USING 87SR/86SR TO IDENTIFY ROCK UNITS THAT SUPPORT GROUNDWATER FLOWPATHS AND FLOWPATH CONNECTIVITY IN DEGLACIATING ALPINE WATERSHEDS.

Alpine glacier meltwater is an important source of recharge supporting groundwater flow processes in the high mountains. In the face of rapid ice loss, knowledge of residence times and response times of mountain aquifers to loss of glacial ice are critical in evaluating the sustainability of alpine water resources for human communities and ecosystems. An important step toward addressing this knowledge gap is to identify the rock units that host flowpaths and how these flowpaths are connected across spatial scales. Here, I use strontium isotopes (⁸⁷Sr/⁸⁶Sr) and geochemical tracers to identify the rock units that host groundwater flowpaths and examine how the flowpaths are connected across spatial scales in Glacier National Park (GNP) and Mount Hood National Forest (MH). MH is comprised of mostly young, reworked volcanic rocks originating from three eruptions in the past 2 Ma. In comparison, GNP has complex geology where older (Precambrian) rock units are thrust over younger (Cretaceous) rocks. The springs in MH show very low variability in ⁸⁷Sr/⁸⁶Sr across spatial scales and aspects (compass direction of slopes) of the mountain. This is not surprising given the low variability in ⁸⁷Sr/⁸⁶Sr in the young volcanic rock. The springs in GNP show greater variability in ⁸⁷Sr/⁸⁶Sr. High-elevation springs are supported by groundwater flow through older sedimentary bedrock units. Springs with relatively low ⁸⁷Sr/⁸⁶Sr represent waters that flow along or through a young volcanic sill that crosscuts the strata. Finally, springs flowing from the alluvium have ⁸⁷Sr/⁸⁶Sr ratios that are intermediate between the two other groups showing a complicated flowpath history. Nearly all the springs that were sampled in GNP emerge on south-facing slopes. This is not an indication of ice preservation, instead it’s controlled by hydrostratigraphy. It’s unlikely that high-elevation groundwater is strongly connected to low-elevation sites due to hydrostratigraphy. There are more springs on south-facing slopes at MH as well; however, they do not preserve an isotopic signature of recharge from glacial meltwater. Springs on north-facing slopes do preserve the signature. Groundwater flowpaths are connected across scales at MH; low-elevation warm springs show an isotopic signature of glacial meltwater. GNP and MH have different hydrogeological characteristics and show different responses to ice loss.