RADIUM ISOTOPES AS NATURAL TRACERS OF GROUNDWATER SEEPAGE IN LOW SALINITY SYSTEMS

In spite of the non-conservative behaviour of radium in fresh water systems, here we present the radium isotope quartet as a tool for tracing fresh groundwater seepage into a fresh water river. We constructed a model based on systematic, repetitive measurements of ²²⁶Ra, ²²⁸Ra, ²²³Ra, and ²²⁴Ra in surface water, groundwater and porewater of a segment of the Neuse River near Garner, North Carolina. The same model was applied separately to each isotope, allowing for an analytical model solution. Since radium is radioactive and surface-reactive, our model accounts for radioactive decay, supply from recoil, and adsorption to mineral surface in both the river channel and the hyporheic zone. We use the ratio of Ra/Cl instead of the absolute Ra concentrations to minimize discreet differences in the measured Ra activities influenced by our selection of surface water sampling locations, and to allow for a regional, statistically viable representation of the groundwater end member. Initial model results indicate the groundwater flow rate to the river in the studied segment is 30-60×10⁶ L/day, less than 4% of the flux of water in the river. It also shows the travel time of groundwater through the hyporheic zone is 2-7days.

We compared our Ra-based calculations with results from a Sr-based model using the ⁸⁷Sr/⁸⁶Sr isotope ratio. Water samples for Sr and Ra analysis were collected simultaneously. We applied a similar model to the one used for radium, which yielded a groundwater flux of ~2.4×10⁶ L/day, less than 0.2% of the flux of water in the river. The similarity between the Ra- and Sr-based models suggests Ra has a great potential of becoming a viable tool for quantifying seepage in low salinity environments.

To examine the importance of this apparently minute groundwater seepage, we applied a mass-balance model to describe the nitrate budget in the river. Our calculation shows that groundwater are the primary source of nitrogen to the river. An increase in nitrate concentration in the local groundwater would thus have a direct effect on the river as a habitat. This stresses the important role groundwater seepage, even in the smallest amounts, can play in sustaining a river ecosystem.