Sources and residence times of water near Mount Shasta determined by tracers and evapotranspiration analysis

Large-volume springs are a significant source of water to communities near Mount Shasta and form the headwaters of the Sacramento River. Aquifers in this region are developed in young volcanic formations and are not well characterized, which makes predicting the impact of drought and climate change on water flow from springs difficult. It is important to understand how the aquifers in this region will react to future droughts and climate change because these events will likely have a significant impact on the amount of water available in the future. To understand the water resources and the hydrology of the region and to better constrain the age of water produced by springs, water was sampled from wells, springs, and streams. Samples were analyzed with a suite of geochemical and isotopic traces to determine the apparent age (residence time) of groundwater over a broad age distribution, from less than one year to tens of thousands of years. Isotopic tracers were applied because of the limited number of sampling points over a large area, leaving traditional hydrogeologic methods such as water levels and pump tests inadequate for a regional study. These tracers help delineate groundwater flow and identify recharge areas, which can help determine vulnerability to drought and climate change. Most samples did not have detections of tracers with short half-lives (5 yrs), such as sulfur-35 (87.5 day half-life) and sodium-22 (2.6 year half-life). Only two non-snow samples had detections of sulfur-35, while there were no detections of sodium-22, tested on a limited subset of samples. Results of tritium analysis (12.3 year half-life) reveal sample ages ranging from less than one year to greater than 60 years; the younger samples were from high elevation springs, while the older samples were from low elevation wells and springs. Of the five samples analyzed for krypton-85 (10.8 year half-life), the ages range from four to 21 years, with one well sample greater than 49 years. The carbon-14 results show a range of 47 to 107 percent modern carbon. Stable isotope results plot close to the global meteoric water line. Recharge elevations were estimated by comparison of 18O results with a lapse-rate line that indicates a -0.23‰ change per 100 m elevation gain (Rose et al., 1996). Results show recharge elevations that range from 2,000 to 2,800 m, indicating a 1,000 to 1,800 m horizontal travel distance from the recharge to discharge areas. Noble gas recharge temperatures range from 0.5 to 8.1°C based on analysis on a VG-5400 Noble Gas Mass Spectrometer. The recharge elevations based on noble gas recharge temperature analysis show a similar range, from 1,600 to 2,900 m, corresponding to a range of 600 to 1,900 m travel distance. Half of all samples analyzed have apparent ages older than 12 years. Sample locations that contain water less than 12 years old are more likely to be affected by future droughts and climate change because they receive most water from recent precipitation. If this precipitation decreases or is removed from the system by increased evapotranspiration, recharge could decrease significantly. Analysis of National Land Cover Database data reveals that evapotranspiration averages about 642 mm/year in the study area, on the southwest portion of the mountain. This relatively low value is due to the amount of reflective surfaces, such as snow and glaciers, as well as the bare exposed rock on the mountain. Based on PRISM (Parameter-elevation Regressions on Independent Slopes Model) data, this portion of the mountain receives about 1,352 mm/year of precipitation, which provides an infiltration rate of 710 mm/year, or 52% infiltration, if runoff is negligibly small. This is a large amount of infiltration, which indicates that a significant amount of water exists within the subsurface. Runoff is minimal because the permeability of the ground surface is high due to the presence of porous volcanic rocks and because most of the recharge is from snowmelt and because areas of snow have low evapotranspiration. A key elevation range for recharge on the mountain occurs between 2,000 to 2,800 m, as determined by noble gas and stable isotope analysis. With future climate change, the treeline will move to higher elevations, decreasing areas covered by snow, which thereby increases evapotranspiration and decreases water infiltration and recharge. In addition, climate change will cause peak snowmelt to occur earlier in spring, likely decreasing late season baseflow at high elevation springs where subsurface residence times are short.