| Abstract |
Data from the U.S. Geological Survey’s National Water Information System (NWIS) database reveal the existence of a number of thermally anomalous areas in the eastern Snake River Plain (ESRP) aquifer, most of them near its margins, whose geochemistry conclusively shows that thermal waters issuing from the hot rhyolitic rocks beneath the ESRP basalts inject heat as well as solute mass into the overlying aquifer. Thermal waters that issue from hot felsic basement in southern Idaho are Na-HCO3 type due to water-rock reactions at elevated temperature and are characteristically depleted in Ca and Mg, typically low in Cl (depending on the source water that reacts), and high in pH, Na, HCO3, SiO2, Li and B (as well as F, if diluted by low-Ca cold waters). These diagnostic tracers are seen in shallow thermal wells (≤44 oC) in late Neogene rhyolites of the Newdale thermal area and in thermal waters (≤57 oC) sampled from INEL-1, a 3.2 km-deep borehole in rhyodacite and welded tuff on the Idaho National Lab (INL). A correlation between SiO2 and Na/Ca molar ratio observed in the Newdale thermal area and dilute, low-temperature mixtures in the ESRP aquifer adjacent to the Newdale area, as well as in warm ground waters of the INL appears to be a sensitive indicator of rhyolite-labeled water where it mixes with ESRP ground water. On the INL, contamination from anthropogenic sources as well as recharge of both cold tributary surface and ground waters along the aquifer’s northern margin renders geochemical tracing of thermal water ineffective in the area of the proposed FORGE test site. The clearest evidence of thermal water entering the aquifer through its base is seen 10-20 km to the southeast, where previously identified thermal and geochemical anomalies are characterized by elevated Na, SiO2, Li, B and slight enrichments in F, as well as high dissolved He concentrations. An estimate of the advective flux of thermal water into the ESRP aquifer in this area was derived from a two-component mixing model. Based on published aquifer flow rate and porosity information, however, the magnitude of the thermal flux so estimated is incompatible with core-scale data on the hydraulic conductivity and vertical gradient observed in INEL-1’s rhyolite. This suggests that advective transport of heat out of the rhyolite is focused along localized preferential flow paths, an interpretation that is consistent with observations of thermal fluid movement in INEL-1’s fractured rhyolite, and which supports the hypothesis that rhyolitic basement in this area hosts preferential flow of thermal water. These findings are incorporated in a geohydrologic conceptual model of the proposed FORGE test site on the INL. |