By Charles A. Cravotta III
Prepared in cooperation with the
PENNSYLVANIA DEPARTMENT OF ENVIRONMENTAL PROTECTION
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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 (
13C),
nitrogen (15N),
and sulfur (34S)
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 13C;
however, most N-sources cannot be distinguished on the basis of 15N
and 34S,
owing to wide ranges and overlap of compositions among different N-source types.
Although values of 15N
for soil and runoff-water samples are qualitatively similar to those of the
applied N source, values of 13C
and 34S
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 13C,
is useful for distinguishing agricultural soils, which have characteristically
high 13C
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.
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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