Runoff Delay Exerts a Strong Control on the FIELD-Scale Removal of Manure-Borne Fecal Bacteria with Runoff.

The microbial safety of surface waters is an ongoing issue which is threatened by the transport of manure-borne bacteria to water sources used for irrigation or recreation. Predictive modeling has become an effective tool to forecast the microbial quality of water during precipitation events, however high uncertainty in model predictions underscores model accuracy. It is therefore vital to determine the extent of the variability of fecal bacteria behavior in the environment. The objective of this work was to assess the interannual variation of the removal of Escherichia coli from a manured field under the simulated rainfall. A unique dataset was obtained during six years (2011 – 2016) in experiments conducted at the OPE3 field site at the ARS Beltsville Agricultural Research Center (BARC). Dairy bovine manure was applied at a rate of about 60 t/ha on a 0.28 ha field in April or May for six continuous years. Following manure application, irrigation was applied to create 2-hours of runoff and then was shut off. Runoff sampling continued for an additional hour. Runoff was sampled at 5 minute intervals. Soil and manure samples were taken prior to- and after each irrigation event from 33 locations across the field. Irrigation and sampling events were conducted on two consecutive weeks. The average recovery of applied water as runoff was 10% implicating infiltration as the dominant method of bacterial transport. On average, 3.9 and 0.2 percent of the applied E. coli was removed with runoff in the first and second weeks of the experiment, respectively. The E. coli export rate, i.e. total number of E. coli removed from the field per unit of irrigation water after runoff began showed the exponential dependence on the amount of irrigation water before runoff began. The determination coefficients of regressions were R2=0.556 for the first irrigation and R2=0.515 for the second irrigation. While small, percentages of E. coli removed in this work equate to up to trillions of cells transported off the field highlighting the importance of understanding field-scale transport processes. The unique long-term experiment of this work has provided first data that show that the introduction of variable export rate has to be researched as means for the performance improvement of field-to-watershed-scale bacterial water quality models.