Microbiological Surface-water Quality of the Río Grande de Añasco Watershed, Puerto Rico

Abstract

Sediments and the persistence of fecal and streptococcal indicator bacteria in surface waters have been identified as the main water-quality impairments in Puerto Rico. Identification of temporal variations in bacterial concentrations and relationships to other water-quality parameters are important in order to understand factors influencing pathogen indicator persistence and sources of contamination. Microbiological indicators (total coliforms, Escherichia coli, and enterococci) were quantified at approximately 15-day intervals in five un-sewered rural subbasins in the Rio Grande de Añasco watershed in western Puerto Rico from May 2002 to December 2003. The agricultural land areas in the subbasins ranged from 2.9 to 20%, and urban/suburban areas ranged from 0.6 to 11.5%. Secondary forest covered the majority of the land-areas studied. Geometric means of total coliform counts, E. coli, and enterococci within subbasins ranged from 34,135 to 85,921, 110 to 531 and 728 and 1,842 cfu/100 mL, respectively. Fecal coliform bacterial concentrations increased from a subbasin outlet to upstream areas along stream transects, suggesting that the bacterial source was groundwater seepage from upstream areas or an unknown point source input. Within the flow regimes evaluated, bacterial transport was positively and strongly associated with suspended sediments and weakly with hydrologic flow and nutrients. Resuspension of bottom sediments during runoff events may serve as a mechanism for coliform and enterococci transport to the water column. Bacterial diversity evaluation showed six genera and eleven species of coliforms and eight species of the genera Enterococcus and suggested that the possible sources of contamination were humans, herbivores and poultry.Runoff from household livestock pens, sewage seepage from septic tanks, and household sewage discharges may be responsible for the water-quality deterioration of the rural subbasins.