Recently Recharged Water

Tritium (³H) has a half-life about 12.4 years and is measured in tritium units (TU)-each tritium unit equals one tritium atom in 10/18 atoms of hydrogen. Prior to 1952, the tritium concentration of precipitation in coastal southern California was about 2 TU. Beginning in 1952, tritium was released to the atmosphere as a result of the atmospheric testing of nuclear weapons, and the tritium concentration of precipitation increased (fig. 4). Because tritium is part of the water molecule, tritium concentrations are not affected significantly by reactions other than radioactive decay. Tritium is an excellent tracer of the movement of water on time scales ranging from 10 to less than 100 years before present. In this report, ground water having tritium concentrations less than the detection limit of 0.3 TU is interpreted as water recharged prior to 1952. Ground water having measurable tritium is interpreted as water recharged after 1952.

Figure 4. Volume-weighted tritum levels
in precipitation at Santa Maria, California.
(Data from International atomic Energy
Agency.)

Tritium concentrations in water from almost 150 wells sampled as part of this study ranged from 9.4 TU to less than the detection limit of 0.3 TU (Izbicki and others, 1995). Ground water containing tritium was present throughout large areas of the upper aquifer system underlying the Santa Clara River valley and parts of the Oxnard Plain near the Santa Clara River (fig. 5). Between 1952 and 1993 about 1.8 million acre-feet of water was recharged in ponds at Saticoy and El Rio. By 1991, this water was present throughout much of the upper aquifer system underlying the Oxnard Plain and had moved more than 6 miles downgradient from the recharge area but had not yet reached the coast. Ground water containing tritium was present only in small areas of the lower aquifer system near Saticoy and El Rio (fig. 6). These data suggest that infiltration of surface water in ponds at these locations is more effective at recharging the upper aquifer system than the lower aquifer system. However, water levels throughout the upper and lower aquifer systems are raised by hydraulic pressure from this recharge water.

Figure 5. Tritium in water from selected
wells.

Figure 6. Geologic section A-A'.

Tritium was present in the upper aquifer system near streams in Las Posas Valley. On the basis of the delta oxygen-18 and delta deuterium isotopic composition, this water was identified as water imported from northern California for public supply and discharged as treated municipal wastewater. Small amounts of tritium also were present in water from wells in the upper aquifer system near the coast as a result of seawater intrusion in those areas (Michel and others, in press).

Tritium was not present in water from most wells in Las Posas Valley and Pleasant Valley, in deep wells at multiple-well sites in the Santa Clara River valley, and in deep wells in most parts of the Oxnard Plain. These data show that water in these areas was recharged prior to 1952. Carbon-14 data were used to determine the age of this older water.

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USGS Water Resources of California