Groundwater flow sensor should aid environmental cleanup

Groundwater flow sensor should aid environmental cleanup – Sandia National Laboratories is developing a groundwater sensor

A low-cost sensor that measures the speed and direction of groundwater is being developed at Sandia National Laboratories.

Called the In-Situ Permeable Flow Sensor, the technology is expected to be useful for toxic waste management and cleanup where groundwater has become contaminated. The sensor can be used to determine groundwater flow patterns, providing valuable information on the size of the contaminated area and how it might be spreading.

The Sandia device is considered to be more accurate than other flow sensors because it is permanently embedded in the ground rather than contained in an open borehole.

The sensor consists of a rod, measuring 30 inches long and two inches in diameter, with a cylindrical electric heater. An array of 30 thermistors (thermal resistors) on its surface measures temperature change as a function of electrical resistance.

The instrument is buried in a location after being lowered inside a hollow stem auger drill. In unconsolidated, or loose, soils, the soil fills in around the sensor when the drill stem is retracted, leaving the surrounding material in direct contact with the sensor.

Other sensors usually remain within their original boreholes, which can alter natural fluid flows and cause inaccurate readings of flow direction and velocity.

Powered by an above-ground power source, the Sandia sensor heats the surrounding soil and groundwater as it flows past the instrument. The thermistors measure the water’s temperature, and the information is transmitted to a data acquisition system on the surface. The direction from which the water is coming is determined by detecting water temperatures – relatively cool water exists upstream, and warmer temperatures are registered downstream as the water is heated by the device. The pattern of temperatures recorded on the probe’s surface indicates the direction of the fluid flow. The magnitude of the temperature variations is used to determine the magnitude of the flow velocity.

In cleaning up sites where hazardous chemicals have seeped into the ground, it must be determined whether they have reached the water table, how widespread the contamination is, and whether fluid flows are causing it to spread. Fluid flows within a contaminated site may be complex and diverse, factors that some flow sensors fail to detect.

The flow velocity derived from the permeable flow sensor is characteristic of a small area, an important factor in remediation, says researcher Sandy Ballard. Numerous sensors can be buried within the boundaries of a contaminated site to produce a highly accurate picture of complex groundwater flow systems.

The sensors are left in the ground, requiring their cost to be kept at a minimum. The production cost of each device is estimated to be less than $1,000. Installation costs, including drilling the hole, would probably cost another $500.

DOE holds the patent (#4547080) to the instrument. Science & Technology Corporation, an Albuquerque company, holds a nonexclusive license from DOE to commercialize the sensor.

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