Is there a ranking of analytes from minimum to maximum difference when sampled?
Is there a ranking of analytes from minimum to maximum difference when sampled by both bailing and low-flow?
In terms of minimum and maximum difference between bailing and low-flow, what we generally see is that turbidity impacted analytes are going to see the greatest difference between them, whereas analytes that are not impacted by turbidity will have the smallest difference.
In the case of metals, for example, if you have bailed samples for metals, and then switch over to the low-flow approach, you often see a significant reduction in total metals concentration, which correlates nicely with the reduction in turbidity that you see. But often at the same time you can have an increase in dissolved metal concentrations, which seems counterintuitive. If the total goes down, why would the dissolved go up? Well, when samples are bailed they are often aerated, and because of aeration you can have precipitation of metal ions out of solution. Whereas, with the low-flow samples, especially when using a device like a bladder pump, those metal ions in solution stay in solution, they don’t precipitate out. Thus, you will see low total metals concentration when compared to the bailed samples, but higher dissolved.
For a ranking of analytes from minimum to maximum, and the differences between them, you would have to look at studies comparing these methodologies. When the diffusion bag sample method was originally developed, the USGS did some testing to see how long it took for these things to equilibrate. And generally, when they equilibrated, they reached the same concentration as what was in the control sample. In terms of field studies, the biggest problem we have is that for some devices the correlation between low-flow samples and bailed samples was very good. In other cases, the correlation was poor. So it depends on the device being used.
In terms of minimum and maximum difference between bailing and low-flow, what we generally see is that turbidity impacted analytes are going to see the greatest difference between them, whereas analytes that are not impacted by turbidity will have the smallest difference.
In the case of metals, for example, if you have bailed samples for metals, and then switch over to the low-flow approach, you often see a significant reduction in total metals concentration, which correlates nicely with the reduction in turbidity that you see. But often at the same time you can have an increase in dissolved metal concentrations, which seems counterintuitive. If the total goes down, why would the dissolved go up? Well, when samples are bailed they are often aerated, and because of aeration you can have precipitation of metal ions out of solution. Whereas, with the low-flow samples, especially when using a device like a bladder pump, those metal ions in solution stay in solution, they don’t precipitate out. Thus, you will see low total metals concentration when compared to the bailed samples, but higher dissolved.
For a ranking of analytes from minimum to maximum, and the differences between them, you would have to look at studies comparing these methodologies. When the diffusion bag sample method was originally developed, the USGS did some testing to see how long it took for these things to equilibrate. And generally, when they equilibrated, they reached the same concentration as what was in the control sample. In terms of field studies, the biggest problem we have is that for some devices the correlation between low-flow samples and bailed samples was very good. In other cases, the correlation was poor. So it depends on the device being used.