Finding of No Significant Impact: Nogales International Wastewater Treatment Plant

Project Location and Description

The U.S. Environmental Protection Agency (EPA) is considering authorizing the award of Border Environment Infrastructure Fund (BEIF) grants to the City of Nogales (City), Arizona and the Comisión de Agua Potable Y Alcantarillado del Estado de Sonora (COAPAES) for the rehabilitation, upgrade and construction of wastewater infrastructure facilities in the communities of Nogales, Arizona and Nogales, Sonora. The project area is located in southern Arizona and northern Sonora, along the U.S.-Mexico border.

The project, which is binational in scope, will improve and expand wastewater treatment and collection infrastructure, situated in Nogales, Arizona, that serve both communities. In Nogales, Sonora, improvements will be made to the collection system and new wastewater treatment and conveyance facilities will be constructed to handle future flows in excess of Mexico's allocation in the existing international treatment plant. Minute 276, between the Governments of the United States and Mexico, established this allocation at 9.9 million gallons per day (mgd) of treatment capacity. The proposed project includes the following specific works.

In Arizona:
Nogales international wastewater treatment plant (NIWTP) - the capacity of the NIWTP will be increased to 22 mgd and the plant will be upgraded by converting the aerated lagoon system to a conventional activated sludge system with nitrogen removal and sludge recirculation.

International outfall interceptor (IOI) - the existing IOI will be replaced with a new, enlarged interceptor.

Wastewater collection system - deteriorated portions of the collection system will be rehabilitated.

In Sonora:
Los Alisos treatment plant - a new 4 mgd treatment plant, whose treatment train consists of a modified advanced pond system including deep facultative lagoons and infiltration basins, will be constructed. The treated effluent will be discharged to the Rio Los Alisos.

Conveyance system - a conveyance system consisting of an interceptor and pump stations will be constructed to transport wastewater flows to the Los Alisos treatment plant. The pump stations will have the capacity to pump only the average daily flows in excess of Mexico's current 9.9 mgd allocation. Peak flows associated with storm events would not be pumped to Los Alisos but would be bypassed to the gravity system and would flow north across the border and be conveyed to the NIWTP.

Wastewater collection system - deteriorated portions of the collection system will be replaced.

Purpose and Need

Currently, domestic wastewater is conveyed northward from Nogales, Sonora and Nogales, Arizona to the NIWTP via a gravity collection system. The IOI, located along Nogales Wash, is the main line which transports the combined flows from both communities to the NIWTP, where treatment takes place prior to discharge to the Santa Cruz river.

Due to deficiencies within the collection systems in Nogales, Arizona and Nogales, Sonora a certain amount of untreated wastewater escapes into the groundwater table and into Nogales Wash, a tributary of the Santa Cruz river. Similarly, the IOI is limited by bottlenecks and periodically, wastewater flows exceed its capacity resulting in overflows of untreated sewage into the Nogales Wash. Further, the IOI and the collection systems are also subjected to significant infiltration.

In addition to the fugitive flow problem, the hydraulic capacity of the NIWTP is near its limit. As populations grow in both communities, additional capacity will be necessary to address the increasing sewage treatment needs. Furthermore, water supply and consumption and the accompanying wastewater production are expected to grow in the near future as COAPAES upgrades and expands the potable water supply system in Nogales, Sonora.

Lastly, the effluent quality limits for the NIWTP are being revised and the proposed National Pollutant Discharge Elimination System (NPDES) permit calls for the reduction of toxicity to aquatic life, which implies a reduction in ammonia concentrations in the effluent. Additionally, concern has been raised about the potential impact of effluent high in total nitrogen concentration since ammonia will likely be transformed to nitrate, affecting the potable water wells located downstream of the NIWTP.

The infrastructure improvements being proposed will address the water quality concerns and treatment capacity issues of the region. Rehabilitation of the collection systems and replacement of the IOI with a larger pipe will reduce fugitive flows of raw sewage and therefore improve surface water and groundwater quality. Future wastewater flows for Nogales, Sonora will be handled by the new Los Alisos plant and conveyance system, which will be expanded in phases as the demand for wastewater service increases. The NIWTP will be upgraded to comply with new regulatory requirements and protect exposed aquatic biota from acute or chronic effects. In addition, the NIWTP will be expanded by 5 mgd to accommodate additional flows from Mexico in excess of its allocation that may reach the NIWTP as a result of storm events, system failures and until later phases of the Mexican wastewater system are implemented.

Environmental Consequences and Conditions

In compliance with the National Environmental Policy Act (NEPA), EPA has prepared an environmental assessment (EA) that analyzes the potential environmental impacts in Arizona as well as the potential transboundary environmental impacts of the proposed infrastructure improvements. After considering a wide range of regulatory, environmental (both natural and human) and socio-economic factors, the EA did not identify any significant impacts to the environment that would result from the implementation of this project.

Environmental impacts in Sonora are addressed in the Manifestación de Impacto Ambiental, Modalidad General, del Proyecto: "Recoleccion Tratamiento y Disposicion Final de las Aguas Residuales de Nogales, Sonora (Environmental Impact Assessment, General Level, for the Project: Collection, Treatment and Disposal of Wastewater in Nogales, Sonora) which was prepared in accordance with the environmental impact review process of the Instituto Nacional de Ecologia. This analysis supports a determination that the project will not have a significant adverse impact on human health and the environment in Mexico.

Public Review

The EA is on file, along with other project materials, and is available for public inspection at the EPA Region 9 office in San Francisco, California. Copies of the EA are also available for public review at the following locations:

Nogales International Wastewater Treatment Plant (NIWTP)
865 Rio Rico Industrial Park, Rio Rico, AZ 85648
(520) 281-1832

City of Nogales Public Works
1450 North Hohokam Drive
Nogales, AZ 85621

Nogales Public Library
518 North Grand Avenue
Nogales, AZ 85621

Interested persons, including those who disagree with this proposal, may submit comments to EPA Region 9 within 30 calendar days from the date this document is issued. No administrative action will be taken on this proposed project prior to the expiration of this comment period. Comments, via letter, fax or email, should be sent to Evelyn Wachtel at the address listed below.

Evelyn Wachtel (wachtel.evelyn@epa.gov)
U.S. EPA Region 9
75 Hawthorne Street
San Francisco, CA 94105
Telephone: (415) 972-3410
Fax: (415) 947-3547

After EPA assesses any comments received, those comments, EPA's responses, and this FNSI will be forwarded to the Regional Administrator for review and signature. If this FNSI is signed by the Regional Administrator, it will not be recirculated for review but will be available to any individual upon request.

Finding

After review of the EA and any comments received, EPA has determined that the proposed project will not have a significant impact on the environment and that an Environmental Impact Statement will not be prepared for this project.

Felicia Marcus
Regional Administrator