The fate of wastewater-derived nitrate in the subsurface of the Florida Keys: Key Colony Beach, Florida

Shallow injection is the predominant mode of wastewater disposal for most tourist-oriented facilities and some residential communities in the US Florida Keys National Marine Sanctuary. Concern has been expressed that wastewater nutrients may be escaping from the saline groundwater system into canals and surrounding coastal waters and perhaps to the reef tract 10km offshore, promoting unwanted algal growth and degradation of water quality. We performed a field study of the fate of wastewaterderived nitrate in the subsurface of a Florida Keys residential community (Key Colony Beach, FL) that uses this disposal method, analyzing samples from 21 monitoring wells and two canal sites. The results indicate that wastewater injection at 18–27m depth into saline groundwater creates a large buoyant plume that flows quickly (within days) upward to a confining layer 6m below the surface, and then in a fast flow path toward a canal 200m to the east within a period of weeks to months. Low-salinity groundwaters along the fast flow path have nitrate concentrations that are not significantly reduced from that of the injected wastewaters (ranging from 400 to 600 lmol kg 1). Portions of the low-salinity plume off the main axis of flow have relatively long residence times (>2 months) and have had their nitrate concentrations strongly reduced by a combination of mixing and denitrification. These waters have dissolved N2 concentrations up to 1.6 times air-saturation values with d15N½N2 ¼ 0:5 5&, d15N½NO 3 ¼ 16 26&, and calculated isotope fractionation factors of about 12 4&, consistent with denitrification as the predominant nitrate reduction reaction. Estimated rates of denitrification of wastewater in the aquifer are of the order of 4 lmol kg 1Nday 1 or 0.008 day 1. The data indicate that denitrification reduces the nitrate load of the injected wastewater substantially, but not completely, before it discharges to nearby canals.