CHARACTERIZING GROUNDWATER HYDROLOGY IN HIGH GRADIENT TERRAINS UNDERLAIN BY COMPLEX GEOLOGY: FOCUS ON THE SAN JUAN MOUNTAINS OF SOUTHWESTERN COLORADO

The San Juan Mountains are an area of glaciated topography with extreme to moderate relief. Aquifer systems include Tertiary volcanic and volcaniclastic rocks and numerous calderas with associated ash flows that are flanked by Precambrian sedimentary and crystalline rocks as well as Paleozoic carbonate through Cenozoic clastic rocks. Extensive hydrothermal alteration and metal-rich mineralization has made parts of the region an historic target for the mining of base and precious metals. Major geologic structures include numerous fault zones, many related to caldera collapse processes, pervasive joints and veins. Tilting and various degrees of folding are also found locally. Surficial materials include glacial, alluvial, colluvial, and landslide deposits.

As is typical of mountainous regions, little is known about the storage and flow of groundwaters and how they interact with surface waters, aquatic ecosystems, and intermontane to basin aquifers. Hydraulic head data are virtually non-existent. Hydraulic properties of geologic materials and features over various scales and the extent to which they are dominated by porous media versus discrete fracture/fault behavior are poorly characterized. Little is know about driving forces such as extreme relief and heat flow anomalies. Infiltration and evapotranspiration, as well as circulation depths and residence times of groundwaters are also poorly constrained. The Animas Watershed is, however, an exception where modern to pre-modern residence times have been documented and in-stream tracer studies have revealed likely shallow groundwater interactions with complex mineralized systems that generate metal and acid loading.

There is a growing need for a better understanding of the hydrologic backbone regions of the intermountain west because they are the primary sources of water to surrounding regions. Future research should focus on development of viable conceptual models based on: 1) detailed geologic and hydrologic mapping; 2) aquifer property and temporal hydraulic head, geochemical, thermal, and environmental tracer data obtained by drilling, testing, and monitoring of representative wells; and 3) the use of numerical inverse modeling to test conceptualizations and quantify uncertainties to provide constraints for sustainable groundwater use.