USGS


Determining Age and Vertical Contribution of Ground Water Pumped from Wells in a Small Coastal River Basin

A Case Study in the Sweetwater River Valley, San Diego County, California

By Wesley R. Danskin and Clinton D. Church

 

Prepared in cooperation with the Sweetwater Authority

 

U.S. GEOLOGICAL SURVEY

Open-File Report 2005-1032

Sacramento, California 2005


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Abstract

   The age and vertical contribution of ground water flowing into four municipal supply wells were determined under pumping conditions in order to better understand the source of water to the wells. Vertical flowrate inside the wells, located in the Sweetwater River drainage basin of San Diego County, California, was measured at selected depths using a dye-injection technique. Flowmeter results indicate that as much as about half the pumped water comes from the alluvial deposits and half from the underlying San Diego Formation. Chemical analysis of water samples collected at specific depths support this vertical distribution of pumped water. Tritium and carbon-14 data indicate that ground water from the alluvial deposits was recharged less than 50 years ago, and ground water from the San Diego Formation is thousands of years old, some as much as 16,000 years old.

CONTENTS

Abstract
Study Area
Methods of Investigation
Measuring flow into a well
Sampling water from a well
Analysis of Flow Data
Analysis of Chemical Data
General Chemistry
Chloride-Boron Ratio
Age Dating
Discussion
References

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