STATISTICAL 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 field.
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.
HYDROGEOLOGIC SETTING
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 data.
The erosional surface on which the Tablazo was deposited was irregular and as a result the Tablazo thickness varies extensively.
PRIOR WORK
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.
ANALYSIS
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 method.
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 expected values.
CONCLUSIONS
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.
REFERENCES
Busk, H.G., 1941, The Geology of the Ancon Oil Field and its Perimeter, with
Notes on Water Supply, unpublished TENEC Report HG B-16Hydrotechnics, 1974, Groundwater resources of the Santa Elena Peninsula,
Ecuador, Albuquerque, New Mexico., unpublished consultants report to
the Empressa Municipal de Agua Potable de Guayaquil under funding
from the World Bank.
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