Abstract
The Folsom Lake Isotopic Model investigates the relationship between location, depth, and isotopic variation in Folsom Lake, California, through stable isotope analysis of liquid water. Folsom Lake is a reservoir roughly 20 miles northeast of Sacramento, California, and is fed by the North and South forks of the American River. Previous studies sampling freshwater lakes and reservoirs have assumed surface samples represent the isotopic average of the entire body. The goals of the project were to: 1) find and identify trends across the reservoir; 2) test whether conventional sampling methods provide an accurate isotopic average of the reservoir; and 3) determine if the ratio of source waters into the reservoir can be derived from samples of the main body of the reservoir. Preliminary results indicate fractionation is occurring linearly with depth, with a strong negative correlation present in the upper 10 meters. From the surface to a depth of approximately 45 meters, water isotope values become more depleted with depth. However, from 45 meters depth to the lake bottom, water isotope values become less negative, returning to values seen at 10 meters water depth. This inversion may be caused by thermal and water density gradients. Furthermore, a shore sample was not representative of the average isotope values of the reservoir, likely due to fractionation from evaporation in shallow water. However, a surface sample taken at the center of the lake proved more representative than expected and may be a viable way to quickly obtain a rough isotopic average for an inland lake or reservoir like Folsom Lake.