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Home > Ag Chemicals & Fertilizers > Spills & Safety > Incident Response Unit > Guidance Documents > Groundwater Sampling Guidance (GD12)

Guidance Document 12 - Groundwater Sampling Guidance

Printable Version (PDF: 223 KB / 7 pages)


Obtaining representative groundwater samples from the formations of concern is a critical goal of any groundwater sampling effort. Appropriately trained and qualified field personnel are necessary to properly collect groundwater samples in conjunction with a well planned sampling protocol. The sampling protocol should define the procedures used to accurately make field measurements, to purge and withdraw samples from the well, fill sample containers and collect blank and replicate samples. The scope of this guidance document is to provide general guidelines to follow when preparing site specific sampling plans for groundwater sampling. Sampling plans are made after a monitoring scheme has been carefully designed and wells have been installed and properly developed.

The following are a summary of recommended sampling procedures to consider when planning a sampling event:

1. Quality Assurance

  • Field Quality Assurance/Quality Control (QA/QC) - There are many potential sources of error in the field that can be difficult to control or assess. In order to minimize the possibility of cross-contamination:
  • When sampling monitoring wells, begin with the one expected to have the best water quality and end with the one expected to have the worst water quality i.e., sample from the least-to most-contaminated well). The sampling order for future event(s) may need to be modified depending on the most recent analytical results.
  • Use dedicated sampling equipment whenever possible. If dedicated equipment is unavailable, a impractical, utilize a separate laboratory-cleaned or new prepackaged bailer (stainless steel, Teflon or polyethylene) for each well. Discard any sampling equipment that cannot be properly de-contaminated (gloves, string, etc.) before proceeding to the next well.
  • Collect field blanks if re-useable sampling equipment is to be utilized for groundwater sampling. Field blanks consist of de-ionized water samples run through the sampling equipment after cleaning. The field blank water samples are handled and analyzed the same way as the well water samples.
  • A minimum of 10% duplicate samples (two samples taken sequentially) must also be collected at each sampling event. All blanks and duplicate samples must be submitted as "blind" samples to the laboratory.
  • Laboratory QA/QC - If the contract laboratory does not have a current quality assurance/quality control plan on file with the Minnesota Department of Agriculture (MDA), the information required in MDA guidance document GD24 Fixed Base Laboratory Quality Assurance/Quality Control Plans must be provided. MDA staff may split samples with any facility to document the accuracy and reproducibility of the analyses and or request additional QA/QC information.

2. Hydraulic Conductivity Tests

Slug or plug tests that estimate the horizontal hydraulic conductivity of the screened formation must be performed once on select new monitoring wells. In order to obtain accurate results, slug or plug tests must be conducted on undisturbed wells before water samples are collected. Please include methodology for conducting slug or plug tests in your Remedial Investigation Work Plan (RI Work Plan).

3. Water Level Determinations

Prior to purging and sampling, measure well water levels in all site monitoring wells to the nearest 0.01 foot from the surveyed reference point (top of well casing).

4. Sampling Procedures

    1. Calibrate all field instruments in the field before collecting samples.
    2. Choose a sampling device based on the following site characteristics: depth to water, parameters of interest, well construction details, and accessibility of the well(s). Ideally, well purging and sampling equipment should be inert, economical to use, easy to clean, reusable, operable at remote sites and versatile. Care should be taken when reviewing the advantages and disadvantages of a sampling device. It may be appropriate to use different devices for purging and sampling a well. The most common example of this is to use a submersible pump for purging and a bailer for sampling (see Attachment 4).
    3. Before a representative sample can be withdrawn from a well, the "old" or stagnant water must be completely removed. Monitoring wells are purged of the stagnant water until certain field water quality parameters have been monitored and stabilized. The following criteria are utilized to demonstrate stabilization:
      • Measure specific conductance, temperature and pH in the field after each well volume is purged until three successive readings yield measurements within the following ranges:
          1. Specific Conductance +/- 5%
          2. Temperature +/- 0.1 degrees Celsius
          3. pH +/- 0.01 units
          4. Dissolved Oxygen (DO): +/- 0.3 mg/L
            (Note: If DO is measured, a flow-through cell or other closed container must be utilized)
        • A stabilization test should be completed on all newly installed monitoring wells during the initial sampling event, after each well volume is purged. The volume of water required to reach stabilization of these parameters should be recorded and used as a minimum purge volume in subsequent monitoring events. Typically the minimum purge volume is between three (3) and five (5) well volumes.
        • For a well that pumps or bails dry, allow the well to recover prior to sampling. If well recovery is slow, the well may be sampled after purging one (1) well volume. Stabilization parameters should be measured after well recovery if there is sufficient water volume to allow for measurement and sample collection.
        • For Long-Term Monitoring sites, you must propose any stabilization parameter measurement frequency changes to MDA Incident Response Unit project staff for approval prior to implementation in the field.
        • Note : Monitoring well construction and development can significantly affect many water quality parameters, including those monitored during well purging. It is normally recommended to wait at least one week after development before collecting the first round of samples. (Newly-installed monitoring wells should be developed until clear, sediment-free water is obtained). Groundwater sampling plans, provided in the RI Work Plan, should include a schedule of sampling events.
        • Submit documentation of purging method, purge volume, recovery rate and stabilization test results to the MDA along with the analytical results.
        • The labels on the water sample bottles must denote:
            1. Type of analysis;
            2. Name of facility;
            3. Monitoring point identification;
            4. Name of person collecting the sample;
            5. Time and date the sample was collected; and,
            6. Whether a preservative was added to the sample or the sample was filtered.
          • Sample Collection:
            1. Use a 1-liter (amber colored) glass bottle with Teflon-lined caps.
            2. Use a plastic bottle for nitrate-nitrogen analysis (a glass bottle may be used for samples analyzed immediately). Keeping these bottles free of extraneous contamination is imperative. Do not drop the cap or place it on the ground or touch the sample bottle to the tap/sampling device. Allow only water from the sampling point and preservatives, if required, to come in contact with the inside of the sample bottles.
            3. Rinse each unpreserved bottle used for sampling pesticides and nitrate-nitrogen three times with water from the well before filling. Rinse bottle caps also. Do not rinse sample bottles that already contain a preservative. To rinse, fill the bottles approximately one-third full, replace cap and shake vigorously. Discard the rinse water after each rinse cycle. Following rinsing, fill the bottles with water from the well, replace the Teflon-lined lid snugly, label and seal the bottles, and place the bottle into a cooler for storage and transport. (Specific labeling, storage and transport procedures should exist for each consultant)
          • Chain of Custody: "Chain-of-custody" tracking is required on all samples. "Chain-of-custody" provides a record of all the personnel responsible for handling the samples. An example of a "chain-of-custody" form is included as Attachment 2.

        5. Transportation

        Store samples, blanks and duplicates at  four (4) degrees Celsius until analyzed. Freezer packs are preferable to ice for short time periods. If ice is used, use Ziploc-type bags to contain ice and meltwater and to prevent meltwater from loosening bottle caps and labels. Coolers should also be packed with vermiculite or other absorbent packing material to prevent breakage. Floating sample bottles or bottles with detached labels may be rejected by the laboratory. When any sample is to be shipped by common carrier or sent through the United States Postal Service, it must comply with the Department of Transportation Hazardous Materials Regulation (49CFR Part 172). The person offering such material for transportation is responsible for ensuring such compliance.

        6. Reporting

        Certain procedures should be followed when submitting groundwater quality data to MDA staff. Please refer to Attachment 1 when compiling information for ground water quality reports.

        Attachment 1 | Attachment 2 Attachment 3 | Attachment 4

        MDA Contact

        651-201-6061 • Fax: 651-201-6112
        Pesticide & Fertilizer Management Division