By John A. Izbicki
U.S. Geological Survey Fact Sheet 126-96
Source and Movement of Ground Water
Oxygen-18 and deuterium are naturally occurring stable isotopes of oxygen and hydrogen. Oxygen-18 and deuterium abundances are expressed as ratios in delta notation as per mil (parts per thousand) differences relative to the standard known as Vienna Standard Mean Ocean Water (VSMOW). By convention the value of VSMOW is 0 per mil.
Most of the world's precipitation originates from the evaporation of seawater. As a result, the delta oxygen-18 and delta deuterium composition of precipitation throughout the world are linearly correlated and distributed along a line known as the meteoric water line (fig. 2). The delta oxygen-18 and delta deuterium composition of a water sample relative to the meteoric water line and relative to the composition of water from other areas provides a record of the source and evaporative history of the water and can be used as a tracer of the movement of the water.
The delta oxygen-18 and delta deuterium composition of water from nearly 240 wells in the basin ranged from -8.85 to -1.75 and -62.5 to -9.5, respectively (Izbicki and others, 1995). The isotopically heavier (less negative) waters result from the mixing of freshwater with seawater or other saline water. The source of these waters has been discussed by Izbicki (1996) and will not be discussed further in this report. Many of the remaining differences in the isotopic composition of ground water are the result of differences in the temperature of condensation (dew point) of precipitation in different parts of the basin. Water recharged from precipitation (or resulting runoff and streamflow) that condensed at higher altitudes and lower temperatures is isotopically lighter (more negative) than water recharged from precipitation that condensed at lower altitudes and higher temperatures. Most other observed differences in the isotopic composition of sampled waters are the result of evaporation, the use of imported water, or (for water recharged many thousands of years ago) long-term changes in climate.
Water from most wells in the Santa Clara River valley and the upper and lower aquifer systems underlying the Oxnard Plain had delta deuterium values less than -50 per mil (fig. 3). (The delta deuterium composition of water resulting from the mixing of freshwater with seawater or other saline water is not shown in figure 3.) Water in these areas was recharged by infiltration of surface water from the Santa Clara River that originated largely as runoff from the higher altitudes of the Topatopa and San Gabriel Mountains.
Water from wells in Las Posas Valley, Pleasant Valley, and near the flanks of the mountains north of the Santa Clara River had delta deuterium values greater than -45 per mil and was isotopically heavier than water from most wells in the Santa Clara River valley or Oxnard Plain. Water in these areas was recharged by direct infiltration of precipitation or infiltration of runoff in small streams that drain the area.
No large areas of isotopically heavy water were present in the upper or lower aquifer systems underlying the Oxnard Plain. These data suggest that the amount of ground-water movement from Las Posas Valley, Pleasant Valley, and near the flanks of the mountains to aquifers underlying the Oxnard Plain is small.
Water from some wells in the upper aquifer system near the northeast corner of the Oxnard Plain have an intermediate isotopic composition (delta deuterium values between -45 and -50 per mil) (fig. 3). As a group, water samples from wells in this area were affected by evaporation and plot to the right of the meteoric water line. These samples were collected near the end of an extended drought when recharge from the Santa Clara River was scant. In this area, there are no thick clay layers separating the upper aquifer system from the surface; and, therefore, irrigation return water can infiltrate to the water table. Isotopically heavier water is not present in the upper aquifer system farther away from the recharge area because low-permeability clay deposits underlying the Oxnard Plain isolate the aquifers from land surface.
Isotopically light water (less than -50 per mil) in the upper aquifer system in parts of Las Posas and Pleasant Valleys near streams(fig. 3) is the result of recharge by treated municipal wastewater. The wastewater originated as water imported from northern California for public supply. This water is isotopically lighter than water from local precipitation and plots along a line parallel to and below the meteoric water line (fig. 2). Northern California water also was present in parts of the Santa Clara River valley, but its presence is less obvious because native water in the Santa Clara River valley is lighter than water in other parts of the basin.
The lightest water (less than -60 per mil) in the basin is from deep wells at the end of long flow paths in aquifers underlying the Oxnard Plain. The isotopic composition of this water reflects a cooler temperature of condensation and cooler climatic conditions than those found in the basin today. It is possible that these waters are very old and were recharged many thousands of years ago.