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FAQs

First of all, the purpose of development is to move fine material out of the sand pack and its surrounding formation after the well is constructed because there will be a lot of artificial turbidity there just from the well construction and drilling process.

If you are in a formation that produces enough water and it’s hydraulically conductive enough that you can easily or readily develop it, then by all means it should be developed. That being said, I have worked at a number of monitoring wells where development won't work. The main reason being that we were dealing with extremely fine formation material and sand pack, and the slot size of the screen was selected without doing analysis of this very fine material.

With extensive well development, as much as 8-10 hours of development in a given well, you can many times get even wells like that to clear up.  But here is one thing about low-flow sampling that’s important to know: low-flow purging and sampling can work really effectively as a mandate to reduce turbidity.  I have worked at sites where turbidity from samples that were bailed or pumped using traditional methods was as high as 1,000-1,500 NTU.  We effectively used low-flow purging and sampling to get turbidity down to 20-50 NTU.

As long as there is not a stated drawdown limit that you have to meet, stay at the highest practical flow rate. Provided that the water level stabilizes very quickly, the preference would be to stay at that higher flow rate and greater drawdown. But, if you have continued drawdown that will not slow down you have no choice but to back off on the flow rate so as not to dewater the well.

That traditional order of bottle-filling starts with what we call the most volatile to the least volatile. VOCs, SVOCs, metals, and filters is based on old research that used predominantly bailers for sampling.  If a bailer is pulled out of a well, you want to take the most volatile sample first since it has the greatest effect of changing that sample with time.  But if you’re pumping from a well at a low flow rate, under steady state conditions, and with stabilized indicator parameters then the bottle filling order no longer matters; what does matter is the flow rate.  So if you start out at a flow rate of 500 or 300 mL per minute and have larger bottles to fill, fill those first, then reduce the flow rate and fill smaller bottles, and finally, pause momentarily to install the filter, and take the filtered parameters.  So the old bottle filling order is still correct if you are using a bailer, but if you are using a low flow pump and a low flow approach, bottle filling order really goes from largest to smallest bottles. 

There is no minimum volume or time requirement for low-flow purging prior to sampling. The purge time and volume are dependent only on achieving indicator parameter stabilization after achieving a stable pumping water level. Typical purge volumes for many 2"-4" wells with screens 5-20 feet long are in the range of 4-20 liters (1-5 gallons) in 15-30 minutes of purging.

Is it ok to sample after as little as 20-30 minutes?

As stated above, as long as both pumping water level and indicator parameters are stable within the established criteria you can sample the well.

That can be a challenge. Because of differential settling on a landfill site you can end up with gas headers that were initially graded so to have spots where the liquid could be dropped out, but now, you have low spots or low-lying areas where the liquid can actually block off the piping. If your issue is liquid accumulating in the pipe because of a low point the only solution is to repair the pipe. It has to be lifted or replaced in order to get that area to flow properly.  But, managing the liquid in the header system is either done through what is called a condensate sump dropout or a drip leg. This is basically a vertical pipe of some sort that is attached to the horizontal headers and drilled down a few feet into the ground.  It’s a point for liquid to accumulate or “drop out” of the header line. Now the liquid moving through the pipe will fall into the dropout tank and you just install a single pump down in that tank to keep the sump itself dewatered.

Not always. We generally see some increase in gas flow, but the amount of increase can vary. It’s dependent on how long the gas well was watered in. If the gas well has been watered in for a fairly long period of time, the accumulation of biosolids and biomass as well as silt within the gas well can control whether or not we can get that well to perform at its previous levels. A couple of papers published on this subject have shown that gas flow increases can range all the way from a 100% original gas output to as much as 20% to 30% of original gas output. If a site is considering using pumps to improve gas collection, there needs to be a measurement of the liquid levels in the existing wells to determine how much of the perforated section of the well is currently exposed.  For example, if you have a well with 30 meters perforated pipe and 20 meters of that pipe are currently watered in, you can do a gas flow test on the existing open portion of the screen and get an estimate of how many cubic meters of gas per hour can be produced with the existing portion of the screen. From there you can estimate how much additional gas might be produced by dewatering. Try to use a conservative factor because it’s not very linear. For example, if we got to 20 cubic meters of gas per hour from a given well and we had two thirds of the well watered in, we might be able to raise that to 30 to 35 cubic meters by dewatering. We may not get as high as 50 or 60 cubic meters per hour.

There have been several papers published in the U.S., as well as a couple of International Solid Waste Association (ISWA) conferences on the topic of sudden gas pressures and problems with leachate seeps both out of the sides of landfills as well as pulling or pounding on top of the landfill. The formation is related to two things, the age of the leachate itself (as it ages it tends to form more), but also the amount of gas that is accumulated in the sides. In general, what we see is an interesting cycle of very high shut-in gas pressure so the wells are dewatered, but when they’re dewatered more gas starts flowing into the wells and then gas and liquids are literally being blown out of the wells, even where the pumps are installed. This sort of a cycle of continued dewatering followed by pulling of gas needs to occur to get the system to settle down.  Eventually the liquid levels get down low enough that there will be a nice and steady gas flow and the foaming and surging problems will be eliminated.

It is possible, and the reason would be accelerating gas production or biodegradation. If you have liquid present, you may have slowed the degradation of waste. If you remove the liquid you could see some increased rate of gas generation or degradation such that you would see an increase in temperature.

There are different types of chlorinated compounds so you have to look at each compounds – it's probably not useful to try to talk about an entire class of compounds. The on-line process model has more than 100 different VOCs available for process simulation.

The best thing to do is to run a pilot study and adjust the Henry's constants accordingly, this way you will get a large enough stripper to deal with the actual conditions.

The VOCs from the water go into the air, which can generally be discharged without any treatment in drinking water applications since the VOC concentrations in water are usually very low (low part per billion range) and greatly diluted by the high air to water ratio.

QED understands the need to ensure compliance and therefore the following methods are available: during the Power-On-Self-Test (POST) process, the date of next service is detailed on the analyzer screen.  The information screen of your analyzer also displays this.  You can check here and go to TRACK YOUR UNIT, plus we will send you a text message and email.  Alternatively, please contact us or your local QED, Landtec or Viasensor representative.

There is no need to book your unit in to the Service Center.  Simply print and complete the Service return form.  For Viasensor G100/G110/G150/G200/G210, use the Service Return form GXXX and enclose it within your unit's case, print out the Service return label and attach it to the case / package to be returned to QED.  

The full factory service and calibration consist of thorough analyzer assessment, service and repair, recalibration, test and verification. Please follow this link for further details or contact service@qedenv.com

QED can provide UL-listed industrial control panels and we have done designs that interface the stripper with existing plant SCADA systems. If you need a control panel with your system, QED will work with you on what’s required to control your specific process.

If you look at the ASTM Standard D6634 in the selection on purging and sampling devices, it cites several studies that show differences in peristaltic pumps in comparison to other devices. In general, peristaltic pumps can provide good samples under certain conditions for VOCs depending on the nature of the VOCs themselves, how readily they partition out of water and into the air (the Henry’s constant for that VOC) and your lift. If the lift is fairly low and you have VOCs that do not partition out readily, you may see virtually no difference between bladder pump and peristaltic pump samples. Whereas, if the lift is greater than 15-20 feet and the VOCs partition out readily, you may see some loss of VOCs from peristaltic pumps. Some states do limit the use of peristaltic pumps for VOCs for that reason. For example, in the Florida SOPs, they limit the use of a peristaltic pump for VOCs and require that the sample be taken from the tubing by removing it from the well and getting the sample. In January 2010, US EPA Region 1 revised their guidelines and specifically included an appendix of low-flow guidelines that includes warnings about the use of peristaltic pumps for VOCs. So, take a look at the literature and your specific conditions; if in doubt, do a comparison of your application using a portable water pump and a peristaltic pump to see if there is a difference.

There are some studies showing that the temperature increase in the water column from some pumps can be fairly significant. Specifically, variable speed, electric submersible pumps can cause a significant amount of heating, not only in the sample collected, but also in the water in the well itself. The best way to avoid this is to use peristaltic pumps if you are within the suction lift limit of about 10 meters/30 feet (on a practical basis, it's really closer to 8 meters/25 feet). The other alternative if the well is deeper than that, is to use an air-powered pump such as a bladder pump. Devices such as bladder pumps and peristaltic pumps can do a better job avoiding any concern at all about losing VOC to heat.

The differences are due to a couple of things. When there are significant differences it can be due to contaminant stratification within the well or formation. But there can be a significant difference in concentration from the top to the bottom of the screen where there is no vertical ambient flow or mixing within the well screen itself. So under non-pumped or ambient flow conditions, where a passive diffusion bag sampler is used in different positions in the well, we could see concentration differences of as much as 50% or more. One reason for this is the contaminants are also stratified in the surrounding formation. Another reason is redistribution in contaminant mass within the well due to ambient vertical flow. For example, studies have shown that many times water will enter one part of the screen zone but exist in another part. And we can see some difference in concentration from top to bottom on the screen due to that. A low-flow purging and sampling method or any method that purges the well should obtain a flow weighted average concentration across the screen, so we may not be able to see the highest possible concentration that exists in the formation surrounding the well, but we’ll get a consistent flow weighted average concentration. That means we are generally not going to see the highest possible concentration that we might see with the PDB, but we are going to get consistent results from sampling provided that the formation hasn’t changed.

Switch over to a bladder pump. Portable or dedicated bladder pumps are air-driven diaphragm style pumps that can be operated at depths down to 1,000 feet or more.  And they are useful for all parameters: volatiles, semi volatiles, inorganic parameters, and radionuclide biological parameters.  There is nothing a bladder pump can’t sample for, it's probably the most versatile pump, and it can get down into wells under ¾-inch diameter (or roughly 18 mm).

There are low-flow purging and sampling devices that get as small as ¾-inch diameter. In fact, there are some companies now making mechanical bladder pumps that can fit into ½-inch monitoring wells (basically 12 mm diameter). The question has come up about larger diameters, in other words, is a 4-inch diameter too big? But, a lot of field studies are conducted in 4-inch monitoring wells. When you start getting into larger wells like 6-, 8- or 10-inch diameter casings, then it becomes an issue of the volume of water required to pump at a low rate in order to get exchange within the screens and the wells.

Some bladder pumps need to be replaced, or at least serviced, it depends on the manufacturer. QED manufactures a bladder pump that has a 10-year warranty including the bladder. But some of the pumps I installed 28 years ago are still in service and have never been repaired or replaced in any way. In fact, QED has installed well over a 100,000 bladder pumps with a failure rate of less than 1%. There is a simple test that can be done at the surface to identify if there is any leakage, or any concern about bladder integrity, QED can email you a troubleshooting checklist for bladder pumps.

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