ENVIRONMENTAL ISOTOPES ASSESS GROUNDWATER FLOW AT AN ALLUVIAL FAN/PLAYA MARGIN INTERFACE, MOJAVE DESERT

Mojave Chub (MC) Spring is located at an alluvial fan/playa margin interface between Soda Lake and the Soda Mountains, Mojave Desert. An alluvial fan formed on the flanks of the Soda Mountains is the likely source of water at MC Spring. Limestone Hill is a carbonate outcrop located between the alluvial fan and the saline playa at Soda Lake. MC Spring flows from the opposite side of Limestone Hill, and is separated from Soda Mountains and the alluvial fan by Limestone Hill. Analysis of groundwater samples for sulfur isotopes, stable O-H isotopes, tritium and carbon 14 identify similar chemical, and age properties at MC Spring and in production wells at the site. Most waters have similar oxygen and deuterium stable isotope signatures, indicating that waters are from the same source. Except at higher elevations in the alluvial fan, tritium is not detected in most shallow wells (2 to 5 meters deep) and deeper production wells (10 to 18 meters deep) at the site. Tritium data suggest sluggish groundwater movement and/or very small amount of active recharge at the Soda Mountains alluvial fan. Carbon 14 is about 70 to 80 percent modern carbon (PMC) in the shallow piezometers at the site but is only about 48 PMC in the deeper production wells, and about 45 PMC at MC Spring. These data suggest a common source of recharge in the Soda Mountains along with the presence of a shallow unconfined aquifer located above a semi-confined aquifer. Water in the semi-confined aquifer provides water to the production wells at the site. Based on almost identical isotopic and hydrochemical signatures between the production wells and MC Spring, the semi-confined aquifer also is the source of discharge at MC Spring.