Hydrogeology of Ash Flow Tuff: A Preliminary Statement

Ash flow tuffs, silicic pyroclastic rocks commonly mapped as silicic lava flows (rhyolite, dacite, or quartz latite) on many pre-1960 geologic maps, cover tens of thousands of square miles in the southwestern states and aggregate between 35,000 and 60,000 mi³ in the Great basin alone. In contrast to silicic lava flows but similar to basalts, ash flow tuffs are sheetlike bodies; individual flows have been traced for as much as 100 miles. Welding, the cohesion of molten glassy shards and pumice fragments during emplacement of these rocks, results in significant vertical variations in physical properties within individual flows. Interstitial porosity and permeability vary from 70% and 2 gpd/ft² (gallons per day per square foot), respectively, in nonwelded portions or zones of ash flows to less than 5% porosity and virtually zero permeability in densely welded zones. Primary and secondary joints spaced several feet apart in nonwelded zones are as close as a fraction of an inch in densely welded zones. Fracture transmissibility of welded zones within ash flow tuffs varies from 100-100,000 gpd/ft. The nonwelded zones are aquitards. Spring groups with discharges ranging from 20-200 cfs (cubic feet per second) emerge from welded tuff in eastern Idaho and Sumatra, Indonesia. The magnitude of the spring discharge attests to the regional hydraulic continuity of these strata. Development of groundwater from welded tuff in the Southwest appears several decades away because of depth of burial, large variations in yield, and locally poor water quality. Nevertheless, further study of these rocks is warranted to test the assumption common to many water budget studies that interbasin movement through and `mountain front recharge' from silicic volcanic rocks are negligible.