## 1 Introduction  
Water erosion and the resulting sedimentation are natural processes caused by water, wind and ice. Some human activities such as deforestation, overgrazing, changes in land use, unsustainable farming practices and global climate change tend to accelerate land impoverishment. The result is landscape degradation, which has negative impacts on soil fertility, crop productivity, water, pollution, and potential effects on global climate, as well as sedimentation in lakes, reservoirs, etc. It is of great importance for sustainable land management to discriminate the main sediment sources in a watershed, thus identifying sites with high levels of erosion and water sedimentation.  
Within the framework of an International Atomic Energy Agency (IAEA) regional project [1], it was possible to apply for the first time the technique of Compound-Specific Stable Isotopes (CSSI) analyzing the fatty acids of the carbon chains present in the watershed area of the Panama Canal. This novel technique allows estimating sediment load contributions associated with different land uses within the study region. Its application can be integrated with other nuclear techniques [2], such as Fallout Environmental Radionuclides (FRN), thus improving the tools needed to assess soil degradation.  
Compound-specific stable isotope (CSSI) analysis based on the use of the stable carbon-13 isotope (\(\delta^{13}\)C) signature of fatty acids or alkanes in soils and sediments has emerged as a very useful technique for tracing the sources and fate of eroded soils in the landscape and distinguish between types of land use, including even forested watersheds [2; 3; 4; 5; 6; 7; 8; 9; 10]. This technique was first introduced to study the origin of estuarine sediments in 2008 [11]. This sediment tracking method (CSSI) can be applied to various biomarkers (fatty acids, n-alkanes, etc.) and different isotopes (\(\delta^{13}\)C and/or \(\delta^{2}\)H) [6; 7; 8; 10].  
This CSSI technique to study the sediments origin in Latin America and the Caribbean region has been introduced in 2014 through a regional project sponsored by the IAEA [12; 13]. As a result of this project, [PERSON] et al. [8] mapped sediment transport in the watersheds of hillside forests in Chile utilizing the CSSI technique, which permitted them to determine the principal sources of soil erosion. To localize the main sediment sources in a highly erodible agricultural watershed in Argentina, [PERSON] et al. [14] suggested the integrated application of two analytical techniques (CSSI and X-ray fluorescence) to choose environmental tracers. Later, [PERSON] et al. [9] combined FRN and CSSI techniques to reconstruct the 67-year sedimentation input history in a forested basin in central Chile.  
The objective of this work is to preliminarily evaluate the role of major soil contributors to the landscape (land uses, river, runoff, slope) as potential of soil contribution relevant to the sediment load within the sub-basin of Alhajuela Lake. For the use of the CSSI technique, taking into account the use of selected landscape indicators, twenty-six (26) soils points selected among different places of relevance in the study area were sampled.