Results of a $1 million investigation into the potential effects of building homes at Napa Pipe were released to the public Friday in a 1,000-page document that delves into everything from traffic congestion to the availability of groundwater.

Commissioned by the county and paid for by developers, the environmental impact report will be used by county officials next year when deciding whether or not to allow a mixed-use development at the south county Napa Pipe site.

The public has two months to comment on the draft report. A final analysis will be available next year.

A summary of the report’s findings:

Traffic

Napa Pipe would add about 17,600 new car trips every day, including 1,600 during morning rush hour and 1,400 during evening peak hour traffic. About half of the traffic from Napa Pipe would flow in and out of the city of Napa.

In most cases, measures can be taken to relieve traffic congestion, and some of the mitigation measures proposed in the report are being considered regardless of whether Napa Pipe ever is built. Many of the intersections affected by Napa Pipe would be congested in the next 20 years with or without Napa Pipe, according to the study.

By 2030, 25 intersections — from American Canyon to the area around Napa Pipe and north to downtown Napa — would be significantly congested if the Napa Pipe project is built (see accompanying chart). Twenty-two intersections will likely be congested by 2030 without Napa Pipe.

Water

Studies show there is more than enough groundwater at the site to support Napa Pipe without siphoning water away from neighboring properties. The water is excellent quality and suitable for drinking.

Napa Pipe would use only about 20 percent of the underground water available to the property. Combined with neighboring properties such as Syar Industries and several vineyards to the east of Napa Pipe, groundwater use in the area would still total only about half of what is available.

Regardless of availability, the county’s General Plan might prohibit the use of groundwater to support the project.

Wastewater treatment

Napa Pipe would either build a new wastewater treatment plant on the property or connect to the Napa Sanitation District sewage system, which currently serves the industrial uses at Napa Pipe.

The Napa Sanitation District has not yet fully evaluated its ability to serve the proposed project, but studies suggest the Napa Sanitation District would not have enough emergency storage capacity to serve Napa Pipe.

The construction of a new on-site treatment plant would not have any significant environmental impacts, but would require a new special district and a sewer management plan.

Flooding

The Napa Pipe project would place housing within a 100-year flood hazard area, and would require that the land underneath the project be raised three to five feet. Raising the site would not increase flooding potential in other areas.

Hazardous materials

The Napa Pipe site is contaminated by hazardous material from the various industrial activities that took place on the site since the 1930s. These contaminants could pose a health risk to residents and employees at the property.

The San Francisco Bay Regional Water Quality Control Board has outlined an extensive cleanup plan designed to make the property safe for residential use, including trucking soil off-site for treatment or disposal, and treating some soil on-site. The developer has already agreed to implement this plan.

Services

Napa Pipe would place additional demands on the Napa County Sheriff’s Office, and could also place unanticipated demands on the city of Napa Police Department.

The project would require 10 additional fire department staff members, a new fire engine and aerial fire apparatus to serve proposed seven-story residential buildings.

Air quality

Napa Pipe would generate new greenhouse gas emissions that would affect long-term air quality.

The project’s greenhouse gas emissions would make up about 2.4 percent of county’s total greenhouse gas emissions in 2020. Cars would account for more than half of the project’s total emissions.

The proposed project includes a number of green features designed to reduce greenhouse gas emissions, but the impacts of the project on air quality are still considered significant.

Endangered Species

Napa Pipe would not directly affect any native habitats. Most of the site is already disturbed by past industrial uses, and sensitive areas would be preserved as part of the project.

Culture

The project would involve demolition of some Basalt Shipyard buildings and structures. Developers are taking steps to document all historic buildings and to implement some into the project itself, but the demolition of historic structures is still considered a significant impact.

Population, jobs

Napa Pipe would add 2,730 housing units (including the senior center), 5,901 new residents and 721 jobs.

Alternatives

State guidelines require that the environmental impact report also studies practical alternatives, ranging from smaller scale developments to a “no-project” alternative.

The industrial/business park alternative, consistent with current zoning, would generate far more traffic than the mixed use development proposed for the site. Lower-density alternatives, including 650-, 950- and 2,050-home projects, would generate less traffic than the current proposal and would provide fewer affordable housing units.

The alternative with the least amount of traffic is the current 2,580-home proposal, but with access to rail.

The draft report analyzes an arrangement with the city in which city water is used for development on the site and groundwater is made available to the city during dry years. This approach could increase the reliability of the city’s water supply.