Tritium (3H) activities in the foliage of forest trees were found to reflect 3H activities in soil water along the perimeter of a retired low‐level radioactive waste disposal site. Spatial variability in foliage 3H activity clearly delineated patterns of 3H in soil water as deep as 3 m. Late summer increases in foliage 3H activity paralleled increases in soil water activity and also suggested greater uptake of water from deep reservoirs after surface soils had dried out. White and chestnut oak (Quercus alba L. and Q. prinus L.) dominated the study area and were good monitors of 3H, regardless of tree size. Spatial patterns of 3H in foliage of these species over a 5‐ha watershed were interpretable in terms of the local landscape and the probable pathways of movement of contaminated water originating on the disposal site. Results of this study suggest that trees and other deeply rooted plants may serve as simple and effective monitors of soil water contamination and may therefore be useful additions to permanent monitoring systems.

Transpiration uptake from a tritiated water source (3H2O) associated with a sandstone bed at 3 m depth was examined in a second-growth Quercus–Carya forest. The effect of utilization of deep water on predawn xylem water potentials (Pstem) was also investigated. The 3H2O saturated the soil contacting the sandstone bed. As surface soils dried during the growing season, 3H2O uptake increased. Although all trees growing over the 3H2O reserve were capable of utilizing some of this water, Q. alba trees from all canopy levels generally exhibited higher foliage tritium (3H) activities than C. glabra (Mill.) Sweet. Higher (less negative) Pstem values were associated with trees having higher foliage 3H activities. The Q. alba tree with the highest foliage 3H activity, 1.2 × 106 Bq•L−1, exhibited the highest Pstem value, −0.875 MPa; the Q. alba tree with the lowest 3H activity also had the lowest Pstem value, 4.5 × 104 Bq•L−1 and −1.75 MPa, respectively. Pstem and the logarithm of 3H activity were highly correlated (r2 = 0.89), suggesting that predawn moisture status was at least partially controlled by the ability of trees to utilize the deep water reserve at or near the fractured sandstone bed.