Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 6.4
PURGING

6.4 PURGING

The purpose of well purging before groundwater sampling is to ensure that the samples will be representative of the groundwater and contaminant levels in the vicinity of the well. If a well is left untended for prolonged periods, the water contained within the well and possibly within the filter pack may not be representative of the surrounding aquifer formation water. For example, water extending above or below the well screen may be stagnant and may have undergone physiochemical changes, such as volatile loss into headspace, or change in oxidation-reduction potential through gas exchange with headspace, etc. Excessive purging may result in biased groundwater samples, since it may dilute or increase the contaminant concentrations at the sampling point (USEPA, 2002b).

Choose a purging device that will not alter the geochemical and physical parameters of the groundwater and dissolved contaminants or increase turbidity. Keep the contaminant characteristics in mind when selecting a purge device, for example, do not choose a purge device that will cause volatilization if the contaminant of concern is a volatile compound. Also ensure that the purging device material is chemically inert and that it does not adsorb contaminants (USEPA, 2002b). Preferred device materials are PVC, stainless steel and Teflon®.

If practicable, use low-flow submersible or positive displacement pumps with variable-speed control. Other purging devices include suction lift pumps or peristaltic pumps, and bailers, though due to the greater potential to alter geochemical and physical parameters of groundwater and other limitations, their use is generally limited to monitoring wells with specific characteristics and/or contaminants. Pumps and other purging/sampling devices are described in Subsection 6.5. Use purging devices dedicated to the well whenever possible to avoid cross contamination. If downhole equipment must be used in more than one well, decontaminate the equipment between wells. Choose decontamination procedures appropriate for the contaminants present.

There are two potential intake positions for well purging: above the well screen or within the well screen. If purging above the well screen (if the screen interval is below the water table), start purging at the water table and gradually lower the pump so that it sits just above the top of the well screen at the end of purging. This will ensure that the stagnant water above the screen has been purged. It also ensures that during sampling, the groundwater has the shortest riser length to pass (USEPA, 2002b).

If the intake is placed within the well screen interval, place the intake into the zone of highest contamination. This method is especially effective if a low-flow purging and sampling technique is used (USEPA, 2002b).

All groundwater extracted from wells during purging must be properly containerized, staged, sampled, and disposed. Do not allow water to flow onto the ground. Place all downhole equipment onto a clean plastic sheet to ensure that the ground around the well is not cross contaminated by groundwater, and to ensure that downhole equipment does not transfer surface materials into the well.

6.4.1 Low-Flow Approach

Researchers have evaluated the relative merits of traditional purging methods; improved methods of purging have evolved out of efforts to improve sample accuracy and precision and to collect more representative samples (Nielsen, 2006). One improved purge method includes low-flow purging (also commonly referred to as micro-purging, low-stress purging, low-impact purging, or minimal drawdown purging). Low-flow purging involves the same approach and equipment as low-flow sampling, as described in Subsection 6.5.1. The HEER Office recommends the improved methods of the low-flow purging and sampling approach be utilized whenever feasible to collect representative groundwater samples.

The purpose of the low-flow purging is to sample a specific depth within a well screen interval. Low-flow purging, whether using portable or dedicated systems, should be done using pump intake located in the middle or slightly above the middle of the screen interval. Placement of the pump at the top of the water column for sampling is only recommended in unconfined aquifers, screened across the water table, where this is the desired sampling point. Carefully place the pump into the well at the selected depth within the well screen to avoid turbulence within the well. Do not use this method with screens exceeding 10 feet in length (USEPA, 2002b).

The approach is based on the assumption that under minimal drawdown the pump will not draw down stagnant water from the well portion above the pump. Therefore, the drawdown in the well must be kept at a minimum and not exceed 0.33 feet or 0.1 meter (Puls and Barcelona, 1996). This involves regular water level measurements, i.e. gauging, throughout purging and subsequent sampling. Gauging intervals can be as short at 30 seconds or as long as 5 minutes, depending on the hydraulic conductivity of the aquifer. Typically, flow rates on the order of 0.1 to 0.5 liters per minute are used; however, this is dependent on site-specific hydrogeology. The use of a variable-speed, low-flow sampling pump greatly assists in changing the flow rates and thus minimizing drawdown. Bailers, suction-lift pumps, and high-flow rate pumps cannot be used for low-flow purging and sampling. The low-flow method is applicable for all types of aqueous phase contaminants and naturally occurring chemicals in aquifers with low to high permeability. Low-flow purging and sampling is generally not appropriate for very low-yield monitoring wells, as noted in Subsection 6.4.3.

Generally, the time or purge volume required for parameter stabilization is independent of well depth or well volumes. Dependent variables are well diameter, sampling device, hydrogeochemistry, pump flow rate, and whether the devices are used in a portable or dedicated manner.

Throughout purging, regularly measure and record water quality parameters. Preferably, water quality parameters should be measured with a device that prevents contact with air, such as a flow through cell. The following criteria are typically achieved during well purging:

  • Well water pH stabilizes to within ±0.1 pH units for three successive readings.
  • Well water temperature stabilizes to within ±1 degree Celsius.
  • Well water conductivity stabilizes to within ±3 percent.
  • Well water oxidation reduction potential stabilizes to within ±10 millivolts.
  • Well water dissolved oxygen concentration stabilizes to within ±0.3 milligrams per liter.
  • Well water is clear to the unaided eye in areas where the local groundwater is known to be clear and the turbidity readings are below 10 NTUs.
  • Turbidity stabilizes to within ±10 percent at concentrations larger than 10 NTU. In areas of known turbid groundwater, the final well water may be turbid to the eye.

The HEER Office recommends the use of flow-through cells when monitoring groundwater stabilization parameters during purging. Flow-through cells contain an inlet at the bottom of a cup that purge water is pumped into. The water is allowed to overflow into a bucket during purging. The field instruments used to monitor stabilization parameters are kept in the flow-through cell to provide a continuous readout.

6.4.2 Well Volume Approach

As noted in Subsection 6.4.1, the newer and more representative low flow purging (and sampling) approach is preferred by the HEER Office. A summary of limitations of the fixed well-volume purging approach is provided in "The Essential Handbook of Ground-Water Sampling" (Nielsen et. al., 2007). The well volume purging approach is described below as a potential option for specific circumstances. Rationale for use of this purging approach should be clearly documented in the SAP or in consultation with the HEER Office.

The purpose of the well volume approach is to remove all stagnant water or non-representative water within the well, the filter pack, and the adjoining formation.

For wells where the water level is above the well screen (i.e., a fully saturated screen interval), start pumping near the water table and lower the pump slowly throughout the purging process. The final position of the pump should be just above the screen interval.

For wells where the water level is within the well screen, set the pump to a level such that the drawdown does not allow air to enter the pump. Set the pump high enough that sediment from the bottom of the well is not introduced into the pump (USEPA, 2002b). Keep the pump rate low enough to avoid turbulent flow within the well; for a 2-inch well that is typically less than one gallon per minute or 3.8 liters per minute (USEPA, 2002b).

Alternatively, use a bailer to remove groundwater from the well. Start bailing near the water table and keep lowering the bailer as purging continues. Do not lower the bailer to a depth where bailing will disturb sediment at the bottom of the well. Avoid introducing turbulence near the bottom of the well that could lead to suspension of sediment into the water column.

Monitor water quality parameters after removal of each well volume (the well volume includes the saturated filter pack volume). Continue purging until a minimum of three well volumes are removed and the water quality parameters have stabilized. The stabilization criteria typically monitored during purging are the same as those listed in Subsection 6.4.1.

6.4.3 Purging Low Permeability Formations

The procedures described in this section are for use in very low permeability formations, where wells are slow to recover.

In wells that are screened below the water table, purge water in storage in the well casing from above the screen, and avoid dewatering and introducing air into the well screen interval. Do not lower the pump into the well screen interval, but pump from the top of the water column, following the water level down to the top of the screen. This procedure requires pumping at low rates to prevent excessive draw down, so bailers or inertial-lift pumps should not be utilized (USEPA, 2002b).

In wells that are screened across the water table, it has been common practice to purge the well dry and let it recover for a minimum of 2 hours and until sufficient water volume is present to take a water quality sample or the well has recovered to 90%. Although it is recognized that purging to dryness may lead to significant problems such as loss of volatiles, increased turbidity, and changes in dissolved gases, alternatives for these low yield wells may be limited, especially in cases with less than 4 feet of water in the well and a depth to water of more than 20 feet (USEPA, 2002b).

Another option is the use of "minimum-drawdown" purging for low yield wells as described in "Low-Flow (Minimal Drawdown) Ground-Water Sampling Procedures" (Puls and Barcelona, 1996). When sampling very low permeability wells, the rationale for purging and sampling strategy selected should be described in the SAP and discussed with the HEER Office prior to the first sampling event.

6.4.4 Well Purging Log

A well purging log should be documented for each monitoring well. The log should include, at a minimum, the following information:

  1. Project name and location
  2. Well designation and location
  3. Well construction including total depth of well and screen length
  4. Well purging technique
  5. Purging device
  6. Average pumping or water extraction rate
  7. Static water level from top of casing before purging
  8. Total well depth and presence/absence of sediment
    1. For low-flow approach: measure total well depth after purging and sampling are complete
    2. For well volume approach: measure total well depth before purging
  9. Water Volumes
  10. For well volume approach: calculate well volume
  11. For low-flow approach: calculate tube volume
  12. A running log of:
    1. Time
    2. Water volume removed both incremental and totalized
    3. For low-flow approach: water level and drawdown
    4. Field measurement of pH, temperature, conductivity, oxidation reduction potential and turbidity
    5. Field measurements of dissolved oxygen
    6. Visual and olfactory observations such as color, clarity, odor, particulates, etc.
  13. Total volume of water removed
  14. Total time needed for purging
  15. 15. IDW inventory including type and number of IDW containers, location of IDW storage

Refer to Table 6-1 in Subsection 6.2.1.7 for methods to determine water quality parameters during purging. Figure 6-11 presents an example Groundwater Sampling Log containing locations for recording purging parameters prior to the collection of groundwater samples.

Figure 6-11

Figure 6-11. Example Groundwater Sampling Log
[Source: US Navy, 2007]