A Radiocarbon Method and Multi-Tracer Approach to Quantifying Groundwater Discharge to Coastal Waters

Carolyn Gramling, Ph.D., 2003
Dan McCorkle, Advisor

Groundwater discharge is important to the transport of dissolved chemical species to coastal waters. D14C was developed as a tracer of one component of total discharge: fresh fluxes from confined aquifers during sampling from November 1999 to April 2002 in two small estuaries in North Carolina. In this region, fresh artesian discharge has a carbonate-derived low-D14C signature. Mixing models were used to evaluate the inputs from potential sources of DIC-D14C to each estuary, including seawater, springs, streams, and respiration DIC. These calculations showed that artesian discharge generally dominated the total fresh water input to these estuaries.

A comparison of these D14C-based SGD estimates with estimates derived from radium isotopes and 222Rn showed how these tracers describe different components of the total SGD. The fluxes of low-D14C and of 222Rn were dominated by artesian discharge. Estuarine 226Ra showed artesian influence, but also reflected the salty SGD processes that controlled the other three radium isotopes. 228Ra fluxes seemed to reflect seepage from the surficial aquifer as well as seawater recirculation through estuarine sediments. 224Ra and 223Ra fluxes were dominated by seawater recirculation through salt marsh sediments. This multi-tracer approach provides a comprehensive assessment of the various components contributing to the total SGD.