**EARTH-SURFACE PROCESSES, MATERIALS USE, AND URBAN DEVELOPMENT \-- A
CASE STUDY OF THE SAN JUAN METROPOLITAN AREA, NORTHEASTERN PUERTO RICO**

W. R. Osterkamp

ABSTRACT

The San Juan Metropolitan Area (SJMA) was selected for case study as
part of an investigation of \"Earth-surface processes, materials use,
and urban development: understanding the human contributions to global
geomorphological change" (ESPROMUD). The ESPROMUD investigation,
commissioned by the Scientific Committee on Problems of the Environment
(SCOPE), has the goal of evaluating and understanding the effects of
urbanization and resource extraction on earth-surface processes.

Puerto Rico, a partially closed system (oceanic island) relative to
water and mineral resources, and its capital city, San Juan, are
struggling to meet the demands for water, building materials, and
infrastructure development by an expanding population and unchecked
urbanization. The SJMA case study suggests a need for long-term,
inventory-based policies and practices regarding the use and
conservation of natural resources to avoid shortages of water and
building supplies for housing and infrastructure expansion. Resource
inventories, both in proximity to the SJMA and island-wide, must be
evaluated relative to projected demand in order to achieve realistic
constraints for a stable urban economy and healthy ecosystems.

INTRODUCTION

In recent decades, stresses imposed by human activities on global
systems have emerged as concerns to planners, earth scientists,
environmental managers, and citizens. Since glacial times, land-surface
changes often have been dominated by human activities, particularly
agriculture. In addition to deforestation, land degradation, and soil
erosion associated with farming, major transformations of landscapes and
of fluvial and coastal systems continue to occur in areas of human
settlements and mineral extraction. The environmental effects of
urbanization and related activities of mining, quarrying, and
infrastructure development are principal factors promoting anthropogenic
change in the nature and processes of the earth's surface.

The ESPROMUD (earth-surface processes, materials use, and urban
development: understanding the human contribution to global
geomorphological change) project is part of the Global Changes cluster
within the scientific program of SCOPE \-- the Scientific Committee on
Problems of the Environment \-- and is designed to evaluate the effects
of urbanization and extractive industries on earth-surface processes. An
objective of the ESPROMUD activity is to compile and evaluate available
information regarding these effects and thus to identify gaps in the
understanding of the impacts. The Río Grande de Loíza Basin/San Juan,
Puerto Rico, exemplifies problems related to urban expansion and was
selected for this pilot study.

Puerto Rico is an island far enough removed from other land areas that
importation of water and rock resources is impractical (fig. 1A). The
island (fig. 1B) can be viewed as a partially closed system relative to
availability of resources for urban development, and therefore the
difficulties of accommodating the needs of an expanding population may
be more easily recognized than they are elsewhere. Moreover, the San
Juan metropolitan area (SJMA) is typical of large cities worldwide by
(1) having a coastal setting of fragile, near-shore ecosystems
vulnerable to the stresses of human development, (2) being dependent on
inland water and rock resources of nearby drainage basins, and (3)
experiencing an urban growth rate greater than the regional rate.

The 802-km2 (square kilometer) Loíza drainage basin, the largest of
Puerto Rico, and the adjacent 67-km2 Río Piedras Basin were selected to
analyze the effects that growth of the capital city, San Juan, has had
on these rivers and the coastal zone into which they discharge (fig.
1C). The growth was intensified after 1953 by the construction of
Carraizo Dam on the Río Grande de Loíza, thereby increasing the
potential supply of municipal water by impounding streamflow in Loíza
Reservoir (or Lago Loíza). The dam and reservoir originally were
intended for the generation of hydroelectricity, but storage of water
and extractions to satisfy as much as 58 percent of the metropolitan
needs of the San Juan area (Fields, 1972) quickly stopped the generation
of electricity. The use of Loíza Reservoir, which collects runoff and
sediment from the upper 540 km2 of the basin, as a storage facility for
a municipal water supply was also complicated by sedimentation resulting
in a 13-percent reduction in storage during the first 10 years and a
47-percent reduction in 1994 (Webb and Soler-López, 1997). Urbanization
was stimulated also, mostly in the 1960s, by ground-water development in
support of industrial expansion. The ground-water resources were
recovered from karstic limestone that occurs in near-coast areas of
Puerto Rico.

Increased land degradation and alteration of the drainage network
prompted accelerated erosion and rapid sedimentation in the reservoir
and the release of solid wastes and chemical contaminants to the
surface- and ground-water systems. Thus, the effects of urbanization and
resource development now are impeding further development of land and
water resources. The Loíza Basin, a principal source of rock and water
for many Puerto Ricans, has had (1) a high rate of urban expansion in
recent decades, (2) active quarrying of building stone and excavation of
sand and gravel to support population increase, and (3) abundant
documentation of the impacts that urbanization has exerted on the
fluvial and coastal systems. These features make the Loíza Basin/San
Juan area an instructive case-study example.

CHARACTERISTICS OF PUERTO RICO AND THE RÍO GRANDE DE LOIZA AND RIO
PIEDRAS WATERSHEDS

Puerto Rico, an oval-shaped island, lies in the West Indies chain
between Hispaniola and the Virgin Islands. Centered at latitude 18 15'
N, longitude 66 30' W, Puerto Rico has an east-west length of 176 km
(kilometers) and a north-south width of 56 km; the area is about 8780
km2. Population centers are mostly within 7 km of the coast, a zone in
which about 85 percent of the 3.8-million inhabitants live (Rodriguez,
1994). A central mountain chain, the Cordillera Central, extends
eastward through much of Puerto Rico's length and typically exceeds 600
m (meters) above sea level (fig. 1B); peaks are as high as 1338 m. An
eastern segment, the Sierra de Cayey, forms part of the southern
drainage divide of the Loíza Basin, and western portions of the Sierra
de Luquillo provide runoff to the eastern part of the basin (fig. 1C).

Physiography, Geology, and Soils

Major physiographic units of Puerto Rico, largely based on geology, are
Upland, Northern Karst, and Coastal Plain (Monroe, 1980). The upper
four-fifths of the Loíza Basin are in the mountainous Upland province,
an erosional landscape of granodiorite and volcaniclastic rocks of late
Cretaceous and early Tertiary age (fig. 2, B and C). In the Piedras and
lower Loíza Basins, thick limestones with basal clastic beds of Tertiary
age (fig. 2, D) unconformably overlie the fractured igneous rocks, and
in some near-coast areas are covered by a veneer of silt and sand (fig.
2, E). The clastic and carbonate beds occur locally along the southern
coast of Puerto Rico, but the limestones that define the Northern Karst
province are mostly exposed as a band, up to 23 km wide, along the
northern coast west of the Piedras and Loíza Basins (Glover, 1971;
Monroe, 1980).

The Coastal Plain province consists predominantly of surficial material
resting unconformably on the older rocks. The sediment, mostly poorly
consolidated Quaternary-aged silt, sand, and gravel, is a variety of
flood-plain, coastal-plain, and beach deposits (fig. 2, E). In the lower
Loíza Basin, Coastal Plain deposits generally blanket karstic limestone,
thereby minimizing surficial expression of the Northern Karst province.

Most soils of the Upland province are acidic, clay-rich, and highly
erodible, reflecting deep weathering of the volcanic rocks from which
they are derived. The southeastern Loíza Basin has loamy soils derived
from weathered granitic rocks. Where flood-plain, terrace, and
coastal-plain sediment overlies volcanic or carbonate rocks of the Loíza
and Piedras Basins, soils are well drained and sandy. Detailed
descriptions of soil characteristics of the upper Loíza Basin were given
by Hunt (1976), and Boccheciamp (1978) mapped the soils of the San Juan
area.

Climate

Puerto Rico has a tropical marine climate of average temperatures
ranging from 20 C in the mountains to 26 C in coastal areas (Picó,
1969). The central mountain range is a barrier to northeast trade winds
and casts a rain shadow over most of southern Puerto Rico. Rainfall in
the northern island averages about 2000 mm (millimeters) annually,
whereas southern areas average about 1100 mm (Calvesbert, 1970). Annual
precipitation in the Loíza Basin generally averages between 1570 and
1700 mm; the basin-wide mean is about 1600 mm but rainfall locally
reaches about 2500 mm due to an orographic effect of the mountains. San
Juan, representative of the Río Piedras drainage basin, has a mean
annual precipitation of about 1530 mm.

Rain in Puerto Rico typically falls as short-duration showers of 5 to 10
mm throughout the year, the 4-month period of August through November
generally accounting for nearly half of the annual total. Hurricanes and
other tropical storms, such as Hurricane Hortense, September, 1996, may
yield as much as 70 percent of total rainfall for a year during a one-
or two-day period (Palm and Hodgson, 1993; Larsen, 1997).

Population, Land Use, and Economy

The population of Puerto Rico, presently about 3.8 million, rose 84
percent from about 1.9 million in 1940 to 3.5 million in 1990, and in
the Loíza Basin population rose from 93,200 in 1971 (Hunt, 1976) to
350,000 in 1990. From 1950 to 1990 the population in the Caguas area
increased 460 percent, from about 34,000 to 191,000. About 1.22 million
people, mostly in the Río Piedras Basin, lived in the San Juan, Bayamón,
Guaynabo, and Carolina districts (fig. 1C) in 1990 (U. S. Department of
Commerce, 1992). By 1984, 75 percent of the Piedras Basin was urban, and
projections suggest that the entire basin will be developed by the year
2000 (Gellis, 1991).

The rapid rise in Puerto Rico's population, and that of the SJMA in
particular, is related to economic changes of recent decades. Through
nearly 70 years following American occupation in 1898, about
three-fourths of Puerto Rico, including the Loíza Basin, was cropland
and pasture converted from forest. Sugar cane, pineapple, coffee, and
tobacco accounted for 31 percent of net income and 80 percent of exports
in 1940 (Brockmann, 1952). In the 1950s, tax-exemption programs aided
industry and manufacturing, such as pharmaceutical companies, and by the
mid-1960s northeastern Puerto Rico especially had changed from an
agrarian to an industrial economy. The shift caused abandonment of many
farms and thus re-conversion to a 35-percent forest cover by 1985
(Gellis and others, 1998). In the Upland and Northern Karst provinces,
urbanization into adjacent woodlands or renewed agriculture will be
hampered by topography. Half of Puerto Rico has slopes greater than 45
percent, which, if developed, would be unstable and highly susceptible
to failure and erosion (Cerame Vivas, 1989; Larsen and Torres-Sánchez,
1996; Larsen, 1997).

The changes in the economic base prompted migration to urban areas,
especially San Juan and Caguas (fig. 1C). By 1991, the Loíza Basin was 8
percent urban (Larsen and Torres-Sánchez, 1992), resulting in congestion
in San Juan, spillover into nearby towns, and decentralization of
industry. In 1940, 70 percent of the island's population was rural as
opposed to about 21 percent now. Furthermore, many rural residents
commute to city jobs so that few in northeastern Puerto Rico escape the
urban influence (Hunter and Arbona, 1995).

FLUVIAL SYSTEM, RIO GRANDE DE LOIZA AND RIO PIEDRAS

Puerto Rico is drained by streamflow except in the karst area, where
much of the runoff infiltrates and recharges limestone aquifers. The Río
Grande de Loíza, which enters the Atlantic Ocean about 10 km east of San
Juan (fig. 3), supplies much of the water for the SJMA. The less
extensive and largely urbanized Río Piedras Basin (figs. 1C; 3),
immediately west of the Loíza Basin, contributes a small proportion to
the water needs of the SJMA. Data summarizing mean discharge, Q, in
cubic meters per second (m3/s), unit discharge, Qu, in cubic meters per
second per square kilometer (m3/s/km2), and sediment yield, in (metric)
tons per square kilometer per year (Qsy), are listed by U. S. Geological
Survey (USGS) station number (Sta. No.) for gage sites of the two basins
in Table 1. Also given are station names and the drainage area (DA) of
each, in square kilometers. Instantaneous peak discharges (Qp) are
provided for selected gage sites.

Streamflow

Unit runoff from the Loíza and Piedras Basins, respectively, averages
about 0.025 and 0.035 m3/s/km2 (table 1). Compared to average
precipitation for the two basins, these rates suggest that about 50 and
72 percent of rainfall, respectively, results in streamflow in the two
basins. The remainder of rainfall is mostly lost to the atmosphere as
evapotranspiration. The relatively high unit discharges of the Piedras
Basin are assumed to be caused in part by high runoff rates typical of
urban settings with extensive paved areas. Mean discharge into Loíza
Reservoir is about 13 m3/s and recent releases of water from Loíza
reservoir have averaged about 6.4 m3/s (table 1, number 15); the
difference is mostly extractions averaging about 3.5 m3/s for municipal
water use (Gellis and others, 1998), infiltration, and
evapotranspiration. Based on runoff relations for non-urbanized areas of
the Loíza Basin, mean discharge from the Piedras Basin prior to
urbanization was about 1.7 m3/s. Extrapolating recent discharge records
for the gage site Río Piedras at Hato Rey, PR (table 1, number 19),
runoff to the ocean from the Río Piedras Basin now averages about 2.3
m3/s. Thus, urban growth of the SJMA has, through extractions for public
water supplies, reduced streamflow in the Río Grande de Loíza, but has
increased flow in the Río Piedras through increased waste water
originating in the Loíza Basin, reduction of infiltration, and increased
runoff from paved areas.

Floods

Flooding in the Loíza and Piedras Basins can occur any time but is most
likely during the hurricane season. An exception occurred in January,
1992, when flooding was due to a cold front combined with an upper-level
trough (Torres-Sierra, 1996). Instantaneous peak discharges of the last
36 years in the Loíza Basin at the Caguas, Juncos, Gurabo, and Carolina
gage sites (fig. 3; table 1, numbers 6, 12, 13, and 16) occurred during
Hurricane Donna on September 6, 1960. Peak discharges during a hurricane
of August 4, 1945, probably exceeded those of 1960 at many sites in the
Loíza Basin, but accurate records are not available (Fields, 1972).
Other peak discharges listed in Table 1, for numbers 1, 4, 5, 6, 7, 9,
and 10, occurred during the frontal storm of January 6, 1992
(Torres-Sierra, 1996). Estimates of peak discharges of September, 1996,
during Hurricane Hortense suggest that most flows did not exceed those
of a return period of 50 years (Torres-Sierra, 1997).

Streamflow records at the Río Piedras at Hato Rey gaging station (table
1, number 19) began in 1970 and indicate a peak discharge on June 17 of
that year. For the period 1960 through 1993, 49 and 57 percent of the
floods, respectively, at the Caguas and Gurabo gage sites occurred
during the August through November period (Gellis and others, 1998).

Mean discharges for 18 gaging stations and instantaneous peak discharges
for ten gaging stations are given in Table 1; at several sites, numbers
4, 5, 7, 9, 10, and 16, the period of record is short and poorly
representative (at number 16, mean discharge is unavailable owing to
tidal conditions of measurement). Nevertheless, at eight sites of the
Loíza Basin, the peak discharge ranged from about 160 to 1050 times mean
discharge; peak discharge for a 26-year period at the Río Piedras at the
Hato Rey site (number 19) was about 170 times mean discharge. This
measure of discharge variability signifies the flashy streamflows that
are typical of areas in the tropics that are subject to intense storms
and hurricanes.

Sediment Yield

Sampling for suspended sediment at two gage sites in the Loíza Basin
began in 1984, and therefore undisturbed conditions are not represented.
Sediment yields in the Loíza and Piedras Basins are listed in Table 1
for 18 sampling sites. Average annual yields for the Caguas and Gurabo
sites, numbers 6 and 13 (table 1) are given for the 1984 through 1994
water years. Most other sediment yields are for the 1991, 1992, and 1993
water years only, a period of slightly deficient runoff and sediment
discharge in Puerto Rico (Gellis, 1991; Gellis and others, 1998).
Sediment yields for Río Cayaguas (no. 3) and Río Canóvanas (no. 17) are
for the 1991 through 1995 water years (Larsen, 1997).

Sediment-transport curves developed by Gellis and others (1998) and
Larsen (1997) show that 80 percent or more of sediment moved by the Río
Grande de Loíza typically occurs naturally during the several days each
year of highest streamflow. The anomalously high sediment yield measured
in Río Piedras Basin at El Señorial (table 1, number 18) is for 1988 and
1989, when active housing construction accounted for about 6 percent of
the basin area (Gellis and others, 1998). Number 19 (table 1) refers to
a very small watershed of a construction area and also illustrates the
erosion and sediment movement that are possible in northeastern Puerto
Rico under disturbed conditions (Gellis, 1991). Thus, the greatest
potential for destructive erosion and large amounts of sediment
delivered to and deposited in Loíza Reservoir is when a hurricane or
other intense-rainfall event occurs in disturbed areas.

Sediment-yield data of Table 1 are too variable and not numerous enough
to show strong relations to high-intensity storms or land disturbance,
particularly because comparative data from relatively pristine drainage
basins are sparse. In the now undisturbed Luquillo Experimental Forest
(fig. 2), which borders part of the Loíza Basin to the east, the
sediment yield, 1991 through 1995 water years, from a 3.3-km2 watershed
of the upper Icacos River was 525 tons/km2/yr. In a 17.8-km2 watershed
of the Mameyes River, which flows northward from the experimental
forest, the sediment yield was 117 tons/km2/yr (Larsen, 1997). Compared
to sediment yields of the Río Grande de Loíza, from watersheds of
similar size, these yields may not be high. Relative to yields from
basins in other tropical areas, the sediment yields of the Icacos and
Mameyes Rivers are high and possibly reflect the effects of landslide
disturbance along road corridors and of recent hurricanes (Larsen, 1997;
Larsen and Parks, 1997).

Using erosion-prediction technology (the Universal Soil Loss Equation),
Molinelli (1982) estimated soil loss in the Loíza Basin for forested,
pasture, cropland, and construction areas, respectively, to be 1950,
2490, 7750, and 7870 t/km2/yr. Gellis and others (1996) measured
sediment discharges from small plots at construction sites that yielded
sediment averaging about 30 to 40 times that of forest and pasture sites
and 10 times that of croplands. These estimates, which are partly
supported by data of Table 1, are useful for distinguishing variation in
sediment yield with land use, but do not account for sediment entrained
by overland flow and later stored at the bases of hillslopes or on
bottomland surfaces. Hence, measured sediment yields are generally lower
than estimates of erosion rates. The estimates suggest that sediment
yields of the last century in the Loíza Basin may have been greatest
prior to 1950, when farming dominated land use. Sediment yields during
the last 30 years possibly decreased as farms were abandoned and the
lands reverted to forest. As development related to the urban areas of
San Juan and Caguas continues in the Loíza Basin, accelerated sediment
discharge from construction sites may in turn accelerate the
sedimentation rate in Loíza Reservoir and thereby further threaten the
water supply that has facilitated urbanization.

Water Quality

Due to a generally granitic/volcanic geology and moist climate,
dissolved-solids concentrations in runoff of northeastern Puerto Rico
are typically less than 300 mg/l (milligrams per liter); pH values of
streamflow are consistently in the range of 7.0 to 7.9 (Díaz and others,
1995). Concentrations of specific dissolved constituents generally do
not exceed health-standard limits, but locally and for short durations
the concentrations of constituents such as manganese, iron, and various
organic compounds may exceed recommended limits (Díaz and others, 1995).
Where ground-water inputs to streams have been contaminated by municipal
landfills, hazardous-waste dump sites, and various illegal disposal
sites, streamflow may be highly contaminated with mercury and other
heavy metals, solvents, and pesticides (Hunter and Arbona, 1995).

High concentrations of fecal-coliform (FC) and fecal-streptococci (FS)
bacteria are a major surface-water quality concern throughout Puerto
Rico, but the problem is most acute in streams draining urban areas,
industrial parks, and suburban communities lacking sewage systems. As
noted by Hunter and Arbona (1995, p. 1336), the combined processes of
urbanization and industrial growth have \"been so rapid as to create a
serious lag in the provision of infrastructure services; unauthorized
domestic hookups always exceed already-saturated plant capacities. Thus
at the majority of sewage facilities, the supply of human waste
outstrips treatment capacity and raw sewage is consequently discharged
into streams. The risk to human health is obvious."

Díaz and others (1995) found that the highest concentrations of FC and
FS in surface waters of Puerto Rico generally occur in streams draining
densely populated and industrialized areas of the island. In 1994 the
highest levels of surface-water contamination by FC and FS occurred at
sites in the SJMA. At the Quebrada Blasina near Carolina, PR,
water-quality sampling site (fig. 3, number 0603), in the eastern San
Juan area, for example, concentrations of FC and FS in water samples
collected during the 1994 water year were as great as 530,000 and
120,000 colonies per 100 milliliters, respectively. Similarly, at the
Río Piedras at Hato Rey (fig. 3, number 0491; table 1, number 19),
water-sample concentrations of FC in 1994 varied from 37,000 to 600,000,
and FS ranged from 7,200 to 730,000. These concentrations compare to a
standard maximum microbiological-contaminant level of 1000 colonies FC
per 100 milliliters for raw recreational water in the United States
(Hunter and Arbona, 1995).

In addition to receiving wastes in runoff from the metropolitan area,
streams of San Juan also receive waste-laden effluent from
sewage-treatment plants and untreated sources in the upper parts of the
Loíza and Piedras drainage basins. Elsewhere in Puerto Rico, the main
sources of contamination in surface-water systems are liquid wastes from
industrial and municipal sources (Díaz and others, 1995).

ENVIRONMENTAL ISSUES OF THE LOIZA BASIN

Puerto Rico is susceptible to earthquakes, tsunamis, hurricanes and
coastal floods, riverine flooding, landslides, and subsidence (Palm and
Hodgson, 1993); natural hazards are exacerbated by urbanization. Water
pollution, accumulation of solid wastes, and erosion accelerated by
development, all direct consequences of urban growth, however, may be of
greater economic and social significance than are natural hazards.
Cerame Vivas (1989, p. 109) commented that the \"main ecological problem
of Puerto Rico by the year 2000 will be population. All of the
environmental problems that we confront emanate from our
overpopulation."

Natural Hazards

Puerto Rico is near the subduction zone and boundary of the North
American and Caribbean tectonic plates and thus is susceptible to large
earthquakes and tsunamis (Hays and Gori, 1984). In 1918 a magnitude-7.5
earthquake destroyed many buildings on stream-valley alluvium and
coastal-plain sediment and was accompanied by a tsunami on the west
coast (Reid and Taber, 1919). Parts of Puerto Rico most subject to
damage by earthquakes and tsunamis are coastal and flood-plain areas of
Río de Bayamón (in the Toa Baja, Cataño, and Bayamón districts, fig.
1C), Río Piedras, and Río Grande de Loíza, where urban expansion has
been most intense (Molinelli, 1987).

Hurricanes are common in Puerto Rico and other coastal areas of the
Caribbean region where they cause storm surge, high wind and rainfall,
tornadoes, and flooding. Storm surge (a rapid rise in sea level) is the
main danger to coastal population centers of Puerto Rico and accounts
for a large majority of hurricane-related deaths (Pielke, 1990, p. 59).
Storm surge of hurricanes and other tropical storms also causes coastal
and bottomland flooding. Effects of high wind and heavy rain, as much as
400 mm in a day, include extensive erosion, flooding, and plant-biomass
reduction (Scatena and others, 1993).

The most recent major storm to affect Puerto Rico was Hurricane
Hortense, September 9 and 10, 1996, which yielded up to 600 mm of rain
in the Sierra de Luquillo (fig. 1C) and generally in excess of 400 mm
throughout the rest of eastern Puerto Rico (Torres-Sierra, 1997).
Flooding was extensive along the Río Grande de Loíza and other streams
draining northeastern Puerto Rico, causing major damage to urban areas
and coastal-plain infrastructure. In some urban areas the flooding and
resulting property damage were compounded by inadequate or clogged
drainage systems. Channel alterations in the Loíza Basin included scour,
aggradation, and widening. Estimates suggested that storm-related
sediment loads in the Loíza Basin, partly due to numerous debris flows,
soil slips, and landslides, equaled the mean annual sediment yield. Much
of the sediment was deposited in Loíza Reservoir, further reducing its
storage capacity. Damage to forests was minor, but crop damage was
extensive. Power and water were disrupted to nearly all of Puerto Rico,
and damage to streets, highways, and distribution systems for food and
water was also substantial. Coastal-plain flooding caused only limited
damage to the San Juan municipal landfill, but movement of solid and
toxic wastes to the fluvial network from other landfill sites may have
been extensive (M. C. Larsen, USGS, written commun., 1996; F. N.
Scatena, USDA Forest Service, oral commun., 1996).

Hazards Aggravated by Urban Development

Deep weathering and soil development in eastern Puerto Rico causes
steep, unstable slopes that are vulnerable to mass movement, especially
when induced by heavy rainfall and treefall. Hurricane Hugo, with
rainfalls exceeding 200 mm in September, 1989, triggered numerous
landslides; 71 percent of the landslides that occurred during and
shortly following the hurricane in the Sierra de Luquillo were also
related to road construction (Larsen and Torres-Sánchez, 1992).
Landslide activity was absent in the Loíza Basin, immediately to the
west, where rainfall did not exceed 200 mm (Larsen and Torres-Sánchez,
1992). Hundreds of landslides, however, were triggered by Hurricane
Hortense in the upper Loíza Basin (M. C. Larsen, USGS, written commun.,
1996). Road construction and related urban disturbances strongly
increase sediment yield through mass-movement processes as well as by
normal hillslope and fluvial erosion (Molinelli, 1984; Larsen and Parks,
1997).

Dissolution of limestone by ground-water circulation occurs naturally in
the Northern Karst province, resulting in caverns and sinkholes. In
urban areas of Puerto Rico these processes are enhanced by fracturing
induced during earthquake activity, and collapse may be induced where
ground-water withdrawals or drainage of wetlands reduce pore pressures
in the aquifer. The imposed weight of large buildings, highway
structures, and artificial fill can be sufficient to cause bedrock
failure in karstic areas (Griggs and Gilchrist, 1983).

Sediment is volumetrically the biggest contaminant of water in Puerto
Rico. Sediment increases water turbidity of reservoirs and lakes, which
degrades water for consumptive use, fouls water-distribution networks,
and destroys aquatic habitat. Riverine sedimentation decreases the
runoff capacity and affects flood frequency, fills navigable channels,
and reduces soil fertility on agricultural land. Sedimentation in
reservoirs throughout Puerto Rico is reducing storage capacity and water
availability (Molinelli, 1982; Webb and Soler-López, 1997).

Environmental Effects of Urbanization

Among the inevitable by-products of urban expansion are loss of wildlife
habitat, solute loading of water supplies, air pollution, soil loss, and
the generation of solid and toxic wastes. As previously noted, pollution
of surface and ground waters in the Loíza and Piedras Basins is in some
cases extreme, and accelerated erosion due to soil disturbance adversely
affects agricultural production, generation of hydropower, and public
water supplies. In the San Juan area, environmental degradation due to
air pollution is minimized by strong trade winds and consequently is
probably less intense than for most urban areas of the world
(Calvesbert, 1970).

Besides the impact on water resources, the most problematic
environmental effect of urbanization in Puerto Rico may be the handling
of solid wastes, including toxic substances. The population of 3.8
million, about 430 persons/km2, generates a daily average of about 5000
tons of solid waste, one of the highest amounts, per capita, in the
United States (Hunter and Arbona, 1995). Almost all of these wastes are
derived from imported foods, raw materials, and manufactured goods, and
about a third, by weight, mostly metals, glass, plastics, and some wood
products, is not readily subject to biodegradation (Cerame Vivas, 1989).

Thirty of 61 landfills in Puerto Rico, including the San Juan landfill,
will be full within 4 years and nearly two-thirds may be unusable within
10 years. The San Juan landfill, in an urban marsh, receives about 1000
tons of solid waste daily and has reached its legal height limit of 22
m; a section of the city's main water-supply pipe underlies the
landfill, causing concerns of contamination by toxic leachates (Hunter
and Arbona, 1995). At least 14 dumps, two of which are Superfund sites
(Cerame Vivas, 1989), containing hazardous waste are in the Loíza Basin;
materials in the dumps are not well documented but probably include
pharmaceutical chemicals, petrochemicals, and wastes from paint
production and the apparel industry (Hunter and Arbona, 1995).

The area of Puerto Rico and its distance from other potential disposal
or recycling sites, the population density, and an undeveloped recycling
capability combine to cause a steady increase of solid wastes. Assuming
a 1989 cost of \$60 per person/year to handle and dispose of wastes, the
annual cost, adjusted for inflation, for refuse management by the year
2000 may approach \$1 million per day (Cerame Vivas, 1989). Much of this
expense will be borne by the SJMA, and does not include the disposal of
presently stored wastes in landfills of the Loíza Basin.

IMPACTS OF QUARRYING AND MINING, LOIZA AND PIEDRAS BASINS

Historic mining activity in Puerto Rico has included the extraction of
lime for cement but started with the mining of gold placers by Spanish
settlers and the development of iron and copper deposits (Cardona,
1984). In the Loíza Basin, known metalliferous deposits are limited to
lenses of magnetite in beach sands (Monroe, 1977) and small amounts of
copper, molybdenum, gold, and silver associated with sulfide minerals
disseminated in quartz diorite and volcanic rocks or concentrated along
shear zones (Pease, 1968). Potential metallic ores in the Loíza Basin
may include iron and copper skarns and copper or copper-gold bearing
porphyries (Orris and others, 1992). The only significant recent
extractions of metal ores in the Río Grande de Loíza Basin have been at
the Keystone iron-skarn and Island Queen copper-skarn mines near Juncos,
in the southeastern part of the basin (fig. 3). About 8 km apart, the
two mines yielded a total of about 200,000 tons of ore in 1951 through
1953. Assuming a weight of 2 tons/m3, the extracted ore amounted to
about 100,000 m3, and a similar volume of deposited wastes (Hooke,
1994).

Most quarrying in Puerto Rico presently is related to the production of
cement and concrete, and is nearly restricted to industrial
(non-metallic) minerals, limestone, volcanic rocks, and sand from
weathered igneous-intrusive rocks. Continuing urbanization and
conversion to an economy based on tourism and industry depend on the
availability of construction materials. Especially during a construction
boom of the 1950s and early 1960s, alluvial deposits along the Río
Grande de Loíza and elsewhere (including beaches) were extensively mined
(Rodriguez, 1994). Downstream from Loíza Reservoir and along the lower
reaches of the Río Guaynabo (in the Río Bayamón Basin) and the Río
Piedras, large volumes of sand and gravel were taken from steadily
expanding pits dug into alluvium of the bottomlands; an analysis of 1990
aerial photography showed 64 sand and gravel pits in the basin (Gellis
and others, 1998).

By 1969, when the estimated rate of extraction of sand and gravel from
the channels and flood plains of the Río Grande de Loíza stream network
was 51,500 m3/month, channel deposits were nearly depleted and
operations were shifting to alluvial-terrace sources (Hunt, 1976).
Despite claims only 2 decades ago of \"virtually unlimited supplies of
sand and gravel" from alluvium of the lower Loíza Basin (Monroe, 1977),
the onshore resource of Puerto Rico has been essentially depleted, and
increasingly the coastal and offshore supplies are being developed;
projections suggest that terrestrial sources of sand and gravel will be
exhausted by the year 2000. Results have been inadequate supplies of
coarse fluvial sediment to replace the bottomland and coastal-zone
losses, beach erosion, and environmental degradation that is proving
harmful to the tourist-based economy (Rodriguez, 1994).

The depletion of sand and gravel resources has led to the development of
alternative sources of building materials. Crushed stone is mined from
large open-pit quarries in unweathered volcaniclastic rocks near Loíza
Reservoir, and sand is extracted from highly weathered granodiorite in
the vicinity of San Lorenzo (fig. 3). An inevitable consequence of
mining sand from saprolitic horizons of the granodiorite is release of
fine sediment to the upper Río Grande de Loíza and thus to Loíza
Reservoir. In lower parts of the Loíza Basin, limestone of the Northern
Karst province yields construction stone. The limestone, however, is
also a principal source of ground water in near-coast areas, and
quarrying of the limestone has reduced ground-water availability and the
potential for urban and industrial development.

Availability of industrial minerals has been essential to the growth and
development of Puerto Rico. Data from the Puerto Rican Department of
Natural Resources indicate that the material moved from sand and gravel
pits and rock quarries active in 1996 ranged between 100 and 3000
m3/day/pit and averaged a total of about 17,000 m3/day. Assuming a mean
density of 2 tons/m3 and 250 working days per year, about 34,000 tons of
material were moved daily and about 8 x 106 tons, or 4 x 106 m3, are
extracted annually. Dividing these estimates by the population of the
SJMA, approaching 1.3 x 106, suggests an annual per-capita consumption
of industrial minerals of 6 tons, or 3 m3, about a sixth that of the
per-capita consumption in the United States (17 m3).

Most of the population growth of Puerto Rico presently occurs in the
SJMA, and therefore the major use of building and construction materials
also occurs in the San Juan area. If a mean thickness of 0.2 m of
concrete, building stone, and other rock construction materials is
assumed for the San Juan area (K. G. Renard, Consulting Engineer, oral
commun., 1997), a total volume for the metropolitan area is roughly 1 x
108 m3, or nearly 80 m3/person. Based on recent estimates of population
increases (U. S. Department of Commerce, 1992) and a population density
approaching 3300 persons/km2, the annual increase in size of the SJMA is
about 15 km2, and the annual use of industrial minerals is about 4 x 106
m3, the same volume, estimated from records of the Department of Natural
Resources, of sand, gravel, and rock extracted annually from the Río
Grande de Loíza Basin.

A 1992 mineral assessment (Orris and others, 1992) indicated that there
were 19 active quarrying sites and about 40 sites of previous activity
in the Loíza Basin. Recent data, acquired from the Puerto Rico
Department of Natural Resources, list 29 active quarries in the basin.
In addition to sand (from weathered granodiorite) and limestone, current
extractions are crushed and broken rock of several kinds, volcanic
aggregate, and surficial \"fill". The 1992 assessment indicated that
other sources of construction materials are calcareous clastic rocks,
beach and swamp deposits, and landslide and breccia areas. Marine
volcaniclastic rocks are quarried at various sites in the Loíza Basin.

Importation of industrial minerals to support construction has proven
costly to neighboring islands in the Caribbean region (Rodriguez, 1994).
In contrast to most metals, industrial minerals are rarely traded on the
world market. Transportation costs of such bulk materials require that
they be obtained within reasonable proximity to the location of their
intended use. This economic aspect poses a particular burden for small
island nations with rapidly growing economies and populations.
Governmental officials of Puerto Rico are beginning to realize
prospective limits to growth imposed by depletion of its
mineral-resource base.

URBAN GROWTH IN THE LOIZA AND PIEDRAS BASINS

The Río Piedras Basin is almost totally urban, and coastal-plain
portions of the Río Grande de Loíza Basin are rapidly approaching total
urbanization. Presently, however, about 8 percent of the Loíza Basin,
mostly near the Atlantic coast, is urban, the remainder, in the Upland
and Northern Karst provinces having forest (about 21 percent),
agriculture and pasture (about 58 percent), and other rural land uses
(about 11 percent) (Gellis and others, 1998). Further urbanization of
the Piedras Basin is not feasible, and urban expansion in the Loíza
Basin during the next few decades is likely to be concentrated in
presently suburban communities of the Coastal Plain province east of San
Juan and Carolina.

Table 2 lists projected water-use needs, relative to those of 1993, for
the years 2000, 2020, and 2050; the projections were compiled by Ocasio
(1996). Entries for the San Juan metropolitan area (SJMA, table 2) are
sums of anticipated needs for the Bayamón, Canóvanas, Carolina, Cataño,
Guaynabo, San Juan, Toa Baja, and Trujillo Alto districts of the
combined urban area. Residential water use, reflecting expected
population increase, is likely to increase the most from 1993 to 2050,
an average of 46.8 percent for all of Puerto Rico. Although some of the
increase, 28.3 percent to 302,000 m3/day, will be in the SJMA, much of
the remaining expected increase will be in areas that become urbanized
during the next half century. Like that of the Río Piedras Basin, water
use in the present SJMA cannot increase greatly because the area is
already almost totally developed. There is, however, significant
potential for increased urban growth and water use in parts of the Loíza
Basin to the east of San Juan.

Presently available water resources are inadequate to accommodate
significant increased usage in the next 50 years by industry, whereas
commercial use necessarily must increase to support an expanding
population. Projected total water needs will not increase as strongly as
will residential needs, in part because progressively smaller
percentages of a finite water resource will be used for other purposes
(table 2). If new technologies for water development, such as
desalinization of sea water (Zack and Larsen, 1993), are implemented
(which, owing to cost, is unlikely in the near-term), water use by the
various components of the population and economy could increase
significantly.

Although Puerto Rico is a water-rich island, present water demands
during periods of normal streamflow are approaching the capacity of
those streams to supply water of adequate quantity and quality. Recent
drought conditions have demonstrated the precarious nature of water
availability and its distribution in Puerto Rico and to the SJMA. The
New York Times (June 22, 1997) reported that \"\.....Puerto Rico ordered
water rationing in San Juan and nearby areas last week because of low
rainfall, heavy use of water, and limited reservoir space. It is the
second time that rationing has been ordered in three years." The report
continued that officials of the Puerto Rico Aqueduct and Sewer Authority
began \"\.....to shut off water service every other day in sections of
San Juan and nearby Carolina, Trujillo Alto, and Canóvanas. Government
officials estimated that about 500,000 people would be subject to the
rationing." The water crisis prompted statements by the governor, Pedro
Rossello, that he would press for long-term projects to remedy the
problem, but the newspaper did not record how the projects might
alleviate water shortages or increase the available supply of water.
Recently, a \$1.8 billion program of the Puerto Rico Commonwealth was
initiated to alleviate water shortages through surface-water and
ground-water extractions (M. C. Larsen, USGS, written commun., 1998).

IMPACT OF URBANIZATION ON EARTH-SURFACE PROCESSES

The main changes to earth-surface processes by urbanization in the Río
Grande de Loíza and Río Piedras drainage basins have been land
disturbance, erosion, sediment transport (including the movement of
contaminants adsorbed on sediment), and elimination of slope-stabilizing
vegetation. Landfills and dump sites contain materials susceptible to
fluvial transport during high-intensity storms, and quarries become
sites of stored sediment and generally have areas of bare surface
vulnerable to erosion. Urbanization creates impervious surfaces that
yield high rates of runoff during rainfall, and thus tends to increase
flood peaks.

The impacts to surface processes by changes in sediment discharge and
storage are self evident and have been described in previous sections of
this discussion. The consequences of flashy streamflow include increased
flooding, possible channel erosion, and flood-plain deposition. The
effects of these changes to biota are unclear, but the entrainment and
bottomland deposition of contaminated sediment certainly may be
injurious to human health, the food network, and agricultural
productivity.

QUANTITATIVE CHANGE TO RIVER SYSTEMS

The main modification made to the channel network of the Río Grande de
Loíza, was the construction of Carraizo Dam in 1953 about 22 km upstream
from the river mouth. Results have been storage of water and sediment in
Loíza Reservoir, channel scour downstream from the dam, possible
sedimentation in the lower reaches of the Río Grande de Loíza, and
interbasin diversion of streamflow to the San Juan area. Regulation of
streamflow has minimized flood peaks except at times of extreme events
(such as Hurricane Hortense), assured downstream water users of low-flow
discharges during periods of drought, and increased the potential for
flooding through induced sedimentation in the lowest reaches of the
Loíza channel. Reduced flows below Loíza Reservoir, by decreasing
channel conveyance, have increased flood potential in densely-populated
bottomlands along the downstream reaches.

The original storage capacity of Loíza Reservoir was about 26.9 million
m3, and was increased to nearly 30 million m3 by flashboard additions in
1977, but by 1995, due to sedimentation, had been reduced to about 14.2
million m3 (Webb and Soler-López, 1997). In recent decades the reservoir
has provided nearly 60 percent of the water needs for the SJMA, but that
percentage is likely to decrease as new engineering approaches to import
water to San Juan are implemented. Among these plans are the development
of a large aqueduct to San Juan from the western end of the island and
construction of water-supply dams and reservoirs upstream from Loíza
Reservoir on the Río Grande de Loíza at Quebrada Arenas, Río Cayaguas at
Cerro Gordo, and Río Valenciano upstream from Juncos (fig. 3).

Lago Las Curias, in a headwater part of the Piedras Basin, provides
minor regulation of Río Piedras streamflow, and all urban reaches of the
lower Río Piedras have been channelized or otherwise altered. A
water-intake and water-treatment plant on the Río Piedras provided the
original source of municipal water for San Juan. Except during periods
of water shortage, these facilities are no longer used owing to
pollution of the Río Piedras (F. N. Scatena, USDA Forest Service, oral
commun., 1996).

COASTAL PROCESSES AND TRANSFORMATIONS

Puerto Rico's north coast at San Juan is a microtidal, storm-dominated
area of Quaternary alluvial and nearshore deposits overlying the
Tertiary limestones of the Northern Karst province. Where they remain,
sandy pocket beaches, lagoons, mangrove forests, and grass marshes
protect inland coastal-plain areas from wave erosion. Geomorphic
features include coral reefs and eolianite and cuspate-beach ridges near
the mouths of Río Grande de Loíza and other major streams entering the
Atlantic Ocean (Webb and Morton, 1996).

Mangrove species concentrate fine sediment and organic matter in
tidal-flat and shallow-water lagoonal environments. The mangroves
provide high species diversity of marine fauna and flora, form habitat
for native and migratory birds, help to buffer inland areas from
storm-surge and related wave-action processes, and provide recreational
and tourist income to the Puerto Rican economy.

Principal lagoons and estuaries of the San Juan area are Bahia de San
Juan, Caño de San Antonio, Laguna del Condado, Laguna San José, Laguna
La Torrecilla, and Laguna de Piñones. These shallow-marine
brackish-water bodies, prior to development, were generally adjacent to
and interactive with mangrove swamps. In large part their positions are
determined by coastal-zone outcrops of eolianite, ancient beach sand
that is mildly cemented by calcium carbonate. Fine sediment of the
lagoons, and to a lesser degree of the deeper Bahia de San Juan estuary,
is subject to continuous reworking and resuspension by tidal processes;
thus, the lagoons appear to be partially self-cleaning. Sand spits and
other barrier islands do not occur along the north coast of Puerto Rico,
probably because shelf and coastal-plain areas are too narrow and the
gradient from the island to deep water of the Puerto Rico Trench is too
great.

Under natural conditions, coastal sand dunes of northern Puerto Rico
were at least as high as 10 m above mean sea level and formed a band as
wide as 160 m or more (Nichols and others, 1987). The dunes provided a
barrier between the Atlantic Ocean and the coastal plain, offering
protection from waves, flooding, and in particular storm surge related
to hurricanes and other tropical cyclones. In recent decades much of the
protection provided by the dunes has been eliminated by mining of the
dune sand for a variety of purposes.

Near-shore coral reefs are extensive off the north coast of Puerto Rico,
many being within a few tens of meters of San Juan. Marine waters in
proximity to the reefs support abundant fish and crustaceans and are
also important for recreation and tourism. Adding to the protection
afforded by mangrove swamps and sand dunes, the reefs form a breakwater
against wave erosion of beach deposits and provide a setting in which
deposition augments beach sand lost during hurricanes and by
high-intensity storms. Where reefs adjacent to coastal-plain areas of
Puerto Rico have been destroyed by streams discharging large loads of
fine sediment and other contaminants, as much as 1 km of inland
shoreline retreat has occurred (M. C. Larsen, USGS, written commun.,
1998).

QUANTIFICATION OF COASTAL CHANGE

Urbanization in the San Juan area of Puerto Rico has had measurable
effects on coastal dunes, tidal-zone stability, and lagoons. The effects
of urban growth on marshes, mangroves, and coral reefs are apparent but
are difficult to quantify.

Mining of Coastal Sand

Following decades of sand mining along beaches of the north coast of
Puerto Rico, few natural dunes persist. Especially from the beach of
Carolina, in the eastern part of the SJMA, enormous quantities of sand
were removed in the 1950s to construct the San Juan airport. Within a
few years accelerated beach erosion necessitated the construction of
artificial breakwaters to protect the tidal zone. Estimates of dune
volumes for the north coast of Puerto Rico decreased from 27 million m3
in 1950 to 16 million m3 in 1980 (Castillo and Cruz, 1980).

Where beach and coastal-plain sand of leeward areas separating shoreline
dunes from lagoonal mangrove forests has been removed, dune heights have
been reduced as much as 12 m, and widths of foredune ridges have been
narrowed to 8 m or less. Storm waves now easily overtop the beach sand
and breach the destabilized ridges that remain. Washover by storm-wave
swash has altered the ecological conditions of the mangroves by
redepositing sand in the mangroves, in some cases suffocating them.
Repair to roads and other public facilities in the SJMA is needed
frequently after storm erosion and deposition of sand and debris in
low-lying areas. Annual erosion rates at a line of dunes along the shore
at Carolina, for example, over periods of 27 to 40 years, averaged 2.0 m
(Nichols and others, 1987). When overtopping and dune-face erosion
occurs, mixing of saltwater ponded in the excavated lowland areas
contaminates freshwater of the coastal-plain aquifer, continuing
shoreline retreat is likely, and buildings, marshes, mangrove forests,
and other areas of the tidal zone become vulnerable to damage or
destruction (Zack, 1986; Nichols and others, 1987).

Various measures of protection, such as seawalls and revetments, have
been employed where erosion damage has been pronounced. A result has
been a narrowing of the dry-beach width for stabilized shorelines versus
greater widths along destabilized shorelines (Wright and Pilkey, 1991).
Many sites of San Juan that are protected by seawalls, for example, have
no beach, and unprotected shorelines of Carolina, where sand has been
extracted, have narrow beaches that have been migrating inland and
displacing mangrove forests.

Dredging and Filling of Lagoons

All of the lagoons of the San Juan area except for Laguna de Piñones,
which is protected by mangrove forests, have been extensively modified
by shoreline structures, dredging of sand from the interior of the
lagoon or channels entering it, filling in shoreline areas, and
construction of railroads, bridges, and buildings for housing and
commercial purposes. Laguna La Torrecilla, at the eastern end of the San
Juan airport, increased in volume by an estimated 110 percent due to
dredging of about 3.1 million m3 of sand and silt from only 25 percent
of the lagoon area. Later filling along the shorelines reduced the area
of the lagoon by about 10 percent. In places Laguna La Torrecilla was
deepened to as much as 18 m, whereas Laguna San José and Laguna del
Condada, both surrounded by urban area of San Juan, were deepened to
about 11 m (Ellis, 1976). The dredging was accompanied by destruction of
adjacent mangrove swamps to permit the construction of hotels and marine
clubs in the vicinities of the airport and the old section of San Juan.

DISCUSSION

The social, economic, and environmental problems that have accompanied
urban growth in Puerto Rico are obvious. Solutions, too, are easily
identified if only specific objectives are considered, but remediation
of an environmental problem may exacerbate a social or economic problem.

Urbanization and Resource Availability

Water development, especially the construction and filling of Loíza
Reservoir, encouraged expanding urbanization, and incentives for drug
industries provided jobs that resulted both in massive contamination of
surface-water and ground-water supplies and population increases in the
Río Grande de Loíza drainage basin. Construction of new homes and
businesses increased erosion and sedimentation in the reservoir. Erosion
and pollution that accompany urbanization, moreover, are seriously
impairing the ability of the reservoir to provide water of adequate
quantity and quality.

Despite hazards such as landslides, hurricanes, and earthquakes,
sedimentation is the greatest threat to continuing water storage in
Loíza Reservoir. As an expanding urban population in Puerto Rico
increases its dependence on stored water for domestic supplies and power
generation, the rate of reservoir sedimentation becomes pivotal to the
social and economic well being of the urban community (L'vovich and
others, 1990). For any reservoir, this rate is indicative of general
basin conditions \-- climate, geology and soils, topography, vegetation,
and especially land use. Reservoir sedimentation, however, is also
influenced by specifics such as rainfall intensity and distribution,
tectonics, fire frequency, and the extent to which soil erosion due to
human disturbance is minimized. The longevity of present and planned
reservoirs in Puerto Rico, those in support of urban growth, can be
increased if presently available erosion technology and land-use
planning are applied in a thoughtful and conscientious manner.

Future discoveries of metallic-mineral resources in Puerto Rico, based
on identification of permissive geologic environments, could include
various types of deposits containing copper, gold, iron, zinc, lead,
manganese, and molybdenum (Orris and others, 1992). In the Loíza Basin
in particular, porphyry copper-gold and copper-iron skarn deposits were
deemed permissive by the geologic setting and characteristics. Were such
deposits to be discovered and developed, additional demands and impacts
on the fluvial and ground-water systems would be anticipated.

Environmental impacts of mining sand, gravel, and other building
materials extend beyond the effects of development and denudation to
modifications in stream regimen. Loss of alluvial-terrace and
flood-plain deposits has an adverse effect on agriculture and other
riparian-zone land uses. Alluvial barrow pits adjacent to river courses
are subject to flooding and piracy of the stream channel with obvious
impact on river-bank property. Perhaps most serious is the loss of sand
and gravel for beach nourishment and replenishment.

Assessment of the environmental costs of exploiting industrial minerals
and sand and gravel has been slow in Puerto Rico. The principal concern
at present appears to be the identification of additional resources for
infrastructure development, with little effort directed toward remedial
work at active or abandoned quarry sites. Attention is being directed
toward the exploitation of lagoons and off-shore deposits of sand and
gravel (Rodriguez, 1994), which could create new environmental problems
for coastal and shallow-marine areas. Mitigation of the environmental
impacts by regrading sites and revegetation, as well as by restrictions
in site development, would help to alleviate continued erosion and
reservoir siltation. Dredging of sediment from Loíza Reservoir is
planned to extend the useful life of the reservoir (Webb and Morton,
1996), an activity that also may provide a small supply of sand and
gravel relative to urban demand.

Problems related to mining and quarrying experienced in the Río Grande
de Loíza drainage basin are not unique. In many urban areas of the
world, a supply of industrial minerals at reasonable cost for continued
urban growth is of increasing concern. In the San Juan area of Puerto
Rico, as elsewhere, the available resources that were unencumbered by
site problems or land-use conflicts have been depleted. Scars remaining
are in need of remediation, and limestone that comprised a major
fresh-water aquifer has been extensively quarried and in some areas the
aquifer severely degraded. The rapid expansion of urban development
further restricts the resource base while increasing demand for the
resources. Runoff increases and ground-water recharge decreases. The
amount of construction material moved and used, on the order of 8
million tons annually, has significantly affected the fluvial and
coastal environments in the Loíza Basin, and these changes have serious
potential consequences for the tourist industry.

Constraints imposed by the coastal position of San Juan are
representative of those that affect mainland coastal urban centers as
well. The World Resources Institute (1996) estimates that by the year
2000, 3.3 billion people \-- more than half of the global population \--
will live in urban areas. About 60 percent of the global population
presently lives within 100 km of a coastline, and fully half of the
world's coastal ecosystems are at significant risk of degradation from
development-related activity. By 2025, when the globe will support a
projected 8 billion people, the urban population is expected to be more
than 5 billion and 90 percent of that increase will occur in developing
countries. Coastal urban centers are experiencing unprecedented growth,
the sort of expansion that requires vast quantities of resources from
outlying areas. As demonstrated by growth of the San Juan metropolitan
area, the ecological footprints of cities extend far beyond their
geographic boundaries. As world population heads toward 10 billion in
the next century, we are obliged to consider the enormity of the demand
that burgeoning urbanization and coastal infrastructures will impose on
such basic and commonly used resources as water, rock, and sand and
gravel.

Controls on Urban Expansion

The example of the San Juan area, Puerto Rico, seems to suggest that
urbanization tends to exploit available resources in such a manner as to
be self-limiting. Puerto Rico, a water-rich island, continues to be
burdened with a deficiency of municipal water supplies; importation of
water to the SJMA may insure controls on growth of other urban areas in
Puerto Rico. A purposeful change from an agrarian economy to that having
an industrial base has promoted rapid urban-population increases, and
infrastructure in support of the population growth has been inadequate
to keep pace. Thus, deficiencies in the municipal water-delivery and
sewage systems may be discouraging further urban growth.

Global population, however, continues to increase. Where water and
mineral resources of urban areas such as San Juan are limited, the
increasing demand for available resources by an expanding population
inevitably will impose disproportionate impacts on urban areas by normal
climate variability and possibly by induced climate change. Agriculture
and other land-use activities of rural areas normally adapt to temporary
excesses or deficiencies in water supply, but a dependence on
infrastructure limits the ability of urban areas to maintain similar
flexibility with changes in water availability. Recent periods of water
rationing due to drought in Puerto Rico provide stark evidence of this
unfortunate situation.

Growth is also being slowed by a dwindling resource of construction
materials. Limestone, for cement and concrete, is abundant in
northeastern Puerto Rico, but quarrying this resource restricts further
urban expansion. Fluvial sand and gravel supplies, once thought to be
nearly infinite, are essentially exhausted (Rodriguez, 1994), and the
mining of limited off-shore deposits will be expensive and may result in
unacceptable environmental degradation. In Puerto Rico, frame housing is
subject to damage by termites and hurricanes, and therefore cement
construction is common; the lack of sand and gravel may have a serious
retarding effect on future housing construction on the island.

Importation of industrial minerals to support construction has proven
costly to neighboring Caribbean islands such as Saint Thomas (Rodriguez,
1994). In contrast to most mineral resources, industrial minerals are
not, with few exceptions, traded on the world market. Transportation
costs of these bulk materials require that they be obtained within
reasonable proximity of their intended use. This economic aspect poses a
particular burden for small islands with rapidly growing economies and
expanding populations. Puerto Rico is beginning to experience the limits
to growth that are imposed by the prospective depletion of an industrial
mineral-resource base.

In recent decades, an important part of the economy of Puerto Rico has
become dependent on tourism. The adverse environmental effects of poorly
planned urban growth \-- water pollution, deforestation, inadequate
infrastructure, accumulation of solid wastes, beach erosion, reef damage
\-- will impact the tourist industry, and further constrain urban
growth.

COMMENTS AND RECOMMENDATIONS

As noted by Larsen (1997, p. 45), \"Puerto Rico is a microcosm of the
developing world.", and as such, stresses on resources due to
development are similar to those being experienced in other parts of the
world. In Puerto Rico, and elsewhere, it seems likely that many problems
attendant upon urban growth, such as inadequate supplies of construction
materials and depletion of sand and gravel resources, could have been
minimized by zoning policies and long-range planning designed to permit
growth in a manner consistent with available resources. Anticipation of
resource depletion could have led to more enlightened land-use planning.

Policy and planning conforming with an anticipated expansion of any
urban area should be consistent with the available resources. Through
the 1940s, Puerto Rico largely had an agrarian economy. Policy changes
in the 1950s encouraged industrialization, which resulted in mass
population migration from rural to urban settings. Economic policies
that stimulated industrial growth were not accompanied, however, by
social policies that regulated the growth and protected environmental
attributes.

Regardless of the accuracy of this historical interpretation, the San
Juan/Loíza Basin example demonstrates a need for urban planners to
embrace a long-term, cause-and-effect perspective if resource shortages
and urban infrastructure deficiencies are to be avoided. Projections of
future growth of the SJMA, leading to urban policy and planning, should
be developed within a context of social and economic goals for the
entire island of Puerto Rico. To be effective, long-term planning by
managers of San Juan must be done in concert with neighboring
communities, such as Bayamón, Toa Baja, Guaynabo, and Carolina, and with
upstream communities of Caguas and San Lorenzo. Inventories of renewable
and non-renewable resources, both in the Loíza Basin and island-wide,
must be weighed against projected needs for those resources if urban
stability is to be achieved.

Various actions can reduce present shortages of rock and water
resources, but these actions should be applied cautiously so as not to
re-stimulate an increase in urban population and thereby perpetuate a
resource shortfall. For example, where sand and gravel can be extracted
from off-shore sites without causing adverse environmental damage,
pressure to deplete remaining terrestrial and coastal supplies can be
reduced. Dredging of impounded sediment from reservoirs and lagoons, as
is presently occurring at Loíza Reservoir, can (1) supplement dwindling
supplies of sand and gravel needed for cement, (2) increase the
fresh-water storage potential of the reservoir or the utility of the
lagoon, and (3) provide replacement fill at riverine and marshland sites
where sediment was extracted previously.

Short-term supplies of water in the SJMA can be enhanced through at
least three strategies: reduction of line loss, conservation incentives,
and recycling (F. N. Scatena, USDA Forest Service, oral commun., 1997).

1\. An estimated 40 to 59 percent of water treated for delivery to the
SJMA from 1988 through 1993 did not reach intended users, presumably
owing to the combined effects of seepage loss from pipes, loss from
damaged pipes, inadequate monitoring of water-delivery, and illegal
tapping or diversion of water from supply lines (Morris, 1994; Dopazo
and Molina-Rivera, 1995). Measures to reduce these losses have been
limited, but it is estimated that water loss can be reduced to about 15
percent by increased surveillance and infrastructure repair (Morris,
1994).

2\. The use of tax advantages to encourage water conservation in the
SJMA could lead to savings by both homeowners and businesses. Included
for water conservation might be incentives for the collection of water
from rooftops and other high-runoff surfaces for use in gardening,
evaporative cooling, and waste disposal. Streamflow diversion for
storage and water treatment should be scheduled to maximize ecological
processes, such as spawning patterns of aquatic wildlife.

3\. Treated sewage effluent can be used advantageously for watering at
sites such as parks, golf courses, and preserved wetlands. Most treated
waste water released by the SJMA presently is discharged to the Atlantic
Ocean. This water typically has concentrations of nutrients that could
be beneficial to green areas but might be injurious to corals and other
marine biota.

A comprehensive, long-term policy for urban change in the SJMA may
require definition and perhaps adjustment of an environmental ethic. If
a resource evaluation suggests that urban expansion cannot be sustained,
policies of resource use must be adopted that recognize this constraint.
For example, as recently as about a decade ago, years after general
acknowledgement that fluvial sand and gravel in northeastern Puerto Rico
were nearly depleted and that removal of coastal dunes was resulting in
disastrous environmental damage, Nichols and others (1987) wrote that a
prohibition of sand-dune mining along coastal areas of Puerto Rico
was\....."not acceptable because sand is needed to support the local
construction industry and alternative sources as river sand and crushed
rock aggregate, are scarce or costly." Obviously, if limited supplies
dictate an unsustainable use of resources, policies must change
accordingly.

An environmental ethic for Puerto Rico and the SJMA cannot be imposed on
the population by government, but government can, through policy,
promote a prudent use of resources. One possible approach is to modify
present inducements that encourage industrialization and an economy
dependent on global markets. National and multi-national corporations,
such as pharmaceutical companies, that were attracted to Puerto Rico
after World War II have had little incentive to be protective of the
island's natural-resource base. Island-based industries and agricultural
cooperatives, especially those that produce foodstuffs intended
primarily for local distribution and consumption, are much more likely
than global conglomerates to be attentive to long-term care and
management of the land and water upon which their continued production
depends. Although neither San Juan nor any other large city reasonably
can be or should be detached from world trade, the relatively isolated
position of Puerto Rico suggests that the more that foods, goods, and
services are provided locally, the more efficient will be the economy
and the more protective of resources will be the population. Thus, a
renewed emphasis on production for Puerto Rico as opposed to export may
help the economic self-sufficiency of the island, and slow the trend of
recent decades toward urban growth at the expense of the rural
population.

A resource inventory, or an accounting of the likely availability of
water and rock, must ultimately serve as a guide for the limits of urban
growth and population in the SJMA. An inventory of available resources
must be compared to resources needed to support projected increases in
population and urbanization with the realization that before the
depletion or exhaustion of any required resource occurs, the effects of
over-consumption will impose constraints on continued growth. Just as
extraction of coastal dunes has resulted in environmental and economic
degradation, further appropriations of surface-water and ground-water
resources, beyond those needed to maintain biophysical processes of
fluvial and coastal systems, will destroy the very features upon which
an urban economy depends. Whether accomplished through policy, which
seems preferable, or through exploitation of resources, which seems
irresponsible, the limits on growth in cities such as San Juan will be
determined by the population that can be supported. Regardless of how
efficiently resources are managed and conserved or how well urban
officials anticipate growth, no city can accommodate expansion
indefinitely.

ACKNOWLEDGEMENTS

This case-study analysis of resources and urbanization in Puerto Rico
has been a team effort, with various but substantial contributions to
the fieldwork and manuscript by Carroll Ann Hodges, Matt Larsen, Rick
Webb, and Allen Gellis (USGS); Fred Scatena (USDA Forest Service);
Robert Morton (Texas Bureau of Economic Geology); and Maria Angeles
Alonso (Madrid, Spain). Significant field assistance, publications, and
data were provided through the USGS Water, Energy, and Biochemical
Budgets (WEBB) project. The USDA Forest Service/Institute of tropical
Forestry, Rio Piedras, PR, provided housing and transportation during
field activities.

REFERENCES

Blanco de Galinanes, M. T., 1977, Geovision de Puerto Rico. Editorial
Universitaria, Universidad de Puerto Rico.

Boccheciamp, R. A., 1978, Soil survey of the San Juan area of Puerto
Rico: U. S. Department of Agriculture, Soil Conservation Service, 141 p.

Brockmann, V. W., 1952, The rural land classification program of Puerto
Rico: Northwestern University Studies in Geography Number 1, p. 115-161.

Calvesbert, R. J., 1970, Climate of Puerto Rico and U. S. Virgin
Islands. *In* Climates of the States: U. S. Department of Commerce,
Environmental Science Services Administration, Climatography of the
United States No. 60-52, 28 p.

Cardona, W. A., 1984, El Yunque mineral prospects, eastern Puerto Rico:
Caribbean Journal of Science, v. 20, no. 1-2, p. 79-87.

Castillo, J. B., and Cruz, H. M., 1980, Sand study, subtask 4.3, Coastal
Management Program: Puerto Rico Department of Natural Resources, 144 p.

Cerame Vivas, M. J., 1989, Puerto Rico 2000: Acta Científica, v. 3, no.
2-3, p. 109-112.

Commonwealth of Puerto Rico, 1984, Water quality standard development
division water quality area. Evaluation of the water quality
classification of Río Grande de Loíza Basin with respect to dissolved
oxygen, 39 p.

Census of population and housing, 1980, Puerto Rico nos. 45-53, U. S.
Dept. of Commerce, Bureau of the Census.

Díaz, P.L., Aquino, Z., Figueroa-Alamo, C., Vachier, R. J., and Sánchez,
A. V., 1995, Water resources data, Puerto Rico and the U. S. Virgin
Islands, water year 1994: U. S. Geological Survey Water-data Report
PR-94- 1, 516 p.

Dopazo, Teresa, and Molina-Rivera, W. L., 1995, Estimated water use in
Puerto Rico, 1988-89: U. S. Geological Survey Open-File Report 95- 380,
30 p.

Ellis, S. R., 1976, History of dredging and filling of lagoons in the
San Juan area, Puerto Rico: U. S. Geological Survey Water-Resources
Investigations 38-76, 25 p.

Fields, F. K., 1972, Floods at Caguas, Gurabo, Juncos, and San Lorenzo,
Puerto Rico: U. S. Geological Survey Hydrologic Investigations Atlas
HA-438.

Gellis, A. C., 1991, Construction effects on sediment for two basins in
Puerto Rico: Proceedings, Fifth Federal Interagency Sedimentation
Conference, Federal Energy Regulatory Commission, p. 4-72 to 4-78.

Gellis, A. C., Webb, R. M., Wolfe, W. J., and McIntyre, S. C. I., 1998,
Effects of land use on upland erosion, sediment transport, and reservoir
sedimentation, Lago Loíza Watershed, Puerto Rico: U. S. Geological
Survey Water Resources Investigations (in press).

Glover, Lynn, III, 1971, Geology of the Coamo area, Puerto Rico, and its
relation to the volcanic arc-trench association: U. S. Geological Survey
Professional Paper 636, 102 p.

Gomez-Gomez, Fernando, 1984, Sinkhole development in limestone areas as
related to rainfall and ground-water development in Puerto Rico. *In: *
Walter Hays and Paula Gori, eds, A workshop on Geologic Hazards in
Puerto Rico, Reston, Virginia U.S. Geological Survey, Open-File Report
no. 84-761, pp. 101-103.

Griggs, G. B., and Gilchrist, J. A., 1983, Geologic hazards, resources,
and environmental planning, 2nd edition: Hedsworth, Belmont, CA, 502 p.

Guzmán-Ríos, Senén, 1989, Suspended-sediment data in the upper Río de
Loíza Basin, Puerto Rico: U. S. Geological Survey Open-File Data Report
88-324, 42 p.

Hays, W. W., and Gori, Paula, 1984, Background and summary of the
workshop on geologic hazards in Puerto Rico, *In*: A workshop on
geologic hazards in Puerto Rico: U. S. Geological Survey Open-File
Report 84-761, p. 12-24.

Hays, W. W., and Gori, P., 1985, A workshop on \"Reducing Potential
Losses from Earthquake Hazards in Puerto Rico". Reston, Virginia: U. S.
Geological Survey Open-File Report 84-761.

Hooke, R. LeB., 1994, On the efficiency of humans as geomorphic agents:
GSA Today, v. 4, no. 9, p. 217.

Hunt, J. L., 1976, Sedimentation of Loíza Reservoir, Puerto Rico: U. S.
Department of Agriculture, Soil Conservation Service, SCS-TP-153, 19 p.

Hunter, J. M., and Arbona, S. I., 1995, Paradise lost: an introduction
to the geography of water pollution in Puerto Rico: Soc. Sci. Med. v.
40, no. 10, p. 1331-1355.

Larsen, M. C., 1997, Tropical geomorphology and geomorphic work: a study
of geomorphic processes and sediment and water budgets in montane
humid-tropical forested and developed watersheds, Puerto Rico:
Unpublished PhD dissertation, University of Colorado, Boulder, CO, 341
p.

Larsen, M. C., and Parks, J. E., 1997, How wide is a road? The
association of roads and mass-wasting disturbance in a forested montane
environment: Earth Surface Processes and Landforms, v. 22, p. 835- 848.

Larsen, M. C., and Torres-Sánchez, A. J., 1992, Landslides triggered by
Hurricane Hugo in eastern Puerto Rico, September, 1989: Caribbean
Journal of Science, v. 28, no. 3-4, p. 113-125.

\_\_\_\_\_, 1996, Geographic relations of landslide distribution and
assessment of landslide hazards in the Blanco, Cibuco, and Coamo basins,
Puerto Rico: U. S. Geological Survey Water-Resources Investigations
Report 95-4029, 56 p.

L'vovich, M. T., and others, 1990, Use and transformation of terrestrial
water systems, *In*: Turner, B. L., II, and others, (eds.), The earth as
transformed by human action \-- global and regional changes in the
biosphere over the past 300 years: Cambridge University Press,
Cambridge.

McIntyre, S. and Gellis, A., 1993, Effects of land-use changes on
reservoir sedimentation, Lake Loíza, Puerto Rico. 13th International
Symposium of the North American Management Society, Seattle, Washington.

Molinelli, José, 1982, Soil erosion, sediment sources, and sediment
loadings in the Loíza and La Plata lake watersheds Puerto Rico. A Study
Prepared For the Environmental Quality Board, 34 p.

\_\_\_\_\_, 1984, Rapid mass movement as a geologic hazard in Puerto
Rico, * In*: Proceedings, Workshop on geologic hazards in Puerto Rico:
U.S. Geological Survey Open-File Report 84-761, p. 80-85.

\_\_\_\_\_, 1987, Earthquake vulnerability study for the metropolitan
area of San Juan, Puerto Rico: Proceedings, Hazard-mapping meeting,
Kingston, Jamaica, p. 71-86.

Monroe, W. H., 1977, Geologic map of the Carolina Quadrangle, Puerto
Rico: U. S. Geological Survey Miscellaneous Geologic Investigations Map
I- 1054, scale 1:20,000.

Monroe, W. H., 1980, Some tropical landforms of Puerto Rico: U. S.
Geological Survey Professional Paper 1159, 39 p.

Morris, G. L., 1994, Ten concepts on water supply and drought in Puerto
Rico: Dimension, Segundo Trimestre, p. 7-14.

Nichols, M., Cerco, C., Clinton, G., and Martinez, R., 1987, Coastal
dunes for protection and sand resources: Proceedings, Fifth Symposium on
Coastal and Ocean Management, v. 2, Washington, D. C., p. 1302-1311.

Ocasio, F. M., 1996, Estudio de necesidad de producción de agua para
Puerto Rico: Autoridad de Acueductos y Alcantarillados, Departamento de
Recursos de Agua, Puerto Rico, 64 p.

Orris, G. J., Carbonaro, Marguerite, Allen, M. S., Bliss, J. D., Ruiz,
A. H., Kibbe, Richard, Paidakovich, M. E., Page, N. J., Pierce, H. A.,
and Staude, J. M. G., 1992, Puerto Rico: industrial mineral mines,
prospects, and occurrences in the Mineral Resources Data System (MRDS):
U. S. Geological Survey Open-File Report 92-0244, 300 p.

Palm, R. I., and Hodgson, M. E., 1993, Natural hazards in Puerto Rico\--
attitudes, experience, and behavior of homeowners: Monograph 55,
Institute of Behavioral Science, University of Colorado, Boulder, CO,
151 p.

Pease, M. H., Jr., 1968, Geologic map of the Aguas Buenas Quadrangle,
Puerto Rico: U. S. Geological Survey Miscellaneous Geologic
Investigations Map I-479, scale 1:20,000.

Picó, Rafael, 1969, Nueva geografia de Puerto Rico\--fisica, economica,
y social: Editorial Universitaria, Universidad de Puerto Rico, San Juan,
460 p.

Pielke, R. A., 1990, The hurricane: Routledge, London, 220 p.

Reid, H. F., and Taber, Stephen, 1919, The Porto Rico earthquakes of
October-November, 1918: Bulletin of the Seismological Society of
America, v. IX, No. 4, p. 95-127.

Rodriguez, Rafael, 1994, Sand and gravel resources of Puerto Rico, In:
Selected Issues in the USGS Marine and Coastal Geology Program: U. S.
Geological Survey, San Juan, Puerto Rico, 2 p.

Scatena, F. N., 1989, An introduction to the physiography and history of
the Bisley Experimental Watersheds in the Luquillo Mountains of Puerto
Rico. Gen. Tech. Rep. SO-72. US Department of Agriculture, Forest
Service, Southern Forest Experiment Station, 22 p.

Scatena, F. N., Silver, W., Siccama, T., Johnson, A., and Sánchez, M. J.
1993, Biomass and nutrient content of the Bisley Experimental
Watersheds, Luquillo Experimental Forest, Puerto Rico, before and after
Hurricane Hugo, 1989: Biotropica, v. 25, no. 1, p. 15-27.

Smith, R. M., and Ambruna, F., 1955, Soil and water conservation
research in Puerto Rico, 1938 to 1947: University of Puerto Rico
Agricultural experiment station Bulletin 124, 51 p.

Torres-Sierra, Heriberto, 1996, Flood of January 5-6, 1992, in Puerto
Rico: U. S. Geological Survey Open-file Report 95-374, 13 p.

\_\_\_\_\_, 1997, Hurricane Hortense \-- Impact on surface water in
Puerto Rico: U. S. Geological Survey Fact Sheet FS-014-97, 4 p.

U. S. Department of Agriculture, 1990, Loíza Lake Water Quality Project
Puerto Rico: Soil Conservation Service 34 p.

U. S. Department of Commerce, 1992, 1990 census of population, general
population characteristics, Puerto Rico: Bureau of the Census CP-1- 53,
p. 1-30.

Webb, R. M. T., and Morton, R. A., 1996, Impacts of mining and
urbanization along the north coast of Puerto Rico: a case study of the
San Juan metropolitan area: Abstracts with Programs 1996, 109th Annual
Meeting, Geological Society of America.

Webb, R. M. T., and Soler-López, L. R., 1997, Sedimentation history of
Lago Loíza, Puerto Rico, 1953 to 1994: U. S. Geological Survey Water
Resources Investigation Report 97-4108, 18 p.

World Resources Institute, 1996, World resources \-- a guide to the
global environment, 1996-97: World Resources Institute, Washington, D.
C., 365 p.

Wright, E. E., and Pilkey, O. H., 1991, The effect of shore-parallel
hard stabilization on the shoreline of Puerto Rico: Abstracts with
Programs, 1991, 26th Annual Meeting, Northeastern Section, Geological
Society of America, Baltimore, Maryland, p. 152.

Zack, Allen, 1986, Effects of sand removal on the shallow aquifer in the
vicinity of the Camuy mangrove forest, Puerto Rico: Proceedings, 3rd
Caribbean Islands Water Resources Congress, Puerto Rico Water Resources
Association, p. 41-48.

Zack, Allen, and Larsen, M. C., 1993, Puerto Rico nad the U. S. Virgin
Islands: Research and Exploration, National Geographic Society, Water
Issue, v. 9, p. 126-134.

Table 1.\--Water and sediment discharges at gage sites, northeastern
Puerto Rico. The periods of record are variable but mostly range from 5
to 20 years. Except as noted, data for stations 1 through 17 are from
Gellis and others (1998); data for stations 18 and 19 are from Gellis
(1991). (DA = drainage area; Q = mean discharge; Qu = unit discharge;
Qsy = sediment yield; Qp = peak discharge for period of record.)

Sta. No. DA Q Qu Qsy Qp

(km2) (m3/s) (m3/s/km2) (t/km2/yr) (m3/s)

1 50051150 8.4 .20 .024 350 210

2 50051180 9.7 .29 .030 670

3 50051310 26.4 1.23 .047 1540a

4 50051800 64.8 3.80 .059 670 1150

5 50053025 18.5 .53 .029 340

6 50055000 233 6.09 .026 1080 2030

7 50055100 13.7 .31 .023 430 50

8 50055170 21.4 .46 .022 1200

9 50055225 30.3 .77 .026 360 380

10 50055390 13.2 .21 .016 140 45

11 50055750 57.8 1.12 .019 380

12 50056400 42.5 1.39 .033 690 1050

13 50057000 156 3.71 .024 1050 1700

14 50058350 19.5 .35 .018 840

15 50059050 541 6.43 .012 440

16 50061000 629 5580

17 50061800 25.5 .76 .030 142a

18 50048770 19.4 .68 .035 11,600

19 50049100 39.9 1.42 .036 (7600) 245

Río Grande de Loíza drainage basin

1: Quebrada Blanca at El Jagual

2: Quebrada Salvatierra near San Lorenzo

3: Río Cayaguas at Cerro Gordo

4: Río Grande de Loíza at Highway 183 near San Lorenzo

5: Río Turabo above Borinquen

6: Río Grande de Loíza at Caguas

7: Río Cagüitas near Aguas Buenas

8: Río Cagüitas near Caguas

9: Río Cagüitas at Villa Blanca at Caguas

10: Río Bairoa at Bairoa

11: Río Gurabo below El Mango

12: Río Valenciano near Juncos

13: Río Gurabo at Gurabo

14: Río Cañas at Cañas

15: Río Grande de Loíza below damsite

16: Río Grande de Loíza at Carolina

17: Río Canóvanas near Campo Rico

Río Piedras drainage basin

18: Río Piedras at El Señorial

19: Río Piedras at Hato Rey

a Larsen (1997)

Table 2.\-- Recent and projected water use, San Juan Metropolitan Area
(SJMA) and Puerto Rico (PR), in cubic meters per day

1993 2000 2020 2050

Industrial

SJMA 19,400 19,500 19,400 19,800

PR 26,200 26,600 28,200 28,700

Commercial

SJMA 64,500 56,300 63,600 76,700

PR 85,000 74,700 86,000 108,300

Residential

SJMA 235,400 252,500 280,800 302,000

PR 383,500 418,000 491,400 563,000

Total needed (does not include government, water loss, etc.)

SJMA 620,600 650,200 685,400 719,100

PR 950,200 1,003,000 1,089,600 1,183,300

(Data from Ocasio, 1996)

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Figure 1. \-- Maps showing: Puerto Rico relative to the Atlantic Ocean
and Caribbean Sea, and to parts of North, Central, and South America
(A); Puerto Rico, with elevation contours generalized at 200-m intervals
(B); and portions of east-central Puerto Rico with drainage divides
(indicated by dashed lines) of the Río Piedras and Río Grande de Loíza
Basins, and urbanized districts (bounded by solid lines) of San Juan,
Toa Baja, Cateño, Bayamón, Guaynado, Carolina, and Caguas (C)

Figure 2. \-- Generalized geologic map of Puerto Rico. Letters refer to
a serpentine-amphibolite-chert complex (A), volcaniclastic rocks of
late-Cretaceous and early-Tertiary age (B), granodiorite and other
igneous intrusive rocks (C), carbonate and clastic beds of
middle-Tertiary age (D), and surficial silt, sand, and gravel of
Quaternary age (E); LEF identifies the Luquillo Experimental Forest
(modified from Glover, 1971, and Monroe, 1980).

Figure 3. \-- Map of Río Piedras and Río Grande de Loíza drainage
basins, with locations of stream-gage and sampling sites by the U. S.
Geological Survey (modified from Díaz and others, 1995).