Shallow groundwater influences on the water quality of eastern Long Island's coastal embayments

Background/Question/Methods

The water quality of coastal environments is influenced by both surface and subsurface transport of anthropogenic contaminants. Sewage contamination can introduce allochthonous microbes, nutrients, and pharmaceuticals that in combination stimulate microbial succession in the subsurface environment and promote evolution of antibiotic resistance. Nationwide, there is growing concern about the interactions of increasing population density, aging sewage infrastructure, and recreational use of the coastal environment. In eastern Long Island, most homes rely on on-site wastewater disposal systems. The sandy nature of Long Island’s shallow aquifer allows for efficient groundwater transport, and contamination from these on-site systems is commonly associated with the degradation of estuarine water quality through nutrient enrichment, expansion of harmful algal blooms, and bacterial contaminants. Using cultured-based assays for fecal indicator bacteria (FIB) and antibiotic resistant bacteria (ARB) we evaluated the extent to which sewage has an impact on shallow groundwater quality at 15 locations where groundwater discharges into coastal embayments. The goals of this study are to: 1) determine the extent of bacterial sewage contamination; 2) examine ARB assemblage abundance, taxonomic diversity and potential for influence by septic discharge; and 3) compare groundwater antibiotic resistant assemblages to those observed in the region’s urban and suburban surface waters. We hypothesized that elevated FIB and ARB concentrations would be detected downgradient of communities that utilize on-site wastewater disposal systems and that ARB would be positively correlated with FIB abundance.

Results/Conclusions

Interestingly, FIB concentrations suggested minimal impact on bacterial water quality with Total coliforms, E. coli, and Enterococci undetectable in most groundwater samples. ARB concentrations spanned more than three orders of magnitude across our study area with extensive spatial variability. Sulfamethoxazole and ampicillin resistant bacteria were consistently more abundant than tetracycline resistant bacteria. Sulfamethoxazole, ampicillin, and tetracycline resistant bacteria were all positively correlated with total heterotrophic bacteria (p<0.01, p<0.01, p<0.05, respectively). Taxonomic identification of resistant isolates using sequencing of 16S rRNA genes, currently in progress, will help to evaluate the extent to which ARB are of enteric origin, the likelihood of multi-resistance as an explanation for the correlated abundances, and how the assemblages of ARB from regional surface water and groundwater differ.  The lack of FIB in the sampled groundwater combined with the prevalence of on-site wastewater systems supports the argument that non-human surface sources, including wildlife, may be the dominant factor in the degradation of eastern Long Island coastal embayment water quality.