Except for studies of temperature data related to ground-water developments that induce infiltration from streams, little attention has been given to the possibility of using temperature fluctuations as a tool for studying the elements of the hydrologic cycle involving ground water.

The temperature of the water discharged from large installations that induce river infiltration through alluvial deposits depends primarily on the following factors: (a) the porosity of the aquifer, (b) the specific heat of the rocks or mineral grains making it up, (c) the temperature of the ground water in storage, (d) the temperature of the river water, and (e) the amount of mixing that occurs as a result of pumping. In six installations of this type, where the annual rivertemperature fluctuations ranged from 44° to 52°F, the average range in groundwater temperature was 22 °F. The cycles of ground-water temperature lagged behind the river-temperature cycles by 1 to 5¹/₂ months. Under the conditions that existed, the lag of the minima of the cycles was much greater than that of the maxima, largely because of variations in pumpage and of the effect of viscosity differences on the rate of flow.

An experimental study was made at Worthington in southwestern Minnesota to determine whether temperature fluctuations could be used to study rates and directions of ground-water movement or to evaluate recharge conditions. Three shallow glacial-outwash aquifers were studied by measuring temperatures at approximately monthly intervals in three municipal-supply wells whose average yields were about 60 to 150 gpm (gallons per minute). Temperatures were read with an accuracy of 0.01° to 0.02°C, and the data were analyzed graphically and correlated in detail with lake levels, lake temperatures, precipitation, and groundwater levels.

Thermographs for 2 wells, 1 completed in a semiconfined aquifer about 200 feet from Okabena Lake and the other in a water-table aquifer about 850 feet from the lake, indicate that pumping induces water to move from the lake into the aquifers. A larger percentage of cold lake water was mixed with ground water from January to March 1958 than from January to March 1959. On the assumption that the cold or warm lake water was a distinct mass, the average time required for water to move from the lake to the well 200 feet away, under the prevailing hydraulic gradient, was 2 to 4 months; and to the well 850 feet away, 5 to 7 months.

The thermograph for a well 1,800 feet from the lake, completed in an artesian aquifer that is confined by relatively impervious glacial till, indicates that the till acts as an insulating medium. However, despite the apparently low permeability of the till, the thermograph suggests that the lowering of artesian pressure, which results from pumping, induces warmer water to move downward through the till.

The infiltration of relatively warm spring and summer rainfall can be detected on the thermographs of all the wells.

The precise measurement of fluctuations in ground-water temperature, based on monthly readings in shallow glacial-outwash aquifers (up to about 70 feet deep), is useful in the study of ground-water movement and recharge. In addition to the study of natural phenomena in the hydrologic cycle, thermometry may be used as a tool in making detailed studies of (1) the effects of inducing the infiltration of surface water, (2) artificial recharge, (3) the effects of injecting petroleum products or radioactive or other wastes into the ground, and (4) ground-water movement in mines.