RADIUM ISOTOPES AS TRACERS OF FRESH GROUNDWATER SEEPAGE INTO LAKES OF VARIOUS SALINITIES: THE NEBRASKA SAND HILLS AS A “NATURAL EXPERIMENT”

Radium isotopes, whose half lives span from 3.6 days to 1600 years (²²⁴Ra – 3.66 days; ²²³Ra – 11.43 days; ²²⁸Ra – 5.76 years; ²²⁶Ra – 1600 years), have been traditionally used to quantify fresh groundwater seepage into saline coastal waters. Here we explore the use of radium isotopes as tracers for groundwater seepage in a variety of salinity interfaces, where mostly fresh groundwater enter a range of fresh to hypersaline lakes in the Nebraska Sand Hills. While applying Ra as a tracer allows calculating the groundwater seepage rate into various lakes in the Nebraska Sand Hills, the different salinity interfaces allows a unique opportunity to examine and demonstrate the effects of salinity on Ra geochemistry.

11 groundwater samples were collected to represent the regional groundwater component, as well as samples of two fresh water lakes and three saline lakes. The average total dissolved solids content, ²²⁶Ra activity and the ratio of ²²⁶Ra:Cl in the groundwater were 311mg/L, 0.205 dpm/L, and 0.053, respectively. In the fresh water and saline lakes the TDS ranged between 550-840 mg/L, and 16,300-100,170 mg/L, respectively. The ²²⁶Ra activity and the ratio of ²²⁶Ra:Cl ranged between 0.158-0.211 dpm/L and 0.007-0.012 in the fresh water lakes, and between 0.133-1.656 dpm/L and 0.0004-0.0009 in the saline lakes.

The higher ²²⁶Ra:Cl ratio in the groundwater as compared to the surface water fed by those groundwater is typical, though different mechanisms control radium removal at different salinities: in the low salinity lakes Ra is preferentially removed through adsorption to suspended particles and mineral surface, while in saline lakes the preferential Ra removal is through co-precipitation with BaSO₄.

Since Ra mostly originates from water-rock interactions in aquifers, its activity in groundwater tends to be higher than in a lake fed by those groundwater. Here a unique situation occurs, where the radium activity in the lakes is similar to or higher than the activity in the groundwater. This is likely the result of the extensive regional evaporation rate. Independently solving a radium mass balance equation for each of the four Ra isotopes has the potential to allow an independent assessment of that evaporation rate.