Tracers Reveal Recharge Elevations, Groundwater Flow Paths and Travel Times on Mount Shasta, California

Elizabeth Peters,Ate Visser,Bradley Esser,Jean Moran

doi:10.3390/w10020097

Elizabeth Peters, Ate Visser + Show 2 more

Open Access

https://doi.org/10.3390/w10020097

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Abstract

Mount Shasta (4322 m) is famous for its spring water. Water for municipal, domestic and industrial use is obtained from local springs and wells, fed by annual snow melt and sustained perennially by the groundwater flow system. We examined geochemical and isotopic tracers in samples from wells and springs on Mount Shasta, at the headwaters of the Sacramento River, in order to better understand the hydrologic system. The topographic relief in the study area imparts robust signatures of recharge elevation to both stable isotopes of the water molecule (δ18O and δD) and to dissolved noble gases, offering tools to identify recharge areas and delineate groundwater flow paths. Recharge elevations determined using stable isotopes and noble gas recharge temperatures are in close agreement and indicate that most snowmelt infiltrates at elevations between 2000 m and 2900 m, which coincides with areas of thin soils and barren land cover. Large springs in Mt Shasta City discharge at an elevation more than 1600 m lower. High elevation springs (>2000 m) yield very young water (<2 years) while lower elevation wells (1000–1500 m) produce water with a residence time ranging from 6 years to over 60 years, based on observed tritium activities. Upslope movement of the tree line in the identified recharge elevation range due to a warming climate is likely to decrease infiltration and recharge, which will decrease spring discharge and production at wells, albeit with a time lag dependent upon the length of groundwater flow paths.

Highlights

A warming climate will bring drastic changes to hydrologic systems in the headwater basins of the major rivers in California
Examination of the location and rate of recharge and delineation of groundwater flow in high elevation hydrologic systems is critically important because the warming climate will likely have a significant effect on the timing, amount and form of precipitation and on the amount of evapotranspiration (ET) over an elevation range that is important for recharge
The two independent tracer approaches to estimating the recharge elevation, δ18 O and fitting the noble gas recharge temperature to the atmospheric lapse rate, agree remarkably well for the majority of the samples (Figure 10)

Summary

Introduction

A warming climate will bring drastic changes to hydrologic systems in the headwater basins of the major rivers in California. Runoff in these rivers fills the reservoirs that sustain cities and agriculture through the dry months, while cool, late season groundwater discharge to streams is critical for sustaining subalpine ecosystems and fish habitat. Prediction of upslope or downslope movement of the tree line and related changes to evapotranspiration (ET) is more complex, as the combination of higher temperatures and changes in precipitation, a longer growing season and higher atmospheric CO2 drive ET and soil water storage [8,9]

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