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U.S. GEOLOGICAL SURVEY
Data Series 285 (ver 1.1, August 2018)

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Methods

This section discusses the methods used in this study to select wells, collect and analyze ground-water samples, and present the data in this report. The methods used were selected to obtain representative samples of the ground water in the study area and to minimize potential biases to the data.

Well Selection

The primary objectives in the selection of wells for study-area assessments were (1) to attain a sampling density of at least one well per 100 km², (2) randomly select at least 10 wells per study area whenever possible, and (3) minimize variability in well type (Gilliom and others, 1995). The objectives were intended to assure an adequate and unbiased assessment of the quality of ground-water resources used for public-supply. In the Southern Sacramento Valley GAMA study unit, public-supply wells were the preferred well type, but in cases where public-supply wells were not available, domestic, irrigation, or monitoring wells were substituted in the selection process. Wells that had available construction information, including well depth, depth of perforations, and date constructed, were selected, when possible, for sampling, but this information was not always available.

Well selection within each study area was based on a grid pattern of equal-area cells (fig. 3; Scott, 1990). A geographic information system (GIS) was used to generate 10 cells in the SUI study area, 15 cells in the NAM, SAM, and QPC study areas, 18 cells in the YOL study area, and 20 cells in the SOL study area. In each cell, wells were randomly assigned a rank. The well with the lowest random rank that met selection criteria (having a sampling point before a storage tank and for which permission could be obtained) was chosen for sampling. Using this method, one randomly ranked well was chosen to occupy each grid cell in each study area (referred to as grid wells). A total of 67 grid wells were sampled. Analytical results from grid wells were statistically representative of ground-water quality in that study area. To enhance the understanding of ground-water movement through the study area, 16 additional wells were selected along two approximate, regional flowpaths across the study area (fig. 3) and were referred to as nongrid or flowpath wells.

Wells were sampled for a “fast,” “intermediate,” “slow,” or “depth dependent” list of analytes (table 1; all tables are shown in back of report). The fast-analyte list was collected at all wells sampled in this study and provides an initial assessment of ground-water quality in the area. Slow-analyte-list wells were selected along ground-water flowpaths and were sampled for all analytes, including those on the fast list. Selected grid wells across the study areas had additional analytes added to their fast list (becoming intermediate-list wells) to add to the data available for understanding ground-water flow. Four depth-dependent samples were collected in one public-supply well near a flowpath and were sampled for a subset of the slow list of analytes. Monitoring wells along the flowpaths were sampled for the slow list, with the exclusion of microbial constituents. Additional wells (nongrid wells) that were sampled were located along flowpaths or in specific locations within the basin and were sampled for slow, intermediate, fast, or depth-dependent lists of analytes.

Samples collected in the six study areas of the Southern Sacramento Valley GAMA study unit were assigned a GAMA identification (GAMA-ID) number that was based on study-area designation and sequence number in the order of sample collection (fig. 3). The samples collected at nongrid wells along ground-water flowpaths were identified by the study area acronym followed by the abbreviation “FP” and a sequence number in order of collection; for example, the first flowpath sample collected from the NAM study area was designated NAMFP-01. Depth-dependent samples collected in one well in the NAM study area were identified by the study acronym followed by the abbreviation “DD” as NAMDD and a sequence number in order of collection. ">Samples collected in the six study areas of the Southern Sacramento Valley GAMA study unit were assigned a GAMA identification (GAMA-ID) number that was based on study-area designation and sequence number in the order of sample collection (fig. 3). The samples collected at nongrid wells along ground-water flowpaths were identified by the study area acronym followed by the abbreviation “FP” and a sequence number in order of collection; for example, the first flowpath sample collected from the NAM study area was designated NAMFP-01. Depth-dependent samples collected in one well in the NAM study area were identified by the study acronym followed by the abbreviation “DD” as NAMDD and a sequence number in order of collection.

Table 2 provides the GAMA-ID number and the date of collection for each sample in the Southern Sacramento Valley GAMA study unit. The table also provides the constituent list and well-construction information. Eighty-three public-supply, domestic, irrigation, and monitoring wells were sampled for this study from March 2004 to June 2005; 4 depth-dependent samples were collected from one of the public-supply wells in June 2005. Of the wells sampled, 19 were in the NAM study area, 12 located in the SAM study area, 13 located in the SOL study area, 19 located in the YOL study area, 5 located in the SUI study area, 15 located in the QPC study area.

Field Methods

Ground-water samples were collected from the selected wells using USGS protocols (U.S. Geological Survey, 1999) and the protocols described in Shelton and others (2001), Ball and McClesky (2003a and 2003b), and Eaton and others (2004). Public-supply, domestic, and irrigation wells were sampled with Teflon tubing attached to a sampling point on the well discharge pipe as close to the well as possible using brass and stainless-steel fittings. When a site was sampled on a fast or intermediate list, samples were collected at the sampling point using a foot-long section of Teflon tubing. When a site was sampled on the slow list, Teflon tubing with stainless-steel fittings was connected to the sampling point at the well and conveyed the water to a sampling manifold in a water-quality lab vehicle, where the samples were collected. Monitoring wells were sampled using a portable, stainless-steel submersible pump attached to Teflon tubing with stainless-steel fittings. All equipment was cleaned after each use and stored in plastic bags until used at the next site. At slow-list wells and monitoring wells, field parameters (pH, specific conductance, dissolved oxygen, water temperature, and turbidity) were monitored during purging of the well before sample collection as described in U.S. Geological Survey (1999). At fast- and intermediate-list wells, specific conductance and temperature were measured once before sample collection, as most of the wells sampled on this list were regularly pumped and assumed to be purged at the time of sample collection.

For depth-dependent samples, ground water was pumped to the surface using a gas-displacement, small-diameter sample pump to collect samples at discrete depths within the well bore (Izbicki, 2004). This sampling equipment consisted of two 1/8-in. diameter Teflon tubes bundled together side-by-side to form a single line that was mounted on a motorized reel. Once lowered to the desired depth, compressed ultra-high purity (grade 5) nitrogen gas was used to displace water from one tube into the other while one-way flow valves prevent the displaced water from flowing back toward the sample pump at the lower end of the hose (Izbicki, 2004). Repeatedly pressurizing and depressurizing the lines slowly brought the water to the surface where it was collected following the protocol described for the fast and intermediate schedules. The field parameters measured in depth-dependent samples were pH, specific conductance, and water temperature.

When possible, samples were collected before any type of water-system filtration, or chemical treatment, such as chlorination. At two sites, the sampling point was located after the chlorination point; at one site, the chlorinator was shut off, and samples were collected after free chlorine was measured and found to be nondetectable; and at the other site, samples were preserved with a dechlorinating agent (ascorbic acid) to prevent formation of disinfection by-products in the sample after collection.

Laboratory Methods

Tables 3A–J list the chemical and microbial constituents analyzed for in ground-water samples collected as part of the Southern Sacramento Valley GAMA study unit. These tables also list the USGS parameter code (numerical identifiers for constituents stored in the USGS database), the Chemical Abstracts Service (CAS) number, the reporting level, the high threshold and type of threshold (see following section for more explanation), and an indication as to whether that constituent was detected in ground water in this study.

In addition to the 88 target VOCs that were analyzed in this study, nontarget analytes found during analysis were tentatively identified by searching the National Institute for Standards and Technology (NIST) Library. Tentatively identified organic compounds (TIOCs) were reported as approximate concentrations, and actual concentrations may be an order of magnitude higher or lower (Connor and others, 1998, p. 46) ">In addition to the 88 target VOCs that were analyzed in this study, nontarget analytes found during analysis were tentatively identified by searching the National Institute for Standards and Technology (NIST) Library. Tentatively identified organic compounds (TIOCs) were reported as approximate concentrations, and actual concentrations may be an order of magnitude higher or lower (Connor and others, 1998, p. 46)

Table 4 lists the analyte, analytical methods, laboratory at which the analyses were conducted, and method references. The analytical methods used include those developed at the USGS’s National Water Quality Laboratory (NWQL), the U.S. Environmental Agency’s (USEPA) standard analytical methods, and research methods currently under development.

Data Reporting

The following types of data were reported for this study: analytical results, QC analyses, comparisons with selected high threshold values, and detection frequencies for selected constituents in grid wells.

Reporting Levels

Analytical results reported in this report use laboratory reporting levels (LRLs), long-term method detection levels (LT-MDL), method detection limits (MDLs), Minimum Reporting Levels (MRLs), Method Uncertainties (MUs), and Single Sample Minimum Detectable Counts (SSMDCs). These reporting levels minimize the reporting of false positive results.

MDLs — “Minimum concentration of a substance that can be measured and reported with 99-percent confidence
that the analyte concentration is greater than zero. It was determined from the analysis of a sample in a given matrix containing the analyte (U.S. Environmental Protection Agency, 1997). At the MDL concentration, the risk of a false positive is predicted to be less than or equal to 1 percent” (Childress and others, 1999).

LT-MDLs — “A detection level derived by determining the standard deviation of a minimum of 24 MDL spike sample measurements over an extended period of time. LT–MDL data are collected on a continuous basis to assess year-to-year variations in the LT–MDL. The LT–MDL controls false positive error. The chance of falsely reporting a concentration at or greater than the LT–MDL for a sample that did not contain the analyte is predicted to be less than or equal to 1 percent” (Childress and others, 1999).

LRLs — “Generally equal to twice the yearly determined LT-MDL. The LRL controls false negative error. The probability of falsely reporting a non-detection for a sample that contained an analyte at a concentration equal to or greater than the LRL is predicted to be less than or equal to 1 percent… These values are re-evaluated annually based on the most current quality-control data and may, therefore, change” (Childress and others, 1999, p. 19).

MRLs — “Smallest measured concentration of a constituent that may be reliably reported by using a given analytical method” (Timme, 1995).

MUs — The method uncertainty generally indicates the precision of a particular analytical measurement, and therefore indicates the range of values wherein the true value will be found (Moran and others, 2002).

The reporting levels for selected radioactive constituents (gross-alpha­ radioactivity, gross-beta radioactivity, radium-226, and radium-228) are based on an SSMDC, a critical value (also sample specific), and the combined standard uncertainty (CSU; U.S. Environmental Protection Agency and others, 2004a). A result above the critical value represents a greater-than-95-percent certainty that the result is greater than zero (significantly different from the instrument’s background response to a blank sample), whereas a result above the SSMDC represents a greater-than-95-percent certainty that the result is greater than the critical value (U.S. Environmental Protection Agency and others, 2004a). Using these reporting level elements, three unique cases were possible when screening the raw analytical data. First, if the analytical result was less than the critical value (case 1), the analyte was considered not detected, and a value is presented in the table as less than the SSMDC. If the analytical result was greater than the critical value, the ratio of the CSU to the analytical result (relative CSU) was calculated as a percent. For those samples with results that have a relative CSU of less than 20 percent, the analytical result is reported unqualified (case 2). For those samples with results that have a relative CSU greater than 20 percent, the analytical results were qualified as estimated values and are preceded in the table below with an “E” (case 3). For clarity, only the screened results are included in this report. The table provided below gives an example of the screening process for each of the three cases described above.

¹Estimated

Depth-Dependent Samples

Concentrations reported here for depth-dependent samples have not been corrected for flow values. Because these samples were collected at specific depths in the well and represent only part of the discharge from this well, the concentrations measured in these samples were not equivalent to the concentrations reported for samples collected at the surface sampling point, which represent all of the discharge for this well. The sum of concentrations from individual depth-dependent samples may not equal the concentration in the total discharge collected at the surface sampling point, as the relative contributions from the different depths can vary because of differences in aquifer properties.

Detection Frequency

Detection frequencies were reported only for grid wells, which were representative of ground-water quality in this study unit, and only for those VOC and pesticide compounds that were sampled at all grid wells. Detection frequencies are listed by study area and for the study unit as a whole. The detection frequency is equal to the number of detections divided by the number of samples multiplied by 100 percent. A constituent is considered frequently detected if it was 
found in more than 10 percent of samples analyzed for that constituent.

Selection of Threshold Values

Constituents detected in sampled ground water were compared with selected threshold values to provide context for these results. These high thresholds were set using the CADHS and USEPA drinking-water standards (California Department of Health Services, 2005a, 2005b, and 2005c; U.S. Environmental Protection Agency, 2005); concentrations detected in this study that were higher than their selected threshold are marked in the tables. The threshold values were selected in the following priority: State and Federal primary Maximum Contaminant Levels (MCLs), CADHS notification levels (NLs), USEPA lifetime health advisories (HA-Ls), the risk-specific dose at a cancer risk level equal to 1 in 100,000 or 10E–5 (RSD5), all of which are health-based standards, and lastly, State and Federal Secondary Maximum Contaminant Levels (SMCLs), which are set for aesthetic concerns. Comparisons of raw ground water to MCLs, SMCLs, NLs, HA-Ls, and RSD5s were made for illustrative purposes only and do not indicate a drinking-water violation or noncompliance with drinking-water regulations. Explanations of the levels used in this report were provided as follows:

MCLs — Legally enforceable standards that apply to public-water systems and were designed to protect public health by limiting the levels of contaminants in drinking water (U.S. Environmental Protection Agency, 1974).

NLs — Health-based advisory level established by CADHS for chemicals in drinking water that lack MCLs. If a chemical was detected above its NL, State law requires timely notification to the local governing bodies and recommends consumer notification (California Department of Health Services, 2005d).

HA-Ls — The concentration of a chemical in drinking water that was not expected to cause any adverse noncarcinogenic effects for a lifetime of exposure. The USEPA lifetime health advisory assumes consumption of 2 L of water per day over a 70-year lifetime by a 70-kg (154 lb) adult and that 20 percent of exposure comes from drinking water (U.S. Environmental Protection Agency, 2004a).

RSD5 — The concentration of a chemical in drinking water corresponding to an excess estimated lifetime cancer risk of 1 in 100,000, hereinafter referred to as the risk specific dose at 10E–5 (U.S. Environmental Protection Agency, 2004b).

SMCLs — Nonenforceable contaminant concentration level that affects the aesthetic qualities of drinking water, such as the taste, odor, and color (U.S. Environmental Protection Agency, 1974).

Constituents Analyzed by Multiple 
Analytical Methods

Twenty-four constituent samples collected in the Southern Sacramento Valley GAMA study unit were analyzed by more than one analytical method. A preferred analytical method was chosen by NWQL on the basis of lab performance of the available methods. Only the detections determined by the preferred analytical method are listed in this report.

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