Estimating environmental conditions affecting pathogenic protozoal removal in wetland systems using a multi-scale model-based approach

Background/Question/Methods

Protozoal pathogens causing waterborne disease are distributed worldwide and empirical evidence suggests wetlands are effective at reducing the concentrations of these pathogens under certain environmental conditions.  The goal of this study was to evaluate if environmental conditions of wetlands in the Monterey Bay region of California facilitate the removal of Cryptosporidium parvum, Giardia lamblia, and Toxoplasma gondii and how rising sea level and temperatures from climate change may impact removal.  This was achieved using multi-scale modeling coupled with a series of lab and field experiments.  To examine the effect environmental conditions had on protozoal transport, observational experiments were conducted at three primary spatial scales: settling columns, re-circulating wetland mesocosm tanks, and an experimental research wetland.  The environmental conditions examined were turbidity, vegetation density, salinity concentration, and water temperature.  A pollutant transport model was simultaneously developed to simulate each of these systems.  We expected there to be a high degree of uncertainty in the model predictions and field observations and therefore we implemented the model within a Bayesian statistical framework. 

Results/Conclusions

From our analysis, we found protozoal removal increased when wetlands had more vegetation cover and higher levels of turbidity, salinity, and temperature. Vegetation had the strongest evidence for enhancing protozoal removal in wetland settings when emergent vegetation was present at 2% cover. Our results provide evidence that vegetation within a wetland channel greatly enhances protozoal pathogen removal and that sea level and temperature rise along coastal California may affect protozoal removal in coastal wetlands by increasing salinity-induced settling and temperature-induced settling and inactivation of protozoal pathogens.  Drawing from these results we find wetlands, when adequately vegetated, are a practical consideration for addressing protozoal pathogen pollution in Monterey Bay watersheds and that an increase in vegetated wetland area would likely correspond to an increase in water quality improvement for the region.