The Relationship between oak tree roots and groundwater in fractured rock as determined by tritium tracing

Abstract

The depths to which trees can extend their roots to obtain water have commonly been estimated to equal the height of the aerial portion of the plants. A detailed hydrogeologic investigation on a foothill pilot watershed has included diamond core drill holes which penetrate the fractured and jointed metamorphic rock well into the saturated zone. These holes provide observation wells for groundwater measurements and access for the injection of tritiated water into the groundwater zone. Over a 3-year period, injections have been made at various locations where the groundwater occurs at different depths from ground surface. In all the injection tests, significant amounts of tritiated water have been found in the water extracted from the fractured rock by the oak trees. Size and species of the oak trees do not influence the uptake of the injected tritiated water from the groundwater. Results of the 1962 experiment show that the oaks extend roots through the fractured rock to depths in excess of 70 ft. The number of oak trees in which the uptake of injected tritiated water has been detected indicates that all oak trees in the study area use water from the capillary zone immediately above the water table during the summer and fall months, a fact supported by observations of the decline of ground-water levels during these seasons. The decline occurs at nearly the same rate regardless of the depth to groundwater or the location of the well on the watershed.