HYDROGEOCHEMICALLY DERIVED TOOLS FOR THE DIFFERENTIATION AND MAPPING OF MULTIPLE AQUIFERS

Using fingerprint diagrams and graphic representations of normative mineral reconstructions has enabled the identification and correlation of individual aquifers in the multiple aquifer Roswell groundwater basin of southeastern New Mexico.

The stratigraphically complex Roswell groundwater drainage basin is subdivided by strike-slip faults and contains surface streams, unconfined and confined alluvial, redbed and carbonate aquifers that cover more than 18,000 km2 (7,000 mi2). As many as 14,000 wells are estimated to have penetrated the aquifers during the past 110 years. Well depths vary from a few meters to over 300 m (1,000 ft.). Well capacities range from windmill output to over 34 m3/min (9,000 gpm). Geophysical logs and sample descriptions for detailed mapping are generally absent or inadequate. Administrative classification of wells as producing from shallow, artesian, or undifferentiated aquifers is inadequate for hydrogeological investigations. Elevations of water levels and potentiometric surfaces are of no comparative value unless they are from the same aquifer.

Using conventional analyses of well water samples to distinguish between Tertiary-Quaternary alluvial and Permian redbed or carbonate aquifers has been found to work exceptionally well. Ion concentrations of sodium, potassium, calcium, magnesium, bicarbonate, sulphate, chloride, nitrate, and silica allow the lithologies of the aquifers to be closely approximated using normative mineral reconstructions and readily correlated using fingerprint diagrams.

This work is based upon the concept that the chemical character of a water is dependent upon the lithologies of the rocks over and through which it has flowed. In the Roswell groundwater basin the correlation of fingerprint diagrams and graphic representations of normative mineral reconstructions has allowed delineation of three Tertiary-Quaternary alluvial aquifers, three confined Permian carbonate aquifers, and one Permian redbed aquifer that is unconfined in the north and confined in the south of the area. The variety of aquifers present suggests these tools should be applicable to many areas.