Mike Gritzuk, Director of Water Services

Paul Kinshella, Wastewater Engineering Superintendent

Robert Hollander, Compliance and Regulatory Affairs Administrator

City of Phoenix, Arizona

Andrew Richardson, Partner

Frank Turek, Hydrologist

Juliet Johnson, Staff Engineer

Greeley and Hansen, Phoenix, Arizona

The U.S. Environmental Protection Agency (EPA) plans to review NPDES permits on a watershed basis rather than individually and to incorporate Total Maximum Daily Load (TMDL) waste load allocations into future permits. This will allow EPA to assess NPDES permits and non-point source discharges in a comprehensive framework. All NPDES permits and discharges within a defined watershed will be evaluated concurrently in five year watershed cycles. The Arizona Department of Environmental Quality (ADEQ) divided Arizona into 10 watersheds. The Middle Gila River Watershed (MGRW) contains about 6,000 square miles including the Phoenix metropolitan area. ADEQ will initiate the MGRW assessment in 1999; however, EPA needed the assessment information in 1997 to develop U.S. Fish and Wildlife Service Endangered Species Act Section 7 consultations for the 91st Avenue Wastewater Treatment Plant (WWTP) and other Phoenix area discharges.

The City of Phoenix and its Subregional Operating Group (SROG) partners, Glendale, Mesa, Scottsdale and Tempe, operate the 91st Avenue WWTP which discharges highly treated effluent to the Salt River. EPA approached Phoenix and requested SROG complete an assessment of the MGRW to facilitate issuance of the 1997 NPDES permit. SROG recognized there were short-term benefits to completing the assessment related to the 1997 NPDES permit. They also recognized a long-term benefit that could be realized by providing good data that EPA could use as the foundation to establish TMDLs. This benefit was demonstrated by SROG in a 1996 ultra-clean mercury analysis pilot study. EPA was facing a lawsuit to set a mercury TMDL for the Salt and Gila Rivers based on the supposition of special interest groups that the 91st Avenue WWTP was discharging excessive mercury. The ultra-clean program demonstrated the actual mercury concentration was much less than the enforcement standard and eliminated the need for SROG to construct $30 million in WWTP improvements to remove mercury. EPA was given the data to develop a mercury TMDL based on good science. SROG recognized another long-term benefit associated with developing stormwater pollutant models based on good data to maintain BMPs rather than new TMDLs that would require treatment.

It was not realistic for SROG to assess the entire MGRW within the EPA’s 1997 NPDES permit schedule. The section of the MGRW selected for assessment was the river channel from Granite Reef on the Salt River, 75 miles downstream to Gillespie Dam on the Gila River. This reach met the needs of EPA and provided a foundation on which ADEQ could build the assessment for the entire MGRW in 1999. SROG with input from EPA, ADEQ, and Arizona Game and Fish Department developed the scope for the prototype watershed assessment model.

The MGRW assessment model used a GIS platform to inventory water quantity inflow into the study reach, water quality analysis, habitat types, and endangered species. Monthly meetings allowed stakeholders and regulators to monitor the study progress and adjust the scope. Initial review of the amount of water quality data allowed reduction to focusing on a list of 22 pollutants of concern.

Data assessment identified gaps where ADEQ needed to initiate sampling for the 1999 study. It also showed where conventional analysis did not have the sensitivity to quantify concentrations and recommended new monitoring programs to develop accurate data. The team approach to scope development and study progress monitoring allowed for consensus building during the process and facilitated NPDES permit preparation and review. It also resulted in a model that could be applied by EPA to other watershed areas and provided a foundation for ADEQ to follow in the MGRW.

Keywords: watershed, GIS, NPDES, TMDL

The U.S. Environmental Protection Agency (EPA) plans to review NPDES permits on a watershed basis and to incorporate Total Maximum Daily Load (TMDL) waste load allocations into future NPDES permits. This will allow the EPA to assess NPDES permits in a comprehensive framework. All NPDES permits within a defined watershed will be evaluated in five-year watershed cycles. The City of Phoenix and its Subregional Operating Group (SROG) partners, Glendale, Mesa, Scottsdale and Tempe, operate the 91st Avenue Wastewater Treatment Plant (WWTP) which discharges highly treated effluent to the Salt River. SROG and the 91st Avenue WWTP are in the Middle Gila River Watershed (MGRW).

SROG initiated the 91st Avenue WWTP Reclaimed Water Study in 1992 in response an increase in Surface Water Quality Standards and NPDES permit monitoring requirements. The Reclaimed Water Study was to identify cost effective, environmentally sound alternatives to constructed improvements at the 91st Avenue WWTP. As a part of the Reclaimed Water Study, 63 effluent discharge alternatives were evaluated including 45 that involved the Tres Rios Constructed Wetlands. The Tres Rios River Management Plan team (TRRMP) was organized to study Tres Rios alternatives in more detail. The TRRMP team consisted of representatives from Federal, State and local agencies, SROG, residents of the Holly Acres area near the WWTP, and others. SROG and the TRRMP developed a large amount of information about the 91st Avenue WWTP and the Tres Rios area, defined as the reach of the Salt River extending 7 miles downstream from the 91st Avenue WWTP.

The EPA goal of reviewing NPDES permits on a watershed basis revolved around the five-year cycle for NPDES permits. The 91st Avenue WWTP NPDES permit was to be reissued in 1997 and the EPA wanted to conduct the review of the 91st Avenue WWTP and other Phoenix area discharges on a watershed basis. The Arizona Department of Environmental Quality (ADEQ) divided Arizona into ten watersheds, but the MGRW characterization by the ADEQ was scheduled for 1999. The ADEQ schedule did not meet the needs of the EPA.

EPA approached Phoenix and requested SROG complete an assessment of the MGRW to facilitate issuance of the 1997 NPDES permit. SROG recognized there were short-term benefits to completing the assessment related to the 1997 NPDES permit. They also recognized a long-term benefit that could be realized by providing good data that EPA could use as the foundation to establish TMDLs. This good data benefit was demonstrated by SROG in a 1996 ultra-clean mercury analysis pilot study. EPA was facing a lawsuit to set a mercury TMDL for the Salt and Gila Rivers based on the supposition of special interest groups that the 91st Avenue WWTP was discharging excessive mercury. The ultra-clean program demonstrated the actual mercury concentration was much less than the enforcement standard and eliminated the need for SROG to construct $30 million in 91st Avenue WWTP improvements to remove mercury. EPA was given the ultra-clean mercury study data to develop a mercury TMDL based on good science. SROG recognized another long-term benefit associated with developing storm water pollutant models based on good data to maintain BMPs rather than new TMDLs that would require treatment.

The MGRW defined by ADEQ contains about 6,000 square miles and was much larger than was needed by the EPA to facilitate the 91st Avenue WWTP NPDES permit review. The initial study area developed for this MGRW study focused on the Salt River extending from Granite Reef Dam downstream to the junction with the Gila River and the Gila River from the Salt River junction downstream to Gillespie Dam. The study area extends about 70 river miles and includes the lands within the 100-year floodplain. The MGRW area was divided into five reaches to conform with designated use standards defined in the Arizona Administrative Code, Title 18, Chapter 11. Each reach had different designated uses and different standards for contaminants. The five reaches were:

• Reach 1 - Granite Reef Dam to 2 km downstream in the Salt River
• Reach 2 - 2 km downstream to the Interstate 10 Bridge over the Salt River
• Reach 3 - Interstate 10 Bridge to the 23rd Avenue WWTP outfall to the Salt River
• Reach 4 - 23rd Avenue WWTP outfall to the confluence of the Salt and Gila Rivers
• Reach 5 - Confluence of the Salt and Gila Rivers to Gillespie Dam on the Gila River

Stressors that represented water quantity and contaminant inflow to the MGRW study area included:

The total list of contaminants cited in NPDES permits and Surface Water Quality Standards was reduced to 22 pollutants of concern. The list contained inorganic and organic contaminants, pesticides and others, such as total dissolved solids selected because they often exceed standards, prompt fish consumption advisory notifications, and were important to wildlife.

A major effort in the MGRW study was to assess the quality of the available information—simply stated, to determine if the information represented good data, bad data, or questionable data. The laboratory techniques were not at issue, the quality of the data was evaluated as it related to identification of exceedances of standards. Exceedances can trigger violations, enforcement, and development of more stringent standards. The purpose of the analysis was to document the total number of potential exceedances that could be projected and to divide them into questionable and bad data exceedances and good data verifiable exceedances. Questionable data exceedances demonstrated the need for more sensitive monitoring and laboratory analyses. This was an essential part of the study because the MGRW study was provided to the EPA and ADEQ to establish future watershed water quality standards.

The MGRW study was a prototype for watershed characterization and a key was development of databases for analysis and GIS platforms. Stressor data existed in paper and electronic mediums, in different software formats and several GIS coordinate formats. Initial analyses involved converting the data to a common format for analysis and screening the information to focus on the pollutants of concern. A detailed analysis of the information in which the reported pollutant of concern concentrations were compared to the standards documented that the majority of potential exceedances were not based on verified concentration but rather were related to questionable data. These were concentrations where the contaminant was detected and exceeded the standard, but the concentration was not at the quantitation limit or where the concentration was reported as a "less than (<)" value that was greater than the standard.

The MGRW also required developing a format for GIS data modeling. GIS projects were developed for:

• Designated Uses
• Surface Water Quality
• Stormwater Quality
• Agricultural Drainage
• Groundwater Quality
• Fish Tissue/Sediment Quality
• Surface Water Quantity
• Habitat/Species

Each pollutant of concern was identified as a layer in each project to allow analysis of the data related to that specific pollutant of concern from all sources. GIS permitted analysis of pollutant of concern trends through the MGRW study area and the database tables allowed monitoring changes in concentration over time. The GIS analyses demonstrated where data gaps existed. These were identified as locations where new monitoring locations could be needed in future studies. The GIS analysis also demonstrated areas where more detailed monitoring is needed to identify the source of contaminants.

The results of the MGRW study were:

• Development of an inventory of available water quantity, water quality and habitat/species data and other information sources necessary to characterize existing water quality and degree of beneficial use protection/attainment in the study area.
• Development of water quantity and water quality databases for pollutants of concern used to conduct a trend analysis and other relevant statistical interpretations so as to characterize existing and alternative future water quality conditions.
• Analysis of the sources and the fate and transport of pollutants of concern. Recommendations for additional data/information needed to more precisely quantify/model these pollutants.
• Development of a database of species abundance and distribution (with particular emphasis on threatened and endangered species), relative to habitat areas.
• A qualitative assessment of the adequacy of existing data to characterize existing water quality conditions.
• Recommendations for additional data needs and potential data-gathering sources.

The initial purpose of the MGRW study was to provide the EPA with information needed to conduct an Endangered Species Act Section 7 consultation with the U.S. Fish and Wildlife Service as a part of the NPDES permit review. The MGRW study also provided information to meet the following additional uses:

• Provide the GIS database framework to ADEQ to form the foundation for the characterization of the Middle Gila River Watershed and a model to be used in other watersheds in Arizona.
• Provide a model for EPA to use to assess the quality of information available that EPA could use to develop TMDLs.
• Provide a water quality, water quantity, and habitat database of the Tres Rios area that could be used by the TRRMP Steering Committee and Technical Committees as a part of their analyses.
• Provide a water quality, water quantity, and habitat database to the U.S. Army Corps. of Engineers for use in their Tres Rios, Arizona Feasibility Study.