CONTROLS ON N2O DYNAMICS AT A CARBONATE-SILICATE CRITICAL ZONE BOUNDARY OF NORTH-CENTRAL, FLORIDA

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 S_R) 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 S_R) 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.