Data Series 301
U.S. GEOLOGICAL SURVEY
Data Series 301
Methods used for the GAMA program were selected to achieve the following objectives: (1) Design a sampling plan suitable for statistical analysis; (2) collect samples in a consistent manner; (3) analyze samples using proven and reliable laboratory methods; (4) assure the quality of the ground-water data; and, (5) maintain data securely and with relevant documentation.
The wells selected for sampling in this study reflect the combination of two well selection strategies. Thirty-five wells were selected to provide a statistically unbiased, spatially distributed assessment of the quality of ground-water resources used for public drinking-water supply, and fifteen additional wells were selected to provide greater sampling density in several areas to address specific ground-water quality issues in the study unit.
The spatially distributed wells were selected using a randomized grid-based method (Scott, 1990). SOSA had relatively few public-supply wells, and these wells were not evenly distributed. To minimize the number of cells without any wells, only the portion of SOSA in close proximity to a public-supply well was included in the gridded area. Locations of wells listed in statewide databases maintained by the CDPH and USGS were plotted and 1.86-mi (3-kilometer) radius circles were drawn around each well. The area encompassed by the circles was then divided into forty 10-mi2 grid cells (fig. 3). The objective was to select one public-supply well per grid cell. Thirty-five of the forty grid cells were sampled in SOSA; the other five grid cells did not contain accessible wells. If a grid cell contained more than one public-supply well, each well was randomly assigned a rank. The lowest ranking well that met basic sampling criteria (for example, sampling point located prior to treatment, capability to pump for several hours, and available well-construction information), and for which permission to sample could be obtained, was then sampled. If a grid cell contained no accessible public-supply wells, domestic and irrigation wells were considered for sampling. An attempt was made to select domestic and irrigation wells with depths and screened intervals similar to those in public-supply wells in the area. In this fashion, a well was selected in each cell to provide a spatially distributed, randomized monitoring network for each study area. Wells sampled as part of the randomized grid-cell network are hereafter referred to as “grid wells.” Grid wells in SOSA were numbered in the order of sample collection with the prefix “SOSA.”
Additional wells were sampled along the axis of the Tehachapi basin (SOSAFP-05, -04, -15, -03, -01, -06, -14, -13), in the central portion of the Cummings basin (SOSAFP-07, -08, -09, -10, -11, -12), and along the Kern River (SOSAFP-02 (fig. 4). These wells were sampled to assess movement of water and dissolved constituents along ground-water flow paths in these areas in combination with selected wells that were sampled on the grid. Wells sampled as part of these studies for better understanding were not included in the statistical characterization of water quality in SOSA. These additional wells were numbered in the order of sample collection with the prefix “SOSAFP” (“FP” indicating “flow path”). ">Additional wells were sampled along the axis of the Tehachapi basin (SOSAFP-05, -04, -15, -03, -01, -06, -14, -13), in the central portion of the Cummings basin (SOSAFP-07, -08, -09, -10, -11, -12), and along the Kern River (SOSAFP-02 (fig. 4). These wells were sampled to assess movement of water and dissolved constituents along ground-water flow paths in these areas in combination with selected wells that were sampled on the grid. Wells sampled as part of these studies for better understanding were not included in the statistical characterization of water quality in SOSA. These additional wells were numbered in the order of sample collection with the prefix “SOSAFP” (“FP” indicating “flow path”).
Table 1 (all tables shown in back of report) provides the GAMA alphanumeric identification number for each well, along with the date sampled, sampling schedule, well elevation, and well-construction information. Ground-water samples were collected from 40 public-supply wells, 5 irrigation wells, and 5 domestic wells during June 2006.
Well locations and identifications were verified using GPS, 1:24,000 scale USGS topographic maps, comparison with existing well information in USGS and CDPH databases, and information provided by well owners. Driller’s logs for wells were obtained when available. Well information was recorded by hand on field sheets and electronically using specialized software on field laptop computers. All information was verified and then uploaded into the USGS National Water Information System (NWIS). In order to maintain confidentiality of well owners and well locations, the standard USGS protocol for identifying sites in NWIS with a site-id containing latitude and longitude was modified for new sites established in this study, and all data are currently inaccessible from NWIS’s public website.
The wells in SOSA were sampled using a tiered analytical approach. All wells were sampled for a standard set of constituents, including VOCs, pesticides and pesticide degradates, pharmaceutical compounds, perchlorate, hexavalent chromium, stable isotopes of water, and dissolved noble gases and tritium/helium age dates. The standard set of constituents was termed the “fast” schedule (table 2). Wells on the “intermediate” schedule were sampled for all the constituents on the fast schedule, plus NDMA, 1,2,3-TCP, nutrients and dissolved organic carbon, major and minor ions and trace elements, and tritium. Wells on the “slow” schedule were sampled for all the constituents on the intermediate schedule, plus radioactive and microbial constituents (table2). Fast, intermediate, and slow refer to the time required to sample the well for all the analytes on the schedule. Generally, one slow or two intermediate or four fast wells could be sampled in one day. In SOSA, twenty-eight of the ground-water wells were sampled on the fast schedule, fifteen were sampled on the intermediate schedule, and seven on the slow schedule.
Samples were collected in accordance with the protocols established by the USGS National Water Quality Assessment (NAWQA) program (Koterba and others, 1995) and the USGS National Field Manual (U.S. Geological Survey, variously dated). These sampling protocols ensure that a representative sample of ground water is collected at each site and that the samples are collected and handled in a way that minimizes the potential for contamination of samples. The methods used for sample collection are described in the Appendix.
Tables 3A–L list the compounds analyzed in each constituent class. Ground-water samples were analyzed for eighty-five VOCs (table 3A), eight gasoline oxygenates and degradates (table 3B), sixty-three pesticides and pesticide degradates (table 3C), fourteen pharmaceutical compounds (table 3D), sixty-two wastewater-indicator compounds (table 3E), three constituents of special interest (table 3F), five nutrients and dissolved organic carbon (table 3G), ten major and minor ions and total dissolved solids (table 3H), twenty-five trace elements (table 3H), arsenic, iron, and chromium species (table 3I), stable isotopes of water, eight radioactive constituents, including tritium and carbon-14 (table 3J), five dissolved noble gases, and helium stable isotope ratios (table 3K), and four microbial constituents (table 3L). The methods used for sample analysis are described in the Appendix.
The methods and conventions used for reporting the data are described in the Appendix. Nineteen constituents analyzed in this study were measured by more than one method at the USGS National Water Quality Laboratory (NWQL), but only the results from the preferred method are reported. Arsenic, iron, and chromium concentrations, and tritium activities were measured by more than one laboratory, and both sets of results are reported.
The quality-assurance plan used for this study follows the protocols described in the NWQL quality-assurance plan (Maloney, 2005; Pirkey and Glodt, 1998) and the protocols used by the USGS NAWQA program (Koterba and others, 1995). Quality-control (QC) samples collected in the SOSA study include source-solution blanks, field blanks, replicates, and matrix and surrogate spikes. QC samples were collected to evaluate contamination, and bias and variability of the water chemistry data that may have resulted from sample collection, processing, storage, transportation, and laboratory analysis. The quality-assurance plan is described in the Appendix.
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/ds/301
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