Civil Engineering ETDs

Publication Date

Fall 11-15-2017

Abstract

In aquifers consisting of fractured or porous igneous rocks, as well as conglomerate and sandstone products of volcanic formations, silicate minerals actively dissolve and precipitate (Eby, 2004; Eriksson, 1985; Drever, 1982). Dissolution of hydrated volcanic glass is also known to influence the character of groundwater to which it is exposed (White et al., 1980). Hydrochemical evolution, within saturated zones of volcanic formations, is modeled here as a means to resolve the sources feeding a perched groundwater zone. By observation of solute mass balances in groundwater, together with rock chemistry, this study characterizes the chemical weathering processes active along recharge pathways in a mountain front system. Inverse mass balance modeling, which accounts for mass fluxes between solid phases and solution, is used to contrive sets of quantitative reactions that explain chemical variability of water between sampling points. Model results are used, together with chloride mass balance estimation, to evaluate subsurface mixing scenarios generated by further modeling. Final model simulations estimate contributions of mountain block and local recharge to various contaminated zones.

High explosive compounds, dispersed in a zone of perched intermediate groundwater at Los Alamos National Laboratory, are contaminants of potential concern. Remediation strategies that may be put in place are largely dependent on the complex subsurface hydrology of Technical Area-16, adjacent to a wet canyon on the western margin of the Pajarito plateau. Distribution of recharge water as it crosses the Pajarito fault zone, to the regional aquifer directly, to lateral subsurface pathways, and to springs and alluvium, is not well understood. A network of monitoring wells in various saturated zones has informed extensive sets of water chemistry data. Geological observations and whole rock geochemical data, often gathered during the development of these wells, are used to constrain and evaluate models.

Validated mixing model simulations indicate that mountain block recharge sources dominate both regional and intermediate water in most sample locations. The upper intermediate zone of groundwater inhibits mixing and appears to act as a barrier between local recharge and the regional aquifer in some places, though this is not uniformly the case.

Keywords

geochemistry modeling recharge flowpaths PHREEQC

Sponsors

Los Alamos National Laboratory ER-ES

Document Type

Thesis

Language

English

Degree Name

Civil Engineering

Level of Degree

Masters

Department Name

Civil Engineering

First Committee Member (Chair)

Kerry Howe

Second Committee Member

Jose Cerrato Corrales

Third Committee Member

Brent Newman

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