A NEW CONCEPTUAL MODEL: GROUNDWATER FLOW IN STRATIFIED MOUNTAINOUS TERRAIN

We have developed a new conceptual model of groundwater flow for mountainous terrain. The model describes groundwater flow regimes in terms of active and inactive groundwater flow regimes. Although the model is based on an analysis of interactions between surface-water and shallow and deep groundwater in the 240 km long Wasatch Range and Book Cliffs, Utah, USA, it has applicability in other mountainous regions. Active zone groundwater flow paths are continuous, responsive to annual recharge and climatic variability, and have groundwater resident times “ages” that become progressively older from recharge to discharge area. In the study area active zone groundwater systems discharge at thousands of springs that issue from the 700+ m thick, gently dipping, clastic bedrock formations. Springs waters contain appreciable ³H and anthropogenic ¹⁴C.

In contrast, inactive zone groundwater has extremely limited or no communication with annual recharge, and has groundwater mean residence times that do not progressively lengthen along the flow path. Groundwater in the inactive zone may be partitioned, occur as discrete bodies, and may occur in hydraulically isolated regions that do not have hydraulic communication with each other. In the study area inactive zone groundwater is encountered in-mines (coal-mines 300-700 m bgs) where groundwater discharge rates decline rapidly and the waters have δ²H and δ¹⁸O compositions that are distinguishable from near surface groundwater. In general, deep waters have no ³H and have mean ¹⁴C residence times of 500 to 20,000 years (45.9 to 4.9 pmc). Chemical evolution modeling, porosity-permeability core plug analysis, and in-mine hydrographs also indicate hydraulic partitioning.