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TRANSMISSIVITY DISTRIBUTION OF THE SHALLOW ALLUVIAL AQUIFER IN THE ROSWELL ARTESIAN BASIN OF SOUTHEASTERN NEW MEXICO, U.S.A.  

By Dr. William M. Turner 

INTRODUCTION 

As part of the effort to develop computer models of the Roswell Artesian Basin for the purpose of managing water rights transfers, the Technical Section of the Office of the State Engineer for the State of New Mexico in the United States evaluated the transmissivity distribution within the shallow alluvial aquifer.  The material that follows is adapted from Rao (1991).

HYDROGEOLOGIC SETTING  

The Roswell Ground-Water Basin covers a large area in the Pecos River valley in southeastern New Mexico (Figure 1).  The basin is bounded on the west by the Sacramento Mountains, about 80 miles (129 km) distant from the Pecos River; on the east by the foot of the High Plains of Texas, about 25 miles (40 km) from the Pecos River; on the south by the Seven Rivers Hills.  The northern boundary of the Roswell Basin is near Vaughn, New Mexico about 90 miles (145 km) north of Roswell, New Mexico. 

The surface elevation within the Roswell Basin declines from about 10,000 feet (3,048 m) at the crest of the Sacramento Mountains on the west to about 3,500 feet (1,067 m) at the Pecos River east of Roswell, New Mexico.  From the Pecos River, the surface rises eastward to 4,000 feet (1,219 m) at the foot of the High Plains of Texas (Hantush, 1957). 

The geologic formations involved in the transmission, storage and confinement of ground water in the Roswell Basin are the shallow alluvium deposits called the Shallow Alluvial aquifer, the Chalk Bluff Formation, the San Andres Formation and the Yeso Formation (Hantush, 1957). 

Alluvium deposits form the Shallow Alluvial aquifer.  The Chalk Bluff Formation forms the upper semi-confining aquitard overlying the San Andres Limestone artesian aquifer.  The Yeso Formation forms the semi-confining aquitard below the San Andres Limestone.

PRIOR WORK 

Hantush (1957) reported transmissivities ranging from 31,100 to 139,000 gpd/ft (386 to 1,724 m2/d) based on seven aquifer performance tests (APTs) in the Shallow Alluvial aquifer and recommended a representative transmissivity of 100,000 gpd/ft (1,240 m2/d) for it. 

Mower et al. (1964) presented a set of transmissivity values for the shallow alluvial aquifer from (APTs), specific capacities of wells and application of Darcy's Law to ground-water movement.  Their estimate of average transmissivity for wells in the cultivated area outside of bottom land is 102,000 gpd/ft (1,265 m2/d), which agrees with the estimate of 100,000 gpd/ft (1,240 m2/d) by Hantush (1957). 

Mower et al. (1964) also conducted a series of 12 APTs of shallow wells in bottom land - an area about one mile (1.6 km) wide on either side of the Pecos River.  Average estimated transmissivity for the Shallow  Alluvial aquifer in the bottom land is only 12,000 gpd/ft (149 m2/d).  According to Mower et al. (1964), the valley-fill alluvium in the bottom land consists principally of fine-grained sediments and, therefore, has a much lower transmissivity.  They also estimated that the average transmissivity for the area between cultivated upland and fine-grained bottom land sediment is about 42,000 gpd/ft (521 m2d). 

Of the Shallow Alluvial aquifer transmissivity estimates of Mower et al. (1964), Kinney et al. (1968, p.24) commented: 

"Test data from 12, small-diameter wells in the bottom lands of the Pecos River indicate that the average coefficient of transmissivity of the alluvium immediately adjacent to the river is about 12,000 gpd/ft (149 m2/d).  These small diameter wells did not fully penetrate the alluvial aquifer and were drilled into silty sand on the present flood plain of the Pecos River.  The average coefficient of transmissivity of the shallow aquifer, based on all available APT data, is probably on the order of 100,000 gpd/ft (1,240 m2/d)." 

Saleem and Jacob (1971) present 52 values of transmissivity for the shallow alluvial aquifer estimated from step-drawdown tests.  

Appendix A is a listing of available transmissivity data for the Shallow Alluvial aquifer. The data were analyzed to study the pattern of their spatial distribution.

ANALYSIS  

The natural logarithms of the transmissivity values are plotted in Figure 2.  The scatter of the data points about the theoretical lognormal-distribution line closely follow the theoretical normal-distribution line.  This means that the variation of the shallow alluvial aquifer transmissivities in the Roswell Basin can be satisfactorily explained by a log-normal probability density function. 

Rao (1991) conducted a X2 goodness-of-fit test was performed to determine whether the observed transmissivity values would be expected based on the lognormal distribution.  The null hypothesis, Ho, is that at the 90 percent confidence level the transmissivities are lognormally distributed.  The results of the X2 analysis in Table 1 failed to reject the null hypothesis that the transmissivities of the San Andres Limestone Artesian Aquifer in the Roswell Basin are lognormally distributed.

CONCLUSION 

We conclude that the transmissivity of the shallow alluvial aquifer in the Roswell Basin is lognormally distributed in space.

REFERENCES 

Hantush, M.S., 1957, Preliminary quantitative study of the Roswell ground-water 
     reservoir, New Mexico, New Mexico Institute of Mining and Technology, State 
     Bureau of Mines and Mineral Resources Division. 

Mower, R.W., Hood, J.W., Cushman, R.L., Borton, R.L., and Galloway, S.E., 
     1964, An appraisal of potential ground-water salvage along the Pecos River 
      between Acme and Artesia, New Mexico, U.S.Geological Survey Water-Supply 
      Paper 1659. 

Rao, B., 1991, Roswell Basin Analytical Groundwater Flow Model - Users Manual, 
     Report TDH-91-2, New Mexico State Engineer Office, March, 1991 

Saleem, Z.A., and Jacob, C.E., 1971, Dynamic programming model and quantitative 
      analysis, Roswell Basin, New Mexico, New Mexico Water Resources Research 
      Institute, WRRI Report 10. 

Supkow, D., 1973, Transmissivity Distribution in the Tucson Basin Aquifer
     Proceedings of the Arizona Section of the American Water Works Association 
     and the Hydrology Section of the Arizona Academy of Science, May 5-6, 1973, 
     Prescott, Arizona, p. 113-123.

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