Development and Evaluation of the Bacterial Fate and Transport Module for the Agricultural Policy/Environmental Extender (APEX) Model.

The Agricultural Policy/Environmental eXtender (APEX) is a watershed-scale water quality model that includes detailed representation of agricultural management but currently does not have microbial fate and transport simulation capabilities. The objective of this work was to develop a process-based model for simulating the fate and transport of manure-borne bacteria on landscape and in streamflow in the APEX model. The bacteria model utilizes manure erosion rates to estimate the amount of edge-of-field bacteria yields. The total number of bacteria removed by runoff is estimated based on the concentrations of bacteria in manure and the amount of eroded manure. Bacteria survival in manure is simulated as a two-stage process separately for each manure application event. In-stream microbial fate and transport processes include bacteria release from streambeds by sediment resuspension during high flow events, diffusive releases from the streambed sediment during low flow periods, bacteria deposition by sediment settling, and survival. Default parameter values were selected from recently published databases and evaluated based on field observation. The APEX model with the newly developed microbial fate and transport module was applied to simulate the dynamics of fecal indicator bacterium Escherichia coli (E. coli) in the Toenepi watershed, New Zealand, that was monitored for seven years. The stream network of the watershed ran through grazing lands with daily applications of bovine waste deposition. Results show that the APEX with the bacteria module reproduced well the monitored pattern of E. coli concentrations at the watershed outlet. The APEX with the microbial fate and transport module can be utilized for predicting microbial quality of water as affected by various agricultural practices (grazing, cropping, and manure application), evaluating monitoring protocols, and supporting the selection of management practices based on regulations that rely on fecal indicator bacteria concentrations.