INTRODUCTION
Water used in Malibu has been almost entirely imported starting sometime after January 6, 1971, when the present purveyor, Water Works District 29 (WWD 29) obtained the Malibu Water Company (MWC). Until that time, the water supply in Malibu was from original developments of ground water in the local area and, reportedly, some truck importation. However, the conservation of Malibu's ground water per se has never been seriously considered. But now, in view of the current drought and well-publicized concern about a water shortage, it is appropriate to bring to public attention the fact that ground water in Malibu does occur in volumes sufficient to make its conservation worthwhile. Large-scale water conservation is accomplished in either of two ways. One is the impoundment of surface runoff in reservoirs. The other is by distributing surface runoff in spreading basins where it infiltrates to become ground water. Neither seems feasible in Malibu. On the other hand, on a small scale, the conservation of ground water is possible by blocking its movement along some stream channels. Before considering this, however, the present source of and usage of water in Malibu should be understood.

SOURCE OF MALIBU'S WATER
The water Malibu uses is supplied by the Metropolitan Water District of Southern California (MWDSC). MWDSC water comes from the Colorado River, the State Water Project, the latter originally initiated as the "Feather River Project," and the Owens Valley Aqueduct, although this latter source may be entirely under the control of the Los Angeles City Department of Water and Power. MWDSC distributes the water to some 26 agencies including 14 cities, 12 municipal water districts, one utility district, and one water authority. The West Basin Municipal Water District (WBMWD) is one of the MWDSC agencies. It supplies water to Waterworks District 29 (WWD 29), one of its sub-agencies. WWD 29 in turn supplies water to what I would call the sub-districts of Marine del Rey, Topanga, and Malibu. Actually the Malibu sub-district includes not only the City of Malibu but also an area of unincorporated Los Angeles County extending to the County line.

This hierarchy of water authority is somewhat analogous to the distribution of goods in which the MWDSC acts as a manufacturer, agencies as wholesalers, sub-agencies, such as WWD 29, as retailers, and sub-districts as groups of customers. Carrying this analogy one step further, the individual customer's water meter can be regarded as a sort credit card - a concept everyone certainly can understand. However, unlike this goods-distribution analogy, both the agencies and the sub-agencies may have their own sources of water, particularly ground water, and also certain independent managerial or supply functions such as water treatment and recycling.

MALIBU WATER USAGE
The importance of conserving ground water in Malibu is a function of four variables: [i] the cost to conserve it; [ii] the cost of imported water; [iii] the ratio of the volume that can be conserved to the imported volume; and [iv] Malibu's projected growth and accompanying increasing water demand. It is understood from WWD 29 that its average annual importation from WBMD is 10,000 acre-feet (ac-ft.) with 1,500 to Marina del Rey, 1,500 to Topanga, and 7,500, to Malibu. This is confirmed by MWDSC which reports the average annual purchase by WWD-29 over the previous 10 years to be 9,976 ac-ft. Further, the 7,500 ac-ft. to Malibu is calculated as the total that passes through customers' meters. Over the years since WWD 29 became Malibu's water purveyor, the daily consumption has been estimated at about 135 gallons per capita per day (gpcd), a factor that is reasonable based on observations of household domestic uses as well as studies such as that of Savini and Kammerer (1961).

These data present a problem. The population in Malibu in the year 2000 was slightly less than 13,000. One prediction has placed it at 14,254 in 2010. Today it probably is about 14,000. Using the 135-gpcd datum, the total annual usage per person would be a little over 2,000 ac-ft. leaving a total of a little over 5,000 ac-ft per year unaccounted for. Water distribution systems leak and, generally, it is impractical to monitor such leakage. Furthermore, leakage loss in Malibu may be greater than in many other systems because some of the lines are very old, dating back to MWC days. Even so however, a discrepancy of over 5,000 ac-ft is very much greater than any leakage loss imaginable. The circumstances therefore indicate that the gpcd criterion to calculate usage is only indirectly useful to calculate water use in Malibu.

Water use in Malibu is best considered in terms of a classification of users. For example:

1. Single-family residences on small lots allowing for little if any irrigation.
2. Single-family residences on lots with yards that require minor to moderate irrigation.
3. Single-family residences with estate-sized lots and the potential for extensive irrigation.
4. Multi-residential facilities such as condominiums served by package treatment facilities.
5. Multi-residential facilities such as condominiums not served by package treatment facilities,
6. Restaurants.
7. Daily-visitor facilities such as shopping centers, City Hall, and the Hughes Laboratory.
8. Pepperdine.

Furthermore, there are unmetered usages from fire hydrants that are fed directly from WWD 29 tank reservoirs, although it is understood that other than for fire-fighting, hydrant users such as contractors are supposed to obtain a permit and take from a temporarily metered hydrant. Finally, it is uncertain the extent to which the total usage includes Leo Carrillo State Park which may use WWD 29 water to supplement its ground-water supply. In any event, a better analysis of Malibu water usage is desirable, because that could be the basis to determine the extent to which imported water usage might be reduced, and therefore, from an actuarial point of view, whether conserving ground water in Malibu is feasible.

WATER COST AND VALUE
Now in order to understand why I am not an accountant, consider the following. The cost of water to consumers in Malibu is according to a graduated scale based on usage volume. On my bill, it ranges from $4.160 to $12.48 per billing unit of 100 cubic feet, depending on arbitrarily defined "normal," "conservation," and "excessive" usages. Data are not immediately available regarding the extent to which there are, meter by meter, such usages in Malibu, but the total annual cost to the public assuming the normal usage rate is $1,812.10 per acre-foot, and this must include the overage of operation and maintenance costs as well as profit. Consequently, the minimal value of ground water in Malibu should be some function of $1,812.10 per acre-foot, and greater than that amount. Where accounting matters go off the rail is trying to decide the value of Malibu's ground water when one considers the "conservation" and "excessive" usages. Until set straight by WWD 29's nummary gnomes, I am assuming that ground water in Malibu has a minimum value of $2,000 per acre-foot, which, if true, would mean an average annual cost $15 million to Malibu users of WWD 29 water.

RINDGE DAM GROUND WATER STORAGE
Presumably, with formation of the Marblehead Land Company in 1921, the MWC also was established. Mr. Tom Doyle, now deceased, was a life member of the Malibu Historical Society and a resident of Malibu since 1945. According to him (Doyle, 2002), the Rindge dam was constructed in 1924, and by 1926, an 8-inch pipe line brought water from its reservoir to be distributed as part of the MWC system. As an aside, a similar kind of system, but much smaller of course, for a time helped to supply the Zuma Canyon area from a small dam, presumably built by MWC, located about a mile upstream beyond the end of Bonsall Drive. Now it's a nice place for a picnic. I hope some of Malibu's goofy greeners won't want to destroy it. But I digress.

The ground water impounded by the Rindge dam is of especially questionable quality because its source is partly effluent from the Tapia treatment plant. Nevertheless, if the recent Corps of Engineers estimate of 780,000 cubic yards of alluvium trapped behind the dam is correct, the ground-water storage there should be in the range of 97 to 145 ac-ft assuming porosity in the range of 0.2 to 0.3. This implies a value in the range of $194,000 to $290,000, cash on the barrel head, but much more, if used, because of recharge by Tapia continually, and by storm runoff periodically. The great advantage of recovering storage from the Rindge dam alluvium is that it can be accomplished simply by tapping through the dam near its base. Clever, wot? In effect, the Rindge dam alluvium acts as a gigantic storage tank. If that storage were to be developed as a water source, it would constantly receive cost-free in-flow. Absent heavy metals and other deleterious substances that it is claimed Tapia does not transmit to Malibu Creek in significant concentrations, the storage would need only chlorination for potable use, and even at its present level of quality it should be suitable for irrigation.

ZUMA CANYON GROUND WATER STORAGE
No one observing the Zuma Canyon creek in flood can help but wonder at the amount of water that is wasted to the ocean. The total flood flow is easily estimated at the ford in well-named Rainsford Place. During storms there, I have observed flow occurring over a period of hours to be about 50 cubic feet per second. An average flow at this rate for one day would amount to about 100 ac-ft. Parenthetically, the total surface flow from streams in Malibu during storms must be astounding. The reason such flows occur is that the stream channels have reached bank storage and therefore, the streams are no longer are influent. Aside from dramatizing the water loss, such flows demonstrate that alluviums upstream must be approaching or have reached saturation and therefore that the alluvial ground-water storages have reached their maximums.

Ground water is always flowing through the alluvium in Zuma Canyon to be lost to the ocean. To prevent this and hence conserve the ground water, a subsurface barrier could be installed across the channel. For a barrier installed at the end of Bonsall Drive, alluvial storage in the upstream area is fairly easily estimated. Assuming a 65-acre upstream floodplain area and an average alluvial thickness of 40 feet, the volume of the alluvium would be over 113 million cubic feet. Further, assuming a range in porosity of 0.2 to 0.3, the volume of the interstices, and therefore that of stored ground-water at saturation, would be in the range of 520 to 780 ac-ft. The minimum cash value of such storage would be in the range of at least $1.04 to $1.56 million, or about 7 to 10 percent of the estimated annual cost of water now imported to Malibu.

Artificial barriers that restrict ground-water flow are quite common. There are essentially three kinds - injection wells, injection curtains, also called "cut-off walls," and trenches, also call "puddle-clay cut-offs." Injection wells have been used for more than 50 years to prevent sea-water intrusion along West Los Angeles and Long Beach shorelines by maintaining near the shore a fresh-water head consistent with the Ghijben-Herzberg phenomenon to avoid sea-water intrusion. Trenches are installed from the surface to allow replacing excavated permeable materials with impermeable back-fill that physically blocks ground-water flow. The injection well is useless to block ground-water flow in alluvial masses higher than sea level, and the trench would not be cost-effective in Zuma Canyon. The injection curtain, on the other hand, involves injecting a slurry of clay or neat cement in borings drilled for that purpose, and it could easily function as an effective barrier to ground-water flow through in the Zuma Canyon alluvium.

Geotechnical engineer Mark H. Stanley of Kleinfelder, Inc., a well-known engineering consulting firm, has described the installation of injection curtains using a special triple-auger deep-soil mixing (DSM) drilling machine. He reports (Stanley, 2009) that curtains of soil cement and bentonite are being injected to block seepage beneath levees in California's Central Valley as a means to increase their stability. In a recent brief conversation with him, he suggested an installation cost in the range of $15 - 25 per square foot of curtain. A curtain extending 500 feet across the Zuma creek channel just north of Bonsall Drive cul-de-sac with an average height 40 feet up from the alluvium base, would serve to store ground water upstream from it to be available for pumping as required. The installation cost should be no more than about a half-million dollars. Moreover, such a system would be advantageous to the residential properties downstream by facilitating the functioning of septic systems and reducing the risk of flooding by increasing influent conditions downstream of the curtain. It therefore seems worthwhile to consider such a project not only for Zuma Canyon, but also for Trancas Canyon, Las Flores Canyon, Corral Canyon, and, possibly Solstice Canyon. Beyond the City boundaries, the Arroyo Sequit also seems worth considering.

CONCLUSIONS
The ground-water resource in the City of Malibu is very limited. However, in view of the increasing value of water, a program of development and management of ground-water storage in certain stream and floodplain alluviums seems well worth considering not only environmentally, but also in terms of either a potable or an emergency source of supply. Presumably, the City could become an agency served by MWDSC, or a waterworks district as a MWDSC sub-agency. In this regard, a recent rumor that WWD 29 is for sale makes one stop and think. But for present purposes, it seems best that the management of Malibu's ground water should be brought within the purview of WWD 29.

It appears that stream alluviums in Malibu, if properly managed, could provide 15 to 20 percent. of the water volume now imported from WWD 29. Of course, detailed ground-water studies of the channel alluviums would be necessary to demonstrate the feasibility of using them to store ground water. Furthermore, considering the periodicity of recharge to the alluvial aquifers, the most advantageous benefit-cost ratio with regard to conserving such ground water in Malibu requires its continual beneficial use. It is not particularly useful to simply store the ground water for a non-rainy day, so to speak. Actually, it may be found that a certain thickness of unsaturated alluvium should be maintained in order to require room for bank storage as a means to reduce surface flooding during especially intense storms. Beyond this, however, the most efficient use of Malibu's ground water dictates the necessity for some type of separate treatment facilities in order to directly supplement the imported supply more or less on a daily basis.

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