PDFs:

Pneumatic Slug Testing... Do You Know Your "K"?

Hydraulic Conductivity (K) is a measure of the capability of a medium to transmit water. This is an important parameter in comtaminant investigations as hydraulic conductivity is useful in determining the volume of water passing through a given soil cross section, and for estimating the flow velocity of contaminated groundwater.

In the past, hydraulic conductivity was typically measured using pumping tests or by performing “slug tests” of installed wells. Both of these past practices, while having their technical strengths, required the installation of some form of well casing; either of the pumping or monitoring type.

Simulated transducer data during a slug test (well head pressurized, stabilization, and release of pressure to initiate the slug test response).

What is a slug test?
As defined by Fetter (1994) a slug test is “an aquifer test made by pouring a small instantaneous charge of water into a well or by withdrawing a slug of water from the well” (e.g with a bailer). This definition clearly shows how early slug testing was conducted, by quickly adding water to a well or quickly removing a bailer full of water from the well.

When environmental monitoring wells are being tested, you don’t want to add (or remove) water from the well. This either alters the ambient water quality or generates potentially hazardous waste. It quickly became common for people to use a “mechanical slug” to run a slug test. This is simply a 3 to 5 foot long section of PVC pipe filled with sand and capped on each end. A cord is attached to one end of the mechanical slug so it can be quickly lowered below the water level (slug-in test/falling head test) and then later quickly raised above the water level (slug-out test/rising head test). Again the change in water level and time are recorded.

Quickly lowering or raising the mechanical slug in or out of the water may cause splashing in the well. For slug tests lasting several minutes or longer this is not a significant problem. However, for wells that recover from a slug test in less than a minute the splashing will interfere with a lot of the early time data so that a good determination of the changes in the water level can not be made. Because of these problems the pneumatic slug testing method was developed (Prosser 1981, Butler 1997). In this method, the well head is sealed and air pressure is used to displace the water level.

Why calculate the hydraulic conductivity of a formation?
Since the hydraulic conductivity lets you know how fast the groundwater can move through a formation, it is the first step in knowing how fast contaminants may be able to move. This information is used to determine the potential “risk” caused by the presence and migration of contaminants in the subsurface for risk based corrective actions (RBCA). It is also a vital piece of information required to determine if monitored natural attenuation (MNA) is an acceptable remedial option for a contaminated facility (EPA 1998). Additionally, knowing the hydraulic conductivity is a very useful piece of information when implementing or designing a remedial system, be it injection of fluids, pump and treat, or other remedial methods.

Why use Direct Push methods to conduct slug tests?
There are several reasons, but primarily economics is the driver. You can save thousands of dollars and a lot of time using direct push methods to do this work. A summary of specific reasons include:

Installation of permanent monitoring wells is not required. This eliminates the need for drilling, disposal of drill cuttings, cost of well construction, and generation of large volumes of development water.
Tests can be conducted at multiple depths and at several locations across the site in a timely manner to determine vertical and horizontal variations in K.
High-quality depth discrete data on hydraulic conductivity can be obtained in many unconsolidated formations.
Equipment and methods exist for measurement of hydraulic conductivity in formations ranging from very high-K (> 750ft/day, 2.6 x 10-1 cm/sec.) to very low-K (< 0.003 ft/day, 1 x 10-6 cm/sec) materials.

	Where did the slug come from?
	Slug Test. No, it’s not a test to see if you have slimy worms crawling around in your garden! When single well tests to measure hydraulic conductivity first got started, the wells being tested were pretty large in diameter ... 4-in., 8-in. or even larger. There were no neat small pressure transducers, no data loggers, and definitely no portable computers at this time. The tests were completed by pouring a 5-gallon bucket of water down the well casing and measuring the change in water level with a tape, marking time using your wrist watch, and writing down the time and water level in your field book with your No. 2 lead pencil! The 5-gallons of water was simply a big “slug” of water poured down the well. And presto! The “Slug Test” was born.

U.S. EPA recommends use of direct push methods for determining vertical and lateral variations in K so that contaminant migration pathways can be located and assessed. (Monitored Natural Attenuation for Ground Water, EPA/625/K-98/001).

This information is needed for RBCA and MNA investigations and remedial actions.
Knowledge of vertical and lateral changes in K can be used to optimize remediation design and minimize the potential for having to reengineer a poorly planned remedial design.
The same tool (e.g. SP16 or Profiler) can be used repeatedly after decontamination to conduct slug tests which reduces equipment costs.
Minimal contaminated waste is generated (essentially no waste cuttings) and exposure hazards to workers are reduced.
You can provide a new and valuable service to your clients while saving them money and time to get information they need.

Can direct push slug tests provide results comparable to tests conducted in conventional monitoring wells?
In short, YES. When the screens are installed properly and adequately developed, field comparison has shown that the direct push methods provide essentially the same results as conventional monitoring wells screened over the same interval (Butler et al. 2002, McCall et al., 2002).

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