Water Well Purging & Sampling Protocols

A.  Introduction

Summary of Traditional & Industry-Accepted Purging Methods for Groundwater Wells

1.  Purging to Stabilization of Indicator Parameters - While purging a well, water quality parameters, such as pH, Temp., D.O., Cond., ORP and Turbidity, are monitored at regular intervals (typically 5-10 min.)  Once 3 sets of readings fall within pre-determined stabilization criteria, the water's chemistry is stable, representative of natural conditions and is ready for sampling.

2.  Fixed Well Volume Purging - A well's static column of water is purged 3 - 5 times prior to sampling.

3.  Fixed Well Volume Purging via. Packer Isolation - A packer (inflatable device designed to isolate water bearing zones in a well) is installed immediately above the well's primary water bearing zones, isolating the water column above.  This method reduces the volume of water that needs to be purged from a well, prior to sampling.

There are many misconceptions among the general public, environmental consultants and laboratories in regard to proper purging and sampling protocols.  Remember, if a water quality baseline is used to help resolve a dispute of potential groundwater contamination, it must be legally and scientifically defensible.  An improperly collected sample could easily be scrutinized by an expert on the basis that the sample was not collected properly and therefore, may or is not representative of natural groundwater quality.

Based on our first-hand and second-hand knowledge, many (if not most), water well samples collected throughout the Marcellus Shale region are done so in a questionable manner.  This does not mean that ALL of the sample results are inaccurate.  However, without properly implemented and documented purging and sampling protocols, the sample's quality cannot be assessed and inevitably some will NOT be representative of natural groundwater water quality. 

The information provided in the following sections describes industry-standard and regulatory-accepted protocols for properly purging and collecting samples from water wells.  These are documented in a variety of literature, including federal and state regulatory guidance documents, U.S. Geological Survey publications, ASTM industry standards and published books.  In some circumstances, we comment on some additional considerations to further improve the methodologies.

B.  Water Well Purging Methodologies

1.  Purging to Stabilization of Indicator Parameters
      (Appalachia's Preferred Method)

Method Summary
Stabilization criteria for field-based water quality indicator parameters (temperature, pH, conductivity, dissolved oxygen, ORP and turbidity) are defined (see Table I).  Well water is purged, while indicator parameter measurements are monitored and recorded on a regular basis (typically 5-10 minutes) depending on the purging rate.  When three consecutive sets of readings fall within the established stabilization criteria, the water quality has stabilized and well water is ready for sampling.  Table II provides an example of proper stabilization (values in blue indicate water quality stabilization).  The abnormally low temperature is due to the well's shallow groundwater source and recent winter/early spring recharge.

Important Note:  The PA Department of Environmental Protection (2001) states in their Groundwater Monitoring Guidance Manual on page 57, "DEP encourages the use of this method."

Table I - Stabilization Criteria
 Temperature +/- 0.2 deg. C
 pH +/- 0.2 pH units
 Conductivity +/- 3%
 Dissolved Oxygen +/- 10% or +/- 0.2 mg/L 
(whichever is greater)
 Eh/ORP +/- 20 mV
 Turbidity Minimal
Modified after Nielsen & Nielsen (2006)

Table II - Example of Stabilization of Water-Quality Indicator Parameters
 TimeTemp. pH Cond. D.O.  ORPTurb. 

 deg. C.SU mS/cm mg/L mV NTU 
 1705 6.28 6.550.198 5.22 228 0.0 
 1710 5.88 6.740.199  4.67 2140.0 
 1715 5.53 6.840.200  4.34 2050.0 
 1720 5.28 6.92 0.201 4.08199  0.0
 1725 5.11 6.98 0.202 3.90194  0.0
 1730 4.96 7.02 0.201 3.85 190 0.0
 1735 4.72 7.04 0.203 3.85 188 0.0
 1740 4.52 7.05 0.2033.79  1850.0 
 1745 4.35 7.070.204  3.71 182 0.0
 1750 4.13 7.11 0.206 3.60 180 0.0
 1755 4.05 7.12 0.205 3.54 178 0.0
 1800 3.937.13 0.206 3.51 177 0.0 

1.  Purge volumes are specific to the observance of water quality stabilization, which can significantly reduce the volume of water purged from a well, prior to sampling. (i.e. wells that are used less frequently may require greater purging to achieve water quality stabilization; however, those that are in regular use will stabilize much faster, requiring less purging).
2.  Does not require knowledge of the well's total depth, static depth to water, location(s) of principal water-bearing zones or diameter.  This is especially advantageous, when little is known of a well, prior to baseline sampling.
3.  Reduces the time required for baseline sampling and hence, associated cost.
4.  Does not require intrusive work within a well.  Therefore, wells with an existing pump and associated infrastructure can be used with little (if any) modification.

1.  Requires appropriate and calibrated equipment for assessing water quality indicator parameters.
2.  Water quality parameters, such as temperature and dissolved oxygen, are not always effective stabilization parameters, due to well infrastructure and/or weather conditions, while purging.  For example, specialized equipment/procedures are necessary to ensure that indicator parameters are collected from purged water that has had minimal contact with the atmosphere.  Atmospheric interaction with water does increase its dissolved oxygen content.  Also, monitoring water quality parameters under varying conditions (sun/variable cloud cover/seasons) can significantly affect water temperature.  Steps need to be taken to minimize these effects.
3.  A well's decline in water level, due to purging, is not typically monitored.  Therefore, excessive purging rates can induce turbidity, which can affect a sample's water quality.  However, there are means to modify this method to address this issue.

Method Improvements/Modifications
1.  Domestic wells should be initially purged between 2-3 gallons per minute.  Our experience indicates that this purging rate rarely has any adverse impact on the turbidity levels of water derived from domestic wells.  In addition, turbidity measurements should always be a part of a well's stabilization criteria.  If significant increases of turbidity are observed, purging rates should be immediately reduced.  Well records (if available) and well owner inquiry can also provide valuable insight into a well's sustainable yield.  Ultimately, a well's turbidity should be as low as possible to ensure collection of a proper sample.
2.  The use of a flow-through cell or similar set-up should be used to minimize any atmospheric contact with purge water while collecting field parameter measurements.  A 5-gallon bucket or a large-diameter plastic tube can be modified to allow purge water to fill from the bottom to top, while positioning the water quality meter probe(s) in the center.  This is a very good technique for obtaining more accurate dissolved oxygen measurements.
3.  The positioning of the water quality instrument and associated set-up should be sheltered as much as possible from the sun and elements.
4.  Monitoring of  a well's water level, while purging, can help ensure that a well is not over-stressed, similar to the "Low-Flow" sampling methodology.

2.  Fixed Well Volume Purging

Method Summary
One well volume is defined as the column of water present in the borehole under ambient (non-pumping) conditions.  In order to calculate this, the well's diameter, total depth and depth to static (non-pumping) water level must be known.  After calculation, 3-5 volumes are purged from the well to ensure all stagnant water is removed from the well borehole and associated infrastructure.

1.  Easy to calculate and document, provided that the well's total depth, diameter and depth to water are known.

1.  Commonly removes more water than necessary to collect a representative sample.
2.  Significant dewatering or over-purging of the well can lead to degassing of volatile constituents, such as natural gas.
3.  Can induce turbidity (mobilization of sediment particles) in the well water due to over-stressing, which can alter the natural water quality.
4.  Over-stressing can significantly lower the water level in the well, resulting in cascading water from the main water-bearing zones.  This can result in escape of volatile constituents from the water.
5.  For wells that do not have known total depths, static water levels and/or diameters and are not accessible, well volumes can not be calculated.
6.  Can result in significant time-requirements for purging, especially for deeper wells with lower well yield.
7.  Does not consider water quality fluctuations as part of the purging criteria.

3.  Fixed Well Volume Purging via. Packer Isolation

Method Summary
This method modifies fixed well volume purging by use of a packer (device that expands and isolates portions of a well borehole) within the well.  Specifically, a packer is lowered and expanded above the main water-bearing zones of a well.  This isolates the water column above the packer and reduces the water volume that needs to be purged, prior to sampling.

1.  Can significantly reduce the volume of water that needs to be purged from a well, prior to sampling.

1.  Requires knowledge of the depth of the principal water bearing zones, which are commonly unknown to the typical domestic water well owner.  Geophysical techniques would have to be employed by the groundwater professional to assess proper packer installation.
2.  Very intrusive method that would commonly require modifications to an existing well's infrastructure, especially for wells currently being used as a potable water supply.
3.  Requires specialized equipment and setup that are likely to be cost-prohibitive for many of those requesting a water quality baseline.
4.  Does not consider water quality fluctuations as part of the purging criteria.

C.  Sampling Protocols

1.  Sample from an access point that is closest to the well.  Typically, the closest access point is the pressure tank spigot.  In some cases, these locations are not readily accessible or may be in a position that creates unfavorable sampling conditions.  Under these circumstances, the next closest access point can be used, provided that an untreated sample can be collected.
2.  Collect the sample prior to any treatment.  If this is not possible, document it and disable/disconnect the equipment, making sure that it is properly flushed, prior to sample collection.
3.  Purge and sample from the same location.  It is always advisable to purge and sample from the same location.  This practice helps flush out the associated infrastructure.
Naturally, if a hose is used to direct purge water from the pressure tank spigot or some other sampling point, it should be removed prior to sampling.
4.  Minimize aeration while sampling from a spigot.  Aeration at the pressure tank spigot or some other sampling point can cause escape of volatile constituents that may be dissolved in groundwater.  In these circumstances, properly cleaned reducing valves can help reduce the aeration at the time of sampling.
5.  Sample immediately after purging is completed.  Letting groundwater sit after purging can allow volatile constituent de-gassing, as well as the commencement of reactions that can alter the water's chemistry.
6.  Make sure to use nitrile gloves, while handling sample containers and during sample collection.  Proper sanitation is always necessary when handling samples.
7.  Inspect and remove products/containers containing volatile compounds from the sampling area.  Volatile compounds can be released by a variety of common household products, such as building materials, adhesives, paints, solvents, varnishes, gasoline, moth balls, vehicle exhaust and disinfecting chemicals, among others.  It is always advisable to remove these items from the sampling area to ensure that cross-contamination does not result during sample collection.
8.  Use laboratory-supplied bottleware with proper preservative(s).  Different analyses require specific bottleware and preservatives for proper laboratory analysis.  Laboratories should provide both to those sampling.  Sample containers should be immediately labeled and packed in ice-filled coolers after sample collection.

D.  Frequently Asked Questions (FAQs)

Q.  Why does well water need to be purged properly prior to sampling?
A.  The water residing in the plumbing infrastructure and wellbore is generally not representative of natural water quality conditions.

Q.  What causes well water to NOT be representative of natural water quality conditions?
A.  There are a variety of possible factors including:
1.  Increased dissolved oxygen,
2.  Increased aerobic microbial activity,
3.  Lower pH, due to greater dissolved carbon dioxide,
4.  Loss of volatile constituents due to well head-space,
5.  Reactions between well construction materials/infrastructure and water,
6.  Condensate on the inside well casing surface,
7.  Introduction of surface contaminants via. well leakage,
8.  Introduction of volatile constituents via. vadose gases,
9.  Water temperature changes and
10.  Water pressure changes, among others.

Q.  I've been told that purging for a set duration (i.e. 15, 20, 30 minutes) is sufficient to purge a well, prior to sampling.  Is this appropriate?
A.  Short answer....no.  If this is the standard protocol used by the company/individual sampling your water, they are making an assumption that this will be a long enough duration to purge stagnant water from a well, prior to sampling.  There is NO scientific rationale behind this practice and it can not be properly defended.  In some cases, it may be sufficient.....in others, it will not.  The bottom line....there is no way of knowing whether sufficient water has been purged, unless the well volume has been calculated and removed (as specified above) or field water quality parameters have been observed to stabilize.

Q.  Is it appropriate to sample from a kitchen faucet?
A.  Generally...no.  It should be standard practice to sample from an access point that is closest to the well.  Allowing water to travel through household plumbing should be minimized or preferably bypassed altogether by using the pressure tank spigot or well hydrant, if available.  Use of kitchen faucets or other similar types of sampling locations should be minimized and if used, it should be documented as to why such a location was selected.