Master's Theses

Date of Award

Summer 1992

Degree Name

Master of Science (MS)

Department

Geosciences

Advisor

Ken Neuhauser

Abstract

The Ranegras Plain basin is a northwest trending alluvial basin located in western Arizona. Groundwater within the basin occurs in Tertiary deposits and Quaternary alluvium. The Tertiary deposits are composed primarily of clay, probably of lacustrine origin, with smaller amounts of sand and gravel. Also included in the Tertiary deposits, to a minor extent, are semi-consolidated to consolidated volcanic tuffs, ash, and a semi-consolidated conglomerate. The Quaternary alluvium consists of interbedded sands, gravels and clay. In the spring of 1988, depth to water measurements collected in select wells throughout the basin ranged from 28 feet to 455 feet below land surface. Groundwater in the basin generally flows to the northwest and is influenced by large withdrawals for agricultural use. From 1950 through 1988, approximately 657,000 acre-feet of groundwater was pumped from the basin. Between 1975 and 1988, water-level changes recorded in selected wells varied from a rise of I foot to a decline of probably more than 40 feet. Estimates place the amount of groundwater in storage within the basin between 15.4 and 22.2 million acre-feet. Total recharge to the basin may be between 7,000 to 8,000 acre-feet per year. Groundwater quality is generally poor with the majority of sampled wells exhibiting levels of total dissolved-solids and fluoride above the Maximum Contaminant Levels (MCLs) as set forth by the United States Environmental Protection Agency's secondary drinking water standards. High total dissolved-solids concentrations in the central portion of the basin reflect a northwest-southeast trend, with concentrations increasing to the northwest. This pattern is similar to the groundwater flow direction. Three water quality types have been identified in the basin. Sodium chloride type-water is the most prevalent followed by sodium sulfate type water and sodium bicarbonate type water. Unusually high concentrations of hexavalent chromium and selenium, which can be toxic or carcinogenic, are present in a small number of wells. The high concentrations of fluoride, hexavalent chromium and selenium in the Pyramid Peak area may be due to highly alkaline waters associated with clays or Tertiary volcanic deposits.

Rights

Copyright 1992 Bradley Johnson

Comments

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