ENVIRONMENTAL TRACERS IN MOUNTAIN GROUND WATER: BLUE RIDGE OF VIRGINIA AND SANDIA MOUNTAIN VICINITY, NEW MEXICO

Limited ground-water supplies occur in residuum and colluvium (0-24 m) that overlie metabasalts and granodiorites, and in fractures in the crystalline rocks in Shenandoah National Park (SNP), within the Blue Ridge Mountains of Virginia.  Fracture density in the crystalline rocks decreases with depth and few wells yield water from depths greater than 90 m.  CFCs, SF₆, ³H, ³H/³He,¹⁸O, N₂, and Ar indicate ground water of the Blue Ridge Mountains is of shallow, meteoric origin with ages of water from springs typically 0-3 years based on SF₆ and ³H/³He, and a median residence time of 5 years (range 0 to 10 years) estimated from seasonal variations in ¹⁸O data. The young fraction in ground-water mixtures from wells (30-200 m) is generally older (0 to 25 yrs) than water from springs. One water-supply well in fractured metabasalts in SNP has only two water-bearing zones, both intercepting the well bore near the bottom of the well (198 m). The CFC, SF₆, and ³H/³He data indicate that the sample is a mixture of about 47% young water with an average age of 10-11 years mixed with old pre-tracer water, indicating that circulation of young water reaches depths of nearly 200 m. The flushing rates of mobile atmospheric constituents through ground water to streams draining the higher elevations of SNP average less than 3 years in base-flow conditions.  Environmental tracers (³H, CFCs) were detected in mountain springs and shallow ground water in alluvium adjacent to mountains in the semiarid Middle Rio Grande Basin (MRGB), New Mexico.  Pre-tracer ground water of mountain origin was recognized based on chemical and stable isotopic composition.  At Albuquerque, there is a sharp boundary between ²H-depleted water of Rio Grande origin and relatively enriched water of mountain-front origin. Ground water with the isotopic and chemical signature of mountain-front water was mapped along the eastern margin of the basin and observed to depths of as much as 300 m below the water table; radiocarbon ages reach 20 ka.  Age gradients along the basin margin at Albuquerque, adjacent to the Sandia Mountains, indicate an average mountain-front recharge rate over the past 10 ka of about 3 cm/yr. Over the past 20 ka, there has been appreciably less recharge to the MRGB from the eastern mountain front both north and south of Albuquerque.