DOWNSTREAM EVOLUTION OF WATER QUALITY AND QUANTITY IN MOUNTAIN MARBLE KARST AQUIFERS

Understanding natural dynamics of water quantity and quality as water moves through karst systems is essential to manage these resources. Without an understanding of storage properties, water sources, and discharge parameters of the system, it is difficult, at best, to determine how these systems will react to environmental change. The highly karstified nature of marble within Sequoia and Kings Canyon National Parks provide opportunities to document the change in water chemistry along a transect from the upstream sinks through multiple sections of a karst aquifer to the downstream spring of the system. This allows for quantification of the amount and location of diffuse flow into conduit systems from primary and secondary porosity of the aquifer. Two karst systems within these parks were sampled to document downstream variations in water quantity and quality: the Tufa Springs system provided an example of alpine karst fed primarily by seasonal snowmelt and the Big Springs system provided insight into a mid-elevation system fed both by snowmelt and rainfall.

In both systems discharge increased as infeeders and diffuse flow entered the conduits. The chemical variations provided a more complex view of the systems. Specific conductance, temperature, pH, and dissolved ion concentrations did not necessarily change relative to the length of conduit travelled. Instead these changes appear to be related to input of water from diffuse sources. In the Tufa Karst system this is seen by comparing two aquifer segments. In the upstream most aquifer segment, conductivity increases from 8 mS/cm to 20 mS/cm along a length of approximately 1000 m. This segment has only a small increase in discharge, indicating minimal diffuse input. The next segment shows a conductivity increase from 21mS/cm to 70mS/cm over a length of 1800m. Sinking stream input into the downstream spring of this segment only accounts for 65% of the total discharge of the spring, indicating a significant diffuse input into the system. A similar pattern is seen in the Big Springs karst, where water enters the system with a conductivity of 35mS/cm and exits the system with a conductivity of 136mS/cm with a similar amount of water entering the conduit as diffuse flow.