Sources of Nutrients and Fecal Indicator Bacteria to Nearshore Waters on the North Shore of Kaua`i (Hawai`i,USA) |
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Authors: | Karen L Knee Blythe A Layton Joseph H Street Alexandria B Boehm Adina Paytan |
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Institution: | (1) Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA;(2) Environmental and Water Studies, Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA;(3) Institute of Marine Sciences, University of California, Santa Cruz, CA 95064, USA |
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Abstract: | Water quality monitoring in Hanalei Bay, Kaua`i (Hawai`i, USA) has documented intermittent high concentrations of nutrients
(nitrate, phosphate, silica, and ammonium) and fecal indicator bacteria (FIB, i.e., enterococci and Escherichia coli) in nearshore waters and spurred concern that contaminated groundwater might be discharging into the bay. The present study
sought to identify and track sources of nutrients and FIB to four beaches in Hanalei Bay and one beach outside the bay, together
representing a wide range of land uses. 223Ra and 224Ra activity, salinity, nutrient and FIB concentrations were measured in samples from the coastal aquifer, the nearshore ocean,
springs, the Hanalei River, and smaller streams. In addition, FIB concentrations in beach sands were measured at each site,
and the enterococcal surface protein (esp) gene assay was used to investigate whether the observed FIB originated from a human source. Nutrient concentrations in groundwater
were significantly higher than in nearshore water, inversely correlated to salinity, and highly site specific, indicating
local controls on groundwater quality. Fluxes of groundwater into Hanalei Bay were calculated using a mass-balance approach
and represented at least 2–10% of river discharges. However, submarine groundwater discharge (SGD) may provide 2.7 times as
much nitrate + nitrite to Hanalei Bay as does the Hanalei River. It may also provide significant fluxes of phosphate and ammonium,
comprising 15% and 20% of Hanalei River inputs, respectively. SGD-derived silica inputs to the bay comprised less than 3%
of Hanalei River inputs. FIB concentrations in groundwater were typically lower than those in nearshore water, suggesting
that significant FIB inputs from SGD are unlikely. Positive esp gene assays suggested that some enterococci in environmental samples were of human fecal origin. Identifying how nutrients
and FIB enter nearshore waters will help environmental managers address pressing water quality issues, including exceedances
of the state Enterococcus water quality standard and nutrient loading to coral reefs. |
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