Effects of Wet and Dry Weather Events on Bacteria (Enterococci) and the Public Health Threat from the Re-Suspension of Sediment Sequestered Enterococci

Abstract

Approximately 66% of Texas surface waters are impaired with bacteria from fecal waste, including several tributaries and segments within the Galveston Bay system. This study was conducted in the waters of the Marina Del Sol marina on Clear Lake in Kemah, Galveston County, Texas, USA. A series of hypothesis were tested; 1) rainfall and subsequent runoff from stormwater is the primary cause of elevated Enterococcus levels in the waters in Marina Del Sol, 2) hotspots of Enterococcus will be present in the waters in Marina Del Sol and 3) the concentration of Enterococcus will increase from the marina entrance to the rear of the marina. Sampling was conducted at 10 stations between 0800 and 1100 every Monday, Thursday, and Saturday over five weeks in June and July 2013. Enterococci concentrations were quantified using the IDEXX Enterolert method for detection and enumeration estimation, Fluorogenic Substrate Enterococcus Test, Multi-well procedure and three-day rainfall accumulation prior to sampling was recorded from NOAA’s Climate Data Online. Eleven dry weather and four wet weather events occurred during the sampling period with the largest rainfall accumulation at 1.39 inches. The geometric means of wet versus dry weather samples were not significantly different (Mann Whitney). Two hotspots were found yielding geometric means of 42.98 and 41.25 MPN, which exceed the U.S. EPA primary contact recreation limit of 35 MPN. Additionally, the EPA single sample maximums (104 CFU/100 mL) were exceeded at nine out of ten sampling stations at least once, including a spike of 1,445 MPN and 1,198 MPN. A low to high gradient of Enterococci, from the entrance to the back portion of the marina, was evident. The results from the initial summer study indicated that the stormwater retention pond to the west of the marina could be a possible source of Enterococci. The fourth hypothesis, states that sediments are a source of elevated Enterococcus concentrations in the water at the Marina Del Sol marina, was tested during a follow up study. Sediment and water samples were collected on the 13th of November, 2013 between the hours of 0900 and 1400. Six stations in the stormwater retention pond were sampled. In addition, three of the original sampling stations in the marina were sampled. A stormwater outfall was found to be a concentrated source of Enterococci into the retention pond (12,098 MPN/100 mL). Data from these two studies indicate that there are numerous sources that contribute to the concentrations of Enterococci in the marina. A gateway effect is occurring between the increasingly built environment of the Galveston Bay marinas and the natural environment.