Older Water

Carbon-14 has a half-life of about 5,730 years and is measured as percent modern carbon by comparing carbon-14 activities to the radioactivity of National Bureau of Standards oxalic acid prepared in 1950: 12.88 disintegrations per minute per gram of carbon equals 100 percent modern carbon. Carbon-14 is a tracer of the movement and relative age of ground water on time scales ranging from recent to more than 20,000 years before present.

Carbon-14 activities for water from 34 wells in the study area ranged from 89.4 to less than the detection limit of 1 percent modern carbon (Izbicki and others, 1995). Carbon-14 activities were higher in areas where ground water contained tritium and lower near the downgradient ends of long flow paths through the lower aquifer system.

Carbon-14 is not part of the water molecule, and in addition to radioactive decay, carbon-14 activities are affected by chemical reactions that occur between dissolved constituents and the material that composes the aquifer. For example, in the Santa Clara-Calleguas basin, oxidation of organic material (that does not contain carbon-14) within the aquifer deposits during the reduction of sulfate to hydrogen sulfide can reduce the carbon-14 activity of water from wells. Izbicki and others (1992) and Izbicki and Martin (written commun., 1996) evaluated the mass transfer of carbon, sulfur, and other elements between the solid and dissolved phases-and the subsequent changes in delta carbon-13, delta sulfur-34, and carbon-14 composition of ground water-using the computer program NETPATH (Plummer, and others, 1994). Interpreted carbon-14 ages ranged from recent (less than 50 years before present) for water from wells near sources of ground-water recharge along the Santa Clara River to more than 20,000 years before present (fig. 7).

Figure 7. Age of water from selected wells.

Changes in ground-water ages with depth (figs. 6 and 7) were evaluated on the basis of samples collected from 19 wells at 8 sites that were screened in individual aquifers (Densmore, 1996). Interpreted carbon-14 ages increased with increasing depth and increasing distance downgradient from recharge areas. The oldest water was from deep wells at the end of long flowpaths through the lower aquifer system underlying the Oxnard Plain (fig. 6). Large changes in ground-water ages with depth confirm that the deeper aquifers of the lower aquifer system are largely isolated from overlying aquifers and surface sources of ground-water recharge.

Water from most wells sampled in the Pleasant Valley and Las Posas areas also is relatively old, reflecting the smaller quantities of recharge available to aquifers underlying these areas in comparison with aquifers underlying the Santa Clara River valley or the Oxnard Plain. However, water from the wells sampled in Pleasant Valley is not as old (and not as isolated from sources of recharge) as water from wells sampled in the western part of Las Posas Valley or from some of the deeper wells sampled in the Oxnard Plain.

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USGS Water Resources of California