Many of those additional inmates are parolees that would have normally gone back to state prisons after violating terms of their parole.
So far, the county has admitted 259 percent more parole violators than the state estimated, Casten said.
"We have a lot more of these people than we anticipated, and the average custody days they're getting on (parole) violations has increased steadily," Casten said.
Last fall, low-level parole violators in Contra Costa County were sentenced to an average of 31 days in jail under the realignment plan, he said.
Now, that number has more than doubled.
The county's main detention facility in Martinez has been the most heavily impacted since realignment took effect, Casten said.
He said it is the only county jail able to accept "special needs" inmates -- those with gang affiliations or who require prescription medications for mental health issues.
The Martinez facility already houses the bulk of the county's inmates, which Casten said now includes 149 gang members, 307 high-security inmates -- or those who "don't get along with others" -- and 178 inmates who are in protective custody, such as sex offenders, who are especially at risk in jail.
"When you take all those people and the special needs population from AB 109, that causes that count to run higher than we want it to," the undersheriff said.
In January, 639 of the jail's 695 beds were filled.
If the number of incoming inmates under AB 109 who also fall under the "special needs" camp continues to rise, the Martinez facility will inch closer toward full capacity, Casten said.
For now, he said, the greatest fiscal impact is on the day-to-day costs of feeding, housing and clothing county jails' rising populations.
The county has also reopened a long-closed portion of a West County jail building to accommodate new inmates.
Since October, the county sheriff's office has hired seven new deputies and a sergeant using AB 109 funding to help cover the rising inmate load.
Meanwhile, the county's parole department has added four new staff members to handle its additional duties monitoring parolees, according to Philip Kader, the county's chief probation officer.
"The impact on overall probation services has been buffered by adding new staff to these caseloads using AB 109 funds," Kader said in December.
But Casten said that in order to cover the cost of incoming inmates shifted to the county over the next fiscal year, "we would need almost double the funding that we get now."
The state allocated a relatively low dollar amount to Contra Costa County versus other counties based on how many offenders the county sends to state prison on average, according to Casten and Kader.
Since the county has historically sent fewer felons to state prison, opting more often for alternative programs than in some counties, it has received less state funding to manage its additional public safety responsibilities, they said.
As other counties statewide began feeling the pinch of realignment last fall, the California State Association of Counties, or CSAC, proposed a measure to ensure state funding for these added duties. The proposal garnered support from the California Sheriff's Association and Chief Probation Officers of California, according to Jean Hurst, a CSAC legislative representative.
Since then, the governor has proposed a November ballot initiative that if approved, would provide constitutional protection for counties' realignment funding and also proposes a tax increase to fund education.
"It essentially has the same realignment protections that we had in our own measure," Hurst said. "The governor has a broad range of support from labor and business and the education community...so we're hopeful his measure can be successful."
Hurst said that the sheriff's association and chief probation officers group have both voted to support the measure, and that CSAC's board will decide next week whether to back the initiative.
Casten said that while the county gets enough state funding now, some type of protective measure would be critical to continue managing its new public safety role.