Groundwater Sampling

Determining the availability and quality of groundwater has become an issue for an increasing number of businesses and government agencies. Changes in legislation, pressure on aquifer supplies and a heightened awareness of the impact of industrial activity have made groundwater sampling a hot topic. However, collecting a sample from a monitoring borehole which is truly "representative" of the quality of groundwater in the surrounding aquifer is not as easy as it might seem.

Borehole Sampling

The water quality of a groundwater sample collected from a monitoring borehole or well is critically dependent on 2 factors:

  • The length of the screened interval in the borehole.
  • The variability in permeability of the adjacent strata.

Inflows from more permeable strata or fissures will dominate and bias the sample in the borehole and produce a flow-weighted average sample (FWA). This is not a "representative" sample. Mixing mechanisms within boreholes can be very complex and research demonstrates that boreholes with longer screened intervals lead to greater bias, and therefore uncertainty, in the sampling result.

Sources of Groundwater Samples

There are essentially three methodologies that can be used to sample groundwater from a borehole:

  1. Fixed volume purging followed by sampling
  2. Low flow sampling
  3. Passive Sampling (Zero Purge) methods

Fixed Volume Purge & Sample

Typically 3 or more times the volume of water in the well lining is pumped from the monitoring borehole or well prior to sampling using relatively high pumping rates. Purge volumes can be reduced by monitoring for chemical stability during pumping (usually by measuring temperature, pH and EC - sometimes extended to include DO and redox if a flow cell is used).

Fixed Volume Purge

This is currently the default guidance when sampling boreholes and wells in the UK and Ireland.

Good for:

  • Inducing inflow from a larger volume of aquifer
  • Routine monitoring programmes
  • Non specialists

Issues:

  • Generates relatively large purge volume for disposal
  • High pumping rates can create large drawdown in water level
  • High pumping rates can produce non laminar turbid flow, which can impact on some chemical constituents - particularly volatile organic carbons (VOCs)
  • Time and labour intensive

Suitable equipment from In-Situ Europe: Inertial Pumps, Suction Pumps, Submersible Pumps 

Low Flow Sampling

Low flow purging and sampling methods require pumping rates to be maintained at 0.5 litres per minute or less. These methods rely on stability monitoring of chemical parameters using flow cells to determine the end point of purging prior to sampling. This methodology has been developed primarily to optimise sampling conditions for VOC's.

Low Flow Sample

Other key criteria for using low flow methods:

  • There should be no prior disturbance of the water column before pumping (i.e. dedicated pumping systems are preferred) 
  • Must maintain minimal drawdown of the water column (to minimise pressure changes)

Good for:

  • Sampling from short screened monitoring wells
  • VOC sampling
  • Reducing the volume of purge water for disposal.

Issues:

  • Use in long screened boreholes (>6m) is questionable
  • Time intensive
  • Specialist technical knowledge in designing sampling protocols is advisable

Suitable equipment from In-Situ Europe: Peristaltic pumps, Submersible pumps with low flow controller, Bladder pumps, Water quality equipment with flow-through cells. 

Passive Sampling (Zero Purge) Methods

Zero purge methods use passive sampling devices to collect samples at discrete depths in the borehole column with minimal disturbance.

 Passive Sampling

 Good for:

  • Sampling from short screened monitoring wells
  • VOC sampling
  • Analytical results are generally comparable to low flow methods
  • Simple sampling procedure - no purging required.

 Issues:

  • Use in long screened boreholes (>6m) is questionable
  • Specialist technical knowledge in designing sampling protocols is advisable.
  • No purge water generated for disposal

 Suitable equipment from In-Situ Europe: Hydrasleeves, Equilibrators 

How can In-Situ Europe help?

In-Situ Europe has a full complement of equipment for pump/purge, low flow and passive sampling methodologies. Our groundwater sampling training courses (CPD accredited by the Geological Society) are designed to raise understanding and provide hands-on experience of the technologies and processes available for taking the most appropriate sample possible from a borehole or well.