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Assessment of Hydrogeologic Conditions of the Punta de Cabullones Area, Ponce, Puerto Rico

Project Number: 2516-CWV00
Project Chief: Jesús Rodríguez-Martínez
Cooperators: Municipality of Ponce and Department of Natural and Environmental Resources
Period of Project: FY 2007-2008

Introduction

The territorial development plan of Ponce contemplates the setting apart of a significant portion of the municipio for conservation purposes on the basis of esthetical, ecological, and hydrological values. Among the areas of the municipio considered for conservation is the Punta de Cabullones area (see Figure 1).

Problem

The municipio of Ponce is interested in preserving the Punta de Cabullones area as part of the wetlands mitigation effort required in compensation for the negative impact of dredging of the Las Américas trans-shipment port. Similarly, the municipio of Ponce is aware of the potential effects that increased groundwater withdrawals may have on the Punta Cabullones ecosystem(s), a problem presently occurring at the Jobos Bay Reserve. The preservation of the Punta de Cabullones area will not only safeguard this unique natural system for the benefit of the present and future generations but will also serve to comply with mitigation requirements of federal and state agencies. Consequently, the municipio of Ponce needs to develop a management plan, in conjunction with federal and state agencies, which includes land-use strategies that will ensure the preservation of the Punta de Cabullones area. In order to develop such a management plan, the municipio of Ponce requested the U.S. Geological Survey (USGS) to submit a proposal to assess the inter-relation of ground-and-surface water in the Punta de Cabullones area and the source of dissolved solids concentrations in the aquifer and surface water bodies.

Objectives

The main objectives of this study are to define:

• the hydrogeology and the surface-water/ground-water relations within the boundary of the Punta de Cabullones study area
• determine the spatial and seasonal changes in the chemical and physical properties of surface water features (wetland and channels).

Knowledge of the surface-water/ground-water relation will help define the relative importance of ground-water discharge and subsequent evaporation from the water table in the formation and permanence of the salt flats in the study area. The chemical composition of the surface water features is unknown but should be highly variable, as the connectivity of the lagoons with the sea is unknown and the water level in these is affected by rainfall runoff, water-table fluctuations, and coastal sea level surges. The water budget for the study area will be estimated to put in a general perspective the importance of ground-water discharge and evaporation in the Punta de Cabullones area. Finally, the study will provide the basis to improve the knowledge of ground-water flow along the coastal part of the South Coastal Plain aquifer at Ponce and develop a conceptual model of the local aquifer, especially as to regional discharge and to the sea.

Scope

The study will include an area of approximately 10 square miles in the coastal area between the Río Bucaná flood channel and the Río Inabón in the municipio of Ponce (figure 1). The study will be conducted from January 1, 2007 to December 31, 2008. Main study efforts will include:

• Hydrogeology: Defining the stratigraphy, hydraulic properties, and water quality of the aquifer within the study area,
• Surface-water hydrology: Defining the chemical and physical properties of surface water features; conduct synoptic surveys to define the distribution of saline/fresh-water within the surface water bodies, movement of water within channels and the sea, and water quality properties.

Approach

The study will define and relate the hydrogeology, surface-water/ground-water relations, and the chemical and physical properties of surface and groundwater resources of the Punta de Cabullones area to the occurrence of a series of wetlands and salt flats that also occur in this area.

Hydrogeology and surface-water/ground-water relations

The hydrogeologic framework of the Punta de Cabullones area, including the ground-water occurrence and movement, will be determined using water-level measurements in nearby wells, lithologic and ground-water hydraulic data from the U.S. Geological Survey Ground-Water Site Inventory (GWSI) and Caribbean Water Science Center (CWSC) files, and geophysical data. At least two piezometer nests of 2 components each one (a shallow and deep components) will be required to obtain data on the hydraulic head distribution, hydraulic conductivity, ground-water quality, the isotopic (18O, deuterium), CFC’s and SF6 composition, and the lithology in the Punta de Cabullones area. Hydraulic (slug) tests will be conducted at these piezometers to obtain estimates of the hydraulic conductivity of the deposits near the surface-between about 10 and 30 ft-and about 100. These intervals were found to contain strata of sand and fine gravel near the coast at the Jobos Bay National Estuarine Research Reserve and could be related to past sea level stands (Rodríguez-Martínez, USGS, written comm.., 2006). Information obtained on the head distribution, hydrogeology, and hydraulic properties will be used to define the ground-water flow system in the study area. Water samples will be collected from the piezometer and wells near the study area to determine the chemical and stable isotope composition of groundwater. The dissolved constituents and stable analyses will be used in conjunction with the hydrologic and aquifer hydraulic data to define ground-water flow and provide estimates of aquifer evapotranspiration. Salinity sources will be defined from analyses for common dissolved constituents (cations, anions, bromide) and deuterium and oxygen-18 stable isotope ratios. Samples for CFCs and SF6 will be obtained from the 100 ft deep piezometer to estimate the age of groundwater discharging to the wetlands in the study area. The data will provide the necessary information to develop a conceptual model of the South Coastal Plain aquifer within the Ponce area from the foothills to the coast.

Chemical and physical properties of surface water features

The chemical and physical component of the study will include the following field activities:

1. Collection of water samples from the saline water lagoons and channels to determine the main ion and isotopic concentrations and the seasonal variability and relation with relative water level stage; field synoptic surveys for physical properties (dissolved oxygen, conductivity, and temperature) will be conducted on a monthly basis and samples would be collected on a quarterly basis;
2. Collection of daily rainfall data data and monthly rainfall composites and
3. Collection of water-level stage data at selected points in the wetland and at outlet canal.

The surface-water data will be used in identifying the spatial salinity distribution, its sources, and temporal variation during the year. In conjunction with the hydrogeologic data it will contribute with defining the relative importance of salinity distribution in the wetlands and its relation to evaporation from the shallow water-table surface, ground-water flow paths, seawater inflow during periods of high sea level stage and seawater intrusion within the aquifer.

All data collected and their interpretation will be published as a USGS Scientific Investigation Report.

Benefits

This study complies with one of the primary activities of the Water Resources Division, of providing knowledge and expertise to assist various levels of government (Federal, State, and local) in understanding and solving critical water-resources problems. This project is consistent with at least five of the nine Priority Water Resources Issues identified in the USGS 1997-2005 Strategic Plan.

• Effects of urbanization and suburbanization on water resources
• Effects of land use and population on water resources in the coastal zone
• Drinking water availability and quality
• Suitability of aquatic habitat for biota
• Surface-water and ground-water interactions as related to water-resource management

This study also complies with at least two of the major societal issues that the Eastern Region science plan addresses.

• Urban Dynamics-as it relates to urban and population growth in the coastal areas
• Ecosystems and Natural Resources-as it relates to the connection between ground-water discharge and the preservation of particular habitats