Description
Groundwater flow is perhaps the most important mechanism for the dispersal of many types of toxic waste once they have been released into the subsurface. Accurate information about the groundwater flow field is critical to the characterization of waste sites, the monitoring of waste remediation activities, and the monitoring of the post-closure performance of remediated waste sites. In situ permeable flow sensors can measure the full three-dimensional groundwater flow velocity vector at a point in a saturated, permeable, unconsolidated medium using only one hole. With In Situ Permeable Flow Sensors, information about the hydraulic conductivity is not required. The flow sensors measure the velocity characteristic of a very small volume of material, on the order of 1 m³. It is easy to set up a flow sensor and monitor it remotely for extended periods of time. Use of this technology at a site does not preclude using any other technology at the same time or at some future date. Shown below is a schematic of the In Situ Permeable Flow Sensor.
The basic operating principle of this technology is to bury a thin cylindrical heater vertically in the ground at the point where the groundwater flow velocity is to be measured. If the heat flux out of the cylinder is uniform over the surface of the cylinder, then the temperature distribution on the surface of the cylinder will vary as a function of the direction and magnitude of the groundwater flow velocity past the cylinder. In the absence of any flow past the device, the temperature on the surface of the probe will be independent of the azimuth and symmetric about the vertical midpoint of the probe. The vertical midpoint will be warmer than the ends of the probe because heat transfer away from the ends of a finite length cylinder is more efficient than from the midsection of the cylinder. Groundwater flow past the device perturbs the surface temperature distribution with the pattern and magnitude of the temperature variations reflecting the direction and magnitude of the groundwater flow velocity. In essence, relatively warm temperatures will be observed on the downstream side and relatively cool temperatures on the upstream side of the instrument as the heat introduced into the formation by the heater is advected around the probe.
If the groundwater flow has a vertical component, the vertical temperature distribution on the surface of the probe will no longer be symmetric about the vertical midpoint of the probe, but will be skewed in the direction of the flow. The surface of the downstream end of the probe will be warmer than the upstream end. If there is a significant horizontal component to the flow velocity, the surface temperature distribution will not be independent of the azimuth, but rather the surface temperature will vary approximately as the cosine of the azimuth, with the downstream side of the probe being warmer than the upstream side. The magnitude and direction of the three-dimensional flow velocity vector are determined from the magnitude and the pattern of the temperature variations on the surface of the probe, respectively.
Electric power, either from line power or a generator, is required. For remote monitoring, access to a telephone line or cellular phone service is also desirable (data transfer by radio frequency transmission is presumably possible).
Technical Performance
Field Demonstration. Field tests indicate that flow velocities as low as a few meters per year are resolvable. The probes are simple to install and monitor. Data from a number of probes at the same site can be collected and sent via modem to computers at a remote site. Other than for installation and occasional maintenance, the system can be operated remotely for extended periods of time. Once the heater on the probe is activated, a flow velocity measurement can be obtained after about 24 to 48 h. Current prototype sensors last for approximately one year.
Failure occurs when the waterproof coatings ultimately leak, allowing water into the probe where it shorts out the electronics. Failure of the probe does not present any serious consequences other than the fact that useful flow velocity measurements will no longer be available from the probes.
The sensor measures the velocity at essentially a point. Sometimes the average velocity over a wider area is desirable. The standard technique measures a velocity that is an average of the velocity over a much broader region, one whose dimensions are characterized by the separation of the boreholes.
Cost. Purchase of a calibration facility, data acquisition system, and computer for data analysis is estimated at approximately $25K. Each flow sensor is estimated to cost between $500 and $700. In remote monitoring applications, approximately one-tenth of a person's time is required to collect and analyze the data.
Projected Performance
The detailed and relatively inexpensive information gained by this technology will enable a greater utilization of time and resources for characterizing, monitoring, and remediating the ground media at problem sites.
Waste Applicability
Since this technology is specifically used for obtaining groundwater flow velocity information at problem sites, the technology is waste independent.
Status
Virtually all of the components for the sensors and the data acquisition system are available commercially. It is expected that this technology will be commercially available by the end of 1993.
Currently, temperature differences of about 0.01°C can be measured. At this level, flow velocities as low as a few meters per year can be resolved. The probe design needs to be improved to assure long-term reliability of electronics and sensors in groundwater conditions. Flow sensors can monitor groundwater flow for as long as required, until they leak (approximately 1 yr).
Regulatory Considerations
Compliance with the Occupational Safety and Health Administration regulations is required for hazardous waste operations and protection of occupational workers from electrical power. In addition, permits may be required for drilling at hazardous waste sites.
Potential Commercial Applications
This technology can be useful at a wide variety of sites and with several different remediation processes. The In Situ Permeable Flow Sensors can be used at any site where information on groundwater flow velocity is necessary. Such information is critical to the characterization of waste sites, the monitoring of waste remediation activities, and the monitoring of post-closure performance of remediated waste sites.
Baseline Technology
The baseline technology is a standard technique used to measure hydraulic head gradients and hydraulic conductivities in boreholes to determine flow velocity. Four holes are required for a measurement using this standard technique in comparison to only one hole with the In Situ Permeable Flow Sensors. Information about the hydraulic conductivity of the medium is required in the standard technique. This is generally determined using a pump test in which large quantities of water are pumped from the well. At contaminated sites, disposal of this purge water can be difficult and expensive. The flow sensors measure the velocity characteristic of a very small volume of material, on the order of 1 m³. The standard technique measures a velocity that is an average of the velocity over a much broader region, one whose dimensions are characterized by the separation of the boreholes.
Intellectual Property Rights
There is no patent on this technology; it is in the public domain. Sandia National Laboratories has applied for copyrights on engineering drawings and on the software that interprets flow sensor data.
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References
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