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Use of Stable Isotopes of Carbon, Nitrogen, and Sulfur to Identify Sources of Nitrogen in Surface Waters in the Lower Susquehanna River Basin, Pennsylvania

U.S. Geological Survey, Water Supply Paper 2497

By Charles A. Cravotta III

 

Prepared in cooperation with the

PENNSYLVANIA DEPARTMENT OF ENVIRONMENTAL PROTECTION

 

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ABSTRACT

Samples of nitrogen-source material, soil, and water were collected from several small, primarily single-source subbasins in the Lower Susquehanna River Basin during 1988–90 to determine the feasibility of using measurements of stable isotope ratios of carbon (delta symbol13C), nitrogen (delta symbol15N), and sulfur (delta symbol34S) to identify sources of nitrogen (N) in stream water. Chemical and isotopic compositions were measured for six N-source types consisting of rain water, forest leaf litter, synthetic fertilizer, farm-animal manure, municipal-sewage effluent and sludge, and septictank effluent and sludge. Compositions of associated, nearby samples of topsoil, subsoil, runoff water, and stream water were measured to evaluate changes in compositions of transported N-containing materials near the N source. Animal manure, human waste (sewage plus septic), and forest-leaf litter can be distinguished on the basis of delta symbol13C; however, most N-sources cannot be distinguished on the basis of delta symbol15N and delta symbol34S, owing to wide ranges and overlap of compositions among different N-source types. Although values of delta symbol15N for soil and runoff-water samples are qualitatively similar to those of the applied N source, values of delta symbol13C and delta symbol34S for runoff-water and stream-water samples appear to reflect the compositions of relatively large reservoirs of the elements in soil organic matter and minerals, respectively, and not the composition of the applied N source. The ratio of organic carbon to total nitrogen (C-org:N), combined with delta symbol13C, is useful for distinguishing agricultural soils, which have characteristically high delta symbol13C and low C-org:N, from forested soils. The C-org:N values of suspended particulates in runoff or stream waters generally are lower than those of nearby soils, however, and indicate that chemical transformations and resultant isotopic fractionation can be important controls on the compositions of N-containing compounds in the soil and water. In aqueous samples including surface water and liquid N-sources, isotopic ratios commonly differ between coexisting dissolved fractions of NO3-N and NH3-N and between dissolved and particulate fractions of N or S, probably because of isotopic fractionation during transport or N-source processing.

 

Isotopic measurements provide qualitative information about important reactions that can affect N concentrations in surface waters. However, mass-balance computations generally are not sufficiently accurate to estimate the proportions of multiple sources contributing to the N load in the streams studied because of (1) variations in source chemical and isotopic compositions and (2) nonconservative behavior and fractionation during transport over short distances (hundreds of meters). Uncertainties in mass-balance computations are complicated by the propagation of errors associated with measurements of discharge, chemical concentrations,and isotopic compositions of relatively dilute, small streams.

CONTENTS

Abstract

Introduction

Purpose and Scope

Acknowledgments

Terminology and Principles of Isotopic Measurements

Isotopic Mass Balance

Isotopic Fractionation

Use of Stable Isotopes as Tracers

Carbon

Nitrogen

Sulfur

Data Collection and Analysis

Study-Site Selection

Sample Collection and Processing

Laboratory Analysis

Statistical and Graphical Analytical Methods

Intrasample Variations

Method Precision and Accuracy

Sample Components

Organic and Inorganic

Dissolved and Particulate

Characterization of Nitrogen Sources and Associated Soils and Waters

Characterization of Nitrogen Sources

Precipitation

Forest Leaf Litter

Synthetic Fertilizer

Manure

Human Septic and Sewage Waste

Isotopic Differences Among Nitrogen Sources

Characterization of Soil and Water Samples from Different Land-Use Areas

Forest: Stony Creek and Upper Dogwood Run

Fertilizer: Monroe Creek and Bald Eagle Creek

Manure: Brush Run and Conestoga River Field Sites

Septic: Middle Dogwood Run and Berkshire Hills

Sewage: Lower Dogwood Run and Codorus Creek

Estimation of Nitrogen Loads in Stream Waters

Point Sources

Nonpoint Sources

Estimation of N-Isotopic Fractionation

Practicality of Using Stable Isotopes of Carbon, Nitrogen, and Sulfur to Identify Sources
of Nitrogen in Surface Waters

Measurements

Computations

Summary and Conclusions

Selected References

Appendix A: Descriptions of Subbasins and Sample Sites in the Lower Susquehanna River Basin, Pennsylvania

Appendix B: Field, Chemical, and Isotopic Data for Nitrogen-Source and Associated Soil
and Water Samples from the Lower Susquehanna River Basin, Pennsylvania, 1988–90


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