Shallow Ground-Water Quality Beneath Rice Areas in the Sacramento Valley, California, 1997

By Barbara J. Milby Dawson

U.S. GEOLOGICAL SURVEY

Water Resources Investigation Report 01-4000

NATIONAL WATER-QUALITY ASSESMENT PROGRAM

CONTENTS

      Abstract        
Introduction 
Purpose and Scope 
Acknowledgments         
Study-Area Description  
Climate and Population          
Land and Water Use 
Hydrogeology    
Methods of Investigation                        
Well Site Selection     
Monitoring Well Installation    
Well Descriptions, Water Levels, and Lithology 
Water Sample Collection and Analysis 
Data Reporting and Analysis 
Quality-Control Data Analyses           
Shallow Ground-Water Quality    
General Water Chemistry         
Major Inorganic Constituents 
Trace Inorganic Constituents 
Nutrient Constituents 
Dissolved Organic Carbon 
Pesticides 
Ground-Water Redox Condition 
Analyses of Ground-Water Quality Data 
Statistical Analyses            
Correlations Between Shallow Ground-Water Quality and Geomorphic Unit   
Correlations Between Shallow Ground-Water Quality and Depth to Ground Water and 
        Top of Well Screen      
Correlations Between Inorganic Constituents, Land Use, and Pesticides 
Tritium Concentration, Pesticide Use, and Age of Shallow Ground-Water Recharge  
Stable Isotopes and Possible Sources of Shallow Ground Water 
Summary and Conclusions 
References Cited
      
        FIGURES
      
 1.   Map showing location of the rice land-use study area and the Sacramento 
        River Basin, California 
 2.   Map showing locations of wells sampled in the rice areas in the Sacramento
        Valley, California   
 3.   Map showing well sites and geomorphic units in the rice areas in the 
        Sacramento Valley, California      
 4.   Map showing water-level contours in the rice areas in the Sacramento 
        Valley, California, August to October, 1997 
 5.   Trilinear diagram showing major-ion composition of ground-water samples 
        from rice areas in the Sacramento Valley, California   
 6.   Map showing redox conditions in wells sampled in rice areas in the 
        Sacramento Valley, California                
 7.   Graphs showing concentrations of arsenic, boron, chloride, dissolved 
        solids, fluoride, molybdenum, nitrate, phosphorus, potassium, silica, 
        sodium, sulfate, and zinc detected in shallow ground water in rice areas
        in relation to well location and geomorphic unit, Sacramento Valley, 
        California    
 8.   Graphs showing correlations between depth to ground water and number of 
        rice pesticides detected per well, cobalt, manganese, nickel, and 
        nitrite concentrations; and correlations between depth to top of screen
        and number of pesticides detected per well, molybdenum, and sulfate 
        concentrations in wells sampled in rice areas in the Sacramento Valley,
        California    
 9.   Graphs showing correlations between concentrations of bentazon and 
        tritium, and correlations between number of rice pesticides detected 
        per well and concentrations of boron and sodium detected in shallow 
        ground water in rice areas in the Sacramento Valley, California      
10.  Graph showing relation between d2H and d18O in ground water and surface 
        water in parts of the Sacramento Valley, California   
11.  Graph showing relation between dissolved solids concentration and d18O in 
        shallow ground water in rice areas in the Sacramento Valley, California
      
        TABLES
      
 1.   Well construction information for wells sampled in rice areas in the 
        Sacramento Valley, California      
 2.   Laboratory analysis methods for measured water-quality constituents
 3.   Pesticides analyzed and reporting limits in water samples, in the 
        Sacramento River Basin, California    
 4.   Summary of mean relative standard deviations for replicate sample pairs of
        inorganic compounds, nutrient compounds, and dissolved organic carbon 
 5.   Summary of general water chemistry in shallow ground water in rice areas 
        in the Sacramento Valley, California   
 6.   Summary of major inorganic constituents detected in shallow ground water 
        in rice areas in the Sacramento Valley, California   
 7.   Summary of trace inorganic constituents detected in shallow ground water 
        in rice areas in the Sacramento Valley, California   
 8.   Summary of nutrient constituents and dissolved organic carbon detected in
        ground water in rice areas in the Sacramento Valley, California
 9.   Summary of pesticides detected in shallow ground water in rice areas in 
        the Sacramento Valley, California           
10.   Pesticides and degradation products not detected in shallow ground water 
        in rice areas in the Sacramento Valley, California
11.   Reported uses of pesticides detected in rice areas in the Sacramento 
        Valley, California         
12.   Summary of isotopes measured in shallow ground water in rice areas in the
        Sacramento Valley, California
    

ABSTRACT

In 1997, the U.S. Geological Survey installed and sampled 28 wells in rice areas in the Sacramento Valley as part of the National Water-Quality Assessment Program. The purpose of the study was to assess the shallow ground-water quality and to determine whether any effects on water quality could be related to human activities and particularly rice agriculture. The wells installed and sampled were between 8.8 and 15.2 meters deep, and water levels were between 0.4 and 8.0 meters below land surface. Ground-water samples were analyzed for 6 field measurements, 29 inorganic constituents, 6 nutrient constituents, dissolved organic carbon, 86 pesticides, tritium (hydrogen-3), deuterium (hydrogen-2), and oxygen-18.

At least one health-related state or federal drinking-water standard (maximum contaminant or long-term health advisory level) was exceeded in 25 percent of the wells for barium, boron, cadmium, molybdenum, or sulfate. At least one state or federal secondary maximum contaminant level was exceeded in 79 percent of the wells for chloride, iron, manganese, specific conductance, or dissolved solids. Nitrate and nitrite were detected at concentrations below state and federal 2000 drinking-water standards; three wells had nitrate concentrations greater than 3 milligrams per liter, a level that may indicate impact from human activities. Ground-water redox conditions were anoxic in 26 out of 28 wells sampled (93 percent).

Eleven pesticides and one pesticide degradation product were detected in ground-water samples. Four of the detected pesticides are or have been used on rice crops in the Sacramento Valley (bentazon, carbofuran, molinate, and thiobencarb). Pesticides were detected in 89 percent of the wells sampled, and rice pesticides were detected in 82 percent of the wells sampled. The most frequently detected pesticide was the rice herbicide bentazon, detected in 20 out of 28 wells (71 percent); the other pesticides detected have been used for rice, agricultural, and non-agricultural purposes. All pesticide concentrations were below state and federal 2000 drinking-water standards.

The relation of the ground-water quality to natural processes and human activities was tested using statistical methods (Spearman rank correlation, Kruskal.Wallis, or rank-sum tests) to determine whether an influence from rice land-use or other human activities on ground-water chemistry could be identified. The detection of pesticides in 89 percent of the wells sampled indicates that human activities have affected shallow ground-water quality. Concentrations of dissolved solids and inorganic constituents that exceeded state or federal 2000 drinking-water standards showed a statistical relation to geomorphic unit. This is interpreted as a relation to natural processes and variations in geology in the Sacramento River Basin; the high concentrations of dissolved solids and most inorganic constituents did not appear to be related to rice land use. No correlation was found between nitrate concentration and pesticide occurrence, indicating that an absence of high nitrate concentrations is not a predictor of an absence of pesticide contamination in areas with reducing ground-water conditions in the Sacramento Valley.

Tritium concentrations, pesticide detections, stable isotope data, and dissolved-solids concentrations suggest that shallow ground water in the rice-growing areas of the Sacramento Valley is a mix of recently recharged ground water containing pesticides, nitrate, and tritium, and unknown sources of water that contains high concentrations of dissolved solids and some inorganic constituents and is enriched in oxygen-18. Evaporation of applied irrigation water, which leaves behind salt, accounts for some of the elevated concentrations of dissolved solids. More work needs to be done to understand the connections between the land surface, shallow ground water, deep ground water, and the drinking-water supplies in the Sacramento Valley.

      For additional information          Copies of this report can be 
write to:                           purcahsed from:

District Chief                      U.S. Geological Survey
U.S. Geological Survey              Information Service
Placer Hall, Suite 2012             Box 25286
6000 J Street                       Federal Center
Sacramento, CA 95819                Denver, CO 80225-0286