ABSTRACT
CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 808 Why Three-Dimensional Plume Delineation is Necessary . . . . . . . . . . . . . . . . . . . . . . . 812 Measurement of Vertical Hydraulic Heads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814 One Time Sampling versus Permanent Multilevel Monitoring Devices . . . . . . . . . . . 814 Where You Monitor is as Important as How You Monitor . . . . . . . . . . . . . . . . . . . . . . 816 Options for Multilevel Ground-Water Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819
Multilevel Sampling within Single-Interval Monitoring Wells . . . . . . . . . . . . . . . . . 819 Multiple Diffusion Samplers Installed inside Single-Interval Monitoring Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819
Diffusion Multilevel System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 820 Passive Diffusion Bag Samplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 821
Active Collection of Samples from Multiple Depths within a Single-Interval Well Using Grab Samplers or Depth-Discrete Pumping . . . . . . . . . . . . . . . . . . . 821 Grab or Thief Samplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822 Collecting Depth-Discrete Samples by Pumping from Different Depths
in Well Screens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822 Nested Wells (Multiple Tubes or Casings in a Single Borehole) . . . . . . . . . . . . . . . . 823
Bundle Wells Installed in Collapsing Sand Formations . . . . . . . . . . . . . . . . . . . . . 824 Nested Wells Installed with Seals between Monitored Zones . . . . . . . . . . . . . . . . 824
Well Clusters (One Well per Borehole) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 829 Dedicated Multilevel Ground-Water Monitoring Systems . . . . . . . . . . . . . . . . . . . . . 830
Drilling and Installation Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 833 Installations in Open Boreholes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 833 Installations in Unconsolidated Sedimentary Deposits . . . . . . . . . . . . . . . . . . . . 833 Minimizing Cross-Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 836 Development of Multilevel Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 836
Westbay MP System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 837 Solinst Waterloo System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 839 Solinst CMT System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 841 Water FLUTe System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 845
One of the most important discoveries made during the last four decades of groundwater research is that the distribution of dissolved contaminants in the subsurface is spatially complex, especially in the vertical dimension. This is due to a number of factors, including the labyrinthine distribution of residual contamination in most nonaqueous-phase liquid (NAPL) source zones, geologic heterogeneity, and mixing mechanisms (e.g., mechanical mixing and molecular diffusion), that are relatively weak in most ground-water flow systems (National Research Council, 1994). This discovery was made possible by the use of multilevel sampling devices that facilitated the collection of discrete ground-water samples from up to 20 different depths in a single borehole (Cherry et al., 1981; MacFarlane et al., 1983; Reinhard et al., 1984; Smith et al., 1987; Robertson et al., 1991; van der Kamp et al., 1994).
