Articles - ASPECTS OF GROUND WATER IN MALIBU

A basic knowledge of ground water is necessary in interpreting much of Malibu's geology. Though not especially significant as a resource, its importance in terms of its is critical. The City currently is considering an addition to the Local Coastal Program that would establish standards with regard to the installation, maintenance, and abandonment of water wells.

BASIC TERMINOLOGY

Ground water is defined as any water in the subsurface regardless of its source or quality, but generally, it commonly is classified in terms of the manner in which it occurs. All ground water occurs in "interstices," i.e., spaces or openings in rocks, either those between individual grains, or fractures. Subsurface sections through which fluids such as ground water can move are said to be "permeable." A basic distinction is that part of the subsurface where the interstices contain both water and "soil gas," commonly much like air, and that part below in which the interstices are full of ground water. The unsaturated zone sometimes is referred to as the "zone of aeration," or the "vadose zone," whereas the lower is called, simply, the "saturated zone." Water in the saturated zone which is phreatic is that which can be recovered using wells. For this reason, the surface of the saturated zone is sometimes referred to as the "phreatic surface," and because its configuration has a low gradient or may be level, it also is sometimes referred to as the "water table".

GROUND WATER MOVEMENT IN THE VADOSE ZONE

The movement of ground water occurs in three ways, by capillary force and gravitational force both related to the interaction of water and inorganic earth materials, and by osmotic pressure. All have significance in certain special ways with regard to the use of property. Both are significant ion relation to the use of property in Malibu.

Capillary Movement

Capillary force is caused by the polar character of both water and mineral molecules. This causes them to be attracted, much in the manner of a blotter. Such minerals are referred to as "hydrophilic" (water loving). This attractive force is strong enough to resist gravity to a limited extent. On the other hand, materials of non-polar molecules are referred to as "hydrophobic" (water fearing). A repulsion force results where water meets non-polar surfaces. This condition is perhaps most familiarly recognized as the immiscibility of oil, which is non-polar, in water.

Capillary water occurs as thin films that adhere to mineral surfaces. This phenomenon is referred to as "capillary attraction," or simply "capillarity." The attraction is strong enough to resist the gravitational force and, at least under some circumstances, evaporation. In fact, to determine the unit weight of a soil mass, in the laboratory the sample is subjected to oven heat. Terzaghi (1942) gives an excellent early description of the effects of capillarity in granular materials, and most later authors of texts concerning ground water give it at least perfunctory recognition, but it is of more concern in the field of soil mechanics because of its effect on soil strength.

Gravitational Movement

As a result of capillarity and the fact that water molecules attract each other, an air-water surface called a meniscus between mineral grains. This surface has measureable tensional strength. As a result, menisci-bounded volumes can form containing aggregates of water molecules. When the mass of such an aggregate becomes great enough, the associated gravitational force overcomes the tensional strength of the lower menisci and water breaks through to spread lower in the section either as capillary water or as water flowing in response to gravity eventually to enter the saturated zone. As a result of this loss, the aggregate mass is again low enough that the integrity of the menisci-volume is restored. It is this sequence of events by which much water passes through the vadose zone to the saturated zone. Water also can move through fractures in the vadose zone that are open enough to allow normal gravitational flow unimpeded by capillarity.

Osmotic Movement

Capillary water retained in the root zone is also taken by vegetation. The transfer of water to the plants occurs through the roots by osmosis. The water then passes up plant stems to be used on the way for plant growth and other of its metabolic processes. Such water as is not retained in this way passes through the surfaces of leaves in a manner closely analogous to animal sweating. In plants, this is referred to as "transpiration." As a result, depending on conditions, the water then is lost to the atmosphere by evaporation. This mechanism of ground water transfer to the atmosphere through the ET process is most important for present purposes, because it can be a significant factor in the functioning of some septic systems. Many septic systems in Malibu depend to some extent on the ET process.

A somewhat esoteric matter concerning movement of water by osmosis has to do with exchanges of ground water with surface water. The issue, whether ground water causes contamination of stream water, is a major concern in planning. If the formation yields water as springs, in other words, by gravity flow, then such inorganic ionic species as well as whatever bacteria might be present are carried bodily into the stream as part of the spring flow. However, if the formation is of clay so that it acts as an semi-permeable membrane, and, as is usually the case, the concentration of dissolved solids is less in the stream water than in the ground water, osmotic pressure forces water molecules into ground water through the membrane until such time that the ground-water pressure head is equal to the osmotic pressure. In terms of water mass movement, this is a minor matter. However, where the issue of sources of stream contamination arises, as is particularly now the case for Malibu Creek, it is to be considered.

GROUND-WATER MOVEMENT IN THE SATURATED ZONE

Ground water in the saturated zone moves entirely in response to gravity. The movement is in the direction of the decreasing hydraulic gradient. Generally, the movement behaves according to Darcy's Law. There are many good texts that explain various aspects of this subject, among them those by Davis and DeWiest (1967), Lohman (1972) Freeze and Cherry (1976), and Driscoll (1986). Aside from texts explaining the technical aspects of how ground water moves in the saturated zone, there are numerous published studies that consider actual circumstances involving such movement on a regional basis. Locally, the most comprehensive of these are those by public agencies, notably the U.S. Geological Survey and the California Department of Water Resources. One reason for this is their cost which generally can't be justified by private interests. Another is because such studies investigate of one aspect of the nation's resources which is the underlying purpose of these agencies. The reason there is no such study of Malibu is that as a resource, ground water there is so limited it has little economic significance.

The need for local ground-water studies in Malibu has become apparent in order to properly evaluate environmental conditions in certain floodplains. Most notable is the Malibu Creek floodplain. However, there is some indication that those of Trancas Creek and Zuma Creek may be desirable to investigate because of concerns for wetlands or what some regard as lagoons at their mouths. In Malibu, as in many coastal areas, the manner in which terrestrially derived ground water and ground water derived from the ocean or other adjacent saline water body move with respect to one another is of special importance. Commonly referred to as sea-water intrusion, this condition was first recognized independently by Ghijben (1889-89) and Herzberg (1901). It is perhaps best described theoretically by Hubbert (1940, pp. 924 - 926), and it has been extensively investigated in practical terms, for example, by Cooper, et al., (1964). Failure to consider sea-water intrusion has vitiated much of the results of an otherwise important recent study of the Malibu Creek floodplain by SEI Staff (2004).

GROUND WATER IN MALIBU

Information regarding ground water in Malibu is very limited. This is because, generally, there are no extensive aquifers and the public water supply is imported. To date there has been no official well canvass of the sort commonly conducted for areas in which ground water is an important resource.

Ground-water Data

There are very little available data on ground-water in Malibu except for certain local areas. Certain local "one-time" studies may contain data such as boring logs and ground-water levels that might be useful for a current investigation in the same locality. Most useful in this regard are the City's operation and maintenance studies of areas such as Big Rock Mesa, or West Malibu Road. Other than this, however, data are lacking. At one time, Los Angeles County maintained a ground-water network that may have included periodic water-level observations of one or more wells in Malibu, but that is no longer the case. To develop ground-water data for a project in Malibu not near an area for which such data are at hand, a site-specific investigation is necessary. In most cases, the only type of ground-water data commonly developed is that concerning the design of a septic system.

Occurrence of Ground Water in Malibu

Probably all formations in Malibu have saturated zones of ground water at some depth with surface above sea level. However, in very few instances could such zones be regarded as aquifers even in the most limited sense. Any attempt to develop a ground-water supply for a large tract would be unsuccessful. Furthermore, there may be an issue of ground-water rights. The Marblehead Land Company may have reserved such rights when subdivision of Malibu was begun.

Bedrock Formations

None of the sedimentary bedrock formations exposed in the City of Malibu is likely to include an aquifer of any significance because, with the exception of the Sespe Formation, they are generally fine-grained and to some extent clayey so that they are not very permeable. In the case of the Sespe Formation, cementation seems to limit permeability, although locally it is known to contain at least limited zones of perched ground water. To some extent, there may be saturated zones in fractures along the Malibu Coast fault, but none has ever been well documented. In this regard, ground water along the northern side of the fault would be more likely to occur at higher elevations, because the fault should act as at least a partial ground-water barrier. The Zuma Volcanics that Yerkes and Campbell (1980) have mapped at isolated locations south of the trace of the Malibu Coast fault are somewhat more likely to have sections that might serve as aquifers, but even so, storage probably would be so limited that the costs of exploration would be difficult to justify.

However, the only demonstrated aquifers in Malibu are the floodplain alluvial deposits of Malibu Creek and the stream alluviums of Zuma Canyon and Trancas Canyon. In both Zuma Canyon and the Malibu Creek floodplain there are wells most of which have not been operated for many years. The floodplain wells were operated perhaps as early as the 1920s initially by the Rindge interests, and probably until importation by County Waterworks District 29 began. Almost certainly, this was done without a clear understanding of the sea-water intrusion mechanism which, if degradation is to be avoided, limits pumping levels to an appropriate design distance above sea level.

CONCLUSION

The ground-water resource in the City of Malibu is very limited. However, in view of water generally, a program of development and management of the storage in certain stream and floodplain alluviums seems worth considering.

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