Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 6.5
GROUNDWATER SAMPLE COLLECTION METHODS

6.5 GROUNDWATER SAMPLE COLLECTION METHODS

The purpose of groundwater sampling is to collect samples representative for the aquifer at the well location. Consider the characteristics of the contaminant(s) such as volatility, solubility, density (denser or lighter than groundwater) and their resultant fate in the subsurface (adhesion to soil particles, biodegradation etc.) during selection of the sampling approach.

The following sections describe different methods of groundwater sampling, the different types of equipment for each method, and details on the appropriate applications for each method. Table 6-2 presents the potential for representative analytical results for commonly utilized groundwater sampling methods for common contaminants of potential concern.

Table 6-2
Table 6-2 Potential for Representative Analytical Results for Commonly Utilized Groundwater Sampling Methods for Common Contaminants of Potential Concern
  VOC SVOC Metals
*
Pesticide/
Herbicide
Dioxins/
Furans
PAH PCB Explo-
sives
Prod-
uct
Positive Displacement Pump E E E E E E E E NR
Submersible Pump G G E E E E E E NR
Bailer NR P P G G G G G G
Peristaltic Pump P P G G G G G G G

Notes:
E = Excellent
G = Good
P = Poor
NR = Not Recommended
* - for dissolved metals analysis, samples must be filtered

Figure 6-12

Figure 6-12. Low-flow bladder pump within a monitoring well
In this illustration the water level is above the screen interval


Figure 6-13

Figure 6-13. Detail of Field Setup for Purging and Sampling Well
See Figure 6-14 for expanded view.



Figure 6-14

Figure 6-14. Field Setup for Purging and Sampling Well
Purging and sampling of groundwater monitoring well using a portable low-flow bladder pump driven by a portable air compressor and control box.


Figure 6-15

Figure 6-15. Bailers for Purging and Groundwater Sampling
The following representative bailers are shown:
1. Micro Bailer
3. PVC Bailer
5. PVC Bailer
2. Mini Bailer
4. PVC Bailer
6. Teflon Bailer
[Source: Eon, 2008]

Figure 6-16

Figure 6-16. Peristaltic Pump Head
[Source: Environmental Pumping, 2008]

6.5.1 Low-Flow Sampling

The purpose of low-flow sampling is to collect representative groundwater samples for a specific depth within a well screen interval. The method is based on the assumption that given sufficiently low removal rates, a sampling pump will not draw stagnant water from the water column above and below the position of the pump. Therefore the rate of removal must be kept to a minimum in order to minimize drawdown, which must not exceed 0.33 feet or 0.1 meter (Puls and Barcelona, 1996). 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 method is most applicable in aquifers with medium to high permeability. The HEER Office recommends the low-flow sampling approach be utilized where appropriate and feasible to collect representative groundwater samples.

Low-flow sampling is preceded by low-flow purging (see Subsection 6.4.1 Low-Flow Approach). In general, place the pump intake at the depth of highest contaminant levels. These are often the depths of highest hydraulic conductivity. Any other depth must be discussed and justified in the SAP and project reports. Do not move the pump intake location between the end of purging and the beginning of sampling. Any adjustment of the pump depth or pump speed will require renewed purging. Do not pause the pump between purging and sampling. Keep the pump rate throughout sampling low enough that the groundwater flow exiting from the discharge tube is laminar and does not induce turbulence in sampling containers. The types of pumps used for low-flow sampling and purging are positive displacement pumps (Nielsen, 2006). These types of pumps include:

  • Bladder Pump
  • Gear Pump
  • Piston pump

6.5.2 Positive Displacement Pumps

Positive displacement pumps are suitable for collecting samples for all analyses, and particularly when samples are to be collected for analysis of volatile compounds (Nielsen, 2006). These pumps use a pushing motion, induced by the gears, pistons or bladders, and are the least likely of the well sampling methods described in this TGM to impart agitation, turbulence, or heat that could affect sample analysis results, particularly for volatile contaminants.

Until recently, positive displacement pumps were limited to use in wells larger than 2 inches in diameter. However manufacturers of bladder pumps and piston pumps have developed pumps with diameters as small as 0.625 inches that can be used in microwells installed via direct push methods. Gear pumps currently still require a well diameter of at least two inches (Nielsen, 2006). While both bladder and piston pumps can be used in a wide range of applications, piston pumps are relatively more complex and tend to be used more as pumps dedicated to individual wells (Nielsen, 2006). Owing to the relative simplicity of design and ease of field repair, bladder pumps are more widely used as portable units and are the most commercially available of the positive displacement pumps.

A bladder pump consists of a flexible membrane enclosed by a rigid housing. In the most common application, a pump is inserted below the water table in a well, and water enters the housing under hydrostatic pressure through an intake check valve. When the housing chamber is full, the intake check valve prevents the water from escaping back out to the well. The bladder is inflated from a compressed gas source or pump at the surface, and the pressure generated by the inflating bladder pushes the water upwards through another discharge check valve, then through a discharge tube upwards to the surface. An example of a bladder pump set up in a well is illustrated in Figure 6-12.

Setup of bladder pumps in the field typically require a compressed gas source to inflate the bladder and a controller unit to regulate the rate of inflation and, therefore, pumping. Compressed gas may be supplied from a tank or compressor. For deeper applications requiring greater pressures, a compressor driven by a dedicated motor is generally used. For these instances, care must be given to locate the motor upwind and away from the well being sampled. For shallower applications, some manufacturers have developed combined compressor/controller units that can be powered off of a portable battery or the battery of a field vehicle.

An example field setup of a portable bladder pump assembly is illustrated from different points of view in Figures 6-13 and 6-14.

6.5.3 Submersible Pumps

Submersible pumps move water up a well by applying positive pressure; however, instead of a pushing motion, submersible or centrifugal pumps typically use electric-motor driven impellers to drive the water to the surface. The impellor pressure is equal to the hydraulic head in the tubing extending from the pump to the top of the well.

Submersible pumps are portable, require less setup and equipment than positive displacement pumps, and do not require compressed gases or air pumps. They generally have higher discharge capacities than positive displacement pumps and may therefore be more effective as a well volume purging method and for collecting of samples for nonvolatile contaminants. They may also be used to collect samples for volatile contaminants but may not recover samples as representative as positive displacement pumps for volatile contaminant concentrations; the impellor motion may be prone to cavitation, inducing bubble formation that may affect pressure sensitive components such as dissolved gases or VOCs. The motors themselves may generate considerable heat that may also affect more volatile components (Nielsen, 2006). Many submersible pumps, particularly older models, may not be capable of producing sufficiently low discharge rates needed for low-flow sampling. In situations where volatile contaminants are being investigated and the investigation team selects submersible pumps as the method of sampling, the evidence and rationale for selection of this equipment over positive displacement pumps should be documented in detail in the project work plan, and concurrence with the approach should be obtained from the HEER Office prior to field investigation.

Select a pump to match the required lift at the required pumping rate. Do not underrate the pump as it will start running hot, which can alter the sample chemistry. The pump must have a variable-speed control. Select the power source and pump characteristics to allow the pump to run continuously throughout purging and sampling. Select the sample tube and associated equipment to be compatible with the site contaminants. Install a device that will prevent backflow to the pump to avoid groundwater contamination.

Install the pump at a sufficient height above the bottom of the well to avoid pumping sediment from the bottom of the well. Sediment can damage smooth internal surfaces, causing leaching of target analytes (NJDEP, 2005). In wells screened below the water table, set the pumping rate such that the water level is not drawn down into the screened section. Do not pump the well dry except as noted under limited circumstances discussed in Subsection 6.4.3 Purging Low-Permeability Formations.

Keep the pump rate low enough that the groundwater flow from the tube is laminar and does not induce turbulence in sampling containers. Choose sample containers appropriate to the contaminants of concern.

6.5.4 Bailer Sampling

A bailer is a well purging and/or sampling device that may be appropriate for use under limited circumstances. The simplest bailer consists of a rigid tube equipped with a check valve at the bottom and a means to attach a line to the top. The check valve allows water in to the interior chamber of the bailer as it is lowered into the saturated zone. When the bailer is raised the check valve is forced shut and a water sample can be retrieved from the well. Larger diameter bailers may be used for well volume purging, although care must be exercised in purging as described in Section 6.4.

If utilized, the HEER Office recommends dedicated disposable bailers that have a certification of laboratory-verified cleanliness available from the manufacturer. However, bailers may be dedicated to a single well to avoid cross contamination. In instances where well volume purging is also being done by bailer, the sampling and purging must be done with separate bailers intended and designed for the respective tasks.

The purpose of bailer sampling is to collect samples representative of the groundwater at the sampling point. Therefore, bailer sampling must be preceded by well purging. Purging has to follow either the well volume approach or the purging approach for low-permeability formations as described in this document.

Bailers are available in a variety of sizes and construction materials, e.g., PVC, Teflon®, and stainless steel. Several different types of bailer materials, diameters, and discharge applications are illustrated in Figure 6-15.

For final decision-making purposes, sampling for volatile organics using bailers is not recommended due to potential bias introduced during sampling (Pohlmann et al., 1990; Yeskis et al., 1988; Tai, et al., 1991). Use of bailers for sampling of semi-volatile organic compounds (SVOCs) and metals is also generally considered a poor option (See Table 6-2). The use of low-flow pumps is recommended for these contaminants, and in some cases may be required by the HEER Office, particularly in situations where residential or sensitive populations are involved. A potential specific exception to this rule is when the recovery rate of a well can be documented to be less than 0.05 gallons per minute (200 milliliters per minute) and the water column in the well is less than 5 feet. Under these conditions, the "minimum-drawdown" purging and sampling approach as noted in Subsection 6.4.3 or the use of a bailer may be considered for volatile sampling. In low well recovery rate situations where volatile contaminants are to be sampled with a bailer, use one designed for volatile sampling; specifically, a bailer with a double check valve and a bottom emptying device with a control flow check valve.

For periodic, long-term monitoring events the use of a bailer may in some cases prove cost-effective; however, in these instances, an initial comparison between data collected from low-flow pumps and from bailers should be carried out.

When bailer use is proposed, the rationale must be documented in the project work plan, and concurrence with the approach obtained from the HEER Office prior to field investigation.

6.5.5 Peristaltic Pump Sampling

A peristaltic pump operates by a circular motion creating a vacuum in an intake line drawing from the monitoring well. The vacuum draws groundwater up to the pump, where the water is dispensed from the end of the tubing. Tubing used for peristaltic pump sampling should be disposed of after one use. A diagram of the working end of a peristaltic pump is illustrated in Figure 6-16.

Peristaltic pumps have advantages in that they have few moving parts, do not need compressed gas or pumps, are generally very portable, and are easily set up and used; further, the tubing and pump heads can easily be replaced (USGS, 2002). Peristaltic pumps can produce a maximum lift of up to 20 to 25 feet and provide a pump rate of 1 to 2 liters per minute.

Peristaltic pumps do, however, have limitations in use and ability to collect representative samples for some contaminants. The vacuum induced in the downhole tubing can also result in loss of dissolved gases or volatile components. In addition, the tubing could diffuse atmospheric gases sufficiently to affect some target gases. Peristaltic pumps therefore are less likely to result in representative samples for the following analyses (NJDEP, 2005):

  • Volatile organic compounds
  • Semi-volatile organic compounds
  • Dissolved oxygen
  • Oxidation reduction potential
  • Carbon dioxide
  • pH
  • Iron and its associated forms.

A peristaltic pump may be proposed for use primarily if the following conditions exist:

  • Depth to water is less than 15 to 20 feet
  • There is limited recovery of water in a monitoring well
  • Contaminants of concern do not include any on the previous list (USEPA, 2002b)

Under these circumstances the rationale for peristaltic pump use must be documented in detail in the project workplan, evidence presented for the need to use the method in place of other methods, and concurrence with the approach obtained from the HEER Office prior to field investigation.

6.5.6 Others

Other groundwater samplers include passive diffusion samplers for VOCs or SVOCs, HydraSleeve®, the syringe sampler, and the Gore Module™. Application of these devices is generally limited to field screening (see Section 8). Use of these devices in field investigations should be discussed with the HEER Office before use.

6.5.7 Order of Groundwater Sampling

Collect samples in groundwater monitoring wells no sooner than 14 days after well development. This delay applies to newly installed wells as well as to wells that necessitated re-development due to clogging or other conditions. If a different time frame from 14 days is proposed based upon known site conditions (e.g. evidence of high hydraulic conductivity, either for the site itself or immediately surrounding area), overall project considerations, or other pertinent information, the interval, rationale, and evidence supporting the proposal should be noted in detail in the SAP or by consultation with the HEER Office. See Subsection 6.2.1.9 for guidance concerning the timelines for drilling, well installation, development, and sampling.

Sample wells in order of increasing contaminant concentrations as determined from previous sampling events. If the order is not known from previous sampling events, use vapor readings collected at the top of the well casing to aid in determining the correct order. This sampling approach minimizes the potential for cross contamination. In addition, follow proper decontamination procedures appropriate for the site contaminants, and protect the ground surface around each well with plastic sheeting.

If the order of sampling must be changed, it should be described in the SAP and discussed with the HEER Office. The order in which analytical samples should be collected is as follows (NJDEP, 2005):

  1. Volatile organic compounds (VOCs)
  2. Purgeable organic compounds (POC)
  3. Purgeable organic halogens (POX)
  4. Total organic halogens (TOX)
  5. Total organic carbon (TOC)
  6. Base neutrals/acid extractables
  7. Total Petroleum Hydrocarbon (TPH)/Oil & Grease
  8. PCBs/pesticides
  9. Total metals
  10. Dissolved metals
  11. Phenols
  12. Cyanide
  13. Sulfate and chloride
  14. Turbidity
  15. Nitrate and ammonia
  16. Preserved inorganics
  17. Radionuclides
  18. Non-preserved inorganics
  19. Bacteria