QED ABSTRACTS IN THIS ISSUE:

-- Low-Flow Purging Reduces Management of Contaminated Groundwater, Keith E. Schilling

-- MicroPurge® Frequently Asked Questions, Dave Kaminski

-- Low Flow (Minimal Drawdown) Ground-Water Sampling Procedures, Robert W. Puls and Michael J. Barcelona

Low-Flow Purging Reduces Management of Contaminated Groundwater

This is an abstract of an article that originally appeared in Environmental Protection, December 1995. Keith E. Schilling is a senior hydrogeologist at Montgomery Watson, in Des Moines, Iowa.

Purging a monitoring well prior to sampling removes stagnant water in the well casing and ensures collection of groundwater samples representative of formation water. Temperature, pH and specific conductance are routinely monitored during purging to determine when a sufficient volume of water has been removed from the well. When these parameters exhibit stabilized readings within a specified range of values, the water sample is considered representative of formation water.

Now there is an alternative to purging multiple well volumes. This new approach focuses on pumping a well from the well screen at a flow rate below the recharge capacity of the formation. The specific rate of pumping typically does not exceed one liter per minute. By purging at low flow rates, only ground water that enters through the well screen is purged from the well. Because stagnant water located above the pump intake in the well casing is not drawn into the pump, the entire casing volume would not have to be purged prior to sampling.

Water stabilization parameters are monitored in-line from the wellhead by sampling the discharge to recognize quickly when formation water is present. In this manner, the amount of water purged from a well is dependent solely on formation water stabilization rather than on predetermined well volumes.

Purging Equipment and Procedures

Dedicated bladder pumps were installed in monitoring wells at the site to perform quarterly groundwater sampling of 15 bedrock wells ranging in depth from 60 to 350 feet. At each well, a bladder pump was installed at a depth approximately 20 feet below the static water surface. In addition, a drop tube from the pump to the well screen was used to purge and sample ground water from the middle of the well screen. The cost to install the dedicated pumps and tubing in the wells averaged about $1,000 per well. A compressor and controller unit to operate the pumps in all of the wells at the site cost an additional $4,000.

Purge water was directed through a flow cell containing measurement probes for temperature, pH, specific conductance, dissolved oxygen, redox and turbidity. The parameter data were visually displayed and recorded on a portable computer at a rate of one set of readings every 20 seconds.

Discharge from the flow cell unit was typically directed to a graduated bucket where the flow rate and total volume of purge water removed from the well could be measured. The cost of the multiparameter flow cell was about $5,000. The unit can be operated with any MS-DOS portable computer equipped with communications software.

Purge Volumes

Low-flow purging with the dedicated bladder pumps greatly reduced the amount of water generated during purging of monitoring wells.

Each well was purged at an average rate of 0.31 gpm for an average duration of 13 minutes. Based on records from previous sampling activities at the site, an average of 50 minutes per well was needed to purge three casing volumes from each well. The total time required to purge three volumes in this manner (12.5 hours) is nearly four times greater than the time needed for low-flow purging (3.25 hours).

Stabilization Parameters

Stabilization data were visually displayed and recorded by a portable computer in the field.

The measurement of dissolved oxygen (DO) was the most sensitive parameter used during the purging activity at the site, with DO stabilization occurring within four to eight minutes of low-flow pumping.

Testing turbidity and redox also has proven useful in judging the stabilization of purged water. The traditional parameters of pH, temperature, and specific conductance stabilized very quickly during purging and have not been useful as indicator parameters.

The paper concludes with Schilling's economic analysis of low-flow purging based on the excerpted table.

Complete document at www.micropurge.com/schilling.html

or phone QED at 1-800-624-2026 and ask for document #2115.


COMPARISON OF LOW-FLOW AND THREE-WELL VOLUME APPROACH
Economic AnalysisLow-Flow PurgingThree Well Volumes
Time for Purging Wells (a)$500$1,875
Disposal Costs (b)$1,300$3,750
Cost per Sampling Event$1,800$5,625
Sampling costs for Year (quarterly)$7,200$22,500
Sampling Costs for 30 Years$216,000$675,000
(a) Assumes two person crew at $150/hour.
(b) Assumes cost to dispose one drum equals $1,000; $300 for additional drums (one drum = 55 gallons).

Low-Flow (Minimal Drawdown) Ground-Water Sampling Procedures

This abstract of Robert W. Puls and Michael J. Barcelona's paper is intended to provide background information on the development of low-flow sampling procedures and their application under a variety of hydrogeologic settings.

Currently the most common ground-water purging and sampling methodology is to purge a well using bailers or high speed pumps to remove 3 to 5 casing volumes followed by sample collection. This method can cause adverse impacts on sample quality through collection of samples with high levels of turbidity. Sampling-induced turbidity problems can often be mitigated by using low-flow purging and sampling techniques.

In general, the overall goal of any ground-water sampling program is to collect water samples with no alteration in water chemistry. The sampling methodology described in this paper assumes that the monitoring goal is to sample monitoring wells for the presence of contaminants and it is applicable whether mobile colloids are a concern or not and whether the analytes of concern are metals (and metalloids) or organic compounds.

Monitoring Objectives and Design Considerations

The paper suggests monitoring objectives include four main types: detection, assessment, corrective-action
evaluation, and resource evaluation. An important goal of any monitoring program is collection of data truly representative of conditions at the site.

Detailed site characterization is central to all
decision-making purposes and the basis for this characterization resides in identification of the geologic framework and major hydro-stratigraphic units.

Fundamental data for sample point location include: subsurface lithology, head-differences, and background geochemical conditions.

Definition of Low-Flow Purging and Sampling

The paper states: it is generally accepted that water in the well casing is non-representative of the formation water and needs to be purged prior to collection of ground-water samples. However, the water in the screened interval may indeed be representative of the formation, depending upon well construction and site hydrogeology. Low-flow purging, whether using portable or dedicated systems, should be done using the pump-intake located in the middle or slightly above the middle of the screened 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.

Puls and Barcelona define "low-flow" in reference to the velocity with which water enters the pump intake and which is imparted to the formation pore water in the immediate vicinity of the well screen. The objective of low-flow sampling is to pump in a manner that minimizes stress (i.e. drawdown) to the system.

The importance of stabilization of purge water parameters such as pH, specific conductance, dissolved oxygen, oxidation-reduction potential, temperature, and turbidity is discussed. Performance criteria for determination of stabilization should be based on water-level drawdown, pumping rate, and equipment specifications for measuring indicator parameters.

Advantages of low-flow purging are that samples are representative of contaminants present; there is minimal disturbance of the sampling point thereby minimizing sampling artifacts; a reduced need for filtration means less time required for sampling; and a smaller purging volume decreases waste disposal costs and sampling time. Disadvantages of low-flow purging may include a higher initial capital cost, greater set-up time in the field, and increased training needs.

Low-Flow (Minimal Drawdown) Sampling Protocols

Limitations to the collection of "representative" ground water samples include: mixing of the stagnant casing and "fresh" screen waters during insertion of the sampling device or ground water level measurement device; disturbance and resuspension of settled solids at the bottom of the well when using high pumping rates or raising and lowering a pump or bailer; introduction of atmospheric gases or degassing from the water during sample handling and transfer; or inappropriate use of a vacuum sampling device.

Samples should not be taken immediately following well development; low flow rates (<0.5 L/min) should be used during both purging and sampling, causing minimal drawdown in the well; tubing wall thickness should be maximized and tubing length minimized; and water quality indicators should be monitored during purging. Equipment calibration is discussed and resources for detailed recommendations and restrictions are outlined.

A level measuring device should be used which will least disturb the water surface in the casing. Measure well depth after sampling is completed. The water level measurement should be taken from a permanent reference point which is surveyed relative to ground elevation.

The use of low flow (e.g. 0.1-0.5 L/min) pumps is suggested for purging and sampling all types of analytes. There are no unusual requirements for ground-water sampling devices when using low-flow, minimal drawdown techniques. Devices which lend themselves to both dedication and consistent operation at definable low-flow rates are preferred. Clearly, bailers and other "grab" type samplers are ill-suited for low-flow sampling since they will cause repeated disturbance and mixing of "stagnant" water in the casing and the "dynamic" water in the screened interval. Similarly, the use of inertial lift foot-valve type samplers may cause too much disturbance at the point of sampling.

Dedicated sampling devices (left in the well) capable of pumping and sampling are preferred over any other type of device. Any portable sampling device should be slowly and carefully lowered to the middle of the screened interval or slightly above the middle.

Other Topics

The paper also covers:

* an explanation of filtration needs and filter setup.

* monitoring of water levels and water quality indicator parameters.

* the use of sampling containers and bottles and the preservation standards that need to be followed.

* an explanation of blanks, what they are and why they are used.

* the use or non-use of low-flow sampling in low-permeability formations and fractured rock: whether low-flow is suitable, and how to go about sampling in such formations.

The usual practices for documenting the sampling event should be used for low-flow purging and sampling techniques. This should include, at a minimum, information on the conduct of purging operations, field instrument calibration data, water sampling forms, and chain of custody forms. The paper includes example forms and other documentation suggestions and information.

Complete document at www.micropurge.com/EPApuls.html

or phone QED at 1-800-624-2026 and ask for document #2128.