Southern California’s largest water district has secured a $175 million deal to purchase five small islands in the Sacramento-San Joaquin River Delta in a move that frightens some of the state’s agricultural interests, but could serve as a boon to Gov. Jerry Brown’s controversial water tunnel project.

The Metropolitan Water District of Southern California (MWD) — an agency that supplies water to more than 19 million state residents —  has agreed to pay $175 million for five islands that comprise roughly 20,000 acres of agricultural property in the Delta, according to the Associated Press. The water district bought the land from a company controlled by Swiss financial giant Zurich Insurance Group.

The deal gives MWD significant control over the Delta, where the State Water Project (SWP) and federally-operated Central Valley Project (CVP) pump millions of acre-feet of water from Northern California reservoirs to thirsty farms and communities in the south.

The acquisition of the Delta islands has caused significant consternation among some of the region’s water interests, who are fearful that MWD will not honor previous land-use restrictions put in place by the original Swiss owners.

“It may not be comfortable to have your opponent owning the land next door, but that’s what it is,” George Hartmann, a lawyer for Delta water interests, told the Sacramento Bee.

Meanwhile, the move could also significantly bolster the likelihood that Gov. Jerry Brown’s long-hoped-for water tunnel project, California WaterFix, can break ground after years of lawsuits and opposition from the state’s environmental groups. The $15.5 billion plan would see the construction of two underground tunnels in the Delta to ensure reliable water transportation to the southern two-thirds of the state.

Earlier this week, the U.S. Interior Department announced it would open an investigation into allegations that federal money earmarked for fish and wildlife protection was improperly used on research for the tunnel project.

Metropolitan Water District is reportedly one of the leading advocates for the tunnel project.

Follow Daniel Nussbaum on Twitter: @dznussbaum