## 4 Discussion  
The United Nations has identified water quality improvement as a global need [41]. Possible methods range from modelling where possible contaminants are coming from to placement of best management practices to development of new techniques to reduce nutrient loads either before or after they enter the water body [42]. The four main requirements for a passive removal of PO4\({}^{3-}\) are high enough influent concentration (i.e., >0.2 mg/L), placement of the structure/barrier in the proper location, flow through the system at an acceptable rate, and ability to replace spent material [43]. Although the introduction mentioned steel-slag for PO4\({}^{3-}\) removal, the discussion will focus on WTR or systems configured closer to the Gate system. Please refer to [PERSON] et al. [44] and [PERSON] et al. [21] for more details on the effectiveness of steel slag. [PERSON] et al. [39] evaluated use of WTR as a 1-m thick passive barrier to remove PO4\({}^{3-}\) but did not provide any details on the full-sized structure other than that the material could be in place for decades depending on the flowrate and initial concentration. [PERSON] al. [45] placed a 65-cm alum sludge layer in an experimental fed-batch-constructed wetland to treat wastewater from an animal farm in Ireland. It achieved mean monthly removal efficiencies of 75-94% for influent total phosphorus levels of 10.7-33.3 mg/L with a batch 4 h. contact time.Figures 5 and 6 indicated a removal of 0.02-0.74 mg/L PO4\({}^{3-}\) depending on the sample date. This range was lower as grab samples do not provide a continuous evaluation. Since at least half the gates were nearly always submersed to some extent, a potential continuous removal of PO4\({}^{3-}\) was possible thereby increasing the subsequent total amount of PO4\({}^{3-}\) removed. The percentage submergence of each gate (Table 3) depended on the amount of preceding precipitation. For instance, seven of the gates were completely submerged on September 8 after receiving over 9 cm of rain the day before. The fact that at least half the system was in constant contact with water could potentially overcome water velocity issues identified by [PERSON] et al. [23] and [PERSON] et al. [24]. It was not possible to track velocity changes during a severe storm, however even when the velocity after a storm was ~0.18 m/s, more water was still in contact with the Gates as the water level had increased. As mentioned in the Section 3, the water level was slow to dissipate due to the impact of elevation levels in Lake Rockwell. The study conducted by [PERSON] and [PERSON] [46], which was the closest to our Gate system, used suspended reactive curtains (i.e., filters containing reactive materials) for  
Figure 7: Average PO4\({}^{3-}\) concentrations (**a**) 1 m upstream and (**b**) 1 m downstream of treatment gates. Error bars are standard deviation of triplicate samples. binding phosphorous. The material was suspended from a pole placed above a 1-m wide by 2 m long \(\times\) 14 cm water body. The system used three rows of eight filters containing 50 g autoclaved aerated concrete (AAC) per filter and the system decreased PO4\({}^{3-}\) from 1.35 to 0.19 mg PO4\({}^{3-}\)/L and increased pH from 7.61 to 8.72. In order to fully compare [PERSON] and [PERSON] [46] with this study, dynamic similarity was used to bring WTR used to the same scale. For [PERSON] and [PERSON] [46], the system contained 0.28 m\({}^{3}\) of water and was operated for 34 days which translated to 0.13 Kg AAC/m\({}^{3}\)-d. The treatment area of Eckert Ditch is 289.56 m long with an average width of 0.76 m and water depth of 0.61 m. Each gate contained 45.4 Kg WTR for a total 766.76 Kg WTR that was used for 151 days yielding 0.04 Kg WTR/m\({}^{3}\)-d. The WTR used in the Gate system was one order of magnitude smaller than that of AAC. In addition, the WTR did not need to be autoclaved prior to use.