210Pb and 137Cs as tracers of recent sedimentary processes in two water reservoirs in Cuba

Abstract

Hanabanilla and Paso Bonito Reservoirs are the main fresh water sources for about half a million inhabitants in central Cuba. Prior to this investigation precise information about the losses of storage capacity was not available. Sedimentation is the dominant process leading to reduction in water storage capacity. We investigated the sedimentation process in both reservoirs by analyzing environmental radionuclides (e.g. ²¹⁰Pb, ²²⁶Ra and ¹³⁷Cs) in sediment cores. In the shallow Paso Bonito Reservoir (mean depth of 6.5 m; water volume of 8 × 10⁶ m³), we estimated a mean mass accumulation rate (MAR) of 0.4 ± 0.1 g cm^-2y^-1 based on ²¹⁰Pb chronologies. ¹³⁷Cs was detected in the sediments, but due to the recent construction of this reservoir (1975), it was not possible to use it to validate the ²¹⁰Pb chronologies. The estimated MAR in this reservoir is higher than the typical values reported in similar shallow fresh water reservoirs worldwide. Our results highlight a significant loss of water storage capacity during the past 30 years. In the deeper and larger Hanabanilla Reservoir (mean depth of 15.5 m; water volume of 292 × 10⁶ m³), the MAR was investigated in three different sites of the reservoir. The mean MARs based on the ²¹⁰Pb chronologies varied between 0.15 and 0.24 g cm^-2y^-1. The MARs calculated based on the ¹³⁷Cs profiles further validated these values. We show that the sediment accumulation did not change significantly over the last 50 years. A simple empirical mixing and sedimentation model that assumes ¹³⁷Cs in the water originated from both, direct atmospheric fallout and the catchment area, was applied to interpret the ¹³⁷Cs depth profiles. The model consistently reproduced the measured ¹³⁷Cs profiles in the three cores (R² > 0.9). Mean residence times for ¹³⁷Cs in the water and in the catchment area of 1 y and 35-50 y, respectively were estimated. The model identified areas where the catchment component was higher, zones with higher erosion in the catchment, and sites where the fallout component was quantitatively recorded in the sediments.