stream side of the probe. If the groundwater flow has a vertical component then the temperature will no longer be symmetrical about the vertical midpoint. The magnitude and direction of the three-dimensional (3-D) flow velo-
city vector are determined from the magnitude and the pattern of the temperature variations on the surface of the probe. The sensor should be sensitive to groundwater flows as low as a few meters per year.

Technology Needs

Because groundwater flow is per-
haps the most important mecha-
nism for the dispersal of many types of toxic wastes once they have been released to the sub-
surface, accurate information

about the groundwater flow is critical to the characterization of waste sites, monitoring of the waste remediation activities, and monitoring the post-closure per-
formance of remediated waste sites.

The primary, currently accepted method of obtaining flow velocity information is to make water-level measurements in screened bore-
holes to determine the hydraulic gradients in the subsurface. With hydraulic conductivity data the velocity field between the bore-
holes can be modeled. The short-
comings of this technique are as follows:

• To obtain detailed knowledge of the hydraulic conductivity distribution in the subsurface,