## 2 Materials and Methods  
Since water types vary by source and location, three different source waters were collected and used in this study: (1) Tile water from an agricultural production field in Crawford County, IL, USA; (2) pond water collected from a retention basin on near the intersection of Windsor Road and First Street in Champaign, IL, USA; and (3) stream water from the Embarras River just south of Urbana, IL, USA. A five-gallon bucket was used to collect water from each location. The source water samples were refrigated upon collection.  
Additional measurements were taken to determine the pH, turbidity, and conductivity of each source water. To test the pH of the water samples, an Oakton pH/mV/\({}^{\circ}\)C/\({}^{\circ}\)F meter (Oakton Instruments, Vernon Hills, IL, USA) was used after standard calibration was completed using pH standards. For turbidity testing, the Hach DR 900 colorimeter (Hach, Loveland, CO, USA) was used. To measure turbidity, 10 mL of the sample was transferred to a smaller vile where it was then shaken and placed into the colorimeter. Conductivity was measured using the Hach Pocket Pro Tester (Hach, Loveland, CO, USA). The device was calibrated using the Singlet Conductivity Solution, which consisted of KCl 12.88 mS/cm Standard Solution at 25 \({}^{\circ}\)C.  
To ensure detectable concentrations of nitrate-N and orthophosphate, all source water was spiked with 10 ppm nitrate-N and 1 ppm orthophosphate. A 100 mL volume of the spiked source water was distributed into a 125 mL Nalgene bottle, labelled with corresponding names for the source water, time-period to be tested, and which preservation method was used.  
After the source water was distributed into the bottles, the respective preservation method was applied to each sample. The control for each water source received no preservation. The samples receiving sulfuric acid received 0.2 mL of sulfuric acid in each 100 mL sample. The samples receiving sodium omadine received 0.25 mL of sodium omadine in each 100 mL sample. The bottles were capped and distributed into three automated water samplers, separated based on water source, in the shaded area adjacent to the Agricultural Engineering and Sciences Building at the University of Illinois at Urbana-Champaign (Figure 1), to mimic outdoor field conditions without refrigeration. Nitrate-N and orthophosphate concentrations were measured on the following time points: Day 0 (when the respective preservation method was applied to the samples), Day 0.5, Day 1, Day 2, Day 5, Day 7, and Day 14 after preservation, resulting in seven time points. Each sample was prepared and analyzed in triplicate.  
At each time point, the corresponding samples were taken from the autosamplers into the lab. The control samples as well as those receiving sulfuric acid and sodium omadine were immediately placed in the freezer. The filtered samples from each water source were filtered by pouring the water sample through a 0.45 \(\upmu\)m filter on top of an Erlenmeyer flask (Figure 2). A vacuum was used to expedite the filtration process. Upon completion of filtration, samples were placed in the freezer. At the completion of the 14 days, all samples were thawed and analyzed for nitrate-N and orthophosphate.