Management Plan for Water Use to Improve a Wildlife Refuge Surrounded by an Agricultural

Management Plan for Water Use to Improve a Wildlife Refuge Surrounded by an Agricultural Community in Southwest Puerto Rico

Eric Harmsen’s Sections

Justification The Laguna Cartagena is in direct connection with the underlying groundwater system. A portion of the laguna water is derived from the alluvium material deposited near the surface, and a portion from the upward flow of water through the fractured limestone aquifer. Laguna water originating from groundwater recharge within upland agricultural land may contain elevated nitrates and/or pesticides. Laguna water originating within the limestone aquifer may contain elevated dissolved solids. By using a groundwater flow model in combination with a particle tracking technique it will be possible to predict the source of the Laguna Cartagena water quality. Furthermore, the model will be capable of evaluating the effects of various water management practices within the watershed on the Laguna Cartagena water level.

Objective

1. Characterize the surface and subsurface flow systems of the Lajas Valley, with emphasis on the hydrologic interrelationships between these systems and the Laguna Cartagena. 2. Develop a numerical groundwater flow model for predicting groundwater levels and flow directions within the Lajas Valley.

Background

The Lajas Valley is located in the extreme southwest of Puerto Rico (Figure 1). The 130 square km valley is oriented in an east-west direction, 35.4 km in length and 6.4 km in width. Unconsolidated material fills the valley which is flanked on the north and south by volcanic and limestone rocks of Cretaceous age. Historically, the area has had problems due to water logging and high salinity of the groundwater and soils. Anderson (1977) presented a detailed study of the hydrologic resources of the Lajas Valley. A summary of the hydrologic characteristics of the Lajas Valley are presented below.

Figure 1. Location of Lajas, PR (Magaly Revera, 2003). The valley is a closed desert basin similar to those found in the Southwest U.S. Fine grained alluvium exists throughout the middle of the valley, while coarser grained material is found in the alluvial fans along the foothills. Five soil associations exist within the valley including the Fraternidad-Aguirre-Cartagena, Fé-Guánica-Aguirre, Americus-Guayabo-Sosa, Guayama- Aguilita-Amelia, Descalabrado-Jacana-San German Associations. Figure 2 shows the areas associated with the five soil associations. Figure 2 also shows the general layout of the valley, including the location of the Laguna Cartagena and the network of irrigation and drainage channels.

Figure 2. Soil Associations in Lajas Valley, PR.

Rainfall varies from approximately 45 inches per year in the north area to less than 30 inches per year along the southern coast. Evapotranspiration is between 30 in per year in the non-irrigated areas to over 50 inches per year in the irrigated areas. Aquifer recharge comes from intermittent stream that head in surrounding mountains. While some of the floodwaters reach the “playa,” most of the surface runoff enters the alluvial fans where it recharges the alluvial aquifer. Groundwater in the western Lajas Valley principally discharges to the Bahía Boquerón. In the eastern portion of the valley groundwater under artesian pressure, leaks upward through relatively impermeable soil, where it is lost to evapotranspiration from the soil or seeps to drainage canals.

Water table or unconfined aquifers occur generally within the alluvial fans and mountain areas along the north and south. Alluvial fans in the foot hills consist primarily of sand and gravel material. In the northern area, water withdrawn from alluvial fans is of high quality (i.e., low salinity). Alluvium filling the central portion of the valley is mainly silt and clay material interspersed with sand stringers. Water withdrawn from this material tends to be brackish. Aquifers also occur within the consolidated limestone and sandstone units. In La Plata basin in the northeastern part of the valley, wells tapping limestone overlain by less permeable alluvium are confined or artesian (i.e., the wells penetrating this unit are flowing wells). Confined aquifers also occur in the eastern central portion of the valley near the Ciénaga El Anegado- Laguna de Guánica. The volcanic rock which bounds the valley in the north and south are nearly impermeable.

In the Laguna Cartagena area the major aquifer is a buried limestone unit, is highly permeably and considered to be unconfined. Water levels in the vicinity of the Laguna Cartagena in March 1965 were approximately 3 m (above mean sea level), and water levels in the extreme eastern portion of the valley were around 9 m. Water levels in the foothills were around 17 m (Figure 3). Typical annual groundwater level fluctuations are on the order of 1 m. In March of 1986, groundwater elevations were significantly higher near the Laguna Cartagena (13 m), but were lower in the extreme western portion of the valley (2 m) (Figure 4). Figure 4 clearly shows that a groundwater divide corresponding with East-West Drainage Divide.

Figure 3. Groundwater elevation within the Lajas Valley alluvial aquifer, March 1965 (Anderson, 1977). Figure 4. Groundwater elevation within the Lajas Valley alluvial aquifer, March 1986 (Graves, 1991).

As a part of the Lajas Valley study, Anderson (1977) developed a computer model to analyze the potential use of a series of pumping wells to reduce water logging in the vicinity of Laguna de Guánica. The transient computer model simulated flow only in the upper alluvial aquifer. From the report it was difficult to determine the size of the model; apparently it was limited to the area of interested and did not include the entire valley.

Materials and Methods

1. Review of existing literature

As part of Objective 1, the surface and subsurface flow systems of the Lajas Valley will be characterized, with emphasis on the hydrologic interrelationships between these systems and the Laguna Cartagena. Numerous studies have been conducted on the Lajas Valley, considering for example, agricultural drainage and salinity, geology and water resources. A geographic information system (GIS) has recently been developed for the Lajas Valley (Perez Alegria, 2003), which includes information on soils, streams, areas of flooding, salt-effected areas, farms, roads, and the irrigation and drainage canals. The GIS will be of great value in the development of the proposed numerical groundwater flow model. 2. Field Study

It is necessary to determine the current conditions within the valley with respect to groundwater levels, especially in the vicinity of the Laguna Caragena. As part of the scope of work, permission will be obtained for measuring water levels within the approximately fifty existing wells within the alluvial aquifer. Approximately eight wells will be installed in the alluvial aquifer where data is lacking. All new wells will be installed in compliance with EPA standards for chemical sampling and for measuring piezometric head. A groundwater elevation map will be developed for the entire valley for comparison with historical groundwater elevation maps. Water levels will also be obtained within the deeper bedrock aquifer to estimate vertical hydraulic gradients. All wells will be sampled for dissolved ions for the purpose of assessing the distribution of salinity within the groundwater systems. Nitrates will be evaluated to assess loading from agricultural land and a limited number of samples will be analyzed for pesticides. [CARLOS, CAN YOU DISCUSS METHODS TO BE USED FOR CHEMICAL ANALYSIS OF WATER]

3. Groundwater Model

A numerical groundwater flow model will be developed for predicting groundwater levels and flow directions within the Lajas Valley. As an integral part of the model development a conceptual model of the hydrologic environment will be developed. A conceptual model represents our best understanding of the system both qualitatively and quantitatively. The conceptual model includes the following components: topography and soils, climate, geology, hydrologic properties, aquifer properties (e.g., aquifer and confining layer thicknesses, hydraulic conductivity, storitivity, etc.), aquifer recharge, aquifer discharge, groundwater levels (historical trends), tidal effects, groundwater flow directions, distribution of pumping wells and pumping rates, saltwater intrusion soil and groundwater contamination, and aquifer transport properties.

A groundwater model will be constructed using the USGS Modular Three-Dimensional Ground- Water Flow Model (MODFLOW) developed by McDonald and Harbaugh (1984). Because of its ability to simulate a wide variety of systems and its rigorous USGS peer review, MODFLOW has become the worldwide standard groundwater flow model. MODFLOW is used to simulate systems for water supply, contaminant remediation and mine and construction dewatering. MODFLOW has been the recognized standard model used by courts, regulatory agencies, universities, consultants and industry.

The GIS-based user interface GMS (Groundwater Modeling System; Brigham Young University, 1997) will be used to manipulate input and output databases. GMS was developed under the direction of the U.S. Army Corps of Engineers and involved support from the Department of Defense, the Department of Energy, and the Environmental Protection Agency. Tools are provided for site characterization, model conceptualization, finite-difference grid generation, geostatistics, telescopic model refinement, and output post-processing. The groundwater elevation data set of Graves (1991) for March of 1986 will be used to calibrate the model and the groundwater elevations collected during the study will be used to validate the model. These models will simulate steady-state conditions. Additional transient groundwater elevation data collected between 1981 and 1986 by the USGS (Graves, 1991) will be used to validate the model as well. Calibration will be achieved by adjusting aquifer properties within reasonable limits in order to match observed average groundwater levels and discharge rates. Discharges will include base flow to drainage channels which discharge to the Bahía de Boquerón and Bahía de Guánica. These data will be obtained from published reports for 1986 and measured during the study. We will perform the MODFLOW calibration with the assistance of a commercially available nonlinear optimization program called PEST (Doherty, 1994).

REFERENCES

Anderson, H. R., 1977. Ground water in the Lajas Valley: U.S. Geological Survey Water- Resources Investigations Report 68-76, 45 p.

Brigham Young University, 1997. Groundwater Modeling System (GMS), User's Manual, Version 2.1. Engineering Computer Graphics Laboratory.

Doherty, J. 1994. PEST Model Independent Parameter Estimation. Watermark Company.

Graves, R. P. 1991. Ground-water resources in Lajas Valley, Puerto Rico. U.S. Geological Survey Water-Resources Investigation Report 89-4182. 55 p.

Magaly Rivera, 2003. Welcome to Puerto Rico. http://welcome.topuertorico.org/city/lajas.shtml

McDonald, M. G. and A. W. Harbaugh. 1984. A Modular Three-Dimensional Finite Difference Ground-Water Flow Model. U.S. Geological Survey.

Pérez Alegría, L. 2002. Sistema de Informatión Geográfico Reserva Agrícola del Valle de Lajas. Final Report, Puerto Rico Department of Agriculture. Prepared by the University of Puerto Rico Agricultural and Biosystems Engineering Department. July 31, 2002.