Groundwater: Low-flow Purging and Sampling

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Courtesy of Geotech

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.  Recent UK press reports 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.  Even the national average in Britain - 293,000 gallons per head of population - is only slightly more than in Sudan and less than half the figure for Spain, where each person has 610,000 gallons1

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. and modern information exchange 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.

Groundwater quality - 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) of water being purged from each monitoring well on a site and can exceed several hundred gallons per well where wells are deep or large in diameter.  This is volume of purge water 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.
  • 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 purged 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.

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