3 Springs and Groundwaters of the Intermountain West

Wednesday, 17 May 2006
8:00 AM - 11:40 AM

CO2 DEGASSING IN HIGH VOLUME SPRINGS IN THE SOUTHERN COLORADO PLATEAU REGION—UNDERSTANDING DEEP INPUTS AND GEOCHEMICAL MIXING IN REGIONAL GROUNDWATER

Paper number: 3-4
Program: THEME SESSIONS
Time: 9:00 AM – 9:20 AM

Authors

Abstract

A synthesis of the water and gas chemistry of springs in the southern Colorado Plateau- Arizona Transition zone region, coupled with groundwater modeling, shows that groundwater represents a heterogeneous mixing of: 1) surface (epigenic) recharge, 2) slow moving aquifer waters that react with the limestone karst system, and 3) deeply sourced (endogenic) fluid inputs. We concentrate on travertine–depositing CO2-rich springs that are discharging in Grand Canyon and the Arizona Transition Zone, tens to 100 km from recharge areas in the San Francisco Mountains. About 60% of recharge water flows north and northeast and emerges in the incised aquifer system at Grand Canyon. Most of this water emerges through two karst springs (Blue Spring and Havasu Spring) to form the base flow for the Little Colorado River and Havasu Creek, respectively. Flow to the south (~40% of recharge) emerges along a series of springs that are aligned along NW-striking faults and that form the base flow for the Verde River.

Geochemical modeling of major ion composition of regional springs indicates that considerable ‘external' carbon is present (up to 0.044 mol/L). External carbon is computed as the bicarbonate alkalinity minus the calcium and magnesium contents, with an adjustment for possible gypsum contribution to the calcium term. Carbon isotope values (ä13C) of the DIC and CO2 range from -20 to -4.9 PDB. The inorganic carbon derives from a mixture of: 1) biogenic soil gas, 2) dissolution of limestone and 3) deep sources. The deeply-derived component coupled with estimates of the spring discharges is used to calculate CO2 fluxes. These analyses are consistent with previously-reported helium isotope values (0.2 to 1.1 RA) in spring gases from the same region also showing that a component of the deeply sourced gas is derived from the mantle asthenosphere. Contributions of CO2 from both the crust and mantle lithosphere, possibly related to the flux of volatiles that have resided in the lithosphere since Laramide time, may also be significant. This approach represents our initial attempts to quantify CO2 flux due to distributed mantle devolatilization that is taking place in travertine-depositing springs in the southwestern U.S.