Download Subsidence of the Former Texcoco Lake

Land Subsidence (Proceedings of the Fourth International Symposium on Land
Subsidence, May 1991). IAHS Publ. no. 200,1991.
Subsidence of the Former Texcoco Lake
R. MORALES Y M.
Comisiôn Nacional del Agua, Proyecto Lago de Texcoco
R. MURILLO-FERNANDEZ
Comisiôn Nacional del Agua, Instituto Mexicano de
Tecnologia del Agua
A. HERNANDEZ-RUBIO
Comisiôn Nacional del Agua, Proyecto Lago de Texcoco,
Mex i co
ABSTRACT A report is made of the piezometric drawdowns and
of the settlements recorded at the region since 1972 due
to the extraction of water from underground aquifers, as
well as the evolution of the subsidence at the zone where
the largest lake at the Valley of Mexico basin existed.
INTRODUCTION
The Valley of Mexico is actually a closed basin which was provided
with a man-made exit for its waters by means of works constructed in
the past, which in turn induced the dessication of the former lakes.
This basin is divided into 11 hydrologie sub-basins out of which the
most important correspond to those of Mexico City, Churubusco,
Xochimilco, Chalco and Texcoco on account of their economic and
politic importance since the large Mexican metropolis has been
developed above them. In addition, and with the purpose of supplying
drinking water to the inhabitants, the aquifer of the Valley has been
exploited since the end of the 19th century by means of deep wells and
as a result there is an ever increasing drawdown rate of the
piezometric levels, which in turn induces consolidation of the
lacustrine deposits in the lower part of the Valley where the former
lakes of Mexico, Texcoco and Chalco-Xochimilco are located.
The subsidence phenomenon affects buildings and municipal
facilities by inducing cracking, rupturing of conduits, apparent
emergence of piled foundations, differential settlements and loss of
hydraulic gradients.
BACKGROUND
The former Texcoco Lake is located in the lowest part of the Valley of
Mexico to the NE of Mexico City and it is used at present as a
regulating reservoir of the surface runoff in the southern and
southeastern regions of the basin. It forms a plateau at an average
elevation of 2234 m above mean sea level and it is bounded to the East
by the Sierra de Rio Frio, to the West by the Sierra de Guadalupe, and
to the North and South by the former Xaltocan and Chalco lakes,
respectively. Some isolated volcanic cones can be seen as outcrops,
such as those known as Peflôn de los Baflos, Peflôn del Marqués and Cerro
de Chimalhuacân (Fig. 1).
35
R. Morales Y. M. et al.
36
FIG.l Geographical location of the former Texcoco Lake.
GEOLOGY
The basin of the Valley of Mexico is situated at the center of the
volcanic belt that crosses the Mexican Republic from East to West and
it has been subjected to major tectonic stresses and to volcanic
eruptions from the beginning of the Tertiary to recent epochs.
In order to determine the deep geology of the Valley, several
studies have been carried out among which mention should be made of
37
Subsidence of the former Texcoco Lake
the gravimetric survey of the Texcoco Lake performed from 1952 to 1953
as well as the drilling of the deep wells known as PP1 (2065 m) and
PP3 (589 m) in 1967-1968 (SHCP & NF, 1969).
The Texcoco sub-basin corresponds to a plateau underlaid by a
highly compressible clay of lacustrine volcanic origin that covers
alluvial and pyroclastic permeable materials from the Quaternary which
in turn form the main aquifer of the region. Below these materials,
igneous rocks of the Tertiary are encountered and they form the lower
impervious barrier. The basal rock is constituted by marine limestone
of the Cretaceous. The last cycle of vulcanism began during the
Quaternary and there are still some manifestations. This period is
representative of the volcanoes Cerro Gordo, Chimalhuacân and
Chiconautla, among which the Texcoco sub-basin is located.
Deep well PP1
The lithologie profile of the PP1 well is characterized by 60 m of
lacustrine clays interbedded with thin strata of silt and sand; by
sands, silt and clay from 60 to 180 m; from 180 to 505 m by clays,
lutites, sandy clays, sands, sandstone and lacustrine limestone; from
505 to 1437 m by tuffs and igneous rocks, breccias and conglomerates;
from 1437 to 1980 m by lithic tuffs, igneous rocks and sands; from
1980 to 2045 m by clayey anhydrites and marls; and finally, from 2045
to 2065 m by clays, marls and calcareous conglomerates (SHCP & NF,
1969).
UNDERGROUND WATERS
Geophysical surveys point towards the presence of two seismic
refractors inclined to the SE, at depths of 770 and 1850 m,
respectively. Geoelectric studies have identified four units related
to boring PP1 as follows: the first unit shows a seismic wave velocity
of 600 m/s and it corresponds to the surface clay layer saturated with
brackish water which has a saline concentration in excess of
10 000 mg/L, and a thickness of 30 m that decreases to the North; the
second unit or stratum is mostly clayey and it behaves as an aquitard
saturated with brackish water with a saline concentration ranging from
5000 to 10 000 mg/L, has a seismic wave velocity of 1700 m/s and a
thickness varying from 30 to 50 m. The third unit, with a wave
velocity of 2200 m/s, has water of medium quality with a concentration
of dissolved solids equal to 2000 mg/L and a thickness from 300 to
500 m, and it corresponds to the main aquifer and is constituted by a
clayey-sandy alluvial deposit. The thickness of this formation
augments towards the West - Mexico City - and decreases to the East
and South. Finally, the fourth geoelectric unit, which belongs to the
lower refractor, is formed by tuffs and marls with very low
permeability, and therefore it constitutes the lower boundary of the
main aquifer; it has a wave velocity of 4500 m/s and has been
interpreted as faulted blocks (SHCP & NF, 1969; Morales et. al,, 1989).
Magnetic and gravimetric anomalies have been detected which indicate
the presence of a buried volcano at the northern part of the zone
under study, as well as magnetic minimums starting at the crater, that
make evident former erosion courses. The high contents of solids
dissolved in the water of the formations existing in the former
38
R. Morales Y. M. et al.
Texcoco Lake are attributed to the fact that this was the lowest zone
since the valley was closed during the Quaternary and consequently the
floods of the fresh water lakes of Mexico, Xaltocan and
Chalco-Xochimilco were discharged into it and after evaporation the
salts became concentrated during filling of the basin.
Pump i nq
During 1980 a total of 1685 wells were surveyed in the region which
extract a volume equivalent to 12.59 m3/s, from which 5.3 are used to
supply potable water to Mexico City and 0.5 correspond to the
exploitation of brackish water for the alkali industry "Sosa Texcoco".
The distribution of these flows as related to their use is shown in
Table 1 (SARH, 1972).
TABLE 1 Uses of the underground water.
Use
Municipal serviices
Agriculture
Industry
Domestic
Electric power generation
Other uses
TOTAL
Wells
(No.)
Mean flow rate
Cm3/s>
121
431
647
301
2
183
6..817
3..110
1..540
0 .951
0..057
0 .057
1685
12 .590
PIEZOMETRIC EVOLUTION
Static level in wells
At the center of the lacustrine zone wells PP1 and PP3 are located;
their casing is grooved from 213 to 1844 m in depth and from 222 to
527 m, respectively, and they provide an outstanding reference of the
evolution of the piezometric levels at great depths. From 1968 to 1984
an average annual loss of hydraulic head of 0.93 m was recorded
starting at a static level found at a depth of 6.8 m; at depths
ranging from 56 to 95 m the static level showed a drawdown rate of
1.38 m/year. From 1984 to 1990 the static level recedes at a rate of
1.1 m/year at a depth beyond 200 m (Murillo, 1984; 1990).
Piezometric levels
Figure 2 depicts the evolution of the static level at wells PP1 and
PP3 and in piezometers of the Casagrande type mounted at the center of
the region. The water table elevation has varied from 0.0 to 1.2 m in
Subsidence of the former Texcoco Lake
39
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1
-I
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W 7 0 71 72 7S 74 78 70 77 ?0 70 CO 01 0 2 0 0 0<3 0G OB 07 GO 0 0 CO 01
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FIG.2 Evolution of the piezometric levels from 1967 to 1990.
depth and it shows the effects of the shallow pumping from 1979 to
1992. The hydraulic pressure at a depth of 35 m evidences the effect
of pumping from 50 to 65 m in depth at an approximate distance of
3 km, from 1973 to 1982. The piezometer tip placed at a depth of 54 m,
within a silty and sandy stratum, clearly registers the effect of
pumping operations carried out from 1973 to 1982 and it shows a rate
of drawdown equal to 0.4 m/year from 1973 to 1984 and to 0.63 m/year
during the last five years (Murillo, 1984; 1990). It can be observed
in figure 2 the effect of a smaller recharge in 1982 which has been
the driest in the last 18 years.
Overexploitation of the aquifer
It has been assumed at present that the aquifer reaches down below tht:
former lacustrine zone and that its formations emerge towards the
sierras surrounding the basin where the largest part of the recharge
takes place. In 1975 (DDF) a recharging equivalent to 38 m3/s was
reported for the period 1920-1970 as well as an extraction of
30.3 m3/s in 1960 and consequently the geohydrological balance was
positive. However, in the lower part of the basin the piezometric
levels went down. It has been estimated that a flow rate of about
57 m3/s is extracted nowadays with a recharge of 25 m3/s and therefore
the overexploitation amounts to 32 m3/s (Murillo, 1990) and it becomes
evident in the fastest rate of piezometric drawdown and in the
acceleration of the subsidence, in addition to the saline intrusion
toward the periphery of the former lake which was reported since 1972
(SRH).
SUBSIDENCE
In 1972 a W-E leveling axis was established with a precision of
1 mm/km which starts in a fixed reference point. Figure 3 shows the
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R. Morales Y. M. et al.
Elev.
in m 2238 SNA SNB BNC BfêD BNE
above 2236
mean 2234 .
sea 2232
level
g
BNF BNG BNH
BNI
BNJ
BM< BNL
300,
400
500
Settlement
profiles, P e f i o n - T e s c o c o
road
AV. RIO C0NSULAD0
g j ^ R O A D PENON-TEXCOCO
»/BNF
^BW"
BNL
BN^
.PNJ
6*1
INTERNATIONAL
AIRPORT
WELL PP1R.R.
WELL PP 3
£>W TO TEXCOOO
-_ CITY
~M
NABOR CARRIIK:
LAKE
Location of benchmarks along the Penon - TeKcoco rood
FIG.3 Settlement profiles, Texcoco Lake.
topographic profile in 1972, the settlement profiles as of 1990 and
the location of the control points.
The maximum regional subsidence has reached 5 m in 18 years and
the minimum is equal to 3.15 m toward the East, and consequently there
exists a differential settlement of 1.85 m in that same period. It is
worth mentioning that the thickness of the clay strata becomes smaller
in that direction.
41
Subsidence of the former Texcoco Lake
It is evident that several settlement rates exist among the
benchmarks BNA and BND; the mean rate was equal to 16.9 cm/year up to
1984. This zone corresponds to urban areas developed before 1972; the
benchmark BNE showed then a higher rate of 28 cm/year induced by
artificial fills, and urban development occurred from 1968 to 1982.
Among reference marks BNF and BNJ, with an average rate of
21.6 cm/year, no recharges were placed and no pumping took place.
Between points BNJ and BNL pumping in the vicinity has been carried
out from 1973 to 1978 and from 1978 to date and an average settlement
rate of 14.2 cm/year has been recorded from 1972 to 1984. This
irregular behavior in the zone has not been satisfactorily explained.
In general terms, the rate of subsidence has shown an increase since
1978 at the benchmarks placed near Mexico City and from 1981 to date
in other parts CMurillo, 1984; 1990). Table 2 shows the settlement
rates.
TABLE 2 Settlement rate at Texcoco Lake.
Maximum
Average
Minimum
1972-1977
cm/year
1977-1982
cm/year
1982-1987
cm/year
22.8
16.7
12.0
33.4
23.6
14.4
27.8
22.7
16.6
1987-1990
cm/year
33.1
28.0
26.0
Compression of the clay strata
In areas subjected to heavy pumping operations from the pervious
layers found at depths from 30 to 31 m and from 50 to 60 m, with no
superficial surcharge, the contribution of the settlement at the
surface, expressed in percentage, was determined (Table 3 ) .
TABLE 3 Distribution of the compression.
Layer
Depth
Cm)
Contribution
(%)
Upper Clay Formation
Lower Clay Formation
Deep Deposits
0-30
31-50
> 50
15.0
70.5
14.5
Additional observations
At wells PP1 and PP3 it was observed that their casings emerged 1.7
and 2.3 m from 1968 to 1984, which represent emersion rates of 10.6
and 14.5 cm/year, respectively (Murillo, 1984), and they showed an
R. Morales Y. M. et al.
42
emersion of 1.1m from 1984 to 1990 at the PP1 and of 0.9 m from 1984
to 1988 at PP3, i.e. rates of 18.3 and 22.5 cm/year, respectively, for
the last few years. The casing of well PP1 emerges at a smaller rate
than that of the regional subsidence and it is subjected to a strong
negative friction, whereas the casing of well PP3 emerges at a rate
equivalent to the subsidence.
ARTIFICIAL RECHARGE OF THE AQUIFER
The National Water Commission treats 50 L/s of residual waters to a
tertiary level to be infiltrated in the ground by means of absorption
wells, with the purpose of determining the technical and economical
feasibility of achieving the artificial recharging of the aquifer at
an accelerated rate and of reducing its overexploitation. Although
highly permeable zones are found in the surrounding mountain ranges,
the cost of rising with pumping the residual waters located in the
lower part of the basin restrains the use of this alternative, and
therefore it was decided to carry out the experiment in grooved
infiltration wells from 70 to 200 m in depth. The permeability of
those layers ranges from 10 E-2 to 10 E-4 cm/s. The static level in
the recharge wells was found between 23 and 29 m in depth and in the
pumping tests specific capacities from 1.1 to 4.7 L/s/m were
determined for flow rates smaller than 100 L/s CMurillo & Pifton,
1986). The first results of the recharging point toward a rapid
decrease of the infiltration capacity from 27 to 11 L/s in the first
five months and from 11 to 4 L/s in the following five months, with a
recovery of the static level equal to 11 m which in turn diminished
subsequently although at a slower rate (Morales & Pifion, 1990). It has
been estimated that the flow rate during recharging is quite slow
because upon injection of fluorescein this substance was never
detected in the test wells (Leor, 1986-1987).
CONCLUSIONS AND RECOMMENDATIONS
As a result of the overexploitation of the aquifer in the region,
particularly in the perimetric zones, the regional subsidence will
proceed with rates ranging from 26 to 33 cm/year. Out of this
superficial settlement, 70% corresponds to the consolidation of the
Lower Clay Formation found at depths varying from 31 to 50 m.
Since it is not possible to diminish appreciably the water
extraction and therefore the overexploitation of the aquifer, it has
been proposed to carry out an exchange of treated residual waters by
the underground supplies for farming purposes and for industrial uses;
to perform an artificial groundwater recharge with residual water
treated to a tertiary level; and to expand the projects of
reforestation, overturning of the surface layer, terrace construction
and building of infiltration dams to increase the natural recharging
of the sierras surrounding the Texcoco sub-basin.
REFERENCES
DDF (1975) Memoria de las Obras del Si sterna de Drenaie Profundo del
43
Subsidence of the former Texcoco Lake
Distrito Federal (Proceedings of the Works for the Deep Sewer
System of Mexico City), Vol. I, Mexico.
Leor, S.A. (1986-1987) Estudios de transmisibilidad del acuîfero y
segunda aplïcaciôn de trazadores en los pozos de recarga (Studies
of transmissibility of the aquifer and second application of
tracers in the recharge wells), Mexico.
Morales R., Pinôn N., Alvarez A. & Lesser J.M. (1989) Resistividades
en el Ex-Lago de Texcoco (Resistivities in the Former Texcoco
Lake). Topicos Geol6qicos del Valle de Mexico. Sociedad Mexicana
de Mecânica de Suelos, Mexico.
Morales R. & Pi non N. (1990) Resultados de un modulo experimental de
recarga de acuiferos (Results from an experimental module for
recharging the aquifers). Proceedings of the 11th National
Congress of Hydraulics, Asociacion Mexicana de Hidrâulica,
Zacatecas, Mexico.
Murillo R. (1984) Comportamiento regional del Ex-Lago (Regional
behavior of the former lake). Obras Recientes en el Laqo de
Texcoco. Sociedad Mexicana de Mecânica de Suelos, Mexico.
Murillo R. & Pinôn N. (1986) Modulo experimental de recarga de
acuiferos en el Valle de Mexico (Experimental module for
recharging the aquifers in the Valley of Mexico). Proceedings of
the 9th National Congress of Hydraulics, Asociacion Mexicana de
Hidrâulica, Querétaro, Mexico.
Murillo R. (1990) Sobreexplotaciôn del acuifero de la Cuenca del Valle
de Mexico: efectos y alternativas (Overexploitation of the aquifer
of the Valley of Mexico Basin: effects and alternatives). El
Subsuelo de la Cuenca del Valle de Mexico v su Relaciôn con la
Ingénier!a de Cimentaciones a Cinco Aftos del Sismo. Sociedad
Mexicana de Mecânica de Suelos, Mexico.
SHCP & NF (1969) Provecto Texcoco (Texcoco Project).
SRH, Comisiôn Hidrologica de la Cuenca del Valle de Mexico (1972)
Aguas subterrâneas de la zona de Texcoco (Underground waters of
the Texcoco zone), Mexico.