Chemical Modeling of Consequences of Surface-Water Delivery Through Existing City of Albuquerque Infrastructure

Science Center Objects

The City of Albuquerque has historically obtained all of its municipal-supply water from production wells completed in sediment of the Santa Fe Group aquifer system. Like several communities in the Rio Grande Valley and various parts of the arid Southwest, Albuquerque has evaluated the use of surface water as a means to improve sustainability of its municipal water supply. The City currently is in the process of implementing a plan to divert much of its municipal-supply water from the Rio Grande and reduce production from wells starting in 2007. Treated water from the Rio Grande will be delivered to customers mostly through existing infrastructure, sometimes after mixing with ground water.

The existing infrastructure used by the City has previously carried only ground water, in some cases for as long as 50 years. Over this period of time, precipitates from ground water have likely accumulated in pipes and on other surfaces of the water-supply system, including residential plumbing and appliances. Because water from the Rio Grande can differ substantially in chemical characteristics from ground water in the Albuquerque area, delivery of surface water or mixtures of surface water and ground water through the existing infrastructure could potentially result in dissolution/dislodging of precipitates that were deposited (and subsequently remained in equilibrium with) ground water. Depending upon the chemical composition of these precipitates, their dissolution could impart unusual color, flavor, and (or) odor to water that is delivered to customers, and could potentially increase the trace-element concentration of delivered water. In addition, dissolution of precipitates (particularly calcium carbonate) could expose large sections of the infrastructure to corrosion. Mixtures of surface and ground water also could potentially cause precipitation of new minerals in the distribution system, reducing efficiency of water delivery. By performing chemical and mineralogical analysis of existing precipitates and modeling their likely behavior with surface water and surface-water/ground-water mixtures, as well as modeling possible precipitation of minerals from new water mixtures, knowledge could be gained to aid in preventing or alleviating potential problems associated with the change in water source.

OBJECTIVES AND SCOPE
The objective of this project is to determine the potential chemical effects of delivering Rio Grande surface water, in some cases blended with ground water from the City of Albuquerque municipal-supply wells, through the existing distribution infrastructure. In particular, the potential for dissolution of precipitates present on surfaces within the distribution system will be studied. The possibility of precipitation of additional minerals by surface water or mixtures of surface water and ground water also will be investigated.

APPROACH
This study will be done in two phases: collection and compilation of mineralogical and chemical data, followed by modeling of geochemical reactions.

Phase I:
Samples of material precipitated by ground water on surfaces of the existing distribution system will be collected and analyzed for mineralogy and, in some cases, chemical composition. Efforts will be made to compile any existing data of this nature and (or) to obtain previously collected samples from the City. Sample collection/analysis will focus on obtaining information on precipitates in different areas of the City and from different components of the distribution system – in particular, distribution lines, reservoirs, and residential water heaters.

Historical data about the chemical compositions of surface water from the Rio Grande and ground water from City municipal-supply wells will be compiled and examined to determine representative water for chemical modeling. Data on the chemical composition of surface water from the Rio Grande during different seasons and flow conditions will be obtained from the USGS National Water Information Systems database. Data on the composition of ground water from City supply wells will be obtained from a City database. The data will be examined for ranges in chemical composition, as well as ranges in saturation indices of selected minerals as calculated by PHREEQC. Representative samples will then be selected for use in geochemical models involving mixing of water and equilibration with mineral precipitates found to be present in the distribution system; models also will be used examine the potential for formation of new precipitates. Expected changes in the chemical characteristics of Rio Grande water during treatment by the City also will be taken into account in determining water for modeling.

Phase II:
The geochemical modeling package PHREEQC will be used to evaluate the likely results of mixing Rio Grande surface water and City of Albuquerque ground water from various well fields, and of placing these mixtures in contact with mineral precipitates found to be present in the City’s water-distribution system. Specific mineral phases to be included in the modeling will be determined based on minerals expected to precipitate during mixing, as well as on the mineral and chemical analysis of existing precipitates. The availability of water-quality data on redox conditions and minor-element concentrations could limit the mineral phases that can be modeled. The mixtures of surface water and ground water to be used will be based on current knowledge of the likely future operation of the water-supply system. Models will be run using a range of temperatures, redox conditions, and partial pressures of selected gases that can be expected to occur within the distribution system. Results of the modeling will be examined for indications of mineral precipitation during mixing, as well as for the potential that minerals already present within the distribution system will dissolve. Conditions under which precipitation and (or) dissolution is most likely to occur will be evaluated. Also, the general compositions of the resulting water, as well as any new mineral precipitates, will be determined.

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