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

Estuarine, Coastal and Shelf Science

By:
, , and
DOI: 10.1016/S0272-7714(03)00131-8

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Abstract

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 10 km offshore, promoting unwanted algal growth and degradation of water quality. We performed a field study of the fate of wastewater-derived 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 18-27 m depth into saline groundwater creates a large buoyant plume that flows quickly (within days) upward to a confining layer 6 m below the surface, and then in a fast flow path toward a canal 200 m 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 ??molkg-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 N 2 concentrations up to 1.6 times air-saturation values with ??15N[N2] = 0.5 - 5???, ?? 15N[NO3-] = 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 ??molkg-1N day-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.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
The fate of wastewater-derived nitrate in the subsurface of the Florida Keys: Key Colony Beach, Florida
Series title:
Estuarine, Coastal and Shelf Science
DOI:
10.1016/S0272-7714(03)00131-8
Volume
58
Issue:
3
Year Published:
2003
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Estuarine, Coastal and Shelf Science
First page:
517
Last page:
539