Groundwater is a valuable resource that provides drinking water, irrigation water, and industrial water to millions of people around the world. However, groundwater can also be contaminated by a variety of sources, including leaking underground storage tanks, septic systems, and agricultural runoff. Groundwater sampling is an important procedure to determine the quality and attributes of a groundwater source.
Groundwater sampling may be conducted for a variety of reasons including:
Determining suitability as a drinking water source
Determining suitability for industrial and agricultural use
Identifying early stages of pollution from hazardous surface and sub-surface activities such as waste disposal and industrial activity
Assessing the impact of pollutants
Researching and understanding groundwater quality variations
Collecting data for compliance such as in pollution control
What is Low Flow Groundwater Sampling?
Groundwater sampling at low flow rates, defined as rates below 500 millilitres per minute, is a method employed to collect samples while minimising drawdown in the well. Maintaining this flow rate is crucial as excessive drawdown has the potential to disrupt the groundwater, leading to the introduction of contaminants into the sample and risking depletion of the well's water supply to the point of cessation.
There are several types of pumps used in low flow groundwater sampling that provide different benefits and are hence suitable for varying applications. This article will explore the benefits and challenges of low flow sampling, as well as the different types of equipment that can be used for this method.
Advantages of Low Flow Groundwater Sampling
Low flow water sampling offers several differences and advantages over other sampling methods for various applications. These advantages include:
Reduced drawdown: Minimises drawdown in the well, which is important for protecting the well from depletion and contamination from mixing with stagnant water.
Lower sample volume: Collects groundwater samples with a smaller volume of water, saving on laboratory analysis costs. Additionally, purge volumes are also less, reducing costs spent on disposing of potentially contaminated water.
Improved sample quality: Helps to improve the quality of groundwater samples by reducing the amount of disturbance to the water and ensuring that samples taken are representative of the actual conditions in the well.
Groundwater Sampling Regulations in Australia
The Environment Protection Authority (EPA) for each state in Australia is responsible for protecting the environment including regulating groundwater sampling to maintain clean, uncontaminated groundwater. The EPA provides several publications for this purpose including regulatory monitoring and testing guidelines as well as general groundwater sampling guidance as helpful references for environmental consultants, protection agencies, and site owners who need to collect samples. Among the different methods for environmental groundwater sampling, low flow sampling is a widely recognised and approved technique that can be used to demonstrate compliance.
In addition to resources from the EPA, there are also broader Standards which cover best practice for regulation compliance. AS/NZS 5667.11:1998 Water Quality – Sampling is the Australian Standard for groundwater sampling which provides guidance on several aspects including the design of sampling programmes, the handling of water samples, sampling techniques and more to help with regulation compliance. While Standards such as the aforementioned are valuable resources in conducting safe, reliable assessments, specific sampling applications may require specialist hydrogeological advice due to the complex nature of groundwater systems.
Common Low Flow Groundwater Sampling Equipment Types
Bladder Pump: A bladder pump is a commonly used simple and inexpensive low flow groundwater sampling device. The pump consists of a flexible bladder that is attached to a rigid frame. The bladder is filled with water, and the pressure of the water forces the bladder to expand. This expansion creates a vacuum that draws a water sample into the pump. Bladder pumps are small in diameter, easy to use and capable of pumping large volumes of water relative to their size.
Bladder pumps generally cause the least disturbance to the well among the various low flow sampling devices and are hence considered best practice. They are suitable for environmental monitoring, contaminant studies and any groundwater monitoring applications where uncontaminated samples are required for analysis and adherence with regulation, offering a combination of reliability, accuracy and the depth capability for low flow sampling.
Peristaltic Pump: Peristaltic pumps such as the Waterra 1 UP Peristaltic Pump and the Geotech Geopump Peristaltic Pump are low flow water sampling devices that use a flexible tube to create a vacuum that draws water into the pump. The tube is squeezed by a series of rollers, which creates a wave-like motion that moves the water along the tube. Limited to shallow wells, the maximum depth samples can be pulled from using a peristaltic pump is approximately 10 metres at sea level and less at higher elevations, with most pumps reaching less than this.
They are also less suitable for volatile organic compounds as they can disturb the water and cause degassing, causing samples to no longer be an accurate representation of the well. These pumps, however, are easy to use, clean and decontaminate, making them a favoured option in applications they are suitable for.
General Considerations for Low Flow Groundwater Sampling Systems
Choosing suitable equipment for groundwater sampling systems can be complex when considering the various challenges associated with specific pumps and well conditions. When selecting a pump, it is important to consider several factors.
Groundwater Sampling Depth and Standing Water Level The depth of deployment, standing water level and the position of the screen is important information when looking to sample groundwater. Different pumps will vary in range, with standard bladder pumps capable of reaching 60m, and peristaltic pumps reaching 8m at sea-level.
Well Recharge Rate Another consideration is the well recharge rate. When the pumping rate exceeds the recharge rate of a well, purges can occur instantaneously, potentially creating turbulence and turbidity as the well recharges.
Correct Disposal of Purge Water Purged water will also need to be disposed of which can be inconvenient and costly, which is why it is best to minimise the amount of water that must be purged. While low flow sampling requires less purging than some other methods, there are also no-purge methods available which may be preferable depending on the well conditions.
Cleaning and Decontamination Some pumps are harder to clean and decontaminate, which may be inefficient when sampling several wells a day. An alternative may be to install dedicated pumps in each well, which can make sampling easier and attract fewer billable hours.
At Air-Met Scientific, we take pride in simplifying the differences among the products available and providing you with the appropriate gear for your specific application. Feel free to contact our knowledgeable team to discuss your upcoming low-flow sampling project.
