The ground water contamination potential of the Estes Landfill, Phoenix, Arizona has been evaluated by the City of Phoenix and the Arizona Dept. of Health Services. The landfill is located in a recharge zone of the Salt River Valley aquifer. The aquifer is under water table conditions. The depth to ground water ranges from 80 feet to 15 feet. Ground water monitoring wells were installed up- gradient and down -gradient from the landfill. Ground water samples collected from the wells during flow events of the Salt River indicated leachate production from the landfill; a mound of ground water develops and intrudes the solid waste. The leachate characteristics include volatile organics and heavy metals: vinyl chloride, trichloroethylene and barium. Analysis of solid waste borings indicated only small quantities of organics and heavy metals. Currently the ground water is used for industrial and agricultural purposes. However, the ground water could be used as a domestic water supply because it has an acceptable ambient water quality. Ground water monitoring is continuing with the intent of using the data to design a leachate migration control system for the landfill and to distinguish contaminants from an adjacent landfill.

Chlorine-36 is an important radioisotope with which to date old ground water. The initial chlorine-36 in ground water originates in the atmosphere by cosmic ray spallation of argon-40. Following precipitation and infiltration processes, the natural decay of this radioisotope is then used to date ground water. One must consider, however, the production of chlorine-36 in the subsurface. The production reaction of most interest is ³⁵C1 + neutron → ³⁶C1 + gamma. Buildup of chlorine-36 in the subsurface can result from cosmic ray secondary neutrons near the surface and natural radioactivity produced neutrons below the surface. These production mechanisms, if not taken into consideration, will contribute to the error in chlorine-36 age determinations. To predict subsurface production rates, field measurements were made of thermal neutron fluxes for various geologic materials and depths below the surface. Thermal neutron fluxes were found to vary by more than three orders of magnitude. Theoretical calculations of neutron flux were compared to filed measurements. Estimates of chlorine-36 production rates were then calculated and compared to measured values of chlorine-36 in very old ground water, where decay rates have been hypothesized to be equal to production rates.

The future acceptance and utilization of Central Arizona Project water by the City of Tucson Water Utility present many complex technical, economic, institutional, and environmental problems. Since Congressional adoption of the Colorado River Basin Project Act in 1968, Tucson Water engineers have supported the concept of a large CAP raw water storage reservoir near Cat Mountain west of the City. The United States Bureau of Reclamation, in its Stage Two planning for Phase B of the Tucson Aqueduct, has identified four potential storage sites, including the Cat Mountain location, for economic and environmental evaluation in conjunction with two basic aqueduct alignments. Engineers of the municipal water utility have utilized available computer tools to develop a preferred CAP delivery location and elevation economically advantageous to water rate payers. This paper discusses the various factors associated with Tucson's projected need for CAP water storage including reliability, operational flexibility, water quality, shortage, and power management. Each of these factors will affect the degree to which the water utility can successfully assimilate Central Arizona Project water into its groundwater supply system. Although a decision regarding storage location and volume has been postponed for the present, the initial years of CAP usage by the City of Tucson will provide sufficient test to justify the decision for no storage or prove its necessity.

Although the effect cannot be isolated, removal of slash windrows has apparently caused some reduction in annual water yield response from a cleared ponderosa pine watershed. Removal of the slash windrows has eliminated any influence on snow distribution, retention, and melting and has also eliminated any possible influence on losses to soil water recharge and on runoff efficiency of the watershed.

Almost all runoff from small rangeland watersheds in the Southwest is the result of intense thunderstorm rainfall, and the variability of this rainfall is an important runoff-influencing factor in such areas where high intensity rainfall dominates watershed hydrology. Thunderstorm runoff estimates for small rangeland watersheds can be made using a multitude of estimating techniques ranging from simple table and graph procedures to utilizing high-speed computers, and even the most sophisticated models greatly simplify the rainfall input. In this paper, the combined effects of rainfall quantity and intensity, and the rainfall energy factor, EI, in the Universal Soil Loss Equation (USLE), were analyzed, and simple procedures for estimating semiarid rangeland runoff volumes were developed. Equally good correlations with runoff volumes were found for EI, and for total storm rainfall times maximum rainfall intensities for 5, 10, and 30 minutes and the square of the maximum 60-minute rainfall.

Flood flows and water quality in the Southwest are roust dramatically influenced by short, intense rainstorms. Runoff from these storms has been modeled with some success. One key element that has often been overlooked, however, is the importance of intra-storm rainfall distribution on runoff response. Actual storms were modeled for small experimental watersheds in the Southwest using different time increments of intra-storm rainfall. Increments of 5 minutes or less proven satisfactory for accurate hydrograph simulation. As increments became longer than 5 minutes, the ability to simulate actual hydrographs became increasingly difficult. Increments of 30 minutes or longer proved unacceptable for most storms. Hydrologic models must be sensitive to short time increments of intra-storm rainfall to accurately predict peak flows in the Southwest. Watershed treatments will be more cost-effective if their design considers intense bursts of intra-storm rainfall in addition to total storm volume.