Low-Flow Purging and Sampling
A. Method Summary and Application
The purpose of Low-Flow Purging and Sampling (LFPS) is to collect groundwater
samples from monitor wells that are representative of ambient groundwater
conditions in the aquifer. This is accomplished by setting the intake
velocity of the sampling pump to a flow rate that limits drawdown inside
the well. LFPS has three primary benefits. First, it minimizes disturbance
of sediment in the bottom of the well, thereby producing a sample with
low turbidity. Second, LFPS minimizes aeration of the groundwater during
sample collection. Third, the amount of groundwater purged from a well
is usually reduced as compared to conventional groundwater purging and
sampling methods.
Because the method allows collection of groundwater samples with low
turbidity, it was originally used for collecting samples for inorganics
analysis. The method typically allows the collection of samples for total
metals analysis and eliminates the need to filter the samples for dissolved
metals analysis. In addition, since the method minimizes aeration of the
samples, it can be used to collect samples for analysis of volatile and
semi-volatile organic compounds (VOCs and SVOCs), provided that appropriate
pumps are used in sample collection, as discussed below.
Advantages of LFPS are:
- Groundwater samples tend to be more representative of actual aquifer
conditions with respect to mobile contaminants and turbidity
- It causes minimal disturbance of the formation adjacent to the screened
interval
- It is generally less prone to sampling variability compared to other
groundwater sampling techniques (e.g., bailers)
- Smaller purge volumes and associated disposal expense
- Increased sample consistency from dedicated systems and reproducibility
of data due to reduced operator variability
Disadvantages of LFPS are:
- Misconceptions regarding reduced purging and sampling time
- Sampling from non-dedicated systems requires greater set-up time
- Sampling from dedicated systems requires higher initial capital expenses
- Increased technical complexity
- Increased training needs for sampling personnel
- Attractiveness of advantages may lead to improper and inconsistent
application
- Typically not a "first round" sampling option
- Not recommended for wells with long screen intervals unless multiple
samples are collected
- Introduction
The following procedures are specific to LFPS of monitor wells in New
Jersey. These procedures were developed in consideration of the USEPA-Region
I guidance document dated July 30, 1996 (https://www.epa.gov/region01/measure/well/lowflow8.pdf)
and the USEPA-Region II guidance document dated March 16, 1998 (No longer available online).
In addition, the U.S. Geological Survey's (USGS) Techniques of Water-Resources
Investigations, Book 9, National Field Manual for the Collection
of Water-Quality Data was consulted (http://water.usgs.gov/owq/FieldManual/).
The reader is encouraged to review these guidance documents prior to
performing LFPS. The procedures provided in the USEPA and USGS guidance
must be followed except where they differ from the information provided
below.
- Low Flow Policy
In the event that a responsible party is conducting a Remedial Investigation
without Departmental oversight, submittal of a sampling plan is not
required. However, it is highly recommended that the responsible
party seek approval for any deviations from this guidance prior to conducting
LFPS. In the event that a responsible party decides to use LFPS without
submitting a sampling plan and receiving approval, it must be recognized
that any deviations from this guidance may result in rejection of the
data. In addition, when submitting the results of the LFPS event, the
responsible party must include specific details of the LFPS techniques
used which demonstrate that they were consistent with the guidance specified
below. The responsible party shall also provide adequate rationale justifying
any deviations from this guidance whether or not they were previously
approved by the Department.
It is also Departmental policy that LFPS is not an acceptable method
for any wells with screened or open borehole intervals greater than
5 feet in length unless: 1) multiple locations at five-foot
intervals along the screen/borehole are sampled, or 2) the data quality
objectives (DQOs) warrant sampling a specific zone (e.g., the shallow
water table to investigate the potential for vapor intrusion inside
a building) or specific zones where sufficient geophysical (e.g., heat-pulse
flowmeter, caliper and temperature logs, etc.) and hydrogeological information
(e.g., tracer tests) or other evidence (e.g., stained soils or fractures
noted on boring logs) that clearly identifies the depth(s)
at which contaminants are entering the well screen or open borehole.
Once the collection of multiple samples (vertical profiling) in a well
has been completed, long-term sampling of the well may require LFPS
at fewer depth intervals, or even just one depth interval, depending
on the data quality objectives of the sampling and the types of contamination
present in the groundwater (e.g., LNAPL, DNAPL, etc).
- Laboratory Certification (N.J.A.C. 7:18)
N.J.A.C. 7:18 requires that any environmental laboratory*
submitting analytical data to the Department, regardless of quality
level, must be certified by the Office of Quality Assurance. This applies
to those firms using LFPS instruments associated with the "analyze
immediately" category of water quality indicator parameters (WQIPs)
including pH, temperature, and dissolved oxygen. Regardless of whether
or not the equipment in question is rented or privately owned the requirement
for certification can not be ignored. All certification documentation
must accompany the instrument into the field and accompany all WQIP
data submitted to the Department. (*Environmental laboratory
is defined as any laboratory, facility, consulting firm, government
or private agency, business entity or other person that the Department
has authorized, pursuant to N.J.A.C. 7:18, to perform analysis in accordance
with the procedures of a given analytical method using a particular
technique as set forth in a certain methods reference document and to
report the results from the analysis of environmental samples in compliance
with a Departmental regulatory program).
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