## 3 Results
Replacing the cartridges in Gate 1 yielded a total of 136.2 kg WTR used for the season, whereas Gates 2-7 only used 90.8 kg. Four extra mesh bags and cartridges enabled all cartridges in a gate to be replaced at the same time they were removed with fresh WTR in the spare cartridges. This reduced the need for a second visit and only briefly taking the gate out of service (around 15 min for each refresh event). All cartridges were removed from the Gates at the end of October in preparation for winter conditions. Visual inspection of the cartridges and mesh bags indicated that none had experienced any major damage while deployed in field. Figures 5 and 6 contains details on the PO43- concentration immediately before and after passing through the gates (i.e., back) throughout the season for the Gates 1-4 and Gates 5 and 6, respectively. Figure 7 contains the average upstream (1 m before the first gate) and downstream (1 m after the last gate) PO43- concentrations in Eckert Ditch (as indicated in Figure 2), which were used to establish a baseline for the waterbody. Each figure also includes the amount of rainfall that occurred during the time the gates were in the field. As shown in Figure 7, some sample dates depicted mg PO43-/L that were lower downstream than it was upstream (PO43- removal), while others had higher downstream concentrations (increase in PO43-). For instance, on June 16 the upstream and downstream concentrations were 0.31 mg PO43-/L and 0.27 mg PO43-/L, respectively, while August 25 had upstream and downstream levels of 0.25 mg PO43-/L and 0.27 mg PO43-/L, respectively. It is not possible to know exactly how much PO43- was removed by the system by solely evaluating the upstream and downstream concentrations due to variability in field conditions (runoff entering the waterbody at multiple locations, fluctuating water levels and velocity, etc.). The results immediately before and after each gate provide a \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Date**} & \multicolumn{2}{c}{**Total**} & \multicolumn{1}{c}{**Total**} \\ \hline
**Gate** & **12 June** & **26 July** & **29 September** & **Changes** & **WTR kg** \\ \hline
1 & x & x & & 2 & 136.2 \\
2 & & x & & 1 & 90.8 \\
3 & & & x & 1 & 90.8 \\
4 & & x & & 1 & 90.8 \\
5 & x & & & 1 & 90.8 \\
6 & & & x & 1 & 90.8 \\
7 & & & x & 1 & 90.8 \\
8 & x & & & 1 & 90.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Cartridge Refresh Frequency and WTR Used During 2020. “x” denotes date that cartridge was refreshed. snapshot of the phosphorus sequestration at each gate. The PO4\({}^{3-}\) concentration before and after each gate (shown in Figures 5 and 6) was input into Equation (1) in conjunction with the % of the cartridge submerged (Table 3) and the average stream velocity (0.044 m/s) to provide an estimate of how much PO4\({}^{3-}\) was removed by the gates during each sampling event. Using this approach, it was determined that a total of 556.31 g PO4\({}^{3-}\) was removed from Eckert's Ditch. The sampling dates were spread across 151 day (2 June to 20 October). Dividing 556.31 g PO4\({}^{3-}\) by 151 days of operation yields an approximate 3.68 g PO4\({}^{3-}\) removed/d. Figure 5: mg PO4\({}^{3-}\)/L immediately in the front and back of gate during sampling events during 2020. (**a**) Gate 1, (**b**) Gate 2, (**c**) Gate 3 and (**d**) Gate 4. * denotes the date when WTR in cartridges was changed. Error bars are standard deviation of triplicate samples. During individual sampling events, desorption of previously bound PO4\({}^{3-}\) was minimal. Desorption was estimated by comparing the difference between the PO4\({}^{3-}\) concentration of samples collected in front of the gate to those collected in the back of the gate. A negative value would indicate that more was released from WTR than was retained. For individual gates that had a higher 'back' than 'front' PO4\({}^{3-}\) mg/L, the values were in the range of 0.01-0.1 mg/L PO4\({}^{3-}\). [PERSON] et al.