The Efficiency of Phosphate Removal via Shallow Wastewater Injection into a Saline Carbonate Aquifer

Wastewater-derived
phosphate contributes to eutrophication
if the
phosphate is not efficiently removed before it is discharged to surface
waters. In the Florida Keys (USA), shallow injection of treated wastewater
into saline limestone aquifers is a common mode of wastewater disposal.
We assessed the possibility of efficient and permanent phosphate removal
following injection at a wastewater treatment facility in Marathon,
Florida. The concentrations of nutrients, dissolved ions, and anthropogenic
compounds in groundwater and nearshore waters were monitored over
two years, as was the progression of a patch of fluorescent dye emplaced
by the wastewater injection well. The density contrast between the
wastewater effluent and saline groundwater caused the effluent plume
to buoy to the shallow subsurface near the injection well. Soluble
reactive phosphorus (SRP) and sucralose were both detected in nearshore
waters, indicating incomplete removal of contaminants. However, ∼75%
of the SRP is removed from the plume in the first 10 days of transit
by adsorption followed by a slower removal mechanism, bringing the
P removal efficiency above 90%. A positive relationship between excess
calcium and phosphate removal efficiency, together with high levels
of calcium phosphate mineral supersaturation, supports calcite dissolution
followed by calcium phosphate mineralization as this slower removal
process.