Skip to main content
USGS - science for a changing world

Mojave Region Water-Quality Studies

Mapping Selected Trace Elements and Major Ions, 2000-2012, Mojave River and Morongo Groundwater Basins, Southwestern Mojave Desert, San Bernardino County, California by Loren F. Metzger, Matthew K. Landon, Sally F. House, and Lisa D. Olsen

Introduction

The population of the Mojave River and Morongo groundwater basins has grown rapidly during the last several decades, increasing from an estimated population of almost 273,000 in 1990 (Mojave Water Agency, 2004) to more than 453,000 in 2010 (Mojave Water Agency, 2014). Groundwater is the primary source of potable water in both basins (Mojave Water Agency, 2014). Previous studies noted elevated concentrations of several trace elements, nitrate, and total dissolved solids in groundwater in portions of the two basins (Christensen and Fields-Garland, 2001; Ball and Izbicki, 2004; Izbicki and others, 2008; Mathany and Belitz, 2009; Wright and Belitz, 2010; Dawson and Belitz, 2012; and Izbicki and others, 2012). Since 2000, the U.S. Geological Survey (USGS) has collected water-quality data annually from a network of wells and has provided quality-assurance for Mojave Water Agency (MWA) data that are stored in the USGS National Water Information System (NWIS) database. The new data and results from the joint State of California and USGS Groundwater Ambient Monitoring and Assessment (GAMA) program assessments of regional water quality (these data are also stored in NWIS), in combination with ongoing MWA/USGS groundwater-quality monitoring provide a timely opportunity for mapping of groundwater quality in the Mojave River and Morongo groundwater basins. The purpose of this report is to provide maps and time-series plots of concentrations of selected water-quality constituents (arsenic, boron, chromium-6, total chromium, dissolved oxygen, fluoride, iron, manganese, nitriate plus nitrite as nitrogen, total dissolved solids, uranium, and vanadium) in the Mojave River and Morongo groundwater basins using data collected by the USGS and MWA from 2000 to 2012. These maps and plots can be accessed on this website.


Approach

Maps were created to show the distribution and concentrations of arsenic, boron, chromium (as chromium-6 and chromium-total), dissolved oxygen, fluoride, iron, manganese, nitrate plus nitrite as nitrogen (N), total dissolved solids, uranium, and vanadium in groundwater. The maps were assembled by using data from 355 well sites representing 503 discrete sampling points (some well sites accessed multiple depths) in the Mojave River and Morongo groundwater basins, including the portions of both basins that are outside of the MWA Management Area. The set of 503 discrete sampling points for which data were available for 2000-12 represented about 40 percent of the more than 1,200 unique wells and sampling points in this area that have been sampled for water quality by the USGS (1908 to present) and the MWA (2004 to present). Site information for the 503 sampling points is accessible on the USGS NWIS website through the hyperlinked list of sites in appendix A.

The wells used for mapping water quality during 2000-12 included monitoring wells and wells categorized as 'other,' which represents production wells and wells having unknown use or status. Monitoring wells constituted about 50 percent of all the wells used in this study. Monitoring wells were distinguished as 1 of 3 types based on construction:

  1. individual wells, consisting of a single casing in a borehole and generally screened over a single 10-20 foot (ft) interval;
  2. multi-depth wells, consisting of 2 to 5 individual, short-screened (usually 10-20 ft interval) wells, installed in the same borehole, but screened at different depths and separated vertically by borehole seals; and
  3. multi-port wells, consisting of a series of 5 to 10 integrated measurement probes and sampling ports, separated by water-inflated packers installed in a single borehole or casing. The multi-port wells are Westbay System installations (Schlumberger Water Services Corp., 2014).

Most of the remaining wells from which the water-quality data used in this report were collected were long-screened wells located in areas where monitoring wells were unavailable. Samples of untreated water obtained from these 'other' wells could represent a blend of different water compositions from multiple water-bearing zones. Designated well type and selected construction information for the wells included in this report are summarized in table 1.


Table 1. Summary of wells with data used in this report by type and well depth, Mojave River and Morongo groundwater basins, Southwestern Mojave Desert, San Bernardino County, California, 2000-2012.

[ft, feet; <, actual value is less than value shown; >, actual value is greater than value shown]

Well Type Number of Well Sites (Sampling Points)1 Number of Well Sites (and Sampling Points1) by Depth2
< 100 ft 100-250 ft 251-500 ft 501-750 ft 751-1,000 ft > 1,000 ft Unknown3
Monitoring - individual4 73 35 21 9 4 0 0 4
Monitoring - multi-port5 7 (55) (2) (6) (8) (14) (12) (13) (0)
Monitoring - multi-depth6 50 (150) (22) (47) (40) (34) (7) (0) (0)
Other7 225 4 38 55 44 22 10 52
Totals 355 (503)            

1The number in parentheses represents the number of sampling points for multi-depth and multi-port (Westbay) wells.
2Based on completed well depth for individual wells, or bottom of sampling point for multi-depth and multi-port (Westbay) monitoring wells.
3Insufficient information for classification.
4Single-cased well.
5Borehole or cased multi-port (Westbay) well with 5-10 sampling points at different depths, each separated by packers. A total of 55 sampling points among the 7 wells. Each sampling point is represented by a unique site identifier in the USGS National Water Information System (NWIS).
6Multiple wells installed within the same borehole; 50 multi-depth monitoring wells contain 150 individual wells at different depths.
7Includes wells used for production or of unknown use or status.


Methods used for well purging, sample collection, sample analysis, and data reporting for the data collected by the USGS generally followed the procedures given in the USGS National Field Manual for the Collection of Water Quality Data (U.S. Geological Survey, 2014) and Mathany and Belitz, 2009, Wright and Belitz, 2010, and Dawson and Belitz, 2012. Methods used by the MWA for individual wells and multi-depth wells generally followed those used by the USGS; the USGS provided training in field methods approximately annually (written commun., Ronald G. Fay, U.S. Geological Survey, 2010). Methods used by the MWA to develop and sample the multi-port wells are described by the MWA Field Sampling Plan (written commun., Anna Garcia, Mojave Water Agency, 2014). Samples collected by the USGS typically were filtered and were analyzed by the USGS National Water Quality Laboratory in Denver, Colorado. Samples collected by the MWA typically were unfiltered and were analyzed by Test America in Irvine, California. Data from both laboratories were quality-assured by the USGS and MWA as a part of the aforementioned studies. Although results from filtered and unfiltered samples were combined for this mapping efforts, the results from the unfiltered analyses could differ from those from the filtered analyses because of the potential presence of particles larger than 0.45-micron, the pore size of the filters used for the filtered analyses. The difference between unfiltered and filtered water concentrations is likely to be small because procedures for sampling included purging the well before collecting samples until the water was clear and the field parameters (water temperature, pH, and specific conductance) were stable.

The amount of data available varied by sampling point and constituent. The number of sampling points that had data for the 12 mapped constituents ranged from 71 for uranium to 433 for arsenic (table 2). The number of analyses for each sampling point ranged from 1 to 15 during 2000-12. About half of all sampling points used for mapping arsenic, boron, total chromium, fluoride, iron, manganese, nitrate plus nitrite (as N), and total dissolved solids had multiple analyses. In comparison, less than 40 percent of the sampling points used for mapping chromium-6, dissolved oxygen, uranium, and vanadium had multiple analyses during 2000-12. Sampling sites that had multiple analyses from 2000-12 are represented by the median concentrations on maps. The use of median concentrations means that the maps tend to represent the central tendency for each constituent, rather than the minimum or maximum.


Table 2. Summary of benchmarks, detection frequencies, concentration ranges, and mapped median concentrations for selected trace elements, major ions, and dissolved oxygen for wells in the Mojave River and Morongo groundwater basins, Southwestern Mojave Desert, San Bernardino County, California, 2000-2012.

[Reporting level (RL): a censored ("<") value representing an unquantifiable result and the lowest value the laboratory was willing to assign to it. The RL is shown as a range where it has varied over time. Benchmarks: MCL-US, U.S. Environmental Protection Agency (EPA) maximum contaminant level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; SMCL-CA, CDPH secondary maximum contaminant level; NL-CA; CDPH notification level. Detection frequency classification: very high, concentrations five times greater than benchmark; high, concentrations one to five times greater than benchmark; moderate, concentrations less than or equal to benchmark, but greater than 0.5 of benchmark; low, concentrations less than or equal to 0.5 of benchmark. Median concentration includes results reported as 'zero' and 'less than' (below LRL, laboratory reporting level). mg/L, milligrams per liter; µg/L, micrograms per liter; na = not applicable; <, actual value is less than value shown; %, percent]]

Constituent Units Reporting level (lowest - highest) Benchmark Number of sampling points Detection Frequency Classification (%) Range of concentrations for mapped wells Median concentration for mapped wells
Type Value Very High Concentration High Concentration Moderate Concentration Low Concentration
Arsenic µg/L 1.0 - 2.0 MCL-US 10 1 433 4.4 17.3 10.9 67.4 0.1 - 359 µg/L 2.6 µg/L
Boron µg/L 39 - 100 NL-CA 1,000 355 1.7 9.0 11.8 77.5 7 - 22,500 µg/L 97 µg/L
Chromium-6 µg/L 0.1 - 1.0 MCL-CA 10 2 355 1.1 11.3 15.2 72.4 0.6 - 98 µg/L 1.5 µg/L
Chromium, total µg/L 0.07 - 2.0 MCL-CA 50 399 0.2 2.0 2.8 95.0 0.05 - 278 µg/L 1.7 µg/L
Dissolved Oxygen mg/L 0.2 na na 226 na na na na < 0.2 - 16.7 mg/L 4.4 mg/L
Fluoride mg/L 0.1 MCL-CA 2 432 1.4 9.0 8.3 81.3 0.06 - 156 mg/L 0.5 mg/L
Iron µg/L 3.2 - 40 SMCL-CA 300 426 1.9 1.6 2.1 94.4 2.1 - 25,500 µg/L 5.7 µg/L
Manganese µg/L 0.13 - 5.0 SMCL-CA 50 426 3.7 3.3 3.7 89.2 0.1 - 7,180 µg/L 0.80 µg/L
Nitrate + Nitrite (as N) mg/L 0.04 - 0.26 MCL-US 10 3 428 0.2 3.7 6.3 89.8 0.02 - 122 mg/L 0.88 mg/L
Total Dissolved Solids 4 mg/L na SMCL-CA 1,000 5 425 1.4 11.1 14.8 72.7 110 - 19,200 mg/L 330 mg/L
Uranium µg/L na MCL-US 30 71 2.8 1.4 4.2 91.6 0.134 - 1,470 µg/L 1.9 µg/L
Vanadium µg/L 2 NL-CA 50 224 0.4 6.7 13.4 79.5 0.63 - 687 µg/L 11 µg/L

1Revised from 50 µg/L in January 2006. Detection frequency ratings based on revised standard.
2Revised from 50 µg/L in July 2014. Detection frequency ratings based on revised standard.
3MCL-US for nitrate.
4Includes sum of dissolved constituents and residue on evaporation at 180°C, USGS parameter codes 70301 and 70300, respectively.
5State of California SMCL - upper level.


The concentrations in samples are presented on the maps as relative concentrations (RC). The RC is the measured concentration divided by the concentration of a benchmark. An RC of less than 1 indicates that the sample concentration is less than the benchmark concentration; an RC of greater than 1 indicates that the concentration in the sample is above the benchmark concentration. The benchmarks used in this study were drinking-water quality benchmarks established by the California State Water Resources Control Board (SWRCB) Division of Drinking Water (California Department of Public Health, 2010; California State Water Resources Control Board, 2014; the regulation of drinking-water quality was transferred from the California Department of Public Health to the SWRCB Division of Drinking Water on July 1, 2014), or the U.S. Environmental Protection Agency (EPA) (U.S. Environmental Protection Agency, 2014). The EPA maximum contaminant level (MCL-US) benchmarks were used for arsenic, nitrate, and uranium; the SWRCB maximum contaminant level (MCL-CA) benchmarks were used for chromium-6, chromium, and fluoride. MCLs are health-based, regulatory benchmarks. The SWRCB secondary maximum contaminant level (SMCL-CA) benchmarks were used for iron, manganese, and total dissolved solids (TDS). TDS has a recommended and an upper SMCL-CA; the upper SMCL-CA is used in this study. SMCLs are non-regulatory benchmarks established for aesthetic and technical concerns, such as taste, odor, scaling, and staining. The SWRCB notification level (NL-CA) benchmarks were used for boron and vanadium. NL-CA are health-based, non-regulatory benchmarks. Although these benchmarks were designed to be applied to drinking water delivered to consumers, not to untreated groundwater, they can be used to provide context for characterizing the relative quality of potential source waters. Measured concentrations were grouped into four RC categories for mapping (table 3). For dissolved oxygen, which does not have a drinking -water quality benchmark, mapping categories were set relative to a concentration of 0.5 milligrams per liter (mg/L), which can be used to differentiate between water that is oxic (greater than 0.5 mg/L) and is anoxic (less than or equal to 0.5 mg/L).

Table 3. Summary of relative concentration (RC) categories used for mapping the distribution and concentrations of selected trace elements and major ions.

[RC, relative concentration; <, actual value is less than value shown; ≤, actual value is less than or equal to the value shown; >, actual value is greater than value shown]

Relative Concentration Category Map Symbol Color Relative Concentration Range
Low Green RC ≤ 0.5
Moderate Yellow 0.5 < RC ≤ 1
High Orange 1 < RC ≤ 5
Very High Red RC > 5

Reporting levels (RLs) for the analyses varied during the study, typically because of differences in laboratories or instrument performance over time. These variations did not affect the mapping of results because nearly all of the RLs were in the lowest mapping category (that is, less than half the benchmark value). Results with RLs equal to or greater than half the benchmark were anomalous and usually were raised RLs, which can be caused by matrix interferences or laboratory dilution of the sample. For example, six wells included results for iron reported as less than ("<") raised RLs between <60 and <500 µg/L. These anomalously high "less than" results were not useful for identifying patterns in concentration and were excluded from the dataset.

For constituents that had half or more of the results reported as less than the RL (boron, total chromium, iron, and manganese), medians (table 2) were determined by using the Kaplan-Meier nonparametric approach (Kaplan and Meier, 1958) as described by Helsel (2005). The Kaplan-Meier approach was also used for individual wells where the median could not be visually identified—wells that had an even number of values for uncensored (values not qualified with the less than symbol) and for censored (values qualified with the less than symbol) results representing multiple detection limits.

The distribution of concentrations for each constituent is displayed on maps by using graduated colors and symbols. Symbols mark sampling points that had at least one sample during 2000-12. The color and size of each symbol corresponds with the concentration. From lowest to highest, green signifies concentrations that are less than or equal to half the benchmark value, yellow for more than half to equal the benchmark, orange for greater than one to five times the benchmark, and red for more than five times the benchmark (table 3). The larger the symbol, the greater the concentration. For dissolved oxygen, the color and size scheme is reversed; decreasing concentrations are represented by increasingly warmer colors (green, yellow, orange, and red) and larger circles. This reversed scheme was used for dissolved oxygen because lower dissolved oxygen concentrations are commonly associated with higher concentrations of selected constituents, including arsenic (McMahon and Chapelle, 2008). The color scheme also represents a general classification of sample concentration in the context of water-quality standards (benchmarks). Concentrations greater than the benchmark (orange and red) are considered "high" and "very high," respectively. Concentrations between half the benchmark and the benchmark (yellow) are "moderate." Concentrations less than or equal to half the benchmark (green) are "low." These designations are consistent with the GAMA Program classification of relative concentrations (RC)—defined as the sample concentration divided by the benchmark concentration—as high ( RC greater than 1), moderate (RC greater than 0.5 and less than or equal to 1), or low (RC less than or equal to 0.5) (Dawson and Belitz, 2012).

Each interactive map includes a graph of cumulative distribution function (CDF). These graphs show the relation between concentration and the cumulative frequency of samples that had values less than or equal to the corresponding concentration. The entire range of concentrations was used to construct the cumulative distribution function for each constituent; however, the graphs show only those results that are greater than (">") the highest RL on table 2. Results less than the highest RL are not shown because of the uncertainty in the "less than" values, which could be any concentration between zero and the RL and can affect the relative ranks of all results less than the highest RL. The resulting graphs included all concentrations that are greater than one-fifth of the benchmark for each constituent. Iron and manganese data included a large number of values less than or equal to the highest accepted RLs of less than 40 and less than 5 micrograms per liter (µg/L), respectively, resulting in CDF graphs representing less than a quarter of the available data. Most of the results for these two constituents were less than one-fifth their respective benchmarks.

Time-series graphs were created for selected sampling points that had a least 9 measurements during 2000-12: arsenic had 27 sampling points; boron, 25 sampling points; total chromium, 6 sampling points; dissolved oxygen, 5 sampling points; fluoride, 31 sampling points; iron, 30 sampling points; manganese, 31 sampling points; nitrate plus nitrite, 31 sampling points; and total dissolved solids, 31 sampling points. No time-series graphs were created for chromium-6, uranium, or vanadium because these constituents had no sampling point that had at least nine measurements. The color scheme used to display sample concentrations on the time-series graphs denotes whether the concentration was greater than a benchmark (red), was detected at a concentration less than the RL (blue), or was either not detected or was measured at less than the RL (yellow). Results reported as less than the RL were plotted at the RL concentration..

Selected Water Quality Data (2000-2012)

Concentrations of inorganic constituents in some groundwater samples were greater than water-quality benchmarks for all 11 of the mapped constituents that had benchmarks. The frequency of detection for high and very high RCs in water from wells sampled by the USGS and the MWA during 2000-12 ranged from 2.2 percent for total chromium to 21.7 percent for arsenic (table 2). Although water that had high and very high RCs of a few of the constituents was widely distributed, for most of the mapped constituents, the presence of high and very high RCs was focused in a few specific parts of the study area, as described in more detail later. In the discussion that follows, mapped constituents are presented in alphabetical order.

Arsenic was detected at concentrations greater than the benchmark value of 10 µg/L in water from 21.7 percent of the 433 sampling points for which it was sampled during 2000-12. Water from sampling points classified as having high or very high RCs included water from 75 sampling points (17.3 percent) that had a concentration 1 to 5 times the current MCL-US of 10 µg/L (revised from 50 µg/L in 2006) and 19 sampling points (4.4 percent) that had a concentration greater than 5 times the current MCL-US. High arsenic concentrations were widely distributed in the study area, particularly along the Mojave River in the vicinity of Barstow; near the conjunction of the Surprise Spring, Mesquite, and Deadman subbasins of the Morongo groundwater basin near Twentynine Palms; along the Mojave River between Adelanto and Helendale; in the Alto subarea of the Mojave River groundwater basin from Victorville south towards Hesperia; and near Newberry Springs southeast of Barstow. Although different wells and criterion (maximum instead of median values for multiple analyses) were used to map concentrations detected in the 1990s (Christensen and Fields-Garland, 2001), a comparison of the two periods indicated that distribution of moderate to very high arsenic concentrations (greater than 5 µg/L to greater than 500 µg/L) appeared to be comparable during the two periods. One notable exception could be the area near Twentynine Palms, where the few wells that were sampled during the 1990s yielded water that had predominately low arsenic concentrations.

Boron was detected at concentrations greater than the benchmark value of 1,000 µg/L in water from 10.7 percent of 355 sampling points for which it was sampled during 2000-12. Water from sampling points classified as having high or very high RCs included water from 32 sampling points (9.0 percent) that had a concentration 1 to 5 times the current NL-CA of 1,000 µg/L and 6 sampling points (1.7 percent) that had a concentration greater than 5 times the NL-CA. Boron concentrations in groundwater from wells appeared to increase with increasing distance from the mountain front; the greatest number of wells that had water with high or very high RCs were near the divide between the Centro and Baja subareas of the Mojave River groundwater basin on the east side of Barstow. This spatial distribution appears to be similar to the boron concentrations in the 1990s (Christensen and Fields-Garland, 2001). High or very high boron concentrations were also present in groundwater from a few wells in the eastern portion of the Alto subarea near Apple Valley and in the western and eastern margins of the Morongo groundwater basin near Lucerne Valley and Twentynine Palms, respectively.

Chromium-6 (hexavalent chromium) was detected at concentrations greater than the benchmark value of 10 µg/L in water from 12.4 percent of the 355 sampling points for which it was sampled during 2000-12. Water from sampling points classified as having high RCs included water from 40 sampling points (11.3 percent) that had a concentration greater than the new MCL-CA of 10 µg/L (effective July 2014), but less than the old MCL-CA of 50 µg/L. The majority of wells where water high RCs were in upgradient areas away from and to the west of the Mojave River in the Alto and Oeste subareas, the Morongo groundwater basin, and to the east of Barstow. Concentrations greater than 50 µg/L and classified as very high RCs were measured in samples from four sampling points (1.1 percent)—all wells in the Oeste subarea of the Mojave groundwater basin near El Mirage.

Total Chromium was detected at concentrations greater than the benchmark value of 50 µg/L in water from 2.2 percent of the 399 sampling points for which it was sampled during 2000-12. Water from sampling points classified as having high or very high RCs consisted of water from eight sampling points that had concentrations one to five times greater than the current MCL-CA of 50 µg/L and one sampling point that had a concentration greater than five times the MCL-CA. All of the wells where water had high and very high RCs were in the El Mirage area.

Dissolved Oxygen (DO) concentrations were classified in terms of oxygen poor (anoxic, DO concentrations less than or equal to 0.5 mg/L) or oxygen rich (oxic, DO concentrations greater than 0.5 mg/L) conditions. Groundwater in the study area was predominantly oxic and had a median DO concentration of 4.4 mg/L. Anoxic groundwater was detected at 33 sampling points, 15 percent of the 226 sampling points for which DO was sampled. Wells where water had low DO concentrations were primarily between Helendale and Hesperia and extended west to the El Mirage area, near Newberry Springs, and in the Morongo groundwater basin near Yucca Valley and Twentynine Palms. For water from sampling points where DO, iron, and manganese were analyzed (about half of the sampling points sampled in 2000-12), there was a weak correlation between low DO and high iron and manganese concentrations. Using Spearman's method (Helsel and Hirsch, 2002) to calculate the rank-order coefficient (rho) and the significance level of the correlation (p), DO was negatively, but not significantly, correlated with iron (Spearman's rho = -0.258, p = 0.0002) and with manganese concentrations (Spearman's rho = -0.235, p = 0.007)

Fluoride was detected at concentrations greater than the benchmark value of 2 mg/L in water from 10.4 percent of the 432 sampling points for which it was sampled during 2000-12. Water from sampling points classified as having high or very high RCs included water from 39 sampling points (9 percent) that had a concentration 1 to 5 times the current MCL-CA of 2 mg/L and 6 sampling points (1.4 percent) that had a concentration greater than 5 times the MCL-CA. Wells where water had the highest concentrations were in the southern part of the Morongo groundwater basin, and a few high RCs were measured in water from wells scattered along the upper, middle, and lower reaches of the Mojave River. Comparison with Christensen and Fields-Garland (2001) indicated that fluoride concentrations in groundwater on the east side of Barstow could have decreased during 2000-12; water from most wells in the area had low RCs of less than 1 milligram per liter (mg/L).

Iron was detected at concentrations greater than the benchmark value of 300 µg/L in water from 3.5 percent of the 426 sampling points for which it was sampled during 2000-12. Water from sampling points classified as having high or very high RCs included seven sampling points (1.6 percent) that had a concentration one to five times the current SMCL-CA of 300 µg/L and eight sampling points (1.9 percent) that had a concentration greater than five times the SMCL-CA. The majority of wells where water had high and very high RCs were along or near the Mojave River in the vicinity of Barstow and from about Helendale to Victorville. High concentrations of iron were also detected in water from several wells near Yucca Valley.

Manganese was detected at concentrations greater than the benchmark value of 50 µg/L in water from 7 percent of the 426 sampling points for which it was sampled during 2000-12. Water from sampling points classified as having high or very high RCs included water from 14 sampling points (3.3 percent) that had a concentration 1 to 5 times the current SMCL-CA of 50 µg/L and 16 sampling points (3.7 percent) that had a concentration greater than 5 times the SMCL-CA. As shown by the cumulative distribution function (CDF) graph, manganese in groundwater in the study area was at relatively low concentrations, and water from about 80 percent of sampled points had manganese concentrations less than the highest accepted RL of less than 5 µg/L. The distribution of manganese in groundwater showed similarities to several other mapped constituents, including arsenic, TDS, and iron; the highest manganese concentrations were detected along or near the Mojave River in the vicinity of Barstow and between Helendale and Victorville.

Nitrate plus nitrite (as N) was detected at concentrations greater than the benchmark value of 10 mg/L in water from 3.9 percent of the 428 sampling points for which it was sampled during 2000-12. Water from sampling points classified as having high or very high RCs included water from 16 sampling points that had concentrations 1 to 5 times the current MCL-US of 10 mg/L and 1 sampling point that had a concentration greater than 5 times the MCL-US. Water from wells that had the highest concentrations were in the vicinity of El Mirage in the Oeste subarea of the Mojave River groundwater basin, including water from six wells that had concentrations greater than 20 mg/L. Mapped concentrations for water from these six wells, all shallow monitoring wells completed to a depth of less than 100 ft below land surface, ranged from 21 to 122 mg/L. In contrast, the water from wells sampled in the El Mirage area in the 1990s had concentrations of nitrate plus nitrite in groundwater less than 5 mg/L (Christensen and Fields-Garland, 2001). This area of high nitrate plus nitrite groundwater coincided with the area that had the highest total chromium concentrations, but unlike nitrate plus nitrite, high total chromium concentrations were detected in water from wells greater than 100 ft deep. Other areas where water from wells had high RCs during 2000-12 included the Warren and Joshua Tree subbasins of the Morongo groundwater basin and the Barstow area, similar to the distribution of moderate to high concentrations of nitrate plus nitrite in the 1990s.

Total Dissolved Solids (TDS) was detected at concentrations greater than the benchmark value of 1,000 mg/L (SMCL-CA upper level) in water from 12.5 percent of the 425 sampling points for which it was sampled during 2000-12. Water from sampling points classified as having high or very high RCs included water from 47 sampling points (11.0 percent) that had concentrations 1 to 5 times the current SMCL-CA upper level and 6 sampling points (1.4 percent) that had concentrations greater than 5 times the SMCL-CA upper level. Water from an additional 65 sampling points (15.2 percent) had TDS concentrations categorized as moderate RCs (greater than the SMCL-CA recommended level of 500 mg/L). TDS generally increased along the Mojave River as distance from the mountain front increased, and the greatest number of wells where water had high RCs were in the Barstow area. Including high TDS concentrations in water from a few wells in the Lucerne and Apple Valleys, this pattern was similar to the distribution of TDS in the 1990s (Christensen and Fields-Garland, 2001). Unlike the 1991-97 pattern, however, high TDS concentrations during 2000-12 were also detected in water from wells near El Mirage and Twentynine Palms. Areas of high TDS during 2000-12 coincided with moderate to high concentrations of other mapped constituents, particularly arsenic and boron on the east of side Barstow, nitrate plus nitrite and chromium-6 greater than 10 µg/L in the El Mirage area, and arsenic and fluoride in the Morongo groundwater basin near Twentynine Palms.

Uranium was detected at concentrations greater than the benchmark value of 30 µg/L L in water from 4.2 percent of 71 sampling points for which it was sampled during 2000-12. The comparatively small set of sampling points sampled for uranium included one sampling point where water had a concentration one to five times greater than the MCL-US of 30 µg/L and two sampling points where water had concentrations greater than five times the MCL-US. The well that had the single sampling point where water had a high RC of uranium was just east of Barstow in the Centro subarea of the Mojave River groundwater basin. The well that had the two sampling points where water had very high RCs of uranium was at the southern end of the Morongo groundwater basin. Spatial patterns in uranium concentrations were less certain than those of the other constituents because of the limited data available for mapping.

Vanadium was detected at concentrations greater than the benchmark value of 50 µg/L L in water from 7.1 percent of 224 sampling points for which it was sampled during 2000-12. Water from sampling points classified as having high or very high RCs consisted of water from 16 sampling points that had a concentration 1 to 5 times greater than the NL-CA of 50 µg/L and 1 sampling point that had a concentration more than 5 times the NL-CA. Wells that had sampling points where water had high and very high concentrations of vanadium were in upgradient (closer to the mountain front) areas of the Mojave River groundwater basin south of Victorville along and to the west of the Mojave River. Water from several wells also had high RCs in the Morongo groundwater basin near Twentynine Palms. The distribution of wells where water had moderate to very high concentrations of vanadium generally coincided with high to very high chromium-6 concentrations (greater than 10 µg/L).

Time-Series Wells

Overall, few of the up to 31 sampling points used to plots concentrations over time exhibited visually notable changes in concentrations during 2000-12. The five sampling points where water did exhibit apparent changes in constituent concentrations were all in wells categorized as 'other' (uses other than monitoring) that had completed depths between 130 ft (well 10N/3W-27F1) and 360 ft (well 9N/2E-7Q1) below land surface. Water from well 10N/4E-20D1 showed apparent decreases in boron (2,430 to 1,960 µg/L), fluoride (9.7 to 4.6 mg/L), and TDS (606 to 521 mg/L), but an apparent increase in arsenic (37 to 41.2 µg/L). Water from well 10N/1E-28G3 showed apparent decreases in fluoride (8.6 to 2.9 mg/L) and nitrate plus nitrite (3.7 to 2.1 mg/L). Well 7N/4W-7K2 showed an apparent increase in nitrate plus nitrite (3.7 to 7.2 mg/L ) and an apparent trend reversal for TDS from decreasing concentration between 2000 and 2005 (1,090 to 804 mg/L) to increasing concentration between 2005 and 2012 (up to 1,130 mg/L). Water from wells 9N/2E-7Q1 and 10N/3W-27F1 each had one constituent that showed an apparent trend; TDS in water from well 9N/2E-7Q1 decreased (617 to 186 mg/L) and nitrate plus nitrate in water from well 10N/3W-27F1 increased (2 to 10 mg/L). Water from several additional wells, including 9N/1W-11R1 (nitrate plus nitrite and TDS) and 4N/5E-11E3 (arsenic and iron), exhibited trends for specific constituents, but because the data for these particular wells corresponded to only a portion of the 2000-12 period they might not represent temporal trends that are comparable to the other wells.

View the time-series graphs

Suggested Citation:
Metzger, L.F., Landon, M.K., House, S.F., and Olsen, L.D., 2015, Mapping selected trace elements and major ions, 2000-2012, Mojave River and Morongo Groundwater Basins, Southwestern Mojave Desert, San Bernardino County, California: U.S. Geological Survey Data Release, https://ca.water.usgs.gov/mojave/water-quality.html, doi:10.5066/F7Q23X95.