EVALUATION OF LAND-BASED INFRARED THERMOGRAPHY TO IDENTIFY AND QUANTIFY GROUNDWATER DISCHARGE TO A SMALL STREAM

Although aerial thermal infrared (IR) imaging has been used to identify groundwater discharge areas in surface water bodies based on thermal contrasts between the waters, little research has been done to delineate and quantify groundwater discharges to streams using handheld or land-based infrared cameras. It was hypothesized that a handheld IR camera could be used as a non-invasive tool to rapidly identify zones of preferential groundwater discharge in small streams and semi-quantitatively estimate groundwater discharge. In February 2006, a ThermoCAM P25 infrared camera (FLIR Systems Inc.) with a thermal sensitivity of ~0.08 °C was used to measure surface water temperatures along a 184 m reach of stream in Teeterville, Ontario, Canada. The IR camera was mounted on a 5 m high tripod and surface water temperatures were determined from images using emissivity, T-reflected, and atmospheric values. These surface water temperatures were compared to discharge patterns which were determined from 254 streambed temperature measurements, 32 seepage meters, 10 mini-piezometers, 4 wells, 7 temperature dataloggers, and stream flow gauging. This reach of stream flowed at ~ 0.025 m³s^-1 and its streambed consisted primarily of fine sand and was gaining groundwater at rates of 10 to 1500 L/m²d along its entire length.

The thermal imaging quickly and clearly identified a +3 °C temperature anomaly 0.15 m in diameter. Streambed temperature mapping and seepage meter measurements confirmed this anomaly was located at a spring within the highest discharging area along the reach. Other preferential groundwater discharge zones were inferred by IR images exhibiting thermal eddying caused by mixing of different temperature waters. Temporal variations in surface water temperatures during the IR survey and variations in water emissivity values meant a semi-quantitative relationship could not be developed between IR derived surface water temperatures and spatial variations in groundwater discharge. Small increases in surface water temperatures appeared to occur down stream of high discharge zones, but these results were not definitive. The hand held IR camera was a valuable tool for identifying large discharges, tile drain outfalls, and seeps of ground water discharge along the waterline that might otherwise have gone undetected.