THERMAL TRACING OF GROUNDWATER FLOW IN MOUNTAINOUS AREAS

Temperature profiles (TP) measured in Tahoe Basin wells are presented. TP shapes characteristic of mountain areas are associated with specific patterns and rates (Fw) of groundwater flow, closely matching TP simulated by heat and groundwater flow models.

Several TP measured in upland Tahoe bedrock wells are presented. Dynamical scaling and conceptual modeling show the vertical T gradient (Tz) is highly sensitive to underlying sub-lateral Fw. Measurements at depth are not required for reliable inferences on Fw to/from depth; TP data from shallow wells suffice. TP curvature due to areal distributed flow in fracture networks is distinct from TP curvature above flow in discrete fracture/fault zones. A novel analytic approximation is introduced, showing Tz alteration in dense rock above a fracture/fault zone (at all depths up to near ground surface) is determined mainly by dip angle and total Fw.

In Tahoe valley basin-fill sediments, large Tz or inverted TP are present at many sites. Anomalously large Tz near a well bottom indicates an advective heat source below the well; in some areas attributable to slow Fw upward in underlying granitic bedrock. Many T inversions are large and/or deep; due to rapid lateral Fw sourced from cool recharge. A plausible source of abundant cool recharge is seasonal snowmelt focused downward at bedrock/basin-fill margins near the base of mountain-fronts. We introduce a novel macroscopic energy balance model that yields robust bounds on Fw of cool mountain-front recharge, given T data from one or more valley wells and estimates of mean annual surface T (Ts).

We propose that accurate knowledge of major spatial Ts variations and of recharge T (Tr) enables significant reduction of uncertainty in thermal tracing of groundwater flow, and that such accuracy is readily achievable. Data from a soil T monitoring network (>80 sites) in the southern Tahoe Basin exhibits spatial variation in Ts > 5C between sites at the same elevation. Observed Ts elevation lapse < air T lapse, due to identified mechanisms. A novel ground surface energy balance model formulation closely fits observed inter-site Ts variations. At sites of episodic or seasonal (e.g. snowmelt) spatially focused recharge, Tr can differ from site Ts. Results for Ts & Tr are pertinent to inference of recharge elevation using inert dissolved gas levels.