Graduation Year

2010

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Marine Science

Major Professor

Ashanti Johnson, Ph.D.

Co-Major Professor

Barnali Dixon, Ph.D.

Committee Member

Albert Hine, Ph.D.

Committee Member

Mark Luther, Ph.D.

Committee Member

Stacy Nelson, Ph.D.

Keywords

Erosion, GIS, Radionuclide, RUSLE, Sedimentation

Abstract

Enhanced soil loss from the watershed is a major environmental issue. Increased soil loss from a watershed can potentially increase sediment delivery and loading to aquatic ecosystems such as rivers and estuaries. An increase in sediment delivery and loading to freshwater and transitional marine ecosystems can impact water quality and supply specifically by: (1) reducing water clarity, (2) transporting nutrient and pollutant laden sediments and (3) reducing the storage capacity of reservoirs. To address these broader environmental impacts of increased sediment delivery and loading to aquatic ecosystems it is imperative that potential source areas of sediments available for transport are identified in the watershed. It is also important that sediment source areas are linked to sediment transport and delivery to aquatic ecosystems.

This study attempted to establish a link between soil loss from watersheds and sediment delivery in two estuaries on the island of Puerto Rico. The two estuarine systems used in this investigation were the Rio Espiritu Santo (RES) riverine-estuarine system and the Jobos Bay (JB) Estuary. Soil loss from both study watersheds was estimated using RUSLE. Sediment cores and surface grab samples were collected from both estuaries. In addition, soil samples were collected from the two watersheds. Gamma analyses were performed in order to measure activity concentrations of 137Cs and excess 210Pb in sediment cores, surface and soil samples. 137Cs inventories were computed for each core collected from both watersheds. Also, grain-size and LOI were performed on the sediments to describe the sedimentological characteristics of collected sediment cores, surface samples and soil samples.

A conceptual framework was developed and implemented for linking sediment production, availability (supply), transport and delivery to study estuaries. Results from the RUSLE model indicated that soil loss within both watersheds were low with patchy instances of erosional hotspots. These results did not provide any information on sediment supply or insights into the hydrologic connectivity of the study watersheds. 137Cs inventories computed from the RES watershed indicated that sediment cores located further upstream had the highest inventories. With reference to the JB Estuary, statistical analysis showed that location had an effect on distribution of Cs in surface samples within the bay. Sedimentological characteristics varied between cores.

The implementation of the conceptual model in both study watersheds allowed for the identification of potential source areas of sediments that were available for transport and delivery to adjacent aquatic systems. This investigation revealed that to link soil loss to sediment delivery it is essential that key processes and variables (rainfall, soils, LULC and geomorphology) must be included in the analysis. This conceptual model may be a valuable tool for monitoring and managing soil loss within the watershed and consequently, addressing problems of increased sediment delivery to aquatic and transitional marine ecosystems such as estuaries.

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