The Importance of Representative Groundwater Sampling

By: RK Water

In the second installment of “Understanding Dewatering,” we’re diving into the importance of representative groundwater sampling — one of the most undervalued aspects of temporary dewatering.

According to Pinyon Environmental, “The groundwater chemistry data collected for permit applications is often flawed because the collection techniques don’t adequately mimic the turbidity associated with the various construction means and methods. This results in either an under- or over-representation of the geotechnical impacts. And under-represented geotechnical condition could result in [the] under-design of the treatment system and [introduce] potential project delays and unanticipated costs.”

Groundwater sampling is essential because each of the following behaviors has the potential to distort groundwater treatment requirements by inadequately characterizing the groundwater chemistry:

  • Reliance upon one test well for groundwater sampling.
  • Only one groundwater sample analyzed for chemistry.
  • Inadequate purging of the test well.

Be sure to uplevel the sampling process by taking heed of potential sample biases. A sampling event is merely a description of general chemistry at a specific moment in time. When test wells are installed, not only will the groundwater table vary according to each well, but the chemistry of groundwater will also fluctuate, sometimes drastically.  Clay layering, bedrock, and underground rivers are below the surface, which is what causes naturally occurring obstructions; these obstructions can introduce chemistry variances that will be overlooked if only one test well is used.

Sampling activities over an extended period of time will provide a more accurate measurement of fluctuations in the groundwater table and its chemistry.  Furthermore, water that is allowed to stagnate with soil, will naturally contain higher levels of heavy metals and other contaminants. Through proper purge procedure, or even extending beyond the minimum, it is possible to obtain lower levels of heavy metals in the sample.

Once dewatering commences, groundwater will begin flowing downstream according to the natural flow of alluvial groundwater. Research also needs to be conducted regarding existing dewatering permits in the vicinity of the project to validate results from sampling activities. More importantly, permits upstream of the project will identify the chemistry to anticipate through continued dewatering activities. In the end, it’s not always about the chemistry of the groundwater within the confines of your shoring, but what’s in your neighbor’s groundwater that may make or break a project.