CONSTRAINTS ON THE DEPTH OF ACTIVE GROUNDWATER CIRCULATION IN A SEDIMENTARY-ROCK MOUNTAIN BLOCK PROVIDED BY TEMPERATURE, CHEMISTRY, AND AGE DATA

The depth of active groundwater circulation is a primary hydrogeologic variable governing groundwater flow and solute transport within a mountain block but is rarely well constrained. Characteristic circulation depths in sedimentary-rock mountain blocks are particularly uncertain because the small number of studies successfully limiting circulation depth have been performed mainly in crystalline rocks, and sedimentary-rock sequences are typically more heterogeneous and anisotropic. Two relatively deep (46 and 81 m) boreholes and multiple shallow piezometers were drilled in Redwell Basin in the Elk Mountains, CO, as part of a study aimed at better understanding processes controlling the export of metals from mineralized mountain headwater catchments. The mountain block is composed of sub-horizontally bedded sandstones and shales variably hydrothermally altered by Tertiary intrusions. Packer tests were performed on 10-m intervals while drilling, geophysical logs were collected, and boreholes were completed as multi-level monitoring wells. The packer tests indicated zones with typical active-zone K values (10^-7 to 10^-6 m/s) persisting to the bottom of each hole. However, temperature logging in the higher-elevation recharge-zone borehole indicated a nearly linear (conductive) thermal profile below a depth of 20 m with a gradient of 38^oC/km, which theoretically can only be maintained at low vertical groundwater flow velocities (q < 1 cm/yr). The chemistry of deeper groundwater (below 20 m) is substantially different from that of shallow groundwater: pH is 7-8 versus 4-5; specific conductance is 400-600 versus 100-300 μS/cm; and concentrations of multiple constituents (e.g., Ca, Fe) differ by a factor >5. Tritium and He isotope data indicate that the deeper groundwater is dominantly >60 yr old (<0.3 to 2 TU), whereas shallow water is modern (6-8 TU) with apparent ³H/³He ages of 5-15 yr. The deeper water also has high terrigenic He concentrations of 4-8 times solubility, typical for groundwater 100s to 1000s of years old. Available temperature, chemistry, and age data therefore all suggest that, while horizontal K may be high, vertical K is likely low, apparently resulting in a shallow active circulation depth of only about 20 m.