WATER SOURCES FOR SPRINGS IN SAGEHEN BASIN, SIERRA NEVADA RANGE, CALIFORNIA, DETERMINED BY CHEMICAL, ISOTOPIC, AND PHYSICAL PARAMETERS

Young spring waters Sagehen Basin (Sierra Nevada range, California) have heavy stable isotope composition, while older waters are isotopically light. This difference has been attributed to a combination of increased temperatures and changes in global atmospheric circulation patterns during the ~30 year period between the oldest and youngest recharge ages for the springs. If this explanation were correct, individual spring waters should become isotopically heavier over time. Instead, the vast majority of springs with >10 yr sampling range have become lighter over time.

Sagehen springwater δ¹⁸O is correlated to the degree of spring discharge variation (R² = 0.64). A possible control on discharge variation is the flow system supplying water to the spring. Springs sourced from a deep, long-path flow system could be less susceptible to discharge variation than springs sourced from a shallow, short-path flow system. This scenario is supported by discharge variation’s inverse correlation with both springwater age (R² = 0.51) and alkalinity (R² = 0.62), showing that springs with low discharge variation discharge older waters that have had more time to weather minerals in the subsurface; temperature data also suggest deeper circulation depths for low-discharge-variation spring waters. The link between δ¹⁸O and discharge variation could be present because the large volumes of water needed to develop high heads to push recharge into deep flow systems may only be available during snowmelt, which is isotopically distinct from warm-season precipitation.

A weak inverse correlation (R² = 0.45) exists between δ¹⁸O range and mean δ¹⁸O value for a spring. Because light waters tend to discharge from springs with low discharge variations, an explanation for this relationship is that low-discharge-variation springs discharge a mixture of water from both deep and shallow flow systems. As the deep/shallow source ratio varies, the range in isotopic composition would increase; in contrast, high-discharge variation springs may only discharge water from shallow flow systems, minimizing the isotopic variation.

Sagehen spring waters appear to have their isotopic signatures influenced by mixing between waters from deep flow systems recharged by snowmelt and shallow flow systems recharged by both cool- and warm-season precipitation.