Water / Wastewater 37
Important Considerations for Low Flow Sampling
Low flow sampling is by no means a simple process. If proper care and consideration are not taken, there is the risk of inaccurate groundwater samples, damage to the test site and in some cases, a risk to the individuals working on the site themselves.
To minimise these risks and ensure your low flow sampling process is efficient throughout, there are a number of important factors to take on board.
AP-2000
together. The length of tubing required will depend on how far down the water underground islocated, but the type of tubing will also be an important consideration.
Tubing for low flow sampling can be found in various materials, all with their advantages and disadvantages. Polyethene tubing is often chosen due to its strength and durability, while Teflon tubing is sometimes chosen as it provides excellent resistance to most solvents and acids.
To ensure you only get accurate results that are representable of the water as a whole, you will need to follow a specific procedure and use the best water sampling equipment. Equipping yourself with training or the knowledge of how to execute sampling correctly, and the tools to do it, will go a long way to getting accurate results.
How to Conduct Low Flow Sampling
Each test site is different, which is why it is recommended to do a full survey of the site before conducting any low flow sampling. Depending on the site, specific rules will need to be followed to ensure a safe and efficient test.
The first and perhaps most simple is to know the background tothe area itself. If it is on a piece of protected land or is a historical site, you want to make absolutely sure you have permission from the relevant body that you are allowed to conduct tests on the site.
Once equipped with your AP-2000 package, the first thing you will need to do before commencing low flow sampling will be to to purge the water located in the aquifier underground.
This will need to be done at a rate that mimics that of ambient water flow. For reference, a typical flow rate from a pumpduring low-flow purging is approximately 0.1 to 0.5L/min.
There is continued debate as to whether manual or automatic pumping is more effective, while the truth is both have their advantages and disadvantages.
Automatic pumping can make the process easier but requires close monitoring to ensure the groundwater being pumped is not expelled too quickly. On the other hand, manual pumping gives you greater control but requires constant monitoring.
Whether you opt for automatic or manual pumping, ensuring it is done at the correct speed is vital. If done too fast you run the risk of contamination, while doing it too slowly increases the likelihood of the well water and water from the aquifer mixing together.
Once pumped, it’s important to then conduct stabilisation tests on the water before a complete sample is taken. The most common parameters to check when performing low flow sampling are pH, D.O, conductivity and temperature.
To ensure stabilisation is reached, we recommend taking readings every 3-5 minutes.Once three readings have been taken that return the same results within limits, samples can be safely collected. These limits are outlined above, as recommended by the Environment Protection Agency.
The reliability of the sample is greatly dependenton the steps involved to be carried out properly without contaminating the sample with soil and oxygen. Having state-of-the-art equipment, such as the AP-2000 is important but using it correctly is just as important.
Once the sampling has been completed, it is important to clean your equipment before taking another sample or this can cause data errors in the following samples.
When conducting low flow sampling, there is the possibility that the groundwater you are testing contains trace contaminants. Regardless of whether there is or not, it is recommended to always wear a clean pair of non-powdered disposable gloves each time a different location is sampled.If dealing with samples believed to contain high concentrations of contaminants, it is important that they are stored separately from those samples containing low levels or no traces of contaminants.
Furthermore, if working in multiple areas, it is recommended to begin tests in the area least suspected to be contaminated in order to minimise the risk of cross-contamination.
It is also important tohave a solid understanding of what you are testing for. For example, When testing for volatile organic compounds (VOCs) there is a risk of losing them in the collection process if proper pumping procedures are not adhered to. To minimise the likelihood ofloss, it is recommended to avoid using vacuum pumps and to not tamper with the tubing, as this may result in a pressure change.
Hot temperatures and direct sunlight can also negatively impact
Aquaread building
results for low flow sampling. Heating of the groundwater can affect the accuracy of the results, as well as the possible risk of damaging equipment. To avoid this, ensure your tests are conducted during cooler times of the day, or make use of an umbrella or tent to limit the impact of the sun.
Finally, there are a few general handling and preservation tips to bear in mind when working with your collected samples.
All samples should be put into appropriate and clearly labelled containers, although any collected for volatile organic compounds, acidity or alkalinity need to ensure they do not have any headspace in the containers they are kept in. Any other containers should be filled with some space still remaining to allow for any ullage that may occur.
Author Contact Details Craig Harrison, Aquaread Limited • Bridge House, Northdown Industrial Park, Broadstairs, Kent, CT10 3JP • Tel: +44 (0) 1843 600 030 • Web:
www.aquaread.com
www.envirotech-online.com IET September / October 2019
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