COMBINING TRACERS AND HEURISTIC FIELD METHODS TO INFER ANTHROPOGENIC AQUIFER RESPONSES IN NEARSHORE WATERS

Sewage effluent is injected into the karst aquifer along the Florida’s SE coast via >1000 shallow wells (cased depth ~20 m) and numerous high-volume (~200 mgd) deep wells. Harmful macroalgal blooms and coral decline are increasing in those waters. A pilot study evaluated the hypothesis that: 1) deep-aquifer (Floridan) discharges occur in localized areas with dense benthic macroalgae, and coral/seagrass decline, and 2) localized dense benthic macroalgae, associated with submarine groundwater discharges (SGD), exhibit stable nitrogen isotope (δ¹⁵N) ratios indicative of sewage effluent. Sites were selected in nearshore (continental shelf) surface waters in Biscayne Bay (vicinity of Black Point deep-aquifer sewage injections); Card Sound/Barnes Sound; Florida Bay (Everglades National Park); and Florida Keys ocean side (vicinity of >1000 shallow aquifer-injection wells). Dissolved helium (He) anomalies in surface waters were used as tracers of groundwater origin. Excesses of 4He indicate deep-aquifer discharges. Greatest 4He excesses were in the Marquesas Keys, associated with localized coral decline and dense benthic macroalgae, and in N Florida Bay, where 1987 seagrass dieoffs occurred. Benthic macroalgal samples from sites with dense macroalgal growth and localized coral decline had δ¹⁵N ratios indicative of sewage: 1) where effluent disposal was concentrated into shallow-aquifer sewage wells, and 2) in the Marquesas Keys, ~40 km from shallow-aquifer injection wells, septic tanks, and cess pits. Surfacewater signatures indicative of aquifer-injection zones reconfirm breached aquifer confinement, and are the first report of Floridan-aquifer discharges for the Keys ocean side. Remote sites with deep-aquifer signatures, extensive benthic macroalgal growth, and δ¹⁵N signatures indicative of sewage effluent suggest effluent-laden SGD, via karst conduits, may be a significant source of localized nutrients supporting harmful macroalgal blooms.