The study, produced by the Community Water Center, Fresno State and the nonprofit Pacific Institute, shows two of every five households nearly 4,000 customers pay more than they can afford for tap water.
Bills climb even higher for those who also buy bottled water to protect their families from the Valley's underground water contamination.
The study sampled 51 small water systems across Fresno, Tulare and Kings counties.
The costs of water are high in these areas due to old infrastructure problems and because the water supply has contaminants such as nitrates and arsenic.
Many of those communities must blend or treat their polluted water, drill a new well or provide an alternative source, passing on the extra costs to ratepayers. In some communities, where no such options are available, residents pay for contaminated water and must buy bottled water out of pocket.
Researchers hope to convince legislators to craft new laws that help people cope with poverty, water supply contamination and the expense of bottled water.
And experts warn that contaminants such as nitrates will only increase in future years, forcing more water systems to upgrade or increase treatment to ensure safe drinking water. The upgrades and cleanup costs will raise rates in many places where people already can't afford to pay water bills.
Already, several communities in central California are fighting utility rate increases.
Earlier this month, residents of Delano filed a lawsuit challenging the city's decision to raise utility rates. The higher rate, meant to cover debt payments on water system improvements, would raise residents' bills by an average of 75 percent over the next five years.