ISOTOPE HYDROLOGY OF VOLUMINOUS COLD SPRINGS IN FRACTURED ROCK FROM AN ACTIVE VOLCANIC REGION, NORTHEASTERN CALIFORNIA

The more than 1550 km(2) (600 mi(2)) Hat Creek Basin in northeastern C alifornia is host to several first magnitude cold springs that emanate from Quaternary basaltic rocks with individual discharge rates rangin g from 1.7 to 8.5 m(3) s(-1) (60-300 ft(3) s(-1)). Stable isotope (del ta(18)O, delta D, delta(13)C) and C-14 measurements of surface and gro undwater samples were used to identify recharge areas, and to evaluate aquifer residence times and flow paths. Recharge locations were const rained from the regional decrement in meteoric water delta(18)O values as a function of elevation, determined to be -0.23 parts per thousand per 100 m for small springs and creek waters collected along the west ern Cascade slope of this region. In general, the large-volume springs are lower in delta(18)O than surrounding meteoric waters, and are inf erred to originate in high-elevation, high-precipitation regions up to 50 km away from their discharge points. Large spring C-14 abundances range from 99 to 41% modern carbon (pmc), and most show evidence of in teraction with three distinct carbon isotope reservoirs. These reservo irs are tentatively identified as (1) soil CO2 gas equilibrated under open system conditions with groundwater in the recharge zone [delta(13 )C(DIC) approximate to -18 parts per thousand, C-14 greater than or eq ual to 100 pmc], (2) dissolved carbon equilibrated with atmospheric CO 2 gas [delta(13)C(DIC) approximate to +1 parts per thousand, C-14 grea ter than or equal to 100 pmc], and (3) dissolved carbon derived from v olcanic CO2 gas emissions [delta(13)C(DIC) approximate to 0 parts per thousand, C-14=0 pmc]. Many regional waters show a decrease in C-14 ab undance with increasing delta(13)C values, a pattern indicative of int eraction with dead carbon originating from volcanic CO2 gas. Several l ines of evidence suggest that actual groundwater residence times are t oo short (less than or equal to 200 years) to apply radiocarbon dating corrections. In particular, water temperatures measured at springs sh ow that deep groundwater circulation does not occur, which implies an insufficient aquifer volume to account for both the high discharge rat es and long residence times suggested by C-14 apparent ages. The large springs also exhibit rapid decreases in flow during periods of drough t that suggests a high level of aquifer interconnectivity to the recha rge area. The estimated amount of volcanic CO2 dissolved in surface an d groundwater originating from the Lassen highlands is consistent with the conversion of approximately 10% of the geothermal CO2 flux into d issolved inorganic carbon.