Ventura County, so drained of its groundwater supplies that the state threatened to intervene a decade ago, still needs to do more to halt over-pumping that has caused land to sink and seawater to invade coastal wells, a new federal report concludes.
After a 13-year study of the regional groundwater system that stretches from the Santa Clara River near Ventura to the Calleguas Creek headwaters in the east county, the U.S. Geological Survey reported last week that over-pumping has caused parts of the southern Oxnard Plain to sink nearly 3 feet since the 1920s.
Too much pumping by farmers and cities has also sucked seawater inland along the coast, especially at Port Hueneme and under farmlands near Point Mugu in the south Oxnard Plain, the survey found.
The inland push of seawater -- first noticed 80 years ago -- results when more water is pumped from underground pools than rainfall can replenish, a condition called over-drafting. And when too much water is pumped, the land will sink, a process called subsidence.
"New water management strategies will be required to deal with the dual effects of subsidence and seawater intrusion from groundwater pumpage in Ventura County," said Randy Hanson, lead hydrologist for the study.
The Ventura County regional water basin was among the most depleted in California when the federal study began in 1990.
Hanson said local water agencies have implemented many changes in recent years to make the county's groundwater basins healthy again. But more are needed, he said.
Beyond current water replenishment projects -- such as a Santa Clara River diversion dam, settling ponds and recharge basins near Saticoy -- Hanson said Ventura County water agencies need to end coastal pumping during droughts that draws down water tables and allows greater saltwater intrusion.
"I think they're on the right track; they're one of the better sets of water agencies [in California] as far as trying to get something going," Hanson said. "What they still need to do is align their management strategies with climatic cycles."
For example, during the last big drought from 1985 to 1991, well pumping in some coastal areas increased 11%, Hanson said. He said a better response would have been for farmers and water agencies to sharply cut back on pumping near the coast, because freshwater basins there were already low from lack of rain.
Of course, farmers need to pump more water during droughts because so little rain bathes their fields in the winter and spring. So local water officials have built a dam-and-pipeline system designed to capture Santa Clara River water five miles inland and deliver it to the coastal Oxnard Plain and the over-pumped Pleasant Valley area south of Camarillo.
"Replacing water near the coast with inland well water has been our strategy for a long time," said Steve Bachman, groundwater manager for the United Water Conservation District. "The USGS is just confirming what we're attempting to do. We've been working with them."
Getting that done has been a costly and lengthy process.
The centerpiece of the United system is the $31-million Freeman Diversion Dam, completed in 1990, and a two-pronged set of pipelines that deliver river water either directly to coastal farms for immediate use or to settling ponds or gravel pit reservoirs, where the water filters down into underground basins for storage.
The dam is designed to capture about 12,700 acre-feet of Santa Clara River water each year. An acre-foot is 326,000 gallons, or enough water to supply two typical homes for 12 months. The county uses about 480,000 acre-feet of water a year, two-thirds of it on agriculture.
In recent years, much of the captured water has been funneled into the shallow Oxnard Aquifer, which has been substantially replenished in the last decade, Bachman said.
The problem now is United's inability to pump the water out when needed, because the district's wells have traditionally reached into deeper basins where water would still be available during drought. So United last year began a $2-million project to drill four new wells into the Oxnard basin near Saticoy.
Pumping from the shallow basin will allow the area's deeper basins -- which are seriously over-pumped -- to refill, Bachman said. In time, both shallow and deep basins will be replenished, he said. "You just hope that during the good times, you've done enough water management that you can survive a prolonged drought," Bachman said. "If we're not pumping from the coast during wet years, that will slow the [saltwater] intrusion during dry ones."
Very wet years in 1992, 1995 and 1998 have helped Ventura County's water basins. Bachman said conversion of three additional gravel pits near the river as part of the huge RiverPark community planned along Vineyard Avenue will add 10,000 to 15,000 acre-feet of storage.
A state-authorized groundwater management agency has also imposed pumping limits and fines on cities and farmers, cutting pumping substantially from historic levels.