For a project that used a traditional sampling method to develop a baseline, but then later needed to switch to low-flow sampling, how do you reconcile the difference between the two methodologies?

If you use the traditional purging and sampling approach first, you may see no difference at all in data, but if you do get a significant change, there are a couple of things to consider. What was the traditional approach that was used? For example, we have a number of sites in the Midwestern United States where the low-flow purging and sampling protocol we switched to resulted in higher concentrations for dissolved metals, but a lower concentration of totals. The previous method used was a bailer and when we looked at the data the initial concern was: how can totals go up? At first, it seemed to make no sense, but remember, bailing potentially aerates the sample, with this we commonly see that total metals will be high because turbidity has been elevated by the bailer, but dissolved metals are low because once those highly turbid samples have been filtered they not only removed very small solids from the sample, but the sample is being oxidized through aeration. In that situation, we went to the regulatory agency that was requiring us to use statistics to analyze the data and asked them to allow us to take four quarterly sampling events to reestablish the baseline conditions for those wells. After the four quarters we saw very consistent elevated results and now the total metals concentrations were down. The very precise results we had allowed the regulatory agency to see if there truly was any change to groundwater chemistry. So basically, look at your previous method and determine if it would potentially cause sample aeration or some other change that would explain differences between what you see in concentrations with low-flow samples versus those traditionally purged samples.