Transcript Multi-Level Ground-Water Monitoring

Multi-Level Ground-Water
Monitoring
Travis von Dessonneck
Multilevel Ground-Water Monitoring
• Most important discovery in the past 40
years “…is that the distribution of
dissolved contaminants in the subsurface
is spatially complex, especially in the
vertical dimension (Nielsen 808).”
Multilevel Ground-Water Monitoring
• This is due to several factors
– Labyrinthine distribution of residual
contamination in most NAPL source zones
– Geologic heterogeneity
– Weak mixing mechanisms in ground-water
flow systems
Multilevel Ground-Water Monitoring
• Weaknesses of single-interval monitoring
wells
– Plume distortion
– Composite samples mask true vertical
distribution
– Samples strongly biased by the position and
length of the well screens
Why Three-Dimensional Plume
Delineation is Necessary
• Site assessment
– Predicting plume migration
• Long screened wells tend to over estimate risk
– Effective remediation
• Especially with Permeable Reactive Barriers (PRB)
One Time Sampling vs. Permanent
Multilevel Monitoring Devices
• One Time Sampling advantages
– Generally faster
• No installation, development
– No permanent structures
– No long term monitoring requirement
One Time Sampling vs. Permanent
Multilevel Monitoring Devices
• One Time Sampling disadvantages
– Whole story not told
• No information on hydraulic head
– Numerous one time samples to get
contaminant vs. time
• Becomes costly
• Holes usually grouted
– Not long term cost effective
One Time Sampling vs. Permanent
Multilevel Monitoring Devices
• When to install?
– Determining hydraulic head distribution is
necessary
• Changes in head over time
– Monitor remediation practices
– Time series samples
Where You Monitor is as Important
as How You Monitor
Options for Ground-Water
Monitoring
• Multiple Diffusion Samplers installed inside
single-interval monitoring wells
– Installation of diffusion samplers over the
screened interval of a well
• Samplers consist of dialysis cells or polyethylene
bags with DI water
• the DI water is isolated from water in the wells
• Contaminants diffuse through the bag or cell into
the water within
Options for Ground-Water
Monitoring
• The samplers must stay in the well long enough for
the contaminant to reach equilibrium with the
aquifer
• Assume no vertical flow in the well
• Assume that water flows unobstructed through the
well
Options for Ground-Water
Monitoring
– Diffusion Multilevel System
• Same idea as above, but individual dialysis
chambers are isolated to reduce vertical flow
• Can detect:
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Chloride
Nitrate
Sulfate
DO
Tetrachloroethylene
1,1,1-trichloroethane
Organic and inorganic compounds
Options for Ground-Water
Monitoring
• Active Collection of Samples from Multiple
Depths within a Single-Interval well using
grab samplers or depth-discrete pumping
– Grab or Thief Samplers
• Samplers lowered to desired depth and activated
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May yield ambiguous results
Vertical flow in the well
Sampler may cause mixing
Increase turbidity
Time demanding
Options for Ground-Water
Monitoring
– Collecting depth-discrete samples by pumping
from different depth in well screens
• Pumping creates vertical flow within the well
• Only the first sample is good
Options for Ground-Water
Monitoring
• Nested Wells (multiple tubes or casing in a
single borehole)
• Bundle Wells installed in collapsing sand
formations
– Allow formation to collapse around the wells
– Usually sampled using peristaltic pumps
– Tube diameters as small as 0.5 inches
– Up to 20 different wells
Bundle Wells installed in collapsing
sand formations
Bundle Wells installed in collapsing
sand formations
Options for Ground-Water
Monitoring
• Well Clusters
– Can act as vertical
conduit for
contaminants
Options for Ground-Water
Monitoring
• Dedicated Multilevel Ground-Water
Monitoring Systems
– Advantages
• Facilitate the collection of samples and
measurement of head from many more discrete
depths
• Only one pipe/tube per borehole
• Total project costs significantly lowered
– Less drilling
– Less waste
Options for Ground-Water
Monitoring
– Less time spent monitoring and sampling
– Fewer wells for decommissioning
• Little or no purging required
– Less cost
• Little water stored in the well, quicker reaction time
to head changes
• Smaller “footprint” on the ground
Options for Ground-Water
Monitoring
– Disadvantages
• Fewer options for sampling
– Due to size of tubes
• Some training required
• May be more difficult to decommission the wells
Options for Ground-Water
Monitoring
• Currently there are four commercially
available systems
– Westbay MP® System
– Solinst Waterloo™ System
– Solinst CMT™ System
– Water FLUTe™ System
Options for Ground-Water
Monitoring
Westbay MP® System
• Two parts
– Casing system
– Portable probes and tools
• Each monitoring zone is valved
• Probes are lowered inside the casing to
each monitoring zone
– The probes locate and open the valve ports
– Multiple probes can be used at the same time
on different ports
Westbay MP® System
• Notebook accessible
– No limitation on number of ports except space
– Samples taken at formation pressure, no
purge
Westbay MP® System
Westbay MP® System
Solinst Waterloo™ System
• Modular system
– 2 in schedule 80 PVC
• Each port has a 1 or 2 valved stem
connected to:
– An open tube that runs up to the surface
– A double valve pump
– A bladder pump
– Pressure tansducer
Solinst Waterloo™ System
• Can be connected to data logger for continuous
logging
• Typically 3-8 monitors per well
Solinst Waterloo™ System
Solinst CMT™ System
• Continuous Multichannel Tubing
• 1.6 in OD flexible HDPE tubing
• Up to 7 discrete zones
– Ports are staggered around the tube
– Channels are plugged a few inches below
each port
• Samples taken with peristaltic pumps,
small diameter bailers, inertial lift pumps,
or small diameter double-valve pumps
Solinst CMT™ System
Water FLUTe™ System
• Flexible Liner Underground Technology
– Flexible impermeable liner of polyurethanecoated nylon
• More than 20 zones
• Can monitor boreholes 2-20 inches in
diameter
• Intervals inserted at factory
– 0.17 and 0.5 in OD tubing
Water FLUTe™ System
• Tubes have two valves on them to prevent
flow back into the aquifer
• Can be installed into nearly horizontal
wells
Water FLUTe™ System