DISTRIBUTION OF TRANSMISSIVITY IN THE TABLAZO AQUIFER AT THE CHAPUCAL WELL FIELD,
CHAPUCAL, ECUADOR, S.A.
Dr. William M. Turner
The Chapucal well field is situated near the community of Recinto Rio Verde in the
Santa Elena Peninsula of Ecuador, nearly midway between Guayaquil and the resort community
of Salinas. The Chapucal well field supplied all of the domestic and industrial
water requirements for CEPECA and its Tigre and Cautivo refineries as well as for the Gulf
Refinery at La Libertad to the west. Insufficient supplies of water were responsible
for closure of these refineries in the past, causing a national emergency. Figure 1 shows the general location of the Chapucal well
The Chapucal well field contains wells on the western side of the Rio Verde which
belong to CEPECA. The municipality of Salinas owns the wells on the eastern side of
the Rio Verde. Both CEPECA and Salinas were in dire need of additional ground-water
supplies and higher capacity water wells.
The Tablazo is a mixed continental-fluviatile, near-shore, Pleistocene deposit of loose
sands, shelly sandstone, and occasionally well-cemented limestone, deposited over
extensive areas of the Santa Elena Peninsula of Ecuador. Remnants of Tablazo
limestone are seen at San Juan in the Progreso Basin. The area of major Tablazo
development is in the Engunga -Engabao and Chanduy -San Rafael - Juan Montalvo area west
of the Chanduy Hills to beyond Atahualpa and Ancon farther west..
The Chapucal well field is centered within the Chanduy - San Rafael - Juan Montalvo -
Atahualpa area. Here the Tablazo occurs as a pinkish to pale-brown, hard, calcareous
sandstone and coarse conglomerate with very shelly horizons. Between the Rio Verde
and Zapotal, thick, coarse, gravel beds are exposed in a quarry along the Guayaquil -
Salinas road. At Julio Moreno and Atahualpa the Tablazo appears to be comprised of
buff, friable , sandy beds. At Atahualpa, numerous hand-dug wells are constructed
into yellowish, soft Tablazo sands.
The Tablazo was deposited on an old erosional surface. This surface truncates
many older structural features such that the base of the Tablazo in the Chanduy - San
Rafael - Juan Montalvo - Atahualpa area rests in angular unconformity above beds of
San Jose Shale, Atlanta (Azucar) Sandstone, Socorro Formation, and Seca Shale. The
distribution of sediment at the base of the Tablazo was compiled from test-well
The erosional surface on which the Tablazo was deposited was irregular and as a result
the Tablazo thickness varies extensively.
Busk (1941), of British Controlled Oilfields, Ltd., studied the geology of the Ancon
oilfield and its nearby areas. Busk included information on the water-bearing
characteristics of the Tablazo deposits in the Chapucal area.
Water in the Tablazo generally occurs under unconfined conditions. However, notes
made by drillers and geologists suggest that there is a basal conglomerate of the Tablazo
and that some water is confined beneath it. In a letter dated March 4, 1943, the to
General Manager of Ecuador Oilfields, Ltd., Busk mentions that a hand-dug well at Chapucal
"struck water at 38 feet (11.6 m) in sandstone of Oligocene age underlying a very
hard shell bed at the base of the Tablazo."
In June, 1947, L.A. Spens, Resident Geologist for Ecuador Oilfields, Ltd. conducted a
number of aquifer performance tests (APTs) using the Chapucal wells. Of the 12 wells
in existence at that time, water levels rose in all wells above the depth at which water
was first encountered.
Studies carried out by Hydrotechnics (1974) for the Empressa Municipal de Agua Potable
de Guayaquil and the Salinas City Council evaluated all APT data and established the
general direction of ground-water flow at the Chapucal well field from north to
south. The transmissivity values in Table 1 were estimated from specific capacity
data. Where more than one specific capacity value was given for a well, the value
determined at the lowest pumping rate was used.
We ranked the nine transmissivity values from the lowest value to the highest. We
calculated the Weibull plotting position i/(n+1) that equals the average exceedance
probability of the ranked observations and are probability-unbiased plotting
positions. This data is plotted in Figure 2.
To test whether the frequency distribution of the transmissivity values is lognormal,
natural logarithms of the transmissivity data were determined and plotted on the same
graph. The plot of the natural logarithms seems to fit the observed data.
To test the statistical significance of the fit, we used the Kolmogorov-Smirnov test
because we have only nine transmissivity values. The K-S test is an exact
The maximum absolute value of exceedance between the observed and the calculated
transmissivity values was 0.0184. For n = 9 observations, the one-tailed K-S test
statistic for an a = 0.10 is 0.339. The exceedance value
of 0.0184 falls well below the critical exceedance value and we cannot reject the
"Ho" null hypothesis that the observed transmissivity values fall outside of the
We conclude that the spatial transmissivity distribution within the Tablazo Formation
is well characterized by a lognormal probability density function. Successful
ground-water exploration must locate the zones of high transmissivity that occur in the
buried subcrop valleys.
Busk, H.G., 1941, The Geology of the Ancon Oil Field and its Perimeter, with
Notes on Water Supply, unpublished TENEC Report HG B-16
Hydrotechnics, 1974, Groundwater resources of the Santa Elena Peninsula,
Ecuador, Albuquerque, New Mexico., unpublished consultants report
the Empressa Municipal de Agua Potable de Guayaquil under
from the World Bank.