Monitoring piezometers in recharge areas: Is one 'upstream' and two 'downstream' adequate?|
K.U.Weyer, Ph.D., P.Geol. and D. A. van Everdingen, Ph.D.
WDA Consultants Inc., 4827 Vienna Drive NW, Calgary, Alberta, Canada
© 1995 WDA Consultants Inc.
Common practice by regulatory agencies and the groundwater industry is to install one piezometer 'upstream'
and two 'downstream' of contaminated sites. 'Upstream' and 'downstream' usually refer to 'up-gradient' and
'down-gradient' with respect to the groundwater table. The 'up-gradient' piezometer is intended to show the
unaffected chemical background condition of the groundwater; the 'down-gradient' piezometers are intended to
detect subsurface contaminants leaving the disposal site.
This practice is based on the widely held belief that shallow groundwater must flow down-gradient parallel
to the groundwater table. In fact, this is only valid for limited parts of groundwater flow systems between
recharge and discharge areas. Groundwater flow is oriented subvertically downwards in recharge areas and
subvertically upwards in discharge areas. This is especially valid in the commonly occurring case where higher
permeable layers exist beneath less permeable layers in the subsurface.
The example of an industrial waste disposal site in Bielefeld-Brake (Germany) demonstrates the costly mistakes
which result if these unjustified assumptions are applied to monitoring design and remediation measures in
recharge areas. Based on the assumption of lateral flow at this hilly site, a great number of piezometers were
installed around the perimeter of the waste disposal site. Since these piezometers did not detect any contaminants,
it was assumed an unknown, and as yet undiscovered, mechanism had confined the contaminants to the site. As a
preventative measure, a slurry wall and pumping wells were installed around the perimeter of the waste disposal
site at an estimated cost of $20-25 million.
A nearby, previously existing deep well showed downward migration of dissolved contaminants typical of recharge
areas. This downward migration could not have been detected with the piezometers installed to monitor the assumed
lateral contaminant migration. At present, there are no plans to investigate the downward migration of the