Assessment of
Groundwater Resources
On the eastern shore of Baja California, dramatic mountains descend
steeply to a narrow coastal strip along the Sea of Cortez. In the
southern half of Baja California, the municipality of Loreto, Mexico
is located along this narrow coastal strip. The region houses
approximately 15,000 people, the large majority of whom live in the
town of Loreto. In addition to the area surrounding the town of
Loreto, the region is usually divided into four other areas south of
Loreto: Nopoló, Notrí, Puerto Escondido, and the Ensenada Blanca/Ligϋi.
Presently, the only source of potable water for the Loreto region is
the San Juan Bautista Londó aquifer located 30 kilometers to the
northwest (Figure 1).
To ascertain the quantity of freshwater available for proposed
development, a preliminary groundwater model was developed for the
San Juan Bautista Londó (San Juan) aquifer. The Loreto groundwater
model estimates the impact of changes in water demand and land cover
on groundwater storage in the region and forecasts the risk of
saltwater intrusion into the municipal wells for each a set of
growth scenarios. MODFLOW, (McDonald and Harbaugh 1988; 1996) the
most widely used and respected groundwater modeling program, was
used to develop a preliminary groundwater model for the San Juan
aquifer. In constructing a groundwater model, estimates of the
water entering the aquifer (recharge), water leaving the system
(well pumping), and hydraulic properties (an estimate of how fast
the water moves through the system) are entered into the model.
MODFLOW then produces a map of groundwater elevations by numerically
solving a set of partial differential equations.
The San Juan aquifer is surrounded on all side by steep mountain
slopes which act as no-flow barriers except to the east where a
constricted, narrow outlet connects to the Sea of Cortez (Figure
2). The entire San Juan watershed is only 623 km2 in
size and the aquifer or basin floor, itself is only 225 km2;
a very small aquifer by any account. To prevent saline water from
entering and thus contaminating the San Juan aquifer, groundwater
must flow out of the basin into the Sea of Cortez. If this flow
stops and the elevation of the Sea becomes greater than the
elevation of the groundwater, saline water will flow into the
system. With continued pumping saline water will reach the
municipal wells resulting in an irrevocable loss of potable water.
Rainfall
or precipitation is the total amount of water that falls within a
basin. Most of this is lost to runoff and evaporation with only a
small portion infiltrating into the aquifer. The amount of water
that does infiltrate the surface and travel to the groundwater
aquifer is termed recharge. This recharge may come in at the sides
of the basin as run off from the mountains or through underground
fractures. Either way the only source of new water or recharge is
precipitation. Potential evaporation is the amount of water that
could be or “wants to be” evaporated. If potential evaporation is
considerably greater than rainfall, recharge is negligible. In the
Loreto region, potential evaporation exceeds precipitation rates in
every month (Figure 3). Because potential evaporation is greater
than rainfall – the rain that falls on the basin floor is lost to
evaporation and plant uptake.
In 1986 the Institute of Geophysics at the Universidad Nacional
Autónoma de México conducted a report on the San Juan aquifer (UNAM,
1986). This study dealt only with the northern portion of the
basin, focused largely on chemical characterization and assumed a
confined aquifer. Transmissivity values were calculated from a
series of pumping tests conducted in the agricultural region of the
aquifer. A preliminary estimate of recharge was made and a
groundwater depletion of 2.5 m per year was calculated. This
groundwater depletion from agricultural and municipal pumping has
since resulted in the basin becoming unconfined. |
