Tracer Based Flow Surveys In Monitoring Wells To Determine Best Sampling Approach; Low Flow or Passive Grab Sampling

Since the advent of Superfund in 1981 the environmental industry has been in a lengthy debate over use of various sampling methods from environmental monitoring wells.  Purging and sampling paradigms can be divided into three main camps; 1) three to five wet casing volume purges prior to sampling, 2) low flow purging prior to sampling and 3) no purging prior to sampling.

 This presentation focuses on measurement results of flow under both low flow purging and ambient (non pumping) conditions using in-well tracers with an up-hole fluorometer and down-hole laser system, respectively.  In the two conditions studied, the results show that under low flow purging and sampling much of the monitoring well screen is hydraulically engaged - even with minimal drawdown inside of a monitoring well.  Flow measurements under ambient conditions using a down-hole laser demonstrate that slow but detectable vertical movements inside monitoring wells are present; shedding light on some limitations of no-purge sampling systems in terms providing meaningful concentration results from monitoring wells.

 The concentration results themselves of scientific value have considerable economic value when translated into cost for groundwater aquifer restoration.  The extraction ratio (gallons of water removed for each gallon or lb. of contaminant removed from the groundwater) is a key concern to those organizations paying for clean up.  There are inherent problems with both low flow purging and sampling and no purge sampling in terms of deriving how much contaminant mass is actually present in groundwater.  The results of these studies conclude that without a proper, basic understanding of flow dynamics inside monitoring wells with either of these practices, groundwater hydrogeologists and remediation engineers alike are at risk of significant errors in contaminant mass estimates for groundwater cleanup.