The question of the best sources from which Monterrey should be supplied
with potable water was one that had been long under discussion, and was
the subject of many investigations prior to the granting of the present
concession. Several of the original schemes called for an impounding
reservoir in the Canon of Santa Catarina and it was on the assumption
that a dam would be built that a clause was inserted in the concession
for the purpose of making its construction obligatory. The general
character of the physical and geological conditions surrounding
Monterrey has already been referred to. A thorough study of these
conditions proved that no suitable site for impounding the Santa
Catarina River could be found, apart from the fact that periodically
this river is subject to enormous floods which tear through the steep
canon with tremendous velocity.
At the site originally proposed for the dam, a considerable underflow
was found, and later investigations, carried out under the present
concession, proved that, although borings were carried to a depth of 54
m., bed-rock could not be found, the strata being composed of gravels,
conglomerate and sand. Assuming that such a dam could have been built,
the quality of the water draining from a comparatively barren
water-shed, on which many thousands of goats are pastured, would have
made its filtration an absolute necessity before it could be delivered
to the consumers.
The various available sources from which water could be delivered to the
city by gravity were investigated by Mr. F. S. Hyde, in the autumn of
1905, and also by J. D. Schuyler, M. Am. Soc. C. E., who was afterward
retained as Consulting Engineer for the Company. The various
investigations made from time to time showed that the question of a
satisfactory supply was one of extreme difficulty, requiring prolonged
observation and study, more particularly into the character of the
underground sources of supply.
One of the chief characteristics of many of the streams in the State of
Nuevo Leon, is their disappearance and reappearance at different points
along their routes, and the Santa Catarina River, under normal
conditions, as already remarked, is a very notable example of a river
which is very dry at the surface for many kilometers of its length. In
the writer's opinion, the waters of this and similar rivers in the State
pass through many open caverns underground, so that experience gained in
the investigation of underflow waters in other places would be
insufficient to determine the quantity passing at any point along the
river if ascertained by merely computing it from the velocity of the
underflow and the area of the water-bearing gravels. The rainfall on the
water-shed of the Santa Catarina River is probably 25% greater than at
Monterrey, and all ordinary rains sink rapidly into the limestone soils
and quickly disappear. In another water-shed of a very similar
character, namely, that of the Rio Blanco, in the southern part of the
State, the underflow waters appear at the surface at a place called
Mezquital, where a metamorphosed sandstone barrier prevents them from
disappearing underground. At this point the normal quantity of water is
about 5,660 liters (200 cu. ft.) per sec., but it gradually disappears,
and a few kilometers below it has sunk to an insignificant stream,
finally disappearing altogether for about 20 km. In the neighborhood of
Monterrey similar conditions exist with regard to the surface-water
supplies, and investigations, therefore, were directed toward obtaining
unpolluted supplies from springs and underground sources.
_Santa Catarina Sources._--The chief points from which it was thought
desirable to obtain underflow supplies were (1) at the barrier of San
Geronimo, and (2) at the Canon of Santa Catarina, both shown on Plate
II.
Conditions at San Geronimo, which is only 6-1/2 km. west of Monterrey,
were investigated by the State Government in 1892, to determine the
depth of bed-rock, the rock on either side of the valley being shale,
with its original bedding planes standing almost vertical. To determine
this depth, borings were made by driving 2-in. tubes until it was
assumed that bed-rock had been reached, a method which, in strata
containing so many boulders, was obviously unreliable. These borings
indicated that bed-rock was from 12 to 15 m. below the surface. If these
had proved to be correct, there is no doubt that a development of the
underground water at this point, by constructing a submerged dam
combined with an infiltration gallery, would have yielded a large
supply.
In March, 1906, the Company commenced operations at San Geronimo by
sinking a well a few meters north of the then dry bed of the river.
Water was found in considerable quantities a few meters below the
surface, practically at the level of the river, that is, 570 m. above
datum. This supply was used for provisional purposes, and will be
referred to later in describing the San Geronimo gravity supply works.
Between August, 1906, and January, 1907, 4-in. bore-holes were sunk in
the river bed and on the high ground to the north with a "Keystone"
driller outfit. These borings showed bed-rock immediately under the
river bed, at a depth of from 15 to 45 m., but dipping gradually as the
borings were carried northward.
Boring operations were also carried on at Santa Catarina, during
November and December, 1906, and in January, 1907, to determine the
geological conditions, and the results are shown on Fig. 7. From the
area of water-bearing gravels found, it was proposed to tap the
underflow water at the 630-m. level by an infiltration gallery. This
would have necessitated a gravitation tunnel 3,000 m. long, and an
aqueduct of 14 km., which it was proposed to carry to one of two
distributing reservoirs at Guadalupe, on the south side of the river,
opposite Monterrey. In May, 1907, the writer, after making a study of
all the available data which had been accumulated, had additional
borings sunk farther across the valley to the north, and these revealed
a considerable area of water-bearing gravels, and proved that, in former
geological times, the Santa Catarina flowed about 500 m. north of its
present position, and to the back of Obispado Mountain, instead of
through the city. This aspect of the subject was discussed with Mr.
Schuyler, who agreed with the writer that, in the interest of economy,
it was better to tap this supply by an infiltration gallery at the
560-m. level, and bring the water thus obtained to a reservoir to be
placed at the western limits of the city, dividing the city, for
distribution purposes, into two interchangeable systems, a high- and a
low-pressure, the high-pressure system being supplied from Estanzuela,
18 km. south of the city. One advantage to be gained from this change
was that the scheme was capable of considerable extension, and any
future developments at Santa Catarina Canon would form part of the works
to be constructed for both high- and low-pressure districts.
[Illustration: FIG. 7.--CROSS-SECTION OF SANTA CATARINA RIVER AT SANTA
CATARINA.]
The future extension of the Santa Catarina sources, the writer believes,
can be developed best by driving an infiltration gallery 10 m. below the
surface of the Santa Catarina River, a little west of the village of the
same name, and then conveying the water through a comparatively short
gravitation tunnel and pressure conduit to a main reservoir near San
Geronimo having a top water level at an elevation of about 590 m. above
datum.
_Southern Sources of Supply._--The available sources of supply southward
from Monterrey include a number of springs at various points in a
distance of 40 km. Many of these springs are of uncertain quantity, and
some are quite dry during periods of drought. The chief perennial
springs near Monterrey are those which contribute to form the Estanzuela
and Diente Rivers, both tributaries of the Silla, while farther south,
at the Potrero Cerna, near El Porvenir, there are excellent springs, at
a considerable elevation, with a minimum flow of from 170 to 200 liters
(from 6 to 7 cu. ft.) per sec. The total quantity of water available
from all these springs during the driest season would probably not be
less than about 560 or 700 liters (from 20 to 25 cu. ft.) per sec.
The Estanzuela springs issue at the foot of the Sierra Madre Mountains,
and have a normal flow of from 56 to 85 liters (2 to 3 cu. ft.) per sec.
in an ordinary dry year; they probably derive their water, through the
limestone formation, from the neighboring water-shed of Santa Catarina,
as the catchment area of the stream is only 910 hectares, and the stream
has never been known to fail, even in the driest periods of prolonged
drought. The rainfall on the area is about 30 in. per annum, and the
catchment area is well wooded and covered with abundant vegetation. The
El Diente springs have an ordinary dry-weather flow of about 28-1/2
liters (1 cu. ft.) per sec.; but part of the water is carried
underground, and the real quantity is much greater and could be
developed by a small submerged dam carried down to bed-rock.
The elevation and the extreme purity of the water of the Estanzuela
River made its acquisition very desirable, and the Company, therefore,
purchased the Federal water rights owned by various members of the
Estanzuela community, amounting to 91 liters per sec., and has since
acquired a Federal concession to all the flood-waters of that river. It
was decided, therefore, to adopt the Estanzuela River as the first step
toward developing the water to the south of Monterrey for a
high-pressure supply, the advantage of the scheme being that from time
to time extensions could be made to tap other sources by gravity, as the
demands of the city required. The Estanzuela scheme, therefore, is a
preliminary step toward future extensions which will be necessary in
this direction as the city grows. The springs near El Porvenir, and
others which contribute to the San Juan River, can be tapped at a
sufficiently high level to convey them by a gravity pressure line to the
Estanzuela Aqueduct near Mederos.
The two sources definitely decided on in July, 1907, were those from
Estanzuela and San Geronimo. The works were designed to supply
40,000,000 liters daily, which it was assumed would be sufficient for
all future developments for a population of 200,000 at a per capita
consumption of 200 liters per day. The present requirements of the
city's population, assuming that all the water was supplied by the
Company, would be, at that rate, which is a very liberal one, only
18,000,000 liters daily. This, it was thought, would be easily met by
the San Geronimo source alone, as it was estimated that it would provide
not less than 20,000,000 liters, if the infiltration gallery was driven
far enough into the water-bearing gravels.
The question of a high-pressure water supply for domestic use in a city
like Monterrey is not a serious one, as practically nine-tenths of the
houses are of one story. The increase in the number of large commercial
buildings, however, will make the demand greater in the future, and this
point has been kept in mind in arranging the division of the
distribution systems.