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U.S. Geological Survey
WRIR 00-4137
Online version 1.0

Nitrate Source Indicators in Ground Water of the Scimitar Subdivision, Peters Creek Area, Anchorage, Alaska

By Bronwen Wang, Pat M. Strelakos, and Brett Jokela

 

ABSTRACT

A combination of aqueous chemistry, isotopic measurement, and in situ tracers were used to study the possible nitrate sources, the factors contributing to the spatial distribution of nitrate, and possible septic system influence in the ground water in the Scimitar Subdivision, Municipality of Anchorage, Alaska. Two water types were distinguished on the basis of the major ion chemistry: (1) a calcium sodium carbonate water, which was associated with isotopically heavier boron and with chlorofluorocarbons (CFC's) that were in the range expected from equilibration with the atmosphere (group A water) and (2) a calcium magnesium carbonate water, which was associated with elevated nitrate, chloride, and magnesium concentrations, generally isotopically lighter boron, and CFC's concentrations that were generally in excess of that expected from equilibration with the atmosphere (group B water). Water from wells in group B had nitrate concentrations that were greater than 3 milligrams per liter, whereas those in group A had nitrate concentrations of 0.2 milligram per liter or less. Nitrate does not appear to be undergoing extensive transformation in the ground-water system and behaves as a conservative ion. The major ion chemistry trends and the presence of CFC's in excess of an atmospheric source for group B wells are consistent with waste-water influences. The spatial distribution of the nitrate among wells is likely due to the magnitude of this influence on any given well. Using an expanded data set composed of 16 wells sampled only for nitrate concentration, a significant difference in the static water level relative to bedrock was found. Well water samples with less than 1 milligram per liter nitrate had static water levels within the bedrock, whereas those samples with greater than 1 milligram per liter nitrate had static water levels near or above the top of the bedrock. This observation would be consistent with a conceptual model of a low-nitrate fractured bedrock aquifer that receives slow recharge from an overlying nitrate-enriched surficial aquifer.

CONTENTS

Abstract

Introduction

Background and Problem

Purpose and Scope

Hydrogeology of the Study Area

Nitrate

Waste-Water Indicators

Methods

Elevation Survey

Water Chemistry

Water Chemistry Data

Major and Minor Ions, and Nitrate Concentrations

Redox and Nitrate Concentrations

Boron Isotopes and Chlorofluorocarbons

Nitrate and the Ground-Water System

References Cited

Appendix 1. Well-elevation data and characteristics for selected wells in the Scimitar Subdivision

 


 

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U.S. Department of the Interior, U.S. Geological Survey
Persistent URL: http://pubs.water.usgs.gov/wri004137
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Last modified: Thursday, September 01 2005, 05:01:29 PM
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