FAQ: Developing, Sampling, Monitoring

Can CMT wells be used for soil gas sampling?

Yes. Special fittings are available to collect soil gas samples from all channels of the CMT tubing. Contact us for details.


What is the purge volume of the various CMT channels per foot of tubing?

The outer 6 pie-shaped channels of the CMT tubing each hold about 40 ml of fluid per linear foot of tubing. The central channel holds approximately 30 ml per linear foot.


What if I don't want to monitor seven zones? Do I have to use all the channels?

No. You can use as many or as few as you like. Unused channels do not affect the rest of the CMT System. Some people use two channels to monitor a single zone. They dedicate one of the channels to a Micro Double Valve Pump and use the other channel for measuring water levels. If you use two channels to monitor a single zone, however, you will reduce the number of discrete zones that you can monitor by 50%.


What kind of hydraulic tests can I perform in CMT wells?

CMT wells make ideal multilevel observation wells during groundwater pumping tests. Hydraulic tests have also been performed in the wells themselves. Most tests performed in CMT wells performed to date have been slug tests where compressed air is used to depress the water level in the test zone. The compressed air is suddenly released, and the recovery of the interval is monitored by measuring the rising water level over time. Small diameter transducers greatly simplify monitoring of the water level recovery, especially in coarse-grained formations that recover quickly. A good source of information pertaining to hydraulic testing in small-diameter wells can be found at the University of Kansas website: http://www.geo.ku.edu


How do I measure water levels inside CMT wells?

Water levels can be measured with Solinst's Model 101 or Model 102 small-diameter water level tapes. If continuous recording of water levels is desired, you can install Solinst's Model PDCR 35/D Druck transducers. The transducers can be connected to a wellhead datalogger or a telemetry unit for remote reading from a central data collection center. These transducers will only fit inside the outer channels and not the narrower center channel.


What are the best ways to purge and sample CMT wells?

You basically have 3 choices for purging and sampling in narrow diameter applications. The Model 410 Peristaltic Pump can be used where the suction lift is less than 25 ft (7.5 m). Solinst's Mini Inertial Pump (MIP) can also be used. The MIP uses 1/4" (6mm) dia. riser tubing fitted with a "push-in" foot valve. Repeated up and down strokes brings the sample to surface from depths up to 150 ft. (46m). Purge & sampling can also be done with the Model 408M Micro Double Valve pump which is ideal for low flow sampling. The 408M is made of 3/8" (10mm) dia. flexible coaxial tubing available in LDPE for use to 50 ft. (15m) or Teflon for use down to 150 ft. (46m) The 408M uses a drive gas which is delivered through a controller.


Are there any chemical affects of biases associated with the polyethylene tubing or other parts of the CMT system?

There are chemical biases associated with all types of groundwater monitoring wells and sampling pumps. Potential chemical biases associated with the CMT system relate to (1) the use of polyethylene tubing and (2) the sampling devices used to collect water samples. Hydrophobic organic contaminants can sorb to the polyethylene tubing, potentially causing a negative sampling bias. In some situations, those same compounds can diffuse through the polyethylene, either from outside of the well or from adjacent channels, potentially causing a positive sampling bias in some channels. Potential biases with hydrophyllic contaminants, e.g. MTBE or most inorganic compounds is minimal. A thorough discussion of these potential sampling biases is presented in the Einarson and Cherry Paper describing the CMT system that was published in the Fall 2002 issue of Groundwater Monitoring and Remediation. (See the Papers Section of this web site.)


Are groundwater samples collected from CMT wells as good as samples collected from 2-inch or 4-inch conventional monitoring wells?

Groundwater samples from CMT wells are not only as good as samples collected from traditional monitoring wells, they are usually better! Most importantly, samples from CMT wells are discrete samples from the aquifer, not composite samples typical of long-screened conventional monitoring wells. Consequently, if the concentration of a contaminant in a sample collected from a particular CMT channel is low, you can be confident that the concentration in the aquifer at that depth is indeed low, rather than being low because of dilution as may be the case with a conventional monitoring well. Additional discussion of the sampling biases associated with conventional monitoring wells and the technical advantages of multilevel groundwater monitoring is presented in our Papers Section. In addition, water samples collected from CMT wells are often less turbid than samples collected from conventional monitoring wells. The screen slot size and sand pack of a conventional monitoring well is often a compromise owing to the wide range of grain sizes present within the screened interval of most wells. The well screens and sand pack may be too small for the coarser fraction, but too large for the fine-grained layers within the screened zone. This leads to high levels of turbidity in the water samples since the fine-grained sediments are not effectively filtered by the well screens and sand pack. CMT wells, on the other hand, typically monitor short, discrete intervals in an aquifer. The well screen and sand pack in each monitored zone can be optimized for the grain size of the sediments within each interval. Each intake port in a CMT well can have a different well screen and sand pack size depending on the lithology of the aquifer materials in each monitored zone. This flexibility in well construction optimizes the filtration characteristics of the CMT well, resulting in clear, turbidity-free water samples. CMT wells have other advantages over conventional monitoring wells. First, the purge volume of CMT wells is very small. That means that there is less contaminated water requiring treatment or disposal during routine sampling. Take the case of a 4-zone CMT well that has ports at depths of 20, 40, 60, and 80 feet. Assuming that the static water level is 10 feet below the ground surface, the volume of water required to purge two times the "casing volume" of the four channels would be about 13 liters or less than 3.5 gallons! Second, CMT wells detect changes in piezometric pressure more accurately than traditional monitoring wells. Two- or 4-inch-diameter monitoring wells store a lot of water compared to the individual channels in a CMT well. The large amount of water stored in a conventional monitoring well means that the well will be slow to respond to changes in piezometric pressure in the aquifer. This is especially true in low-yield formations, where weeks and even months may be required to recharge the well casings to the static water level. CMT wells, on the other hand, respond and equilibrate quickly because of the low volume of the various channels.


I have flowing artesian conditions at my site. Do you have a way to plug and sample the ports at the wellhead?

Yes. Special expansion plugs are available to seal the various channels at the wellhead. The plugs have optional valves which allow you to collect groundwater samples simply by opening the valves. Pressure gauges can also be attached to the plugs at the wellhead in order to measure the piezometric pressures in each monitored zone.


How do I develop CMT wells?

We have had good success purging the wells with Peristaltic Pumps and Mini Inertial Pumps. You cannot, of course, develop CMT wells the way you would a water supply well, but that type of rigorous development isn't required with small-diameter monitoring wells. The goal of developing CMT wells, which is usually easily achieved, is to establish hydraulic connection with the formation. The well won't be 100% efficient, but the hydraulic head values measured in the well will be accurate, and the well will yield more than enough water to sample (assuming that the formation is reasonably permeable). If you added water when drilling the borehole or constructing the well, the best way of dealing with this is to simply wait several days for the water you've added to "drift" down gradient. For sites with typical groundwater velocities (0.5 to 2 feet per day), the water added during drilling and/or well construction will have drifted away from the CMT intake ports in several days. If the water added during drilling has a different electrical conductivity (EC) than the formation water, you can monitor the EC in water pumped from the well to confirm that the drill water is gone. Some consultants have added potassium bromide (an inert tracer commonly used in groundwater research) as a tracer to the drill/construction water and then monitored the purge water in the CMT well with a bromide-specific electrode to verify that the drill water is no longer in the vicinity of the CMT well prior to collecting samples. Contact us for details.


® Solinst and CMT are registered trademarks of Solinst Canada Ltd.
*Patents #6,865,933 B1, #6,758,274 B2, #2,260,587, #6,581,682, #2,347,702, and #2,381,807