USGS

Evaluation of Volatile Organic Compounds in Two Mojave Desert Basins—Mojave River and Antelope Valley—in San Bernardino, Los Angeles, and Kern Counties, California, June-October 2002

By Jill N. Densmore, Kenneth Belitz, Michael T. Wright, Barbara J. Dawson, and Tyler Johnson

 

U.S. GEOLOGICAL SURVEY

Water-Scientific Investigations Report 2004-5248 Version 1.01— ONLINE ONLY

Sacramento, California 2005

 

Prepared in cooperation with the

California State Water Resources Control Board

 



Complete accessible text of report Version 1.01 (6,917 KB PDF)

Errata_sheet

To view PDF documents, you must have the Adobe Acrobat Reader (free from Adobe Systems) installed on your computer.
(download free copy of Acrobat Reader).

Abstract

The California Aquifer Susceptibility Assessment of the Ground-Water Ambient Monitoring and Assessment Program was developed to assess water quality and susceptibility of ground-water resources to contamination from surficial sources. This study focuses on the Mojave River and the Antelope Valley ground-water basins in southern California.

Volatile organic compound (VOC) data were evaluated in conjunction with tritium data to determine a potential correlation with aquifer type, depth to top of perforations, and land use to VOC distribution and occurrence in the Mojave River and the Antelope Valley Basins. Detection frequencies for VOCs were compiled and compared to assess the distribution in each area. Explanatory variables were evaluated by comparing detection frequencies for VOCs and tritium and the number of compounds detected. Thirty-three wells were sampled in the Mojave River Basin (9 in the floodplain aquifer, 15 in the regional aquifer, and 9 in the sewered subset of the regional aquifer). Thirty-two wells were sampled in the Antelope Valley Basin. Quality-control samples also were collected to identify, quantify, and document bias and variability in the data.

Results show that VOCs generally were detected slightly more often in the Antelope Valley Basin samples than in the Mojave River Basin samples. VOCs were detected more frequently in the floodplain aquifer than in the regional aquifer and the sewered subset. Tritium was detected more frequently in the Mojave River Basin samples than in the Antelope Valley Basin samples, and it was detected more frequently in the floodplain aquifer than in the regional aquifer and the sewered subset. Most of the samples collected in both basins for this study contained old water (water recharged prior to 1952). In general, in these desert basins, tritium need not be present for VOCs to be present. When VOCs were detected, young water (water recharge after 1952) was slightly more likely to be contaminated than old water.

Trihalomethanes (THMs) were detected less frequently in the Mojave River Basin samples than in the Antelope Valley Basin samples. The THMs that were detected in the Mojave River Basin were detected more frequently in the floodplain aquifer than in the regional aquifer and sewered subset. Solvents were detected more frequently in the Mojave River samples than in the Antelope Valley samples. In the Mojave River Basin samples, solvents were detected less frequently in the floodplain aquifer than in the regional aquifer and the sewered subset. Benzene, toluene, ethylbenzene and xylene (BTEX) were not detected in either study area. Methyl tert-butyl ether (MTBE) was detected in one sample from both the Mojave River and Antelope Valley Basins.

The most frequently detected compound (detected in more than 10 percent of the wells) in the Mojave River Basin was chloroform. The two most frequently detected compounds in the Antelope Valley Basin were chloroform and tetrachloroethylene (PCE).

In the Mojave River Basin, aquifer type and land use within 1,640 ft (500 m) of the well head were not statistically correlated with the number of VOCs detected, although VOCs were detected more frequently in the floodplain aquifer than in the regional aquifer and the sewered subset. Depth to the top of the perforations was an explanatory factor for the number of VOCs detected in the Mojave River Basin; the detection frequency was greater for shallow wells than for deep wells.

In the Antelope Valley Basin, neither aquifer type, depth to the top of the perforations, nor land use within 1,640 ft of the well head were explanatory factors for the number of VOCs detected. Although aquifer type and depth to top of the perforations did explain the presence of tritium in the Mojave River Basin, land use within 1,640 ft of the well head was not a statistically significant explanatory factor for the presence of tritium in this basin. Aquifer type, depth to the top of the perforations, and land use within 1,640 ft of the well head were not statistically significant explanatory factors for the presence of tritium in the Antelope Valley Basin.

The areal distribution of VOCs in the Mojave River Basin does not have an apparent spatial pattern that can be associated to one direct source. Although VOCs were detected more often in samples from wells in the floodplain aquifer than in the samples from the regional aquifer and the sewered subset, the VOC detections in wells in the regional aquifer generally contained more than one compound, unlike the VOCs detections in wells in the floodplain aquifer and the sewered subset, which contained only one compound.

The areal distribution of VOCs in the Antelope Valley Basin samples seems to be concentrated in the urban areas and near the mountain front. Differences in VOC detection frequency and distribution between the Mojave River and the Antelope Valley Basins probably were related to differences in urban land use between the two basins.

The oxygen-18 (18O) and deuterium (D), expressed in delta notation (d), isotopic composition of the ground-water samples from the Mojave River and the Antelope Valley Basins indicate that recharge along the Mojave River and in the sewered subset resulted from precipitation from a slightly different storm track and recharge elevation than recharge in the regional aquifer and in the Antelope Valley Basin. The isotopic composition was not related to the occurrence of VOCs.

CONTENTS

Abstract
Introduction
   Description of Study Area
   Hydrologic Setting
Methods of Study
   Well Selection
   Sample Collection
   Determination and Use of Categorical Variables
 		Aquifer Type
 	 	Well Depth
 	 	Land Use 
 	  	Tritium Concentrations in Ground Water
   Quality-Control Data Analyses
 		Surrogate Data Analysis
 		Blank Data Analysis
   Statistical Analysis
Results 
   Detection Frequencies for VOCs, Tritium, and VOC Class
   Detection Frequency of VOC by Compound
   VOC Occurrence by Ground-Water Age
   Evaluation of Aquifer Type, Well Depth, and Land Use as Explanatory Factors
  		Aquifer Type
  		Depth to Top Perforation
		Land Use
		Number of VOCs Detected and Tritium in the Mojave River and Antelope Valley Basins
   Isotopic Compostiton of Mojave and Antelope Valley Samples
   Noble Gases and Recharge Temperature
Summary and Conclusions
References Cited
 

Document Accessibility: Adobe Systems Incorporated has information about PDFs and the visually impaired. This information provides tools to help make PDF files accessible. These tools convert Adobe PDF documents into HTML or ASCII text, which then can be read by a number of common screen-reading programs that synthesize text as audible speech. In addition, an accessible version of Acrobat Reader 5.0 for Windows (English only), which contains support for screen readers, is available. These tools and the accessible reader may be obtained free from Adobe at Adobe Access.

 



Water Resources of California



U.S. Department of the Interior, U.S. Geological Survey
Persistent URL: http://pubs.water.usgs.gov/sir20045248
Page Contact Information: GS Pubs Web Contact
Last modified: Thursday, January 10 2013, 06:40:15 PM
FirstGov button  Take Pride in America button