A multi‐isotope (B, Sr, O, H, and C) and age dating (³H–³He and ¹⁴C) study of groundwater from Salinas Valley, California: Hydrochemistry, dynamics, and contamination processes

The chemical and isotope ( 11 B/ 10 B, 87 Sr/ 86 Sr, 18 O/ 16 O, 2 H/H, 13 C/ 12 C, 14 C, and 3 He/ 3 H) compositions of groundwater from the upper aquifer system of the Salinas Valley in coastal central California were investigated in order to delineate the origin and processes of groundwater contamination in this complex system. The Salinas Valley has a relatively deep, confined “400‐foot” aquifer, overlain by a “180‐foot” aquifer and a shallower perched aquifer, all made up of alluvial sand, gravel and clay deposits. Groundwater from the aquifers have different 14 C ages: fossil ( 14 C = 21.3 percent modern carbon (pmc) for the 400‐foot aquifer and modern ( 14 C = 72.2–98.2 pmc) for the 180‐foot aquifer. Fresh groundwater in all aquifers is recharged naturally and artificially through the Salinas River. The two modes of recharge can be distinguished chemically. We identified several different saline components with distinguishable chemical and isotopic fingerprints. (1) Saltwater intrusion in the northern basin has C1 concentrations up to 1700 mg/L, a Na/Cl ratio less than seawater, a marine Br/Cl ratio, a Ca/Cl ratio greater than seawater, δ 11 B between +17 and +38‰ and 87 Sr/ 86 Sr between 0.7088 and 0.7096. Excess dissolved Ca, relative to the expected concentration for simple dilution of seawater, correlates with 87 Sr/ 86 Sr ratios, suggesting base exchange reaction with clay materials. (2) Agriculture return flow is high in NO 3 and SO 4 , with a 87 Sr/ 86 Sr = 0.7082,δ 11 B =19‰and δ 13 C between −23 and −17‰. The 3 H– 3 He ages (5–17 years) and 14 C data suggest vertical infiltration rates of irrigation water of 3–10 m/yr. (3) Nonmarine saline water in the southern part of the valley has high total dissolved solids up to 3800 mg/L, high SO 4 , Na/Cl ratio >1, δ 11 B between +24 and +30‰, and 87 Sr/ 86 Sr = 0.70852. This groundwater may have acquired its geochemical signature from leaching of sedimentary rocks associated with the Coast Range marine deposits of Mesozoic to early Cenozoic age. The combination of different geochemical and isotopic fingerprints enables us to delineate the impact of salt sources in different areas of the valley and to reconstruct the origin of the SO 4 ‐enriched NO 3 ‐depleted saline plume that is located west of the city of Salinas. We suggest that the latter is derived from a mixture of different natural saline waters rather than from anthropogenic contamination.