Groundwater Monitoring

Low-flow Purging and Sampling

Significant timesavings, improved sample process integrity, quality assurance and minimal quantities of potentially contaminated purge water to manage – are attractive as the focus on groundwater quality sharpens and economics demand best cost- benefit based methods.

Water has become a vitally important resource. Maintaining its quality and quantity and avoiding pollution are set to be watched and regulated ever more closely. As a commodity for investors, water may prove more attractive than oil, natural gas or renewable energy – further fuelling its protection and monitoring. Although water supplies are renewed through the water cycle, avoiding contamination and pollution requires continued vigilance. A recent UK press report stated, “The South-East of England has less water per person than the desert states of Syria and Sudan, according to official figures. The statistics, from the UK’s Environment Agency, show that there are 58,000 gallons of water available for every person in south-east England per year, while in Syria the figure is 95,000 gallons and in Sudan – a country wracked by civil war – 269,000.” (Telegraph.co.uk)

As each country looks more closely at its water resources and maintaining water quality, considerations on water sampling and techniques become more important. For any source, there is an obvious need for sampling process integrity to be able to sample accurately, reliably and repeatably, in compliance with rules and guidelines thus achieving quality assurance – and as economically as possible. One of the most important and sensitive water resources, yet least accessible, is groundwater. It supplies more than half of all drinking water in many countries, even those with vast lakes and plentiful rivers. Being far less accessible to monitor in situ than an estuary, river, reservoir or lake, it is much more difficult to maintain quality assured sample integrity as it is retrieved from boreholes.

New technology, techniques, experience pioneered in the U.S. means low-flow groundwater sampling is available and can meet the objectives for sampling integrity and annualised least-cost, best value economics.

Sampling groundwater is often needed to determine its quality (chemical purity and/or contamination) for human consumption and supporting ecosystems, or identifying if industrial activity or a natural event or disaster may have caused or could cause potential contamination.

Analysis of groundwater chemistry

Groundwater is analysed for a variety of metals, organic chemicals and inorganic elements as well as secondary quality parameters such as clarity (measured as “turbidity”), odour and taste. Sample analyses will vary with the land use above. For example, groundwater near chemical plants and petroleum refineries is most often analysed for the presence of, amongst others; petroleum hydrocarbons and chlorinated solvents including volatile organic compounds (VOCs).

By comparison, groundwater near a coal-fired power plant might be analysed for metal ions such as lead, copper, mercury, boron, arsenic, cadmium and zinc. In addition to anticipated contaminants, general groundwater quality parameters such as pH, conductivity, oxidation-reduction potential (REDOX), dissolved oxygen (DO), biological and chemical oxygen demand (BOD and COD) are often measured at the wellhead using field instrumentation.

Sampling groundwater from monitoring wells has traditionally involved purging the well to remove stagnant and contaminated drilling water within the borehole that may not be representative of in-situ groundwater quality. Scientific and regulatory guidance often recommends purging a fixed volume of water from the well, usually three to five times the volume of water contained in the well casing and screen.

This can result in 20-200 litres (roughly 5-50 gallons) from each monitoring well on a site and can exceed several hundred gallons of purge water per well where wells are deep or large in diameter. This is volume is multiplied by every sampling visit. Many practitioners resort to high pumping rates to remove these large volumes efficiently. In shallow wells, devices called “bailers” (essentially a narrow bucket on a string) are repeatedly dropped into the well and retrieved to remove the purge volume required.

The greater the purged volume the longer time to achieve a sample and the more questionable the representative quality of the sample.

While fixed-volume purging with bailers or high-flow pumps can remove overlying stagnant water from a well and provide a sample of the groundwater near the borehole, researchers have determined that these traditional purging practices pose significant scientific and practical concerns. These include:

• High pumping rates and bailers can greatly increase the turbidity of samples. This can cause biased or ‘false-positive’ analytical results and interfere with sample analysis. Filtering samples to remove turbidity can further alter sample chemistry (Puls et al. 1992; Heidlauf and Bartlett 1993)
• Bailers, while inexpensive to purchase, can introduce further bias or error in sample results due to aeration, sample agitation, surging, and contamination due to handling of the bailer at the wellhead 17
• In low-yield wells, complete dewatering of the well can aerate the sample water; stripping out volatile organic compounds (VOCs) and precipitating dissolved metals from samples thus affecting sample chemistry (Giddings 1983)
• High pumping rates can cause mixing of chemically distinct water zones within the aquifer Such as floating, light non-aqueous phase liquids (LNAPLs), diluting or averaging the sample, and often spreading further contaminants within the aquifer
• Field technicians must properly handle the large volumes of purge water generated. Where the purge water is contaminated or when regulatory requirements specify, the water must be contained in tanks or drums and often removed for off-site treatment or disposal, increasing sampling costs
• Excessive high-rate pumping of monitoring wells can lead to damage of the well filter pack and annular seal

Low-flow purging and sampling

Low-flow purging and sampling is a methodology that does not require removing large volumes of purge water from the well. It can avoid the pitfalls of traditional well-volume purging. Groundwater is pumped at low flow rates from within the well screen (intake), purging only the sampling zone and minimising disturbance to the water in the well and surrounding formation, reducing turbidity in samples (Robin and Gilham 1987; Kearl et al. 1992; Powell and Puls, 1993; Puls and Barcelona, 1996).

Low-flow sampling with bladder pumps that isolate the sample from air and maintain sample chemistry can improve sample and process quality, cut time and minimise purge water handling.

Completion of purging using low-flow methods can be verified and documented using two approaches, stabilisation of selected indicator parameters or comparison of data from conventional sampling and low-rate sampling. In the first method selected water quality parameters such as pH, conductivity, and dissolved oxygen are monitored during low-rate purging, with stabilisation of these parameters indicating when the discharge water represent aquifer water (Barcelona et al. 1994). In the second method, data resulting from low-flow samples is compared to data from conventional fixed-volume purging, with comparable data verifying the equivalency of the two methods (Power and Puls, 1993; Shanklin et al. 1993; Kearl et al. 1994).

Advantages of low-flow purging and sampling

Purging and sampling at low-flow rates offers several advantages over traditional purging methods:

• Sample quality is improved through reduced turbidity, minimised degassing and volatilisation. The need for sample filtration is eliminated, further reducing sampling costs and analytical expenses
• Sample accuracy and precision are also improved, avoiding regulatory issues with suspected contamination and costly re-sampling to explain erroneous results
• Purge volume is typically reduced by ninety percent or more, resulting in significant savings in handling, treatment or disposal costs
• Sampling systems are simpler and less expensive, as the need for high-flow purging pumps in eliminated
• Extends the useful life of a monitoring well and preserves the integrity of the filter pack by reducing the movement of fine sediments into the well

Easy steps to proper low-flow sampling

While low-flow purging and sampling methods may appear more complicated than simply purging well volumes, it can be easily accomplished in three steps:

• Position the pump within the well screen intake based on well construction data or prior well depth measurement, the adjust the flow rate of the pump to match the rate at which the well produces water (the yield rate).
• Measure the water level in the well to achieve a stabilised level and avoid dewatering the well.
• Monitor the selected water quality indicator parameters to determine stabilisation and completed purging, then collect samples.

Indicator parameters for purging

Before taking a sample, indicators are used to determine if the yield has stabilised. DO and C are the most reliable indicators pH stabilises readily, often shows little change. Temperature is easily affected by air temperature and sunlight. Turbidity is not used as an indicator, but should be measured during purging to support sample data and prevent excessive pumping rates. The equipment used for low-flow purging and sampling applications has evolved into an automated, easy-to-use system that is lightweight and highly portable. Where traditional well-volume purging can require the user to carry heavy high-rate pumps, hoses, connecting wiring harnesses and cumbersome generators, low-flow sampling can be accomplished using simple air-powered bladder pumps with small-diameter plastic tubing and microprocessor controls that can be powered by a small petrol-engine or electric compressor or 2-3 kilo (5 lbs) compressed-air or gas cylinder. Water level control can be accomplished automatically through dipmeter tie-in with the controller, and purging indicator parameters can be measured and automatically recorded, with advanced systems indicating when stabilisation has been achieved.

Low-flow sampling has gained wide regulatory acceptance in the U.S. and other countries. In the USA, nearly all 50 states currently have sites with approved low-flow sampling plans, and there are a number of states and federal guidance documents that offer information on proper low-flow purging and sampling procedures. In addition, the US standards organisation, ASTM has the standard D6771 on low-flow sample purging and sampling methods. The practice is now in use at thousands of U.S. sites including solid waste landfills, power plants, manufacturing facilities and military bases.

Purge water disposal in UK

According to the Environment Agency currently in the UK, where the purge water does not pose a risk to the environment, i.e., will not cause pollution, it can be discharged on, or nearby to, the site. The main issue surrounds the safe disposal of contaminated water brought to the surface. Where the effluent may cause pollution, i.e., volumes and concentrations are such that they would pollute if discharged to a drain, ditch or spread on the land surface, the effluent must be managed and disposed of in a safe and approved way.

Each case needs to be considered on a site-specific basis and where necessary advice sought from the local Environment Agency office. Where the effluent is deemed to cause pollution, containing it and then having it treated can carry the penalty of time and cost so the smaller the quantity, the lower the cost. Typically, in UK, costs may run to many thousands of pounds per site per year. The greater the volume of contaminated purge water, the greater the cost.

Field Notes: Case study 1

Low-Flow Sampling Improves Sampling Quality and Productivity for Hydrogeology Firm

As soon as Columbus, Ohio-based Eagon & Associates began implementing low-flow sampling at selected waste management landfill sites in Ohio, their low-flow experience was been positive, based on superior sample quality and productivity improvements. According to environmental scientist Andy Graham, low-flow sampling provides more representative samples and a substantial reduction in water volume compared to conventional purging methods. “Low-flow sampling provides improved sample quality through reduced turbidity. As a result, data accuracy and consistency are improved and artificially high analytical results are avoided. Fewer false statistical ‘hits’ help reduce the costs to our clients to comply with regulatory requirements. Better data translates into better decisions,” said Graham

Low-flow sampling in combination with dedicated bladder pumps has also reduced purge water handling and disposal, and decreased purging time in most wells. “The combination of low-flow sampling and dedicated pumps in the well have produced significant savings in time with up to a 60 percent reduction in overall time required for sampling. Low-flow sampling and dedicated pumps reduce headaches in the field and allow us to better focus on the sampling itself,” Graham stated. “In addition, dedicated pumps have reduced field labour, require virtually no cleaning, and eliminate the need for equipment blanks. And that increases productivity, so we sample more sites in less time. In today’s world of reduced budgets, low-flow sampling helps us to do more with less,” said Graham.

Dedicated Systems: Instead of portable pumps, dedicated in-well sampling systems for projects of greater than two years can offer even better economics for:

• Lower operating costs (operation/maintenance)
• no assembly/disassembly at each well
• no cleaning labour between wells
• no cleaning supplies or reagents
• no cleaning blank samples
• labour typically 30% – 50% lower overall
• Eliminates cross-contamination from sampling equipment
• Reduces potential for operator exposure to contaminants

This is despite higher initial equipment cost (purchase/lease).

Field Notes: Case study 2

On a European site, following mineral extraction, backfilling, some landfill and now post-remediation, the consultancy taking one-year’s borehole groundwater samples said, “Before using low-flow equipment, one borehole could take up to five hours using the old methods which were time consuming and far more labour intensive. With the low-flow method, after getting the portable equipment together, sampling can take on average no more than about one hour per borehole. During the stabilisation period we can utilise this freed up time to prepare the paperwork for that borehole.” When comparing and contrasting work rates between the old method and the low-flow equipment, they said, “We do about five boreholes a day now at depths ranging between 5 to 70m. This is a significant improvement on previous sampling rates.” Asked about ease of use, they said, “The system ‘appears’ initially very complicated but once you use it the benefits and efficiencies quickly become apparent.” The functionality of being able to choose different re-fill and discharge rates is very helpful because of the varying depths of the boreholes. So to maximise the output while maximising the sample draw is a major advantage along with only 3-4 litres typically of purge water. We are very pleased with the sampling results so far.”

Comparison (Case study 2, above with Case Study 3, below)

The 1-year project brief for Case Study 2 makes portable low-flow pumping the best solution. In Case Study 3, which is on site for more than two years, the operational time-cost savings with dedicated pumps develops rapidly from end year two making dedicated pumps the most economic decision.

In increasingly cost-conscious times this calculation may become very much more important.

Field Notes: Case study 3

A European landfill operator with 20-30 boreholes sampled monthly is progressively installing dedicated pumps and has already seen a time saving of around seventy percent on the previous sampling method. Typically each borehole takes no more than thirty minutes including those which took up to six hours before. A technician said, “We arrive and connect, and within 5-15 minutes we have a steady sample showing on the flow cell and then it is just the time to take the sample. Each is done in about half-an-hour.”

Published: 10th Jun 2009 in AWE International