Data Series 285
U.S. GEOLOGICAL SURVEY
Data Series 285 (ver 1.1, August 2018)
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The Southern Sacramento Valley GAMA study unit ground-water-quality data presented in this report are archived in the USGS’s National Water Information System (NWIS) database, except for the following constituents: tritium and noble gases analyzed at the LLNL; chromium, arsenic, and iron speciation analyzed at the USGS’s NRP laboratory in Boulder, Colorado; stable isotopes analyzed at the USGS’s Reston Stable Isotope Laboratory (RSIL); and perchlorate, NDMA, and 1,2,3-trichloropropane analyzed at Montgomery Watson-Harza laboratory; radium 226, 226 Gross alpha–beta radioactivity analyzed at Eberline Analytical Services. These data are available on request. ">The Southern Sacramento Valley GAMA study unit ground-water-quality data presented in this report are archived in the USGS’s National Water Information System (NWIS) database, except for the following constituents: tritium and noble gases analyzed at the LLNL; chromium, arsenic, and iron speciation analyzed at the USGS’s NRP laboratory in Boulder, Colorado; stable isotopes analyzed at the USGS’s Reston Stable Isotope Laboratory (RSIL); and perchlorate, NDMA, and 1,2,3-trichloropropane analyzed at Montgomery Watson-Harza laboratory; radium 226, 226 Gross alpha–beta radioactivity analyzed at Eberline Analytical Services. These data are available on request.
Tables 9–19 show the results of water-quality analyses. These tables are limited in presentation only to those constituents that were detected and only to those wells with one or more detections in each constituent group. Concentrations and activities, which were higher than a selected threshold, are denoted by an asterisk in the tables. Additionally, well locations with selected constituent detections above a threshold value are shown in figures 4–10.
Study areas containing nongrid wells in addition to the randomly selected grid wells have had their constituent detections divided into two categories: grid wells and nongrid wells. Nongrid wells were not included in comparisons made of detections between the six study areas because they could have introduced a spatial bias as a result of increased sampling density in a particular area within the study unit.
The chemical and microbial data presented in this report characterize the quality of the untreated ground-water resources within the Southern Sacramento Valley GAMA study unit and do not represent the drinking water delivered to consumers by water purveyors. The samples collected for this study were not regulatory samples, even when collected from public-supply wells, as sampling and analytical methods used in this study differ from those used for regulatory samples. The chemical and microbial composition of drinking water may differ from ground-water in that drinking water may be subjected to disinfection, filtration, mixing with other waters, and exposure to the atmosphere prior to its delivery to the consumer.
General water-quality parameters are listed in table 9. One well had a pH greater than the USEPA SMCL, and one well had a turbidity measurement greater than the CADHS MCL. Fifteen wells had a specific conductance measurement greater than the lower CADHS SMCL of 900 μS/cm, and one well had a specific conductance measurement greater than the upper CADHS SMCL of 1,600 μS/cm. Ten wells had a total dissolved solids concentration greater than the lower CADHS SMCL of 500 μS/cm.
Table 10A lists the concentrations of VOCs detected in ground water in the Southern Sacramento Valley GAMA study unit. VOCs, including gasoline oxygenates, were collected at all 83 wells sampled, with the exception of 3 gasoline oxygenate constituents, methyl acetate, tert-Amyl alcohol, and tert-Butyl alcohol, which were analyzed in samples from 21 wells. No VOC concentrations were greater than an MCL or other threshold value. Forty-nine of those 83 wells had one or more detections of a VOC (table 10B). The following VOCs were censored on the basis of QC analysis, and the results were not reported: m- and p-xylene and toluene.
Some of the VOC detections in well NAMFP-05 were suspect, but passed QC analysis; of the 15 VOCs detected in this well, only 3 were detected in any other well in the study. Additionally, this was a deep-monitoring well near the axis of the Sacramento Valley, a location that is not expected to have any VOC detections. No other wells near this location or upgradient from this well had that many VOC detections.
Thirty-four of the 88 VOC and gasoline oxygenate analytes were detected in at least one ground-water sample; 21 of these VOCs were detected in grid wells. No VOC concentration in this study was greater than an MCL or other threshold value. The most frequently detected VOCs were trichloromethane (chloroform), tetrachloroethylene (PCE), carbon disulfide, and trichlorethylene (TCE); the most frequently detected class of VOCs were disinfection by-products, followed by solvents.
Five TIOCs were identified and one unknown compound was found in some samples collected in this study (table 11). The TIOC hexafluoropropene was also detected in ground-water samples collected using the depth-dependent method, but all detections were censored because of detections in the field blank. Cyclopentane was found in four grid wells in the SAM and SOL study areas and in one flowpath well in the NAM study area. Sulfur dioxide was identified in two wells, in the SAM and SOL study areas, respectively. Methyl-propanethiol and methyl-naphthalene were found individually in two flowpath wells in the NAM study area. Dichlorofluoroethane was found in one depth-dependent sample, and an unknown compound was found in three of the depth-dependent samples.
Tables 12A and 12B list the concentrations of pesticides detected in ground-water in the southern Sacramento Valley GAMA study unit. Of the 70 pesticide compounds that were analyzed in samples from all wells, 12 were detected at least once (table 12A). Of the additional 59 pesticide compounds analyzed in samples from selected wells, 9 were detected at least once (table 12B). No pesticide concentrations were greater than an MCL or other high threshold value.
Pesticide compounds were sampled in two analyte groups; 1 group was analyzed in samples collected at 82 wells; the other group of pesticide compounds and caffeine (analyzed with the pesticide compounds, but not used as a pesticide) were analyzed in samples collected at 43 of the wells sampled. Of the 82 wells sampled, 34 had at least one detection of any pesticide compound (tables 12A and 12B). The following pesticide compounds were censored on the basis of QC analysis and were not reported here: benfluralin, DCPA, oxamyl, and trifluralin.
The most frequently detected pesticide compounds were 2-chloro-4-isopropylamino-6-amino-s-triazine (deethylatrazine, a degradate of atrazine), atrazine (general application herbicide), molinate (herbicide used on rice), and simazine (general application herbicide) (table 12C).
Nutrient samples were collected at 43 wells. Nitrogen compounds were detected at 42 wells, and orthophosphate was detected at 43 wells (table 13). Nitrite was not detected in any well; all nitrate plus nitrite concentrations are reported here as nitrate. Nitrate concentrations (as N) were lower than the CADHS MCL of 10 mg/L in 42 wells and higher than the MCL in one well (fig. 4). Dissolved organic carbon was detected in 11 of 22 wells sampled, but all concentrations were less than the blank censoring level (table 6) and are considered nondetections.
Major ions were analyzed in samples from 43 wells. Table 14 lists the analytical results for major ions. There are no applicable health-based thresholds for the major ions; however, two of the major ions (chloride and sulfate) have SMCLs that are set for aesthetic qualities rather than for health concerns. Concentrations of sulfate were lower than the SMCL in all 43 wells. One well had a chloride concentration above the CADHS SMCL of 250 mg/L.
Twenty-six trace elements were analyzed in samples from 43 wells (table 15). The following trace element results were censored on the basis of QC analysis: antimony, cadmium, fluoride, and lead (table 6); all values of lead were censored. Only concentrations above the QC censoring level were reported. Of the 18 trace elements with an MCL or other health-based threshold, 3 were detected at concentrations greater than the threshold: arsenic, barium, and boron. Two trace elements, iron and manganese, were detected at concentrations greater than an SMCL (table 15; figs. 5–9).
Constituents of special interest and trace element speciation were analyzed in samples from 43 wells (table 16). Concentrations of the special constituents were generally low: NDMA was not detected; perchlorate was detected in 11 wells, but all detections were below the NL; 1,2,3-trichloropropane was detected in a single sample at a concentration above the NL. Note that total arsenic, chromium, and iron results reported here were different in some wells than the results for these constituents reported in table 15. The results for arsenic, chromium, and iron in table 16 were determined using research methods that were under development and that were not the preferred method (see discussion of Field Methods in the Methods section); they were reported here for comparison with the speciation results. Some detections of total chromium and chromium(VI) were censored because of field blank detections; only the concentrations above the censoring level were reported. Comparison of the concentrations of arsenic, chromium, and iron with threshold values were discussed in the section on trace elements.
Tritium, deuterium, and oxygen-18 results that were analyzed by the USGS laboratories in samples from all wells are reported in table 17. Carbon-13/carbon-12, carbon-14, radium-226, gross alpha radiation at 72 hours and 30 days, gross beta radiation at 72 hours and 30 days, and radon-222 results in samples collected from 20 wells are also shown in table 17. Radon-222 concentrations were greater than the proposed Federal (USEPA) MCL of 300 pCi/L in 13 of the 20 samples analyzed (fig. 10); none of the values was greater than the upper proposed value of 4000 pCi/L.
Tritium, helium-3 to helium-4 ratio, helium-4, argon, neon, krypton, and xenon, were analyzed at LLNL in samples collected from 43 wells; the results are listed in table 18.
The microbial constituents total coliform and Escherichia coliform and the viruses F-specific coliphage and somatic coliphage were analyzed in 11 ground-water samples collected for the Southern Sacramento Valley GAMA study unit (table 19). One well had a detection of total coliforms (1 colony counted). Another well had a detection of somatic coliphage.
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