FDevelopment and evaluation of the bacterial fate and transport module for the Agricultural Policy/Environmental eXtender (APEX) model

Authors: HONG, EUNMI - US Department Of Agriculture (USDA); PARK, YONGEUN - University Of Ulsan College Of Medicine; MUIRHEAD, RICHARD - Agresearch; JAEHAK, JEONG - Texas A&M University Health Science Center; Pachepsky, Yakov

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/21/2017
Publication Date: 1/18/2018
Citation: Hong, E., Park, Y., Muirhead, R., Jaehak, J., Pachepsky, Y.A. 2018. FDevelopment and evaluation of the bacterial fate and transport module for the Agricultural Policy/Environmental eXtender (APEX) model. Science of the Total Environment. 615:47–58.

Interpretive Summary: The Agricultural Policy/Environmental eXtender (APEX) is a watershed-scale water quality model that includes detailed representation of agricultural management practices and is used to evaluate the efficiency of various conservation measures. So far APEX has not been capable of simulating fate and transport of manure-borne microorganisms. This has limited the applicability of APEX in addressing microbial water quality issues that are critical for recreation, irrigation and other freshwater uses. We developed the first microbial module for APEX using the latest knowledge of the fate and transport mechanisms and the latest progress in creating databases of fate and transport parameters. The module works as an add-on to APEX and can be added or removed without any changes in current APEX version. We tested the new microbial module with the unique E. coli bacterium monitoring database compiled with years of data from the well-characterized Toenepi watershed in New Zealand. Testing results appeared to be satisfactory. The new microbial module can be used to improve the efficacy of APEX as the tool for evaluation environmental effects of agricultural management practices.

Technical Abstract: The Agricultural Policy/Environmental eXtender (APEX) is a watershed-scale water quality model that includes detailed representation of agricultural management. The objective of this work was to develop a process-based model for simulating the fate and transport of manure-borne bacteria on land and in streams with the APEX model. The bacteria model utilizes manure erosion rates to estimate the amount of edge-of-field bacteria export. 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 due to sediment resuspension during high flow events, active release from the streambed sediment during low flow periods, bacteria settling with sediment, and survival. Default parameter values were selected from published databases and evaluated based on field observations. The APEX model with the newly developed microbial fate and transport module was applied to simulate fate and transport of the fecal indicator bacterium Escherichia 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 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 will be utilized for predicting microbial quality of water as affected by various agricultural practices, evaluating monitoring protocols, and supporting the selection of management practices based on regulations that rely on fecal indicator bacteria concentrations.