Performing a mass balance of radium isotopes is a commonly employed method for quantifying the flux of groundwater into the sea. However, the spatial variability of 224Ra can compromise the results of mass balances in environmental studies. We address this uncertainty by optimizing the distribution of Ra samples within a surface survey of 224Ra activity in the Lesina Lagoon, Italy.

After checking for spatial dependence, location-allocation modeling (LAM) was utilized to determine optimal distribution of samples for thinning the sampling design. Trend surface analysis (TSA) was employed to interpolate the Ra activity throughout the lagoon. No significant change was found when using all 41 samples or only 25 randomly distributed samples.

Results from the TSA showed a linear trend and bi-modal distribution in surface 224Ra. This information was utilized to perform mass balances in two separate basins (east and west). SGD was found to be significantly higher in the western basin (4.8 vs. 0.7 cm d− 1). Additionally, mass balances were performed using the average 224Ra activity from the trend surface analysis calculated with 41 and 25 samples respectively and total lagoon SGD was found to be 10.4–10.5 m3 s− 1. Results show that SGD is significant in the Lesina Lagoon.