Alluvial Aquifer Heterogeneities in the Rio Grande Valley: Implications for Ground Water Contamination

Spatial variation of hydraulic conductivity has been generally recognized as the dominant medium-dependent control on the transport of contaminants in ground water. An empirical study focusing on the relationship between patterns of sedimentology and the distribution of permeability is being conducted at an outcrop of the Pliocene/Pleistocene Sierra Ladrones Formation located in central New Mexico.

Methods of geostatistics and sedimentary basin analysis are employed to study aquifer heterogeneity. An air permeameter provides a means of obtaining extensive field measurements of air-flow-rate data through the sediments. These flow rates are subsequently used to characterize the permeability distribution of the outcrop. Both the geologic information and the air-flow-rate data provide the basis for aquifer heterogeneity analysis. Preliminary geologic mapping indicates that sediments in the study area are the products of an arid fluvial/interfluvial depositional environment. Probability distribution analysis of the air-flow-rate data suggests that the permeability of these sediments is log-normally distributed. The air permeability data are used to estimate variograms and correlation ranges in both the horizontal and vertical directions. At the scale of tens of centimeters, the horizontal variograms exhibit either exponential or bell- shaped behavior. Variogram analysis of estimated mean permeability at the scale of meters shows evidence of a nested correlation structure in the horizontal direction and a periodic correlation structure in the vertical direction. Results of this study suggest there is a direct connection between observable geologic structures and permeability statistics.