Effect of Water Table Fluctuations on the Fate and Distribution of Escherichia coli in a Freshwater Beach Environment: An Experimental Study

English Abstract:

To investigate the role of water table dynamics on the distribution of E. coli in sandy lake beaches, an automated column system was used to simulate the effect of water table fluctuations. Four undisturbed cores were collected perpendicular to the shoreline at Burlington beach site in Ontario, yielding four sand columns. The water table regime in the columns was designed to mimic the dynamic water table fluctuations observed at variable distances from the shoreline. The columns were inoculated with a naturally occurring nalidixic acid resistant E. coli strain every 48 hrs. A one time inoculation of 1.0µm fluorescent microspheres as bacterial surrogates was also carried out. A mini-auger system was used to periodically retrieve small amounts of sand (< 1.0 g) from the columns, hence making it possible to monitor changes in the E. coli concentrations. Changes in pore water composition were monitored by collecting water with micro-Rhizon samplers from different depths of the columns. The experiment ran for 35 days in total and at the end of the experiment, the columns were sliced and analysed for microspheres, E. coli and solid phase chemistry. High level of E. coli and microspheres were observed at the top of the columns. By the end of the experiment, only less than 5% of the total E. coli introduced to the columns reached 25cm deep below the sand surface. These results suggests that in-situ depth distributions of E. coli in beach environments are strongly controlled by the input and transport properties of the bacteria. Nitrification occurred more intensively at the layers of sand columns with higher E. coli concentrations under aerobic conditions controlled mainly by water table fluctuations. This can be explained by E. coli aging and subsequent die-off releasing organic N in the sand.