Data Series 283
U.S. GEOLOGICAL SURVEY
Data Series 283
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Even though the spring of 2006 was wetter and cooler than is typical, the springs at Pinnacles National Monument discharged water at relatively low rates—from about 0.1 to about 6 gallons per minute. The importance of ground water discharged at low rates from Superintendent’s Spring, Chalone Bridge Spring, and Oak Tree Spring to the health of nearby stream environments is uncertain. Ground water discharging from these three springs and McCabe Canyon Spring No. 1 reinfiltrated the soil after flowing downslope approximately 30 ft, 5 ft, 15 ft, and 70 ft from the respective spring pools. If the infiltrated water reemerges in adjacent stream channels, the springs may provide baseflow to these streams. Although the flow rates of springs were low when they were sampled, the water quality of the sampled springs may be similar to the water quality of other diffuse seepage that provides baseflow to streams.
Water-quality analytical results from spring samplesare presented in table 2. Spring pH was found to be near neutral, ranging only from 6.3 to 7.4. Considering that the springs are in similar environments and relatively closely spaced, there was substantial variability in spring temperature (14.5–19.5°C), specific electrical conductance (171–497 µS/cm), and dissolved oxygen concentration (1.5–7.0 mg/L).
Hydrologists often classify natural waters by chemical water type, that is, by the dominant cation (positively charged ion) and anion (negatively charged ion), as expressed in milliequivalents per liter. For example, samples from Chalone Bridge Spring and Moses Spring contained more than 50 percent sodium (the dominant cation) and more than 50 percent bicarbonate (the dominant anion) on a milliequivalent-per-liter basis. These samples, therefore, were classified as sodium bicarbonate waters. Likewise, the sample from Oak Tree Spring was a calcium bicarbonate water. The sample from Split Rock Spring does not contain a single dominant cation. This sample was classified as a sodium calcium bicarbonate water because sodium and calcium concentrations together (on a milliequivalent basis) comprised substantially more that 50 percent of the dissolved cations. Because the sodium concentration is greater than the calcium concentration (on a milliequivalent basis) the sodium cation is listed first in the water type designation. Likewise, the sample from Superintendent’s Spring was a calcium sodium bicarbonate and the samples from Willow Spring and McCabe Canyon Spring No. 1 were sodium bicarbonate chloride waters. The relative proportion of major cations and anions in each water sample is shown on a trilinear diagram (fig. 18) as a percentage of the total cationic or anionic content measured in milliequivalents per liter. The ionic composition of ground water varied with spring location. Variability in chemical composition of ground water often results from differences in geology, land use, residence time in the aquifer, and other factors.
The proportion of heavy stable isotopes of oxygen (18O) and hydrogen (2H, deuterium) in water molecules can be used to infer the source and evaporative history of water. Atoms of oxygen-18 (18O) and deuterium (2H) have more neutrons and a greater atomic mass than do atoms of the more common isotopes, oxygen-16, and hydrogen. The difference in weight results in differences in the physical and chemical behavior of the heavier, less abundant isotopes. Oxygen-18 and deuterium abundances are expressed as ratios of the heavy isotope to the light isotope, in delta notation (δ), as per mil (parts per thousand) differences, relative to Vienna Standard Mean Ocean Water (VSMOW) (Gonfiantini, 1978). By convention, the value of both oxygen-18 and deuterium abundance ratios in VSMOW is 0 per mil. Oxygen-18 (δ18O) and deuterium (δD) ratios, relative to VSMOW, can be measured more precisely than absolute abundance, and these ratios are useful in hydrologic studies (International Atomic Energy Agency, 1981).
Most ground water is meteoric, that is, it is derived from direct infiltration of precipitation or infiltration of surface water that was derived from precipitation. Most precipitation throughout the world originates from the evaporation of seawater. The ratios δ18O and δD are relatively constant in atmospheric moisture at given latitudes over the oceans. Water molecules composed of oxygen-18 and deuterium atoms need slightly more energy to evaporate than water molecules composed of lighter oxygen (16O) and hydrogen (1H) atoms. Thus, atmospheric moisture over the oceans is slightly depleted in oxygen-18 and deuterium, compared to VSMOW. Thermodynamic processes slightly favor the condensation of water vapor containing oxygen-18 and deuterium. Condensation of water vapor containing oxygen-18 and deuterium occurs at higher temperatures than condensation of water vapor containing 16O and 1H. As water droplets repeatedly undergo evaporation and condensation, the aqueous phase becomes isotopically heavier and the vapor phase becomes isotopically lighter. Generally, as atmospheric moisture moves inland from coastal areas, the ratio of the heavy isotopes to the lighter isotopes decreases. Heavy isotopes are more concentrated during initial precipitation, which occurs at lower altitudes in coastal areas, and most depleted during later precipitation, which occurs inland at higher altitudes. Oxygen-18 and deuterium are affected similarly by this isotopic fractionation process; a plot of the isotopic ratios of oxygen-18 to deuterium in precipitation is a straight line with a slope similar to the global meteoric water line (Craig, 1961; Gat and Gonfiantini, 1981). The position of a stable-isotopic analysis from a precipitation sample, relative to the line of isotopic fractionation, is controlled by air temperature and storm duration.
Often, the isotopic composition of ground-water samples collected from wells and springs in the same general area plot on a straight line (the local ground-water line) representing the variability of isotopic composition of local precipitation. Waters with isotopic compositions that plot low on the local ground-water line likely condensed at cooler temperatures—at a higher altitude or in a cooler climate long ago—than waters with isotopic compositions plotting higher on the line. Isotopic compositions that plot below and to the right of a local ground-water line likely indicate that the water sampled has undergone some evaporation since falling as precipitation. Often, the isotopic composition of ground-water samples collected from wells and springs in the same general area plot on a straight line (the local ground-water line) representing the variability of isotopic composition of local precipitation. Waters with isotopic compositions that plot low on the local ground-water line likely condensed at cooler temperatures—at a higher altitude or in a cooler climate long ago—than waters with isotopic compositions plotting higher on the line. Isotopic compositions that plot below and to the right of a local ground-water line likely indicate that the water sampled has undergone some evaporation since falling as precipitation.
Figure 19 shows the relation between oxygen-18 and deuterium for all ground-water samples collected from springs at Pinnacles National Monument by the USGS. Too few samples were collected to accurately define a local ground-water line, but most samples plot linearly between the global meteoric water line and a line defined by precipitation collected at Santa Maria, California (International Atomic Energy Agency, 1981) about 110 miles southeast from Pinnacles National Monument.
Tritium (3H) is a naturally occurring radioactive isotope of hydrogen that has a half-life of 12.43 years. Tritium is produced naturally by the interaction of cosmic rays with nitrogen and oxygen in the atmosphere. Tritium exists in the atmosphere as tritiated water and is transferred from the atmosphere to the sea-surface through vapor exchange and rainfall. In this report, tritium was measured in picocuries per liter (pCi/L); 1 pCi/L is equivalent to 1 tritium atom in about 1,018 atoms of hydrogen. Prior to 1952, the tritium concentration in precipitation in coastal California ranged from 9.6 to 16 pCi/L (R. Michel, USGS, oral commun., 2006). About 800 kg of tritium was released to the atmosphere as a result of the atmospheric testing of nuclear weapons during 1952–62 (Michel, 1976), and the tritium concentration of precipitation increased to a maximum of about 3,840 pCi/L. After the cessation of atmospheric testing of nuclear weapons in 1962, the tritium concentration of precipitation decreased; present-day tritium levels in precipitation are near the pre-1952 levels. Because tritium is part of the water molecule, tritium is not affected by reactions other than radioactive decay, and—neglecting the effects of dispersion and mixing with older or younger waters—tritium is an excellent tracer of the movement of ground water recharged fewer than 50 years before present. Figure 20 shows tritium concentrations for samples collected from springs at Pinnacles National Monument (table 2). These samples are plotted at zero years on the time scale, that is, present day at the time of the sampling. For comparison with tritium concentrations in the spring samples, time-decayed tritium concentrations were calculated for water that contains levels of tritium similar to background levels measured in precipitation falling in central coastal California (9.6-16 pCi/L). These calculations are plotted as theoretical decay curves (fig. 20) that describe the tritium concentration remaining in water that initially (at year zero) had tritium concentrations of 9.6 and 16 pCi/L. The decay curves illustrate the changing tritium concentration in recharge water that slowly moves through the ground-water-flow system. Because tritium concentrations in the initial waters for the decay curve calculations contained only background levels of tritium the area between the two curves in figure 20 represents the likely tritium concentration range for precipitation that occurred during periods when atmospheric moisture was not affected by nuclear tests. Tritium concentrations plotting below the two background decay curves represent old water that mostly is unaffected by infiltration of post-1952 rainwater.
A statistical summary of water-quality results is presented in table 3, along with U.S. Environmental Protection Agency (USEPA) and California Department of Health Services (CADHS) standards for drinking water. The standards provide a basis for evaluating the results of this study.
In general, concentrations of chemical constituents in spring waters sampled for this study were lower than USEPA and CADHS drinking-water standards and USEPA recommended water-quality criteria for freshwater aquatic life. However, the concentrations of dissolved arsenic in Oak Tree Spring, Split Rock Spring, and Moses Spring of 19.2, 15.5, and 10.9 μg/L, respectively, were above the new USEPA Maximum Contaminant Level (MCL) for arsenic in drinking water that took effect on January 23, 2006 (U.S. Environmental Protection Agency, 2006a). Historically, Oak Tree Spring and Split Rock Spring were used as sources of drinking water at the Monument. Until recently, the concentration of arsenic in these springs would not have exceeded the Federal MCL. The Federal MCL, for dissolved arsenic in drinking water was lowered from 50 μg/L to 10 μg/L in January 2006 (U.S. Environmental Protection Agency, 2006a). The state of California complies with the Federal MCL with respect to monitoring requirements and is expected to adopt a revised State MCL for arsenic of 10 μg/L during 2007. Historically, Willow Spring also has been used as a source of drinking water; the arsenic concentration of water in Willow Spring South Rivulet (7.5 μg/L) was lower than the federal MCL. The concentrations of dissolved uranium in all spring samples were below the USEPA MCL for drinking water, 30 μg/L, although the uranium concentration in samples from Oak Tree Spring (23.8 μg/L), Superintendents Spring (14.6 μg/L), and Chalone Bridge Spring (17.7 μg/L) were higher than in samples collected at the other four springs (median concentration, 1.34 μg/L).
Nutrient concentrations in all samples were lower than USEPA and CADHS drinking water standards and USEPA-recommended water-quality criteria for freshwater aquatic life. The highest concentration of dissolved nitrate plus nitrite was in the sample from McCabe Canyon Spring No. l, 2.93 mg/L (as N), which is close to the estimated national background nitrate concentration in ground water of 2 mg/L (Mueller and Helsel, 1996).
The concentrations of all constituents in the field blank (table 2) were less than minimum reporting limits, except for a chromium concentration of 0.04 µg/L (equal to the reporting level) and a zinc concentration of 0.7 µg/L (reporting level: 0.6 µg/L). The absence of appreciable trace element and nutrient concentrations in the field blank indicates that sampling techniques likely were sufficiently rigorous, and sampling equipment sufficiently clean, to preclude cross-contamination of ground water collected from springs during sampling.
The percent difference in the sum of dissolved cations and anions (in milliequivalents per liter) for all the samples was small (less than ±3 percent), indicating that spring waters are electrically balanced and that laboratory analytical techniques likely produced valid data.
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