Low Flow Ground Water Sampling
Maine DEP encourages the use of low flow ground water sampling (LFS) procedures to retrieve samples that are more representative of groundwater chemistry than samples collected using traditional bailing and purging techniques. The procedures to be used in low flow sampling are clearly outlined in the EPA Ground Water Issue Paper, Low-Flow (Minimal Drawdown) Ground-Water Sampling Procedures (Puls and Barcelona, 1996). DEP refers all interested parties to this paper for the specifics of the procedure. We do not wish to spell out the procedure in a "cook book" method because we believe that the characteristics of each site will determine the details of the sampling procedure. The specifics will be worked out through collaboration with the DEP geologist assigned to the site and will be written into the site sampling plan. In some, but not all, cases DEP will require low flow sampling procedures to be used.
What is Low Flow Sampling (LFS)?
Low flow sampling is a technique to minimize the hydraulic stress on the aquifer during purging and sampling. This is done by using an adjustable rate pump to remove water from the screened zone at a rate that will cause minimal drawdown of the water level in the well. (The use of bailers for purging or sampling is not acceptable in LFS.) Drawdown is measured in the well concurrent with pumping using a water level meter.
LFS does not require a specific flow rate or purge volume. A sample can be collected after the water level and measured field parameters (such as pH or dissolved oxygen or others) stabilize over three consecutive readings taken three to five minutes apart.
Pump Selection: Any adjustable flow rate pump may be used including, in some cases, peristaltic pumps. Although peristaltic pumps may decrease VOC concentrations, they are an improvement over bailers and may be less costly than other pumps.
Well Performance Evaluation: The DEP recommends that slug tests be performed before the initial sampling date and used to estimate optimal sample flow rate. We also recommend field testing the sampling procedure (including field parameters) prior to the first sampling round, and evaluating pump performance on site before purchasing specific dedicated pumps.
Cold Weather Sampling: Flow cells are difficult to use during cold winter weather. If sufficient LFS monitoring data is available, the use of flow cells can be eliminated during the cold weather months. In some cases eliminating winter sampling events may be acceptable to DEP.
Low Permeability Formations: A properly designed, constructed and developed well is extremely important in low permeability formations. No amount of creative sampling technology can overcome a bad well. The use of a hollow stem auger to drill a monitor well in a low permeability formation is not recommended and may not be approved by DEP. If the permeability of the aquifer is so low as to make low flow sampling impracticable, an alternative technique will be approved.
Field Parameters: DEP recommends the measurement of water level, flow rate, pH, Eh, temperature, specific conductance, dissolved oxygen, and turbidity as field parameters to determine when a representative ground water sample can be collected. (Eh and dissolved oxygen must be obtained with the use of a flow cell.) After more is known about the aquifer, the number and types of field parameters may reduced or increased as long as sampling objectives are being met. Monitoring programs should continually be evaluated to determine if the goals are being met. Keep in mind that some field parameters in addition to equilibration testing are useful for chemical characterization.
Vertical Profiling: DEP strongly encouraged the use of low flow sampling to characterize the distribution of contamination within the aquifer. LFS removes the water sample from the aquifer in close proximity to the depth of the pump or the intake end of the sampling tube. By sampling from different depths within the screened interval, the vertical distribution of the contamination in the aquifer can be delineated. This is particularly important for understanding contaminant flow in stratified aquifers.
Misconceptions about Low Flow Sampling
Purge Volume Requirements: The LFS procedure does not use pre-calculated purge volumes to determine when samples can be collected. Samples are collected when water level and field parameters stabilize during pumping.
LFS is only for inorganics or high turbidity wells. LFS is a specific water delivery technique that is designed to produce a sample that most closely resembles the water quality in the screened zone. Other techniques place undue stresses on the aquifer that may alter the chemistry of the sample.
Why do low flow samples sometimes produce results that are no different than those achieved with earlier methods?
Due to chemical and hydrogeologic factors. some parameters are not as sensitive as others to sampling technique. For example, measurement of barium in a turbid sample from a bailed well will be vastly different from the barium concentration in a low turbidity sample collected by LFS techniques. In contrast, a gasoline concentration (GRO) may be similar no matter what turbidity the sampling method induces.
Why does Maine DEP encourage the use of LFS?
LFS is designed to collect a sample that most truly represents the water in the screened section of the aquifer surrounding the monitor well. It does not come from water that is mixed within the well by a bailer or inertial sampler, nor does it come from an average of water that flowed the full length of a long screened interval.
The low flow sample can most often be trusted to best represent the contamination or (lack thereof) in the aquifer because it was produced by a process that minimized stress on the aquifer or well. Low flow sampling also reduces the variability in sampling technique that is inherent in traditional bailing and purging procedures. In summary LFS reduces the physical and chemical stresses, reduces the variability in sample procedures, and reduces the chance that changes in chemical concentrations are induced by the sampling technique.
Is LFS only for sites with Long Term Monitoring Programs?
No. Many investigations would benefit from the field data collected using LFS procedures. The investigator should review the Data Quality Objectives(DQOs) and consult with the DEP site geologist before selecting a sampling technique.
Puls, R.W., and Barcelona M.J., 1996. EPA Ground Water Issue Low-Flow(Minimal Drawdown) Ground-Water Sampling Procedures, EPA/540/S-95/504.