STATISTICAL TRANSMISSIVITY DISTRIBUTION OF THE BASIN-FILL ALLUVIAL AQUIFER IN THE ESTANCIA BASIN OF CENTRAL NEW MEXICO, U.S.A
Dr. William M. Turner
The success of any ground-water exploration program depends on the ability to place a well into part of an aquifer with the highest transmissivity. In our continuing effort to understand the risk of uninformed ground-water exploration, we examine the statistical distribution of aquifer transmissivity of different aquifers. This paper deals with the analysis of the statistical distribution of transmissivity of the Basin-Fill Alluvium in the Estancia Basin of Central New Mexico in the American Southwest. The location of the basin is shown in Figure 1.
HYDROGEOLOGIC SETTING
The Estancia Basin is a closed topographic basin 2,400 square miles in area (3,861 km2) in central New Mexico. The geologic basin is created by uplift on the west of the Manzano and Sandia Mountains, the southernmost extension of the Rocky Mountains; intrusions on the northwest that form South Mountain and San Pedro Mountain; a syncline in the north that plunges into the Galisteo Basin; land surfaces of Precambrian bedrock on the east along the Pedernal Hills-Loco Hills axis; and, a syncline on the south, plunging into Socorro County, New Mexico below the Mesa de los Jumanos (Broadhead, 1997).
Important structural features inside the topographic basin include a bedrock horst at Lobo Hill that separates the basin into north and south parts, the Perro Sub-basin of locally-thick Pennsylvania rocks, and an unnamed fault extending northeast of Lobo Hill marking the east side of the Galisteo Basin.
The basin fill alluvium is deeper than the topographic base level of erosion. The overdeepening is accommodated by removal of rock mass in solution from underlying gypsum beds. The valley floor contains a series of salt playas totaling 20.8 square miles (53.8 km2) in an area about 6,040 feet above mean sea level (ft amsl) (1,841 m). The salt playas contain saturated brine that evaporates and under steady state conditions removes recharge to the basin. The valley floor below 6,200 ft amsl (1,889 m) is underlain by Pleistocene lake bed sediments up to 80 feet (24.4 m) thick that overlie older basin-fill alluvium of Quaternary age.
The Precambrian basement is overlain by Pennsylvanian sandstone of the Sandia Formation and the overlying Madera Limestone. The Madera Limestone grades transitionally upward into the Abo and Yeso Formations of Permian age. Above the Yeso Formation occurs the San Andres Limestone and Glorieta Sandstone. In the northern part of the Estancia Basin, a younger series of Mesozoic rock units including the Chinle Shale and Mancos Shale occur. On the northwestern margin of the basin, a complete sequence of Cretaceous Mesaverde Formation of interbedded marine sandstone and Mancos shale occurs.
The Paleozoic and Mesozoic rock units were truncated by erosion during the Tertiary period and basin-fill alluvium up to 400 feet (122 m) thick was deposited.
The aquifers of interest for development are the Madera Limestone, the Glorieta Sandstone and the basin-fill alluvium.
PRIOR WORK
Meinzer (1911) was the earliest worker who provided data on the initial condition of the basin and a conceptual hydrogeologic framework.
Smith (1957) performed a thorough hydrogeologic study of the southern part of the Estancia Basin and provided a great deal of information on wells, well yield and the specific capacity of wells.
Shafike (1998) presents a summary of transmissivity values for wells completed in the basin-fill alluvium obtained from published and unpublished reports of others. Shafike developed two transmissivity values. Shomaker et al. (1996) reported three transmissivity values. Kilmer (1997) reported eight values. Twenty-eight transmissivity values were calculated by Shafike (1998) from specific capacity data in Smith (1957). These data are given in Appendix A.
Transmissivity data are plotted as cumulative probability of the occurrence of transmissivity values having values greater than indicated on the graph. The plot is shown in Figure 2.
We next calculated the theoretical normal, lognormal and exponential distributions based on the estimates of population mean and standard deviation. For each statistical distribution, we created a Chi-Square (C2) contingency table to test for the goodness-of-fit between the proposed statistical frequency distribution and the observed distribution of the data. The C2 test may be used to test the goodness-of-fit for any statistical distribution. If C2 = 0, there is a perfect match between the observed and the expected values. The larger the C2 value, the greater the departure of the observed from the expected values.
In the present case, we prepared a two-way contingency table representing frequency classes and the number of transmissivity values within each frequency class. The null hypothesis, Ho, being tested, is that the distribution of observed transmissivities follows a lognormal probability density function (pdf).
The C2 value for the contingency table constructed from observed and expected values is 0.337. With 7 degrees of freedom, the C2 statistic at the 99 percent level is 16.08. Because C2 = 0.337 << 16.08, we conclude that the disagreement between the observed and predicted transmissivity valued cannot be rejected at the 0.01 level. We are unable to reject the Ho hypothesis and we are, in the case of the Estancia Basin alluvium, satisfied that the transmissivity distribution is very close to the expected lognormal distribution.
CONCLUSIONS
The statistical distribution of transmissivity in the Basin-Fill Alluvium in the Estancia Basin is lognormally distributed. The average transmissivity is 149,744 gpd/ft (1,857 m2/d). If a well is randomly located within the Basin-Fill Alluvium, the owner has about a 12.8-percent chance of finding part of the aquifer with a transmissivity greater than the average. On the other hand, the well owner has a 87.2-percent chance of finding part of the aquifer with less than the average transmissivity.
REFERENCES CITED
Broadhead, R.F., 1997, Subsurface Geology and Oil and Gas Potential of the
Estancia Basin, New Mexico, New Mexico Bureau of Mines & Mineral
Resources, Bulletin 157.
Kilmer, C., 1997, Geohydrologic Report for Sierra Vista Subdivision (Formerly
Caballo Grande Subdivision), Santa Fe County, New Mexico,unpublished
consultant's report to Associated Development, Inc.Kilmer, C., 1997, Water Availability Assessment, Mountain Ranch Limited
Partnership Well, T.10 N., R. 8 E., and T. 11 N., R. 8 E., Santa Fe County, New
Mexico, unpublished consultant's report to Mountain Ranch Limited PartnershipMeinzer, O.E., 1911, Geology and Water Resources of the Estancia Valley, New
Mexico, with Notes on Ground-Water Conditions in Adjacent Parts of Central
New Mexico, U.S. Geological Survey Water-Supply Paper 275.Shafike, N.G., and Balleau, W.P., 1998, Hydrologic Model of the Estancia Basin,
unpublished consultants report, 72 pp., 21 plates.Shomaker, J., Southwest Land Research, Sheehan, Sheehan & Stelzner, P.A. and
Livingston Associates, Inc., Regional Water Plan Estancia Underground Water
Basin, New Mexico (Draft) Unpublished consultant's report.Smith, R.E., 1957, Geology and Ground-Water Resources of Torrance County, New
Mexico, New Mexico Bureau of Mines & Mineral Resources, Ground-Water
Report 5.
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