew, Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611-2120

The global rise in reactive nitrogen species abundances corresponds with an increase in atmospheric concentrations of the greenhouse gas nitrous oxide (N2O). To understand N2O dynamics in the carbonate critical zone, water samples were collected from selected locations along the 121 km length of the Santa Fe River in north-central FL. In the upper watershed, a siliciclastic confining unit separates the river from the karstic Floridan aquifer. In the lower watershed, the confining unit is absent, allowing extensive surface water-groundwater mixing that affect nitrogen dynamics, including possible N2O production and consumption. Surface water pools in and drains from wetlands in the upper watershed and is tannic (DOC up to 50 mg C/L) with a low median NO3- concentration of 0.13 mg N/L. Spectroscopic values (high HIX, SUVA254 and low FI, BIX, and SR) indicate this DOC is allochthonous with high aromaticity and molecular weight. This water has median N2O concentrations of 0.35 µg N/L, which is slightly elevated over atmosphere equilibration (0.30 µg N/L). Clear water (DOC ≤ 1 mg C/L) in the lower watershed is sourced primarily from the Floridan aquifer as indicated by elevated specific conductivity values and has a median NO3- concentration of 0.91 mg N/L. Spectroscopic values (low HIX, SUVA254 and high FI, BIX and SR) indicate the DOC is autochthonous with low aromaticity and molecular weight and may be more labile and support more denitrification than the upstream DOC. The downstream water has a median N2O concentration of 0.91 µg N/L, approximately 2.6 times that of upstream water. The highest N2O concentrations in the river (~6 µg N/L) occur in an ~10 km reach of the river where it flows across the confined-unconfined boundary of the Floridan aquifer. The N2O concentrations correlate positively with specific conductivity, suggesting mixing of DO-rich surface water and groundwater with labile DOC and high NO3- concentrations may increase N2O concentrations through incomplete denitrification during short term mixing (hours-days). As groundwater input increases downstream, N2O concentrations decrease as a result of longer residence times (years-decades) of discharging groundwaters, where complete reduction of N2O to N2 occurs.