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 Ž 3y. Ž 1y. Ž PO , or bulk unlabeled nitrate NO was added simultaneously with conservative tracers sulfur hexafluoride and 4 3 . 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 mmol m groundwater h, 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 Ž 3 . 3 a relatively small facility 2.6 m wastewater injected per day , while some facilities in the Keys inject as much as 750 m day. Saturation of available adsorption sites and organic substrate availability may limit the efficiency of wastewater nutrient removal under such conditions. q 2000 Elsevier Science B.V. All rights reserved.