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Low flow sampling

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IN December 1995, the US Environmental Protection Agency Office of Research and Development, Office of Solid Waste and Emergency Response, developed and published a document entitled "Low-Flow (Minimal Drawdown) Ground-Water Sampling Procedures".

Since then, the use of low flow sampling in ground water has increasingly been used to support site assessment and remedial performance monitoring objectives, according to ECO Environmental Pty Ltd .

Many state agencies encourage the use of low flow sampling because it's designed to collect a sample that most truly represents the water in the screened section of the aquifer surrounding the monitoring 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 most common ground water purging and sampling methodology is to purge wells using bailers or high speed pumps to remove 3-5 casing volumes followed by sample collection.

Adverse impacts can occur through this method affecting sample quality by increasing levels of turbidity.

An overestimation of certain analytes - namely metals or hydrophobic organic compounds - may affect results with this method through the inclusion of otherwise immobile artifactual particles.

Filtration of these turbid particles has proved undesirable in rectifying the turbidity problem and may, in fact, bias the results of contaminant concentration on the low side by potentially removing mobile (contaminant-associated) particles.

Using low flow purging and sampling to reduce sampling induced turbidity can often mitigate these problems.

Benefits of low flow sampling include:

* Representative samples of the mobile load of contaminants (dissolved and colloid-associated).

* Less operator variability and greater control.

* Reduced stress on the formation (minimal drawdown).

* Less mixing of stagnant casing water with formation water.

* Reduced turbidity, therefore reduced need for field filtration.

* Reduced time and money spent per sample.

* Smaller purging volumes decrease environmental impacts and potential disposal costs.

* Better sampling consistency.

Low flow sampling: the pumping system

In order to minimise the hydraulic stress placed on an aquifer during purging and sampling, the technique of using low flow sampling is recommended.

This is typically done through the use of an adjustable rate pump to remove water from the screened zone at a rate less than the recharge capacity of the well.

This technique will cause minimal drawdown within well and reduce mixing between stagnant casing water and formation water.

The Geotech Low Flow Pumping System incorporates a bladder pump, low flow controller, compressor and 12v battery in one lightweight carry case. This kit allows pumping from 60m in a 50mm well casing.

Low flow sampling: drawdown/water level meter

Low flow sampling does not require a specific flow rate or purge volume. Low flow refers to the velocity with which water enters the pump intake and that is imparted to the formation pore water in the immediate vicinity of the well screen.

To ensure the pump flow rate is less than the recharge capacity of the well, drawn down is concurrently measured whilst pumping. Drawdown is measured with a water level meter.

Water level drawdown provides the best indication of the stress imparted by a given flow rate for any given hydrological situation.

Typical flow rates on the order of 0.1-0.5L/min are used, but this does depend on site-specific hydrogeology. A Heron Water Level Meter is perfect for this task.

Low flow sampling: water quality meter

Low flow sampling also relies on the ability to collect samples after water level and measured field parameters stabilise over three consecutive readings taken three to five minutes apart.

To take accurate parameter readings a water quality meter and sealed flow cell is used. It is important that flow cells are sealed against the atmosphere to ensure dissolved oxygen is not introduced into the sample.

Hydrolab Quanta units, complete with sealed flow cell, allow users to obtain exceptionally accurate results in the field.

It's important to note that parameter selection in monitoring program design is most often dictated by the regulatory status of the site.

However, background water quality constituents, purging indicator parameters, and contaminants all represent targets for data collection programs.

The tools and procedures used in these programs should be equally rigorous and applicable to all categories of data, since all may be needed to determine or support regulatory action.

Low flow sampling: parameter stabilisation

It is recommended that water quality parameters be used to determine purging needs prior to sample collection in each well.

Stabilisation of parameters such as pH, specific conductance (EC), dissolved oxygen (DO), oxidation-reduction potential (ORP), temperature, and turbidity should be used to determine when formation water is accessed during purging.

In general, the order of stabilisation is pH, temperature, and EC, followed by ORP, DO, and turbidity.

It should be noted that turbidity is a very conservative parameter in terms of stabilisation. Turbidity is always the last parameter to stabilise and excessive purge times are invariably related to the establishment of too stringent turbidity stabilisation criteria.

It should also be noted that natural turbidity levels in ground water may exceed 10 nephelometric turbidity units (NTUs).

In-line water quality indicator parameters should be continuously monitored during purging.

Water level drawdown should also be checked periodically as a guide to flow rate adjustment with the goal being minimal drawdown (<0.1m) during purging. Measurements of water quality parameters should be taken every three to five minutes.

Stabilisation is achieved after all parameters being measured have stabilised for three successive readings. In lieu of measuring all five parameters, a minimum subset would include pH, conductivity, and DO (or turbidity).

Three successive readings should be within + 0.1 for pH, + 3% for conductivity, + 10mv for ORP, and +10% for DO or turbidity. These are guidelines provided for rough estimates only.

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