## 3 Results  
As mentioned earlier, samples were taken immediately before and after each gate to determine the PO\({}_{4}\)\({}^{3-}\) concentration (Figure 2). Figure 4 contains a representative field evaluation of two sampling events on 2 June 2020 and 8 September 2020. As expected, PO\({}_{4}\)\({}^{3-}\) concentration varied as water moved downstream from the system. The PO\({}_{4}\)\({}^{3-}\) concentration was higher before and lower after passing through the Al-WTR contained in the four cartridges of Gates 2, 3 and 4 on June 2 (Figure 4a). Water levels in Eckert's Ditch were observed to impact the amount of contact with the Al-WTR in the four cartridges within the gates (i.e., low water level resulted in a lower percentage of the gate and subsequent WTR in the cartridges submerged). As shown in Figure 4b, the mg PO\({}_{4}\)\({}^{3-}\) concentration immediately after the gate was lower than that before the gate for all gates. On 8 September 2020 the amount of each gate area that was in contact with the water (i.e., % submerged) ranged from 0 to 75%, whereas on 8 September 2020 all but Gate 2 were 100% submerged (Table 3). Figure 3: (a) Gate 2 completely above water level on 6 October 2020, (b) Gate 5 with measuring pole on 28 July 2020 and (c) Gate 5 completely submerged on 8 September 2020. Figure 2: Simplified representation of gate locations and sample collections in Eckert’s Ditch. The water level and flow in Eckert Ditch can change dramatically with weather events. Generally, the site has very slow flow, but during even moderate rain events the stream  
Figure 4: mg PO\({}_{4}\)\({}^{3-}\)/L in front of and after treatment gates on (**a**) 2 June 2020 and (**b**) 8 September 2020 to show changes through Eckert’s Ditch. Error bars are standard deviation of triplicate samples. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Date** & **Gate 1** & **Gate 2** & **Gate 3** & **Gate 4** & **Gate 5** & **Gate 6** & **Gate 7** & **Gate 8** \\ \hline
2 June 2020 & 25 & 10 & 40 & 12 & 75 & 0 & 0 & 75 \\
16 June 2020 & 25 & 10 & 40 & 0.09 & 25 & 1 & 10 & 25 \\
30 June 2020 & 25 & 0 & 45 & 0.09 & 25 & 1 & 0.5 & 25 \\
14 July 2020 & 12 & 0 & 50 & 1 & 1 & 25 & 13 & 25 \\
28 July 2020 & 25 & 0 & 50 & 1 & 13 & 25 & 25 & 40 \\
11 August 2020 & 12 & 0 & 50 & 0 & 1 & 25 & 40 & 25 \\
8 September 2020 & 100 & 75 & 100 & 100 & 100 & 100 & 100 & 100 \\
22 September 2020 & 12 & 0 & 50 & 1 & 12 & 13 & 12 & 13 \\
6 October 2020 & 12 & 0 & 50 & 12 & 25 & 50 & 13 & 50 \\
20 October 2020 & 12 & 50 & 100 & 50 & 25 & 75 & 62 & 75 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Percent of cartridge area below water level at time of sampling. can overflow banks, flood the surrounding land and roads, and increase flowrate. The increased water level and flowrate is heavily dependent on the elevation of Lake Rockwell Reservoir as Eckert Ditch is close enough that reservoir conditions affect stream conditions. A combination of low flowrate and high water level are ideal for the system as the passive-reactive adsorption system being used has increased efficiency with increased contact between PO43- and the WTR. If the flowrate is too fast the water will not have adequate contact time with the WTR in the cartridges. During high rain events, the water flowrate will be high. However, as the increased water level in Eckert's Ditch dissipates slowly due to a similar increased water elevation in Lake Rockwell, the flow rate is decreased while maintaining a higher than average water level. In addition to evaluating gate performance at each sampling event, changes of each gate with time and the number of times the cartridges had to be refreshed were also tracked. As expected, the WTR replacement frequency decreased through the system as the first few gates were exposed to the highest PO43- concentrations. The cartridges in Gate 1 were replaced twice while other gates were replaced only once (Table 4). As was shown in Table 1, each cartridge contained 11.35 kg WTR for a total of 45.4 kg WTR per gate.