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Site Remediation News
November 1997 (Vol 9 No 3) - Article 04

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The Low-Down on Low Flow
By: Greg Giles and Jeff Story
Bureau of Ground Water Pollution Abatement

In recent years there has been much discussion of alternative purging and sampling methods in the technical literature, particularly where low flow rates are used. These methods have been termed low-flow, low-stress, minimum-drawdown and micropurging, among others. These terms usually refer to the use of very low ground water extraction rates (pumping rates below 1 liter/minute) during well purging and sampling. Since this method is not addressed in the Department's Field Sampling Procedures Manual (1), this article has been written to discuss this procedure so that people who write and review low-flow sampling plans will be better able to assess when the use of this procedure is appropriate.

History and Significance

Regulators and the regulated community have been at odds over the issue of filtering water samples collected for metals analysis. The regulated community has argued that the incorporation of aquifer matrix material (i.e., silt & clay) into ground water samples result in the generation of ground water metals data that are biased high. Metals are naturally occurring and exist in all subsurface geologic material; the required acid preservation of water samples may release metals previously bound to the surface of aquifer material included in the water sample or dissolve some aquifer material altogether (e.g., metal oxides & hydroxides). Therefore, it is reasonable to assume that the incorporation of aquifer matrix material into water samples, which increases turbidity of the water samples, will result in higher metals concentrations.

The cause(s) of turbidity in a given ground water sample may be from any or all of the following:

  1. Natural turbidity (i.e., mobile colloids) in the aquifer;

  2. The well may have been improperly constructed.
    Considering the nature of the aquifer material the well is installed in, the screen slot size chosen for the well may be too large or the sandpack placed in the annular space around the well screen may be too coarse;

  3. The well may have been improperly or inadequately developed;

  4. The well may have been pumped at too high a flow rate during the purging or sampling of the well. Anytime that a well is pumped at a rate that is higher than the well's recharge rate, or higher than the rate at which the well was previously developed at, sediment may be stirred up within the well casing or released from the aquifer into the well, resulting in the water becoming turbid; and,

  5. The procedures used during well sampling may result in increased well turbidity. The sounding of wells with probes to determine total well depth may stir up sediment that has accumulated at the bottom of the well. The sediment will be pulled up the water column as the probe is reeled up to the surface. The contact of sampling equipment with the well casing or well screen may release material into the well, and the quick movement of sampling equipment within the well (e.g., bailers) may produce turbidity due to hydrostatic stresses between the well and the surrounding aquifer.

A 0.45-micron filter is the industry standard to filter water samples. Puls and Barcelona (2) concluded that the use of a 0.45-micron filter was not useful, appropriate or reproducible in providing information on metals mobility in ground water systems, nor was it appropriate for the determination of truly dissolved constituents in ground water. The argument that well samples should not be filtered for metals analyses is also supported by the Department's Field Sampling Procedures Manual (1) which states (on page 178) that the Department requires "metals analysis to be performed on unfiltered ground water samples pursuant to the requirements of the Safe Drinking Water Act and the Clean Water Act."

With the requirement that water samples analyzed for metals not be filtered, the technique of low-flow purging/ sampling appears to have originated as a means of reducing turbidity in ground water samples. As stated in the April 1996 EPA Ground Water Issue (3), "Sampling-induced turbidity problems can often be mitigated by using low-flow purging and sampling techniques."

While the principal goal of the low-flow sampling technique is the collection of representative ground water samples, one of the major benefits of this technique is the potential cost savings due to the generation of less purge water requiring storage, transportation and disposal. The Department has received an increasing number of requests to allow low-flow sampling, and the potential reduction in the amount of purge water generated appears to be largely responsible for the increase in such requests.

Low-Flow Procedures

The underlying principle of the low-flow technique is that at low pumping rates (less than 1 liter/minute with 0.1 to 0.5 liter/minute being typical), the ground water flow in the area of the sampling device intake approaches horizon-tal linearity (i.e., the ground water that is being drawn into the sampling device should be limited to the sandpack and aquifer in the immediate area of the sampling device intake). According to the EPA document (3) on page 5, low-flow "refers to the velocity with which water enters the pump intake and is imparted to the formation pore water in the immediate vicinity of the well screen....The objective is to pump at a rate that minimizes stress (drawdown) to the extent practical."

Where there is no (or minimal) drawdown during purging and sampling the well is recharged by the aquifer at the same rate the well is being pumped. Under low-flow sampling conditions, suspended solids (e.g., colloids) are considered to be mobile in the aquifer, representative of natural conditions, and not an artifact of sampling or well construction. For this reason, analytical results for metals testing using low-flow sampling are considered to be representative of the total mobile contaminant load in the aquifer (4). Thus, sampling for both "total" and "dissolved" metals is usually not needed.

Using the low-flow sampling technique, wells are purged and sampled at flow rates at or below 1 liter/minute. There is generally no required volume of water to purge from the well before collecting the ground water sample. Instead, the decision on when the ground water sample can be collected is based on the stabilization of ground water-quality parameters (e.g., temperature, pH, Eh, specific conductance, dissolved oxygen (DO) and turbidity). Accordingly, it is important that the stabilization parameters be measured accurately, and that the purging be conducted in such a manner that the sampling procedure does not affect the field parameter values (e.g., some submersible pumps generate a lot of heat when pumping at very low rates; this may cause the temperature reading to drift); sampling devices that operate under negative pressure (e.g., peristaltic pumps) cause degassing of the water which may cause drift in the pH and dissolved oxygen readings.

During conventional purging and sampling the sample can often represent an average of the entire screened interval/bedrock borehole. In contrast, low-flow sampling conceptually results in the collection of a sample drawn from a discrete interval in the well. As such, the collection of a ground water sample using low-flow techniques may be considered somewhat analogous to collecting discrete ground water samples using equipment such as "Geoprobe" and "Hydropunch" samplers and temporary well points which typically have short intakes.


  1. With respect to metals analysis, low-flow samples are typically considered to be representative of the total mobile contaminant load (i.e., dissolved and colloid-associated). This reduces the need for sample filtration. Samples collected using low-flow methods usually contain less turbidity.

  2. Using low-flow procedures, the volume of water purged from the well may be significantly reduced. Costs associated with the storage, transport and disposal of the purge water may be reduced and the amount of time needed to purge the well may be reduced.

  3. Because the same equipment is used for well purging and well sampling, less equipment may need to be used in the field.

  4. Because the low-flow technique generates water samples obtained from very discrete zones, if the contaminant distribution in the section of the aquifer screened by the well is heterogeneous, the sample collected by low-flow procedures may show signifi-cantly higher contaminant concentrations than samples collected at higher flow rates (i.e., using traditional sampling methods).

  5. The reduction in the amount of fine-grained material flowing into the well can increase well life and reduce the need for well re-development.


  1. Not all sampling equipment can be used for low-flow sampling. Pumps used for low-flow sampling should be variable in speed and designed to operate at very low pumping rates. Pumps should preferably operate under positive displacement.

  2. Because the measurement of DO and Eh must be made before the ground water comes in contact with the atmosphere, a flow-through cell must be used to measure these parameters in the field.

  3. The zone sampled within the well by low-flow methods is conceptually limited. If the contaminant distribution in the screened section of the aquifer is heterogeneous, which may be the case in most wells, the sample results obtained by low-flow sampling may be significantly biased low if the sampling device intake is not placed at the same depth as that of the highest contaminant concentration entering the well. Accordingly, for wells contaminated with DNAPL or LNAPL type contamina-tion, the sampling device intake depth could signifi-cantly affect the sampling results. For wells con-structed with long screens, vertical flow gradients within the well may cause mixing of ground water and the samples will not be depth-discrete.

  4. The method requires higher initial capital costs and longer set-up time in the field.


EPA's issue paper (3) provides a very thorough evaluation of the method and its advantages and disadvan- tages. This paper is considered recommended reading by anyone involved in low-flow sampling.

At this time the Department does not have a formalized low-flow sampling procedure/policy in place. However, the Department is currently working to develop a formalized low-flow sampling guidance document. Currently, low-flow sampling can be approved only on a case-by-case basis until the Department develops appropriate guidance. A low-flow sampling plan must be provided to the Department for review and approval. Low-flow sampling plans should be as detailed as possible and take into consideration well construction, contaminant type and distribution in the aquifer, and local hydrogeology.


  1. NJDEP. 1992. Field sampling procedures manual (FSPM).

  2. Puls, R.W. and M.J. Barcelona. 1989. Ground water sampling for metals analyses; Superfund ground water issue. EPA/540/4-89/001. Ada, Oklahoma, R.S. Kerr Lab.

  3. Puls, R.W. and M.J. Barcelona. April 1996. Ground water issue. Low-flow (minimal drawdown) ground water sampling procedures. EPA/540/S-95/504. Ada, Oklahoma, R.S. Kerr Lab.

  4. The Nielsen Environmental Field School. Micropurge low-flow purging and sampling, Edison, New Jersey, May 22, 1997.

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Last revision: 15 January 1998