Study of the Irvine Sub-basin: Using isotopic and general mineral data to identify processes of chemical modification of groundwater.

摘要

Forty-nine groundwater samples were collected from 12 monitoring wells at depths ranging from 45 to 1,404 ft. below ground surface (bgs) in the Irvine Sub-basin. Samples were analyzed for general minerals/major ions and for stable isotope tracers (18O, 2H, 15N) of water and nitrate to investigate subsurface modification of locally recharged waters. In addition, samples were analyzed for tritium-helium isotopic composition (3He/4He) to constrain subsurface residence time. These data demonstrate that the Irvine Sub-basin is geochemically and isotopically stratified. Samples collected <200 ft. bgs have isotopic values that overlap those of modern precipitation. Samples collected from depths >800 ft. bgs show characteristics of "pre-development" water found at depth in the Main Orange County Basin. Samples collected from 200 - 800 ft. bgs have geochemically intermediate values, possibly suggesting mixing of recent recharge with the deep "pre-development" water. One well contains young water at intermediate depths that is over- and underlain by old (>60 yr.) water, suggesting preferential flow underneath an aquitard. Analysis indicates that Irvine Sub-basin groundwaters have dissolved halite, gypsum, and plagioclase. Samples collected near the base of the Tustin Hills have mean apparent 3He/4He ages that exceed 60 years, suggesting low subsurface flow rates, and elevated δ 15N records microbial reduction of nitrate. Subsurface residence ages suggest flow rates are higher along the basin axis.