Fate of wastewater-borne nutrients under low discharge conditions in the subsurface of the Florida Keys, USA

We designed experiments to evaluate the fate of the addition of wastewater-borne nutrients injected into the shallow subsurface in the Florida Keys. During three different experiments, either bulk unlabeled phosphate, radio-labeled phosphate (32PO3−4), or bulk unlabeled nitrate (14NO1−3) was added simultaneously with conservative tracers (sulfur hexafluoride and I-131) into a wastewater injection well on Long Key. Relative concentration changes monitored over time indicated that both phosphate and nitrate acted non-conservatively in the subsurface. Phosphate showed an initial rapid uptake followed by a slower removal, possibly caused by adsorption–desorption reactions. Based on our observations, we estimate that approximately 95% of the phosphate injected into the subsurface could be removed in 20 to 50 h. There was also evidence for some removal of nitrate, possibly due to denitrification. Approximately 65% of the nitrate was removed over several days, suggesting a denitrification rate of 2700 μmol m−3 groundwater h−1, comparable to estimates of denitrification in other groundwater systems. Collectively, our results suggest that nutrients injected in the subsurface are removed rapidly from solution and thus may not have a significant impact on surface waters. However, these experiments were conducted at a relatively small facility (2.6 m3 wastewater injected per day), while some facilities in the Keys inject as much as 750 m3 day−1. Saturation of available adsorption sites and organic substrate availability may limit the efficiency of wastewater nutrient removal under such conditions.