## 3 Results and Discussion
### Nutrient Concentrations from Preserved Samples
#### 3.2.2 Orthophosphate Results
(**a**) Tile water; (**b**) Stream water; (**c**) Pond water. For orthophosphate, the baseline concentration was approximately 1 ppm. Therefore, the slightest variation in concentration resulted in a large percent difference from the baseline. It is important to consider this when determining what is an acceptable change in concentration for a given research study objective. The control treatment maintained relatively constant orthophosphate concentrations for all three source waters. Stream water showed a large decrease (11.87%) on Day 14, but much smaller variations (2.56-6.12%) leading up to Day 14. The filtered treatment showed decreases in orthophosphate concentrations for every time point and large variations between source waters on Day 14, ranging from a 9.50% decrease for stream water to a 15.30% decrease for tile water. Processes like adsorption and immobilization may have converted some portion of the orthophosphate into particulate forms, which would have been removed by the filtering process and caused a decrease in concentrations. Similar to the nitrate-N findings, the consistent decrease in orthophosphate concentrations is in contrast to other findings where orthophosphate concentrations increased by as much as 1.8-fold when samples were left unfiltered for 10-days [4]. As discussed previously, the sulfuric acid preservation method is intended to eliminate microbial activity. Therefore, processes like mineralization or immobilization should not have impacted orthophosphate concentrations for this method. While the sulfuric acid treatment had large variations in orthophosphate concentrations throughout the 14 days of study, it performed quite well on Day 14 for both tile water (1.94% decrease) and stream water (2.37% decrease). The decreasing concentrations for the sulfuric acid method are in contrast to the findings from [PERSON] et al. [3], who found that orthophosphate concentrations increased by as much as 68% when sulfuric acid was used as a preservative. Pond water showed an 8.48% increase, which is a greater change from the baseline concentration than the control, but a smaller change than the filtering or sodium omadine treatments. Similar to the nitrate-N results, orthophosphate showed an initial reaction with the sodium omadine, with tile water and pond water decreasing by 6.58% and 6.82%, respectively. Stream water showed a Day 0 increase in orthophosphate concentration of 3.56%. By Day 14, the sodium omadine treatment concentrations had increased by 11.13% for the tile water and decreased by 8.86% and 13.06% for the stream water and pond water, respectively. Table 6 compares the orthophosphate concentrations over time and presents significant differences (\(p<0.05\)) between the concentration on each day and the baseline concentration. The orthophosphate concentrations were only different from the baseline on Day 5 of the tile water (\(p=0.028\)) and Day 2 of the pond water (\(p=0.022\)) for the Control. When filtering of the samples occurred, the orthophosphate concentrations were different for eight different time points across the three source waters. The addition of sulfuric acid only produced a concentration that was significantly different from the baseline concentration on Day 0.5 of the stream water (\(p=0.037\)). The addition of sodium omadine was not significantly different than the baseline concentration for any time point of the stream water, but was different for six of the 14 time points of the tile and pond water. The addition of sulfuric acid was the best performing method for preserving orthophosphate over the full 14-day period in this study. The addition of sodium omadine was quite good at preserving stream water samples, but significantly varied for several time points for tile and pond water samples. While filtering is the US EPA recommended preservation method for orthophosphate in water, it was significantly different from the baseline concentration for more sampling points than the other methods.