The investigation by the International Boundary and Water Commission found that at least 28 million gallons of sewage were released over a four-day period.
However, the total could have been far worse -- 256 million gallons of sewage didn't reach a water treatment plant during January and February, the report from a commission working group said. Initial estimates were that 143 million gallons of sewage spilled.
The group noted the difficulty in determining the exact scope of the spill, since it was not directly measured.
The mess caused a strong, foul odor throughout San Diego's South Bay neighborhoods, resulting in a flood of complaints to the commission, and elevated levels of E. coli bacteria were noted in the Tijuana River Valley.
According to the report, strong storms and a heavy flow caused the collapse of a wastewater collector on Jan. 1 in Tijuana, resulting in a sinkhole that damaged a roadway and bus shelter. Uncontrolled flows ended up in the Tijuana River, which runs north into the U.S., where it spills out into the Pacific Ocean.
The 28 million gallons of spilled sewage came during repair work in early February, which diverted the flow into the Tijuana and Alamar rivers, the report said.
"This report provides concrete recommendations to help us properly respond to and prevent transboundary spills," said U.S. Commissioner Edward Drusina.
"It also has some reasonable and effective ideas as to how to speed up notifications," Drusina said. "The commission is committed to partnering with other agencies to use the report's findings to make improvements."
Mexican Commissioner Roberto Salmon indicated that officials south of the border are making efforts to prevent such spills from happening, and improving notifications when they do.
Imperial Beach Mayor Serge Dedina agrees with the recommendations and prevention efforts.
“We need to make sure we protect the health of American residents, the IBWC needs to prioritize the interest of American residents and not worry about upsetting people in Mexico,” Dedina said.
Dedina is hopeful money to build these sewage mitigation projects is raised.
“If our president is advocating $15 billion for a border wall, I think we can spend a little of that money making sure we protect our Navy Seals and Border Patrol Agents who are getting sick from sewage,” Dedina said.
The report noted that strong rains began overwhelming Tijuana's aging water collection infrastructure as early as December.