Do you need to ignore historical data during statistical analysis
When determining trends, however, does historical data have to be ignored during the statistical analysis?
If we know that well volume purging, especially at higher pumping rates, is not really giving us representative data then switching over to low flow is going to give us a much better idea of the total mobile contaminant load and give us samples that don’t have issues with artifacts. Now the question becomes: what do you do with all your historical data, will you see changes or not? In some cases we don’t see significant changes between the low-flow samples and the three volume purge samples, and this is highly dependent on lot of things: the contaminants concerned, the nature of their distribution, the well construction, the hydraulic connectivity of the formation, and so on. But in some cases we do see differences and probably the most common one is this: if you have a lot of historical data from three volume purging or from bailing where you are looking at metals in solution and you are looking at both total and dissolved metals, if you switch from bailing or high rate pumping to low flow purging and sampling you see a very unusual thing happen: total metals concentrations with low flow generally come down because turbidity comes down. But, unfortunately, dissolved metals concentrations often increase and can increase significantly. That doesn’t seem very obvious but the key to it is that low flow purging and sampling, because it doesn’t aerate the sample and because it doesn’t contain a high level of turbidity, is the best representation of the true dissolved metal concentration. We had this problem in Illinois and Wisconsin where statistics are used to analyze the data and when we saw great increases in dissolves and a drop in totals, the state was scratching their head and saying what are we going to do with this? We asked them for four quarters to reestablish our baseline conditions for the true dissolved metals concentrations. Once we did that everything flattened out just like it should and the new dissolved metals concentrations were established as the baseline.
If we know that well volume purging, especially at higher pumping rates, is not really giving us representative data then switching over to low flow is going to give us a much better idea of the total mobile contaminant load and give us samples that don’t have issues with artifacts. Now the question becomes: what do you do with all your historical data, will you see changes or not? In some cases we don’t see significant changes between the low-flow samples and the three volume purge samples, and this is highly dependent on lot of things: the contaminants concerned, the nature of their distribution, the well construction, the hydraulic connectivity of the formation, and so on. But in some cases we do see differences and probably the most common one is this: if you have a lot of historical data from three volume purging or from bailing where you are looking at metals in solution and you are looking at both total and dissolved metals, if you switch from bailing or high rate pumping to low flow purging and sampling you see a very unusual thing happen: total metals concentrations with low flow generally come down because turbidity comes down. But, unfortunately, dissolved metals concentrations often increase and can increase significantly. That doesn’t seem very obvious but the key to it is that low flow purging and sampling, because it doesn’t aerate the sample and because it doesn’t contain a high level of turbidity, is the best representation of the true dissolved metal concentration. We had this problem in Illinois and Wisconsin where statistics are used to analyze the data and when we saw great increases in dissolves and a drop in totals, the state was scratching their head and saying what are we going to do with this? We asked them for four quarters to reestablish our baseline conditions for the true dissolved metals concentrations. Once we did that everything flattened out just like it should and the new dissolved metals concentrations were established as the baseline.