Scientific support for reversals in sample collection order
Can you provide some scientific support to back up why the sample collection order has flip-flopped. I noted that the order in which samples are collected has carried over from bailer sampling to low-flow sampling (LFS), but that it should be basically the opposite for LFS. What is the reasoning behind why we should now start with large bottles first and work towards VOCs and then filtered samples. We typically start with VOCs, then do the rest, followed by metals or filtered samples. I'm looking for some support for why we should change our strategy. Once the well is stabilized, it seems as though the order in which the bottles are filled shouldn't really make much of a difference.
Your question about bottle-filling order is a good one, and one that comes up frequently when talking about low-flow sampling procedures due to regulatory or permit requirements that dictate bottle filling order. The traditional order from "most volatile to least volatile" (i.e., VOCs first, then SVOCs, inorganics, etc.) is a carry-over from a time when most sampling was done with bailers. As you stated, the order of bottle-filling shouldn't matter once a well is stabilized, so there isn't necessarily a bottle filling order when using low-flow purging and sampling, at least not from the standpoint of sample volatility. However, one aspect of low-flow purging that is important is that, once the purging rate of the well is established and both the pumping water level and the indicator parameters have stabilized, ideally, you don't want to change the pumping rate for bottle filling. This is because the vertical 'zone of influence' in the formation adjacent to the borehole is affected by the pumping rate – if you increase the rate, you could increase the zone from above and below the well screen zone where water enters the well, potentially changing the chemistry of subsequent samples. Also, if there is any water in the well casing above the screen, increasing the pumping rate will generally increase drawdown, potentially pulling more "stagnant" water downward into the screen and again potentially changing sample chemistry. Therefore, the rate established for purging is also the highest rate available for bottle filling. You can reduce this rate to fill bottles, but you shouldn't increase it.
Based on this, I typically recommend:
Your question about bottle-filling order is a good one, and one that comes up frequently when talking about low-flow sampling procedures due to regulatory or permit requirements that dictate bottle filling order. The traditional order from "most volatile to least volatile" (i.e., VOCs first, then SVOCs, inorganics, etc.) is a carry-over from a time when most sampling was done with bailers. As you stated, the order of bottle-filling shouldn't matter once a well is stabilized, so there isn't necessarily a bottle filling order when using low-flow purging and sampling, at least not from the standpoint of sample volatility. However, one aspect of low-flow purging that is important is that, once the purging rate of the well is established and both the pumping water level and the indicator parameters have stabilized, ideally, you don't want to change the pumping rate for bottle filling. This is because the vertical 'zone of influence' in the formation adjacent to the borehole is affected by the pumping rate – if you increase the rate, you could increase the zone from above and below the well screen zone where water enters the well, potentially changing the chemistry of subsequent samples. Also, if there is any water in the well casing above the screen, increasing the pumping rate will generally increase drawdown, potentially pulling more "stagnant" water downward into the screen and again potentially changing sample chemistry. Therefore, the rate established for purging is also the highest rate available for bottle filling. You can reduce this rate to fill bottles, but you shouldn't increase it.
Based on this, I typically recommend:
- Establish the highest purging rate practical, with consideration given to minimizing drawdown and turbidity.
- Once the water level and indicator parameters have stabilized, if the purging flow rate 250-500 mL/minute or greater, fill the largest bottles first.
- Reduce the flow rate to 100-200 mL/minute to fill smaller bottle and VOCs to minimize VOC losses and sample aeration.
- If filtered samples are needed, momentarily stop the pump to install the filter, and then resume with taking filtered samples at 100-200 mL/minute. If large volumes of filtered sample are needed, you can reverse steps 3 and 4.
- If the purging rate established is around 200-250 mL/minute or less, bottles can be filled in any order desired as long as flow rate isn't increased during sampling.