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The Pah Tempe hot springs discharge ~260 L/s of water at ~40 °C into the Virgin River in the footwall damage zone of the Hurricane fault at Timpoweap Canyon, near Hurricane, Utah, USA. Although these are Na-Cl waters, they actively discharge CO 2 gas and contain signifi cant quantities of CO 2 (~34.6 mmol/kg), predominantly as H 2 CO 3 and HCO 3 -. Because of excellent exposures, Pah Tempe provides an exceptional opportunity to observe the effects of enhanced fracture permeability in an active extensional fault.Pah Tempe waters have been deeply circulated (>5 km; >150 °C) into basement rock as illustrated by the clear water-rock exchange of oxygen isotopes. Waters were probably recharged under colder climate conditions than present and therefore have a prolonged subsurface residence. Discharge of both water and gas in the springs correlates to the density of fractures in carbonate rocks above stream level. This observation suggests that clusters of high fracture density in the faultdamage zone act as pathways from the likely regional aquifer, the eolian Queantoweap Sandstone, through the overlying confi ning unit, the gypsiferous silty Seligman Member of the Kaibab Formation.Mass-balance modeling suggests that the majority of CO 2 discharge is the product of the quantitative dissolution of CO 2 gas at depth within the fault zone. Upon discharge, most of the carbon is released to the surface as dissolved species. It appears that the subsurface production rate of CO 2 is relatively low because Pah Tempe waters are grossly undersaturated in CO 2 at inferred minimum circulation depths and temperatures. Geological and geochemical data also suggest that the CO 2 is dominated by a crustal component complemented by minor mantle contributions.

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We present a new conceptual model of groundwater flow that describes active and inactive groundwater flow regimes. The model is based on an analysis of interactions between surface water and shallow and deep groundwater in the 240-km-long Wasatch Range and Book Cliffs, Utah, USA. Active zone groundwater flow paths are continuous, responsive to annual recharge and climatic variability, and have groundwater resident times ''ages'' that become progressively older from recharge to discharge area. Active zone groundwater systems discharge at thousands of springs that issue from the 700؉-m-thick, gently dipping, clastic bedrock formations. Springs waters contain appreciable 3 H and anthropogenic 14 C.In contrast, inactive zone groundwater has extremely limited or no communication † E-mail: alanmayo@byu.edu. with annual recharge and has groundwater mean residence times that do not progressively lengthen along the flow path. Groundwater in the inactive zone may be partitioned, occur as discrete bodies, and may occur in hydraulically isolated regions that do not have hydraulic communication with each other. Inactive zone groundwater is encountered in-mines (coal-mines 300-700 m below ground surface) where groundwater discharge rates decline rapidly and the waters have ␦ 2 H and ␦ 18 O compositions that are distinguishable from near surface groundwater. In general, deep waters have no 3 H and have mean 14 C residence times of 500 to 20,000 yr (45.9 to 4.9 pmc). Chemical evolution modeling, porosity-permeability core plug analysis, and inmine hydrographs also indicate hydraulic partitioning.

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Shoreline carbonate deposits of Pleistocene Lake Bonneville record the conditions and processes within the lake, including the evaporative balance as well as vertical and lateral chemical and isotopic gradients. Tufas (swash-zone) and tufaglomerates (cemented, subaqueous colluvium or beachrock) on multiple, well-developed shorelines near the Silver Island Range, Utah, also present an opportunity to examine physicochemical lake processes through time. Three shorelines are represented by carbonate deposits, including the 23-20 ka Stansbury stage, 15-14.5 ka Bonneville stage, and 14.5-14 ka Provo stage.Mean 18 O VSMOW values of all three shorelines are statistically indistinguishable ( $ 27 AE 1%), when a few Bonneville samples of unusual composition are neglected. However, differences in primary carbonate mineralogy indicate that the correspondence is an artefact of the different fractionation factors between calcite or aragonite and water. Second, in order to sustain a much smaller, shallower lake during the colder Stansbury stage, the climate must have also been relatively dry. Third, 18 O values in tufa are higher than tufaglomerate by $ 0.5%, consistent with greater evaporative enrichment of lake water in the swash zone. Fourth, mean 13 C values for the Provo, Stansbury and Bonneville shorelines (4.4, 5.0 and 5.2%, respectively) show that carbon species were dominated by atmospheric exchange, with the variations produced by differences in the oxidation of organic matter.Comparisons of shoreline carbonates with deep-lake marls of the same approximate age indicate that shoreline carbonate was much higher in 13 C and 18 O values (both $ 2.5%) during Bonneville time, whereas isotopic differences were minor (both $ 1%) in Stansbury time. In particular, the Bonneville stage may have sustained large vertical or lateral isotopic gradients due to evaporative enrichment effects on 18 O values. In contrast, the lake during the much shallower Stansbury stage may have been well mixed.Differences in the primary mineralogy (Stansbury and Bonneville, aragonite >calcite; Provo, calcite >aragonite) reflect profound differences in lake chemistry in terms of open versus closed-basin lakes. The establishment of a continuous outlet during Provo time probably reduced the Mg 2þ /Ca 2þ ratio of lake water. Curiously, regardless of primary mineralogy, tufaglomerate cements are enriched in Na þ and Cl À and depleted in Mg 2þ relative to capping tufa of the same age. This probably reflects vital or kinetic effects in the swash zone (tufa). We suspect that 'abiotic' effects may have been important in the dark pore space of developing tufaglomerate, where the absence of light suppressed photosynthesis.