The objectives of a good sampling program should be the collection of a “representative” sample of the current ground-water conditions over a known or specified volume of aquifer. Therefore sampling equipment, sampling method, monitoring well construction, monitoring well operation and maintenance, and sample handling procedures should not alter the chemistry of the sample.
Important methods for purging a well include the “Low-Stress Approach” and the “Well-Volume Approach”.
2. Sampling Objectives
Parameters that may impact the design of a groundwater sampling program include:
a) the thickness, lateral extent, vertical and horizontal flow direction, and hydraulic conductivity contrasts of the geologic materials.
b) the types of contaminants present (volatile organic compounds, semi-volatile organic compounds, metals, etc.)
c) lateral and vertical distribution of contamination
d) direction of groundwater flow, ground-water velocity (horizontal and vertical variability)
e) water-level information
Prior to field activities personnel should have specific information including well casing diameter, borehole diameter, casing material, lock number and keys, physical access to wells and length of and depth of well screen. Furthermore the amount of time that has expired between the last sampling effort and the planned sampling effort should be known to decide whether the well should be redeveloped prior to sampling.
After the installation of a well or sampling point time should be allowed for equilibration.
3. Groundwater Sampling Methods
Groundwater sampling methods depend on the site-specific conditions. The types of equipment available for ground-water sampling include hand-operated or motor-driven suction pumps, peristaltic pumps, positive displacement pumps, submersible pumps, various in-situ devices and bailers made of various materials. The same device used for purging the monitoring well should be used for sampling to minimize agitation of the water column.
Low-flow submersible or positive displacement pumps that can control flow rates are recommended for purging wells. Dedicated sampling systems are greatly preferred since they avoid the need for decontamination of equipment and minimize turbulence in the well.
Sampling devices (bladders, pumps, bailers, and tubing) should be constructed of stainless steel , Teflon®, glass or other inert materials). For low-flow sampling short screens (less than 1.6 m) should be incorporated into the monitoring design.
Essentially there are two positions for placement of the sample pump intake, within the screen and above the screen.
4. Purge Criteria
4.1 ‘Low-Stress Approach’
The most common ground-water purging and sampling methodology would be to purge a well using bailers or high speed pumps to remove 3 to 5 casing volumes followed by sample collection.
The low-stress approach requires the use of a variable-speed, low-flow sampling pump. The low-stress method is based on the assumption that pumping at a low rate within the screened zone will not draw stagnant water down. Drawdown should not exceed (0.1 meters). Bailers are inappropriate devices for low-flow sampling. Furthermore the water-quality indicator parameters (purge parameters), pH, specific electrical conductance, dissolved oxygen concentration, oxidation-reduction potential, temperature and turbidity are monitored at specific intervals.
4.2 ‘Well-Volume Approach’
This method is based on proper purging of the stagnant water above the screened interval and the stabilization of water-quality indicator parameters prior to sampling. The pump should not be allowed to touch or draw sediments from the bottom of the well. The well-purging should not be great enough to produce turbulence in the well (commonly no greater than 3.8 litres).
Sampling equipment typically includes:
- Depth-to-water measuring device-An electronic water-level indicator or steel tape and chalk
- steel tape and weight- Used for measuring total depth of well
- Sampling pump – Submersible or bladder pumps with adjustable rate controls are preferred. Pumps are to be constructed of inert material such as stainless steel and Teflon, peristaltic pumps can be used when the depth to water is 20 feet or less.
- Tubing – Teflon ® or Teflon-lined polyethylene tubing is preferred
- Power source
- Flow measurement supplies-flow meter and stop watch
- Multi-parameter meter with flow-through cell
- decontamination supplies
- sample bottles, sample preservation
5. Alternative Ground Water Sampling Techniques
Alternative Ground Water Sampling Techniques include:
- the Screened Auger Tool, which is a five foot length of laser-slotted hollow stem auger, available in 2 to 12 inch ID, constructed of carbon steel through which samples of ground water may be obtained.
- Miniature Drive Point (MDP), which is a retractable screened probe ((3/4) inch OD) connected to small diameter (1/8-<< inch OD) flexible tubing. The tip and tubing are driven into place with a temporary outer casing
- Well point, which is a small diameter (1-2 inch) probe constructed of continuously wrapped stainless steel or wrapped stainless steel gauze screen over perforated carbon steel pipe.
- Passively Placed Narrow Diameter point (PPnDP) is a small diameter (<<-1 inch OD) screened casing passively placed in a borehole
- Small Diameter Direct Push Point (SDDPP), which is <-1 inch OD casing (slotted or blank) which can be driven or pushed through the soil into the ground water (Tracer Research, Geoprobe ®). No filter or gravel pack is used in the installation.
- HydroPunch®, which is a sampling tool constructed of stainless steel and Teflon used for collecting ground water samples.
Each tool and technique has strengths and weaknesses.
Yeskis, D., Zavala, B. (2002): Ground-Water Sampling Guidelines for Superfund and RCRA Project Managers, Ground Water Forum Issue Paper, EPA 542-S-02-001
New Jersey Department of Environmental Protection (1994): Alternative Ground Water Sampling Techniques Guide