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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5156

Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants in the Zone of Contribution of a Public-Supply Well in Modesto, Eastern San Joaquin Valley, California

Prepared in cooperation with
National Water-Quality Assessment Program
Transport of Anthropogenic and Natural Contaminants (TANC) to Public-Supply Wells

By Bryant C. Jurgens, Karen R. Burow, Barbara A. Dalgish, and Jennifer L. Shelton

Abstract

Ground-water chemistry in the zone of contribution of a public-supply well in Modesto, California, was studied by the U.S. Geological Survey National Water Quality Assessment (NAWQA) Program’s topical team for Transport of Anthropogenic and Natural Contaminants (TANC) to supply wells. Twenty-three monitoring wells were installed in Modesto to record baseline hydraulic information and to collect water-quality samples. The monitoring wells were divided into four categories that represent the chemistry of different depths and volumes of the aquifer: (1) water-table wells were screened between 8.5 and 11.7 m (meter) (28 and 38.5 ft [foot]) below land surface (bls) and were within 5 m (16 ft) of the water table; (2) shallow wells were screened between 29 and 35 m (95 and 115 ft) bls; (3) intermediate wells were screened between 50.6 and 65.5 m (166 and 215 ft) bls; and (4) deep wells are screened between 100 to 106 m (328 and 348 ft) bls. Inorganic, organic, isotope, and age-dating tracers were used to characterize the geochemical conditions in the aquifer and understand the mechanisms of mobilization and movement of selected constituents from source areas to a public-supply well.

The ground-water system within the study area has been significantly altered by human activities. Water levels in monitoring wells indicated that horizontal movement of ground water was generally from the agricultural areas in the northeast towards a regional water-level depression within the city in the southwest. However, intensive pumping and irrigation recharge in the study area has caused large quantities of ground water to move vertically downward within the regional and local flow systems.

Analysis of age tracers indicated that ground-water age varied from recent recharge at the water table to more than 1,000 years in the deep part of the aquifer. The mean age of shallow ground water was determined to be between 30 and 40 years. Intermediate ground water was determined to be a mixture of modern (Post-1950) and old (Pre-1950) ground water. As a result, concentrations of age tracers were detectable but diluted by older ground water. Deep ground water generally represented water that was recharged under natural conditions and therefore had much older ages. Ground water reaching the public-supply well was a mixture of older intermediate and deep ground water and young shallow ground water that has been anthropogenically-influenced to a greater extent than intermediate ground water.

Uranium and nitrate pose the most significant threat to the quality of water discharged from the public-supply well. Although pesticides and VOCs were present in ground water from the public-supply well and monitoring wells, currently concentrations of these contaminants are generally less than one-hundredth the concentration of drinking water standards. In contrast, both uranium and nitrate were above half the concentration of drinking water standards for public-supply well samples, and were above drinking water standards for several water-table and shallow monitoring wells. Shallow ground water contributes roughly 20 percent of the total flow to the public-supply well and was the entry point of most contaminants reaching the public-supply well.

Naturally-occurring uranium, which is commonly adsorbed to aquifer sediments, was mobilized by oxygen-rich, high-alkalinity water, causing concentrations in some monitoring wells to be above the drinking-water standard of 30 µg/L (microgram per liter). Adsorption experiments, sediment extractions, and uranium isotopes indicated uranium in water-table and shallow ground water was leached from aquifer sediments. Uranium is strongly correlated to bicarbonate concentrations (as measured by alkalinity) in ground water. Bicarbonate can effectively limit uranium adsorption to sediments. As a result, continued downward movement of high-alkalinity, oxygen-rich ground water will likely lead to larger portions of the aquifer having elevated uranium.

Nitrate concentrations were above the drinking water standard of 10 mg/L (milligram per liter) in two water-table wells influenced by agricultural practices. In contrast, concentrations in most water-table wells influenced by urban practices were less than 5.0 mg/L. All three shallow monitoring wells which were located beneath urban land had nitrate concentrations above the drinking water standard. Co-occurrence of sulfate with nitrate indicated that agricultural practices mainly were responsible for high nitrate in ground water. Denitrification can occur in localized areas; however, attenuation of nitrate was not significant enough to decrease contaminant concentrations throughout the aquifer. As a result, nitrate concentrations in ground water were mostly influenced by nitrogen fertilizer application rates and dispersion processes.

The water chemistry of the public-supply well was strongly influenced by well-bore leakage. During static conditions, anthropogenically-influenced shallow ground water migrated down the gravel pack or through the well screen and down the well into the deep part of the aquifer where it was stored until pumping was resumed. Because deep ground water contributes approximately 25 percent of the total flow, the amount of shallow ground water which could contribute to the total flow could increase from about 20 percent to approximately 45 percent. The relative amount of shallow ground water would then decrease as migrated shallow ground water was pumped from the deep part of the aquifer. As a result, historical water-quality data shows seasonal fluctuations, with higher concentrations of nitrate and uranium during winter months. Increasing the amount pumped during the winter months may improve water-quality during these months; however; continued downward migration of uranium and nitrate will likely negatively affect the long-term sustainability of the local ground-water aquifer as a source of drinking water, as well as the public-supply well itself.

Contents

Abstract
Introduction
Study Design
Methods
Hydrogeologic Setting
Water Chemistry
Factors Affecting the Transport of Contaminants to the Public-Supply Well
Summary and Conclusions
References
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E

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Send questions or comments about this report to the author, Bryant Jurgens, (916) 278-3275.

For more information about USGS activities in California, visit the USGS California Water Science Center home page.