CLASSIFIED GROUNDWATER AGE: BASIN-SCALE ASSESSMENT OF SAMPLES FROM LONG-SCREENED PRODUCTION WELLS

Classified groundwater age, based on whether a sample is predominantly modern, mixed, or old, allows for assessment of groundwater samples obtained from long-screened production wells. In turn, a groundwater basin, or group of basins, can be characterized by evaluating classified age from a set of representative samples. If ≥50% of the samples are in a single age class, the basin is defined strongly by that class. If the number of samples in one age class is larger than the others, but <50% are in that class, the basin is defined weakly by that class. A mixed-age basin is further characterized by the second-most dominant age class: mixed modern or mixed old.

In this study, samples were obtained from 1513 long-screened production wells, primarily public supply wells, distributed across 25 study units in California (study units represent one or more basins). All samples were analyzed for tritium (³H) and noble gases (including helium [He] isotopes); 872 of these were also analyzed for carbon isotopes (including carbon-14 [¹⁴C]). ³H was used to characterize samples as either modern (≥0.2 TU) or pre-modern (<0.2 TU); ¹⁴C was used to characterize samples as modern (percent modern carbon [pmc] ≥97), young (97>pmc ≥80), or old (pmc<80); and terrigenic ⁴He was used to characterize samples as either young (<3% of total ⁴He) or old (≥3% of total ⁴He). The thresholds were chosen to minimize classification conflict (because not all samples were analyzed for ¹⁴C). Classified age then was assigned to a sample based on all 3 tracers, if available, or on 2, if not.

Of the 1513 samples: 460 were modern, 575 were mixed, 449 were old, and 29 lacked sufficient data. Also, 7 study units were modern (1 strongly), 6 were mixed modern, 5 were mixed old, and 5 were old (2 strongly). Two study units were mixed but with equal numbers of modern and old samples. Study units characterized as modern or mixed modern were in areas where groundwater is broadly and intensively used for urban or agricultural supply. Study units characterized as old or mixed old were in areas where groundwater is not as extensively used, particularly in the desert. This study shows that age-dating of samples from long-screened production wells provides meaningful information at the basin scale; this information can be used for other purposes such as evaluation of groundwater quality.