Data Series 301
U.S. GEOLOGICAL SURVEY
Data Series 301
Results from analyses of raw (untreated) ground-water samples from SOSA are presented in tables 4 through 15. Ground-water samples collected in SOSA were analyzed for up to 300 constituents, and 210 of those constituents were not detected in any of the samples (tables 3A–L). The results tables present only the constituents that were detected, and list only samples that had at least one constituent detected. For constituent classes that were analyzed at all of the grid wells, the tables include the number of wells at which each analyte was detected, the frequency at which it was detected (in relation to the number of grid wells), and the total number of constituents detected at each well. Results from the flow-path wells are presented in the tables, but these results were excluded from the detection frequency calculations to avoid statistically over-representing the areas in the vicinity of the flow-paths.
Table 4 includes water-quality indicators measured in the field and at the NWQL, while tables 5 through 15 present the results of ground-water analyses organized by compound classes:
The wastewater-indicator compounds have no summary table because the only compound detected, tetrachloroethene (PCE), also was analyzed on the volatile organic compound analytical schedule (table 3A) that is the preferred method for this compound (see Appendix). PCE results appear on table 5.
Results of quality-control analyses (blanks, replicates, matrix spikes, and surrogates) were used to evaluate the quality of the data for the ground-water samples. Assessment of the blanks resulted in censoring of less than 0.2 percent of the data for the ground-water samples. Matrix-spike recoveries for a number of organic constituents were lower than the lower end of the acceptable limits, which may indicate that these constituents might not have been detected in some samples if they were present at very low concentrations. The quality-control results are described in the Appendix.
Detected concentrations in ground-water samples were compared with CDPH and USEPA drinking-water health-based thresholds (California Department of Public Health, 2007a; U.S. Environmental Protection Agency, 2006). The chemical and microbial data presented in this report are meant to characterize the quality of the untreated ground-water resources within SOSA, and are not intended to represent the treated drinking water delivered to consumers by water purveyors. The chemical and microbial composition of treated drinking water may differ from untreated ground water because treated drinking water may be subjected to disinfection, filtration, mixing with other waters, and exposure to the atmosphere prior to its delivery to consumers.
The following thresholds were used for comparisons:
For constituents with MCLs, detections in ground-water samples were compared to the MCL-US or MCL-CA. Constituents with SMCLs were compared with the SMCL-CA. For chloride, sulfate, specific conductance, and total dissolved solids, CDPH defines a “recommended” and an “upper” SMCL-CA; detections of these constituents in ground-water samples were compared with both levels. The SMCL-US for these constituents corresponds to the recommended SMCL-CA. Detected concentrations of constituents that lack an MCL or SMCL were compared to the NL-CA. For constituents that lack an MCL, SMCL, or NL-CA, detected concentrations were compared with the HAL-US. For constituents that lack an MCL, SMCL, NL-CA, or HAL-CA, detected concentrations were compared with the RSD5-US. Note that this hierarchy of selection of comparison thresholds means that for constituents that have multiple types of established thresholds, the threshold used for comparison purposes may not be the one with the lowest concentration. The comparison thresholds used in this report are listed in tables 3A–L for all constituents and in tables 4–15 for constituents detected in ground-water samples from SOSA. Not all constituents analyzed for this study have established thresholds available. Detections of constituents at concentrations greater than the selected comparison threshold are marked with asterisks in tables 4–15.
Field and laboratory measurements of dissolved oxygen, pH, specific conductance, alkalinity, and associated parameters (turbidity and water temperature) are presented in table 4. Dissolved oxygen and alkalinity are used as indicators of natural processes that control water chemistry. Specific conductance is the unit electrical conductivity of the water, and is proportional to amount of total dissolved solids (TDS) in the water. The pH value indicates the acidity or basicity of the water. Two wells had specific conductance values above the recommended SMCL-CA, although only one well was also above the upper threshold and this well was not a public-supply well. Three wells had pH values outside of the SMCL-US range for pH. Laboratory pH values may be higher than field pH values because the pH of ground water often increases upon exposure to the atmosphere (see Appendix).
Volatile organic compounds (VOCs) are present in paints, solvents, fuels, fuel additives, refrigerants, fumigants, and disinfected water, and are characterized by their tendency to evaporate. VOCs generally persist longer in ground water than in surface water because ground water is isolated from the atmosphere. All detections of VOCs in samples from SOSA were below health-based thresholds, and most were less than one one-hundredth of the threshold values (table 5). Approximately 30 percent of the grid wells sampled had at least one detection of a VOC. The only VOCs detected in more than 10 percent of the grid wells were chloroform, a byproduct of drinking-water disinfection, and tetrachloroethene (PCE), a solvent used for dry-cleaning.
Pesticides include herbicides, insecticides, and fungicides, and are used to control weeds, insects, fungi, and other pests in agricultural, urban, and suburban settings. All detections of pesticides in samples from SOSA were below health-based thresholds, and all were less than one one-hundredth of the threshold values (table 6). Approximately 25 percent of the grid wells sampled had at least one detection of a pesticide. The only pesticides detected in more than 10 percent of the wells were the herbicides atrazine and simazine, and deethylatrazine, a degradate of atrazine. These three compounds are among the most commonly detected pesticide compounds in ground water nationally (Gilliom and others, 2006).
Two pharmaceutical compounds were detected in samples from three wells in SOSA at very low concentrations (table 7). The concentrations were less than one hundred-millionth of the concentration of a typical daily dose dissolved in one cup of water.
Perchlorate, NDMA, and 1,2,3-TCP are constituents of special interest in California because they recently have been found to be widely distributed in water supplies (California Department of Public Health, 2007b). Perchlorate was detected in approximately 10 percent of the grid wells, and all concentrations measured in SOSA wells were less than one-third of the NL-CA (table 8). Only one sample contained 1,2,3-TCP, but the concentration was greater than the NL-CA. NDMA was not detected in any samples.
Unlike the organic constituents and the constituents of special interest, most of the inorganic constituents are naturally present in ground water, although their concentrations may be influenced by human activities.
The nutrients, nitrogen and phosphorus, and the dissolved organic carbon present in ground water can affect biological activity in aquifers and in surface- water bodies that receive ground-water discharge. Nitrogen may be present in the form of ammonia, nitrite, or nitrate depending on the oxidation-reduction state of the ground water. High concentrations of nitrate can adversely affect human health, particularly the health of infants. All concentrations of nitrate, nitrite, and ammonia measured in samples from SOSA wells were below health-based thresholds (table 9). Concentrations of phosphorus and dissolved organic carbon were also low.
The major-ion composition, total dissolved solids (TDS) content, and levels of certain trace elements in ground water affect the aesthetic properties of water, such as taste, color, and odor, and the technical properties, such as scaling and staining. Although there are no adverse health effects associated with these properties, they may reduce consumer satisfaction with the water or may have economic impacts. CDPH has established non-enforceable thresholds (SMCL-CAs) that are based on aesthetic or technical properties rather than health-based concerns for the major ions chloride and sulfate, TDS, and several trace elements.
The concentrations of chloride and sulfate measured in samples from SOSA wells were all below the recommended SMCL-CAs (table 10). Two samples contained TDS above the recommended SMCL-CA, but only one was also above the upper SMCL-CA and this well was not a public-supply well.
Eighteen of the twenty-five trace elements analyzed in this study have health-based thresholds. Detections of all trace elements in samples from SOSA wells were below health-based thresholds, with the exception of arsenic and boron (table 11). Samples from four wells had arsenic concentrations above the MCL-US. One of these samples also had a boron concentration above the NL-CA, although the sample was not from a public-supply well.
Iron and manganese are trace elements whose concentrations are affected by the oxidation-reduction state of the ground water. Precipitation of minerals containing iron or manganese may cause orange, brown, or black staining of surfaces. Iron was detected in less than half of the samples, but two wells had concentrations above the SMCL-CA (table 11). Concentrations of manganese in SOSA wells were typically very low, but three wells had concentrations above the SMCL-CA.
Arsenic, iron, and chromium occur in different species depending on the oxidation-reduction state of the ground water. The oxidized and reduced species have different solubilities in ground water and may have different effects on human health. The relative proportions of the oxidized and reduced species of each element also are used to aid in interpretation of the oxidation–reduction state of the aquifer. Concentrations of total arsenic, iron, and chromium, and the concentrations of either the reduced or the oxidized species of each element are reported in table 12. The concentration of the other species can be calculated by difference. The concentrations of arsenic, iron, and chromium reported in table 12 may be different than those reported in table 11 because different analytical methods were used (see Appendix). The concentrations reported in table 11 are considered to be more accurate.
Stable isotope ratios, tritium and carbon-14 activities, and noble gas concentrations can be used as tracers of natural processes affecting ground-water composition. Hydrogen and oxygen stable isotope ratios of water (table 13) can aid in interpretation of ground-water recharge sources. The stable isotope ratios of water depend on the altitude, latitude, and temperature of precipitation and on the extent of evaporation of surface water or soil water. Noble gas concentrations can be used to aid in interpretation of ground-water recharge sources because the concentrations of the different noble gases depend on water temperature. Noble gas analyses were not completed in time for inclusion in this report; they will be presented in a subsequent report.
Tritium and carbon-14 activities (table 13), and helium isotope ratios can provide information about the age of the ground water. Tritium is a radioactive isotope of hydrogen that is incorporated into the water molecule. Low levels of tritium are continuously produced by cosmic ray bombardment of water in the atmosphere, and a large amount of tritium was produced by atmospheric testing of nuclear weapons between 1952 and 1963. Thus, concentrations of tritium above background generally indicate the presence of water recharged since the early 1950s. Helium isotope ratios can be used in conjunction with tritium concentrations to estimate ages for young ground water. Helium isotope analyses were not completed in time for inclusion in this report; they will be presented in a subsequent report.
Carbon-14 (table 13) is a radioactive isotope of carbon that is incorporated into dissolved carbonate species in water. Low levels of carbon-14 are continuously produced by cosmic ray bombardment of nitrogen in the atmosphere. Because carbon-14 decays with a half-life of approximately 5,700 years, low activities of carbon-14 relative to modern values generally indicate presence of ground water that is at least several thousand years old, or has interacted with carbonate-rich sediments in the aquifer.
Of the inorganic tracer constituents analyzed for this study, the only one with a health-based threshold is tritium. All measured tritium activities in samples from SOSA wells were less than one one-thousandth of the MCL-CA (table 13).
Radioactivity is the release of energy or energetic particles during changes in the structure of the nucleus of an atom. Most of the radioactivity in ground water comes from decay of naturally-occurring isotopes of uranium and thorium that are present in minerals in the sediments or fractured rocks of the aquifer. Both uranium and thorium decay in a series of steps, eventually forming stable isotopes of lead. Radium-226, radium-228, and radon-222 are radioactive isotopes formed during the uranium or thorium decay series. In each step in the decay series, one radioactive element turns into a different radioactive element by emitting an alpha or a beta particle from its nucleus. For example, radium-226 emits an alpha particle and therefore turns into radon-222. Radium-228 decays to form actinium-228 by emission of a beta particle. The alpha and beta particles emitted during radioactive decay are hazardous to human health because these energetic particles may damage cells. Radiation damage to cell DNA may increase the risk of getting cancer.
Activity is often used instead of concentration for reporting the presence of radioactive constituents. Activity of radioactive constituents in ground water is measured in units of picocuries per liter (pCi/L), and one picocurie is approximately equal to two atoms decaying per minute. The number of atoms decaying is equal to the number of alpha or beta particles emitted.
The seven SOSA samples analyzed for radioactive constituents had activities of radium and of gross alpha and beta emitters less than established health-based standards (table 14). Activities of radon-222 in samples from four wells were above the proposed MCL-US of 300 pCi/L, although only one sample had an activity that was also above the proposed alternative MCL-US of 4,000 pCi/L. The alternative MCL-US will apply if the State or local water agency has an approved multimedia mitigation program to address radon levels in indoor air (U.S. Environmental Protection Agency, 1999a).
Water is disinfected during drinking-water treatment to prevent diseases that may be spread by water-borne microbial constituents derived from human or animal wastes. The specific viruses and bacteria responsible for diseases generally are not measured because routine analytical methods are not available. Measurements are made of more easily analyzed microbial constituents that serve as indicators of the presence of human or animal waste in water. Drinking-water purveyors respond to detections of microbial indicators by applying additional disinfection to the water.
Samples from seven SOSA wells were analyzed for microbial indicators. No samples contained the viral indicators F-specific and somatic coliphage and none contained the bacterial indicator Escherichia coli (E. coli), but there were three detections of low levels of the bacterial indicator total coliforms (table 15). The threshold for total coliforms is based on recurring detections; thus, the detections reported here do not necessary constitute an exceedance of the MCL-US.
Future work will interpret the data presented in this report using a variety of statistical, qualitative, and quantitative approaches to assess the natural and human factors affecting ground-water quality. Water-quality data contained in the CDPH and USGS NWIS databases, and water-quality data available from other State and local water agencies will be compiled, evaluated, and used to complement the data presented in this report.
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/ds/301
Page Contact Information: Publications Team
Page Last Modified:
