Looking for a reservoir to store a backup supply, one California district went directly to the source: its aquifer.

by Barbara Wolcott

Engineering advancements in basic clean water delivery have given major impetus to the establishment of great cities and small towns around the globe. However, as the size of the urban enclaves grows, so too does the controversy of water storage for millions of people.

The more densely populated regions have irregular replenishing rains, which require that water be collected in times of abundance in order to serve people when rainfall is sparse. For hundreds of years, the solution has been dams, but they threaten the habitats of endangered animals and plants. Thus, engineers are examining other solutions, including one in progress at the Calleguas Municipal Water District in Ventura County, California.

Like so many creative solutions to engineering problems, this one arose from calamity. As water service companies in southern California struggled to recover from the record-setting drought of the late 1980s and early 1990s, they were blind-sided on Jan. 17, 1994, by the Northridge earthquake.

For the Calleguas Municipal Water District, it was a nightmare, as West Valley Feeder No. 2, the single feeder line into its water system, was cut by the quake. Half a million people were left with less than 10,000 acre-feet of drinking water stored in Lake Bard until the line from the State Water Project was restored. When the line was repaired, the greater demand in the Los Angeles metropolitan area kept water from reaching Calleguas Municipal's system even longer.

The State Water Project of 1957 was one of immense proportion, carrying water from the rain-rich Sacramento River delta near the state capital to semi-desert southern California by way of canals and pump stations that drive water 300 miles south. In one earthquake, water service for millions was in grave jeopardy.

It was a scenario envisioned more than a decade earlier by mechanical engineer Don Kendall, who had a plan to avoid it. As costs for dams and water storage escalated during his tenure at the Metropolitan Water District of Southern California, which runs the West Valley Feeder, Kendall became convinced that recharging aquifers was a solution to manage growing water needs.

Kendall was general manager of the Calleguas district (a member of the Metropolitan system) at the time the quake hit. He completed his initial study for the Las Posas Basin Aquifer Storage and Recovery Project by the end of summer 1994, and the draft environmental report was finished three months later.

Recharging on a Unique Scale

The idea of recharging an aquifer is not new, nor is it untried. The city of Oxnard, Calif., and other coastal municipalities have had water injection wells for a number of years in their efforts to deal with the problem of saltwater intrusion into the water table. The idea was discussed in engineering circles in the 1960s, then abandoned because it was thought to be more difficult than creating dams, which capture water faster.

What makes the Las Posas Project unique is that it has never been attempted on such a scale for drinking water. It has the capacity of more than a quarter-million acre-feet. An acre-foot is equal to water covering one acre at a depth of one foot. That comes to 43,560 cubic feet, 325,851 gallons, or 1,233 cubic meters.

The Calleguas Municipal Water District serves an area of approximately 385 square miles and provides water for residential, commercial, industrial, and agricultural uses in the cities of Moorpark, Simi Valley, Thousand Oaks, Camarillo, Port Hueneme, and Oxnard, to part of the Point Mugu Naval Base, and to surrounding unincorporated areas. The district, CMWD for short, receives all incoming water from the State Water Project, and distributes it through 20 retail water purveyors.

The industry recommends two-year water storage capacity to meet a prolonged drought. The topography of Ventura County and the position of water treatment facility discharge points made a reservoir impractical for meeting the district's storage needs.

Besides, the Calleguas watershed is home to 16 threatened or endangered species and 37 others that are candidates for listing.

The construction of another feeder line from the State Water Project could not ease the problem. It might be feasible if a new feeder line could tap into water coming from the Colorado River, which contributes to the metropolitan system. That distribution stream is well south of Ventura County, however, so a new line would be just as vulnerable to earthquake as the present one from the northeast.

Appealing to Farmers and Ranchers

The construction of tanks as another alternative was not feasible. It would have taken 5,600 new storage tanks to meet the district's minimum storage requirements.

Ventura has seven groundwater basins, but not all of them are candidates for recharging. Some areas of aquifer are contaminated with pesticide and fertilizer residues.

Four basins in the region were unacceptable because of uncertain capacity and volcanic rock composition. Other aquifers in the area have high salinity and agriculture contamination of salts, iron, manganese, and sulfates, all very costly to clean up. Only one aquifer, the Las Posas Basin, met all the requirements for the project.

At first, the project was met with suspicion by agricultural interests because Metropolitan has a history of taking water, not giving it back. When engineers explained how increased water in the lower aquifer would raise the level of their wells, thus making it less costly to pump, farmers and ranchers welcomed the plan.

Don Kendall invited environmental groups to help with the environmental impact statement from the start and the support of the Sierra Club, Planning and Conservation League, Environmental Reform, and CAL Trout helped speed permit approval.

The aquifer storage and recovery wellfield in Las Posas Basin lets Calleguas Municipal Water District bank water below ground and retrieve it for distribution.

s While they were waiting for the permits to come from municipal, county, state, and federal agencies, Calleguas engineers designed a wellhead system for drilling a well airtight from the time the first drill bit the dirt. They also completed a working pump that generates electricity as water is gravity-fed into the aquifer, then reverses to extract water when the need arises. Each well takes two to four weeks of drilling 24 hours a day to reach the lower aquifer, which is about a thousand feet below the surface.

Every wellhead is unique and must be individually designed for the varying porosity of the shaft by means of a lithologic log done with precise electronic measurements. The open hole has as many as 24 sensors for specific measurements such as spontaneous potential, a naturally occurring static electrical potential in the earth arising from the diffusion of ions through pore space in rocks, or by natural flow of a conducting fluid, such as briny water.

The gamma ray log is a measurement of the presence of gamma rays and is particularly important because shales and sandstones typically vary in their gamma ray signatures. However, other rocks are radioactive, such as carbonates and feldspar-rich rocks, which is why the well drilling has a variety of other sensors for a combined wellhead picture.

Another measurement rates variable density by way of an acoustic waveform. In an open hole, it is sometimes used to detect fractures.

Temperature logs correlate to other logs for accurate appraisal, such as fluid resistivity, to see if a well is producing water with varying salinity from different zones in the well.

Beyond the location of prime sites for inflow and outflow, a flowmeter calculates vertical flow in a well under pumping, nonpumping, or injection conditions. Each perforation zone in a particular well has a custom shaft based on the greatest efficiency of inflow and outflow.

Each lithographic log is the blueprint for design of the shaft, the exact placement of holes in the piping, and the type of gravel surrounding pipe in the hole. Mechanical engineers work closely with geologists and the pump manufacturers to ensure that each well will function at its greatest efficiency.

Electric Logging: It's Been Around

Electric logging has been used commercially since 1929. In France, the Schlumberger brothers were doing early resistivity logs on oil wells and accidentally discovered that the equipment measured a signal when the power to the unit was off. The equipment signal registered a potential generated in the borehole and gave rise to spontaneous potential, which remains a part of every log currently done.

Each measurement of clay, sand, and rock ensures that fluid and porosity information matches the pore space and geologic sequence in the area. Water added to the process, stray electrical currents, and cathodic protection or drilling mud must also be taken into consideration when the readout is interpreted. Taking into account other variables, including the characteristics of the mud and the effect of temperature on instruments, logs provide a picture for pump and pipe manufacturers to follow in custom designing each well. Injecting water must be done carefully so that air doesn't displace water and clog an aquifer.

The Las Posas project may finally have as many as 30 wells like this one. The first few wells have stored more than 25,000 acre-feet of water.

The Las Posas project will have a capacity of 300,000 acre-feet of water with 25 to 30 wells when it is complete. Since 1961, 18 wells have been drilled, and four are up and running. They have already stored more than 25,000 acre-feet of water. More wells are being drilled, while others are awaiting the installation of pumps and motors.

Kendall said, "We took basic technology and moved it to the next level. Mechanical engineers have to work with geologists and hydrogeologists because not only are you designing a well for maximum efficiency, a hole in the ground, steel casing with a liner and a gravel pack that has a specific bead size for a schematic, but all of it has to be designed to allow water to go in and out. You can measure the efficiency of that design, irrespective of the pump and motor to run at their most efficient operating point ... you also have to develop designs for well efficiency."

Each pump is designed to produce electricity as water is injected. The amount of power generated is more than enough to run the project and excess power is sold back to the grid.

The Las Posas Project records the amount of water injected, and the plan is to never extract more water than it stores, leaving the aquifer with a net gain. It will comply with requirements to filter or cover water supplies without additional treatment facilities and the loss of water to evaporation is virtually nil.

Doing the Math

The best argument in favor of the project is the math—300,000 acre-feet of water stored in a $70 million project, compared with Metropolitan's new dam east of Los Angeles to store 800,000 acre-feet of water at a cost of $2 billion. That comes out to $233 an acre-foot for Las Posas and $37,500 for Metropolitan. Even if an aquifer is contaminated, the additional costs to purify the water may still be cheaper than to construct and maintain a dam with equal capacity.

Word of the project has been getting around. Visitors from other parts of California, as well as from Holland and Asia, are interested in applying the engineering technology developed by Calleguas to aquifers in their own areas.

Kendall was invited by the Thai government to discuss the project in 2001. "They are interested in the same thing, aquifer storage and recovery," Kendall said. "They have big problems there, and it all comes down to what to do with untimely delivery of surface water and how to store it for future use."

Copyright © 2006-21 Claud "Sonny" Rouch, all rights reserved. Website by OACYS Technology. Cover photo by Roberts Engineering.