The girl was sipping a smoothie with her friend at The Dog restaurant at 1:15 p.m. when they heard a "pop" and she felt a sting in her thigh. The victim and her mother, who asked that they remain anonymous out of fear she would be targeted again, believe the gun was fired from a passing car.
A surveillance video from the restaurant shows the teenager and her friend chatting one minute, then frantically using napkins to dab at the blood the next.
After the injury, she was taken to Kaiser Permanente Medical Center in Walnut Creek, where a doctor determined it would cause more damage to her muscle to remove the pellet.
An X-ray later indicated the champagne cork-shaped lead capsule traveled 2 inches up her thigh and was lodged 1.5 inches deep in her flesh.
"It hurt my heart to see her go through this," her mom said. "I would never want this to kill her spirit."
The shooter didn't appear to have specifically targeted the girl, who had come straight from church, she said. The family is "assuming it's a kid" who doesn't understand how much damage the weapon can do.
"We can only believe it was some random thing. I don't want her to be angry in life," she said.
At this point, the police have no leads to go on, making it difficult to determine if the shot was intentional, Alamo Deputy Elmer Glasser said.
Glare from the afternoon sun makes passing cars difficult to view on the surveillance tapes and there are otherwise no clues or witnesses pointing to the responsible party, Glasser said. There are no records of pellet gun injuries on file with the police department, he added.
"Stuff like this doesn't happen here," he said.
The carbon dioxide-powered gun is capable of causing death and serious bodily injury, and this case is being handled as an assault with a deadly weapon.
Pellet guns, like BB guns, use compressed air to fire projectiles. Most guns manufactured today are 177 caliber and can be loaded with either spherical BBs or sharper pellets. In these pistols, the damage done depends on the shape and weight of the projectiles along with other factors, like distance from the target.
Although fatalities from pellet and BB guns are rare, an 18-year-old boy in Guam died earlier this month as a result of a pellet gun wound. The pellet was fired through his lungs and heart - puncturing his aorta. The police reports showed the death was caused by a single shot.
In this Alamo case, the 16-year-old and her family find themselves feeling simultaneously thankful and outraged.
"It could have made her blind or hit her neck. People who say pellet guns are no big deal are wrong," her mom said. Ultimately, they feel oddly lucky, she explained.
The teenager took time off from soccer, school activities and her job to recuperate and is still adjusting to the idea of having a pellet in her leg for the rest of her life. She lost her father when she was young and she knows what it's like to overcome emotionally traumatic experiences, her mom said.
"I want don't this to be a sob story ... I want my daughter to be able to have a smoothie on a Sunday and not worry something is going to happen to her."