Net export of E. coli from the Toenepi wetland cannot be explained by growth of naturalized E. coli in the water column

Runoff from agricultural land is recognized as an important source of contaminants—nitrogen, phosphorus, sediment and Escherichia coli—that impact water quality. Constructed wetlands have been promoted as a green infrastructure approach to attenuate these contaminants. A considerable amount of work has been carried out on nitrogen, phosphorus and sediment attenuation through wetlands and these processes are reasonably well understood. There has been much less research on the attenuation of faecal microbes, such as E. coli, through wetlands. A research/demonstration constructed wetland was established in the Toenepi catchment, Waikato, New Zealand (NZ) to investigate nutrient removal from sub-surface drainage from grazed dairy pasture. As a part of the earlier nutrient monitoring of the Toenepi wetland some samples were analyzed for E. coli concentrations. The surprising result of the E. coli testing was the frequently higher concentrations of E. coli in the outflow than the inflow to the wetland, indicating a “net export” of E. coli from this wetland. This apparent net export of E. coli from the Toenepi wetland led to a hypothesis that some E. coli strains were becoming naturalized and hence growing in the wetland. “Naturalized E. coli” is a term used to describe subtypes of Escherichia that persist/grow in aquatic environments, and hence, do not represent “recent faecal contamination”. A recent genome study identified potentially naturalized strains of faecal E. coli and nonfaecal E. coli-like Escherichia in the Toenepi wetland. E. coli isolates from fresh and aged faecal inputs and a naturalized Escherichia strain isolated from the Toenepi wetland were used in a series of microcosm (in the lab) and mesocosm (placed in the wetland) studies to assess if the isolates could grow in the water and if so, determine their potential growth rate. In this study, a model of the putative E. coli concentrations in the water flowing into and out of the 2-cell wetland system was developed based on a 15-minute time step and assuming first order growth rates in the water column. Modelled water flows were based on measured inflow rates over the winter of 2006 and model outputs compared to the E. coli concentrations measured during that winter flow period. The mean and maximum growth rates measured for any of the E. coli isolates were 0.1 and 0.2 (ln day-1), respectively. When these growth rates were used in the model for outlet water concentrations, the modelled E. coli concentrations in the outlet were always less than the average measured outlet E. coli concentrations. To fit the model to measured outlet concentrations required increasing the growth rate to 0.3 day-1. This high E. coli grow rate appears to indicate that E. coli growth in the water column alone cannot explain the net export of E. coli from the Toenepi wetland system. Furthermore, the model predicted that the E. coli concentrations in the outflow would progressively increase during the low flow period between major flow events and then decrease as the wetland water was effectively diluted with low E. coli concentrations in the storm inflow. Detailed examination of the measured E. coli concentrations showed that the outflow concentrations did not increase during the low flow periods but increased dramatically at the beginning of the high flow periods and often even before the flow rate in the wetland increased. This dynamic response in the wetland further confirms that E. coli growth in the water column cannot explain the net export of E. coli from this wetland complex. The rapid increase in E. coli concentrations in the outflow, coinciding with or preceding the increase in flow rates, indicates that the source of E. coli may be attached to the surfaces of the plants or leaf litter in the wetland. This new hypothesis will require further investigation.