Geochemical and statistical evidence of deep carbonate groundwater within overlying volcanic rock aquifers/aquitards of southern Nevada, USA

Abstract

Samples collected from springs and wells in southern Nevada were analyzed for major solutes and trace elements as part of a larger study to characterize the geochemical signatures of these groundwaters. In this study, principal component analysis (PCA) was used to reduce the large data sets, including the four major cations (Ca, Mg, Na, K) and 27 trace elements, analyzed in these groundwater samples. Principal components analysis of the major cation data indicates that groundwaters from Cenozoic felsic volcanic rock aquifers/aquitards of southern Nevada exhibit strong chemical associations to each other but weak relationships to groundwaters from the regional carbonate aquifer (which were instead chemically similar to each other). However, PCA of the trace element data demonstrates that some groundwaters from the volcanic aquifers/aquitards are chemically similar to those of the underlying regional carbonate aquifer. The PCA also reveals that these groundwaters from the volcanic aquifers/aquitards have significantly different trace element compositions than perched groundwaters contained within similar felsic volcanic rocks. Moreover, rare earth element (REE) data from groundwaters collected from wells finished in the volcanic aquifers/aquitards of southern Nevada have similar concentrations and similar shale-normalized patterns to the carbonate aquifer groundwaters as well as local carbonate rocks. These same southern Nevada well waters do not exhibit REE concentrations or shale-normalized signatures that resemble the perched volcanic groundwaters or the tuffs of southern Nevada. The REE data and trace element PCA, along with previous carbon isotope analyses, water temperature data, hydraulic head relations, and results of a recent pump test of a well near Yucca Mountain, suggest close contact of the regional carbonate groundwaters and groundwaters from the overlying volcanic rocks of southern Nevada and possible upwelling of the carbonate groundwaters into the overlying volcanic rock units in the vicinity of Yucca Mountain.