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USGS Mendenhall Postdoctoral  Research Fellowship Program

14-4. Testing and improving nutrient source and process models in aquatic systems using multi-isotope techniques

High nutrient concentrations are degrading national surface and groundwater resources, and questions related to land use impacts on water quality and ecosystems -- as well as the identification of sources, transport mechanisms, and biological cycling of nutrients and other contaminants -- are priority issues for the USGS. Regulation of non-point nutrient sources is a major concern across the country, and identifying and understanding these sources is crucial to making good management decisions. Multiple natural and anthropogenic nutrient sources are often present in impacted surface waters, and contributions from non-point sources may be particularly difficult to identify and quantify.

The USGS is engaged in many large-scale modeling and sampling projects aimed at understanding and predicting nutrient sources, distributions, and impacts in surface and groundwater. These efforts include several large-scale monitoring networks (e.g., NASQAN and NAWQA) that monitor concentrations and loads of various constituents including nutrients and organic matter within selected watersheds throughout the country, along with the development of new tools and novel applications of existing techniques in order to better identify, understand, and predict the sources and impacts of nutrient loading. One such tool is the Spatially Referenced Regression on Watershed Attributes (SPARROW) model ( ) which integrates watershed data over multiple spatial scales to estimate contributions of nutrients and contaminants from different point and non-point (land use) sources, and to predict the fate of these contaminants.  The SPARROW model has been used to predict N, P, and C loads to the Gulf of Mexico (Figure 1) and at many other locations in the USA (Alexander et al., 2008). Because many of the important sources have distinctive isotopic signatures, multi-isotope approaches can be used to test and improve model predictions over different spatial and temporal scales, and to identify critical missing sources or processes which can then be incorporated into modeling efforts.

SPARROW pie charts

Figure 1.  SPARROW model predictions of the relative percentages of N, P, and C from different sources to the Gulf of Mexico from the Mississippi River. The N&P pie diagrams are based on Alexander et al. (2008), from: The C pie diagram is based on data from: Shih et al. (2010).

This Mendenhall research topic presents an opportunity for a strong scientist to utilize new multi-isotope techniques and the wealth of chemistry and hydrologic data available through USGS river monitoring programs and other USGS studies to investigate nutrient and organic matter sources and interactions in rivers. The applicant will have the flexibility to develop her/his own scope of research related to the topic of using new and established isotope techniques to improve characterization of the isotopic signatures of different sources and land uses, develop new source or process models, and test existing source or process models.  Within this broad theme, there is a wide range of specific topics that could be explored by the candidate, including but not limited to the following ideas:

This project requires a strong background in biogeochemistry, with experience in environmental isotope analysis. The USGS Menlo Park Stable Isotope Laboratory has extensive facilities for processing and analysis of environmental isotope samples, and has the capabilities for the following isotope analyses: δ18O & δ2H of water; δ15N, δ13C, δ34S of POM; δ15N & δ18O of NO3; δ15N of NH4; δ13C of DIC and DOC; δ18O of dissolved oxygen; δ34S and δ18O of SO4; δ18O of PO4; δ15N, δ13C, and δ34S of DOM (method modification required for simultaneous analysis). Our research group engages in many collaborative field research projects (Kendall et al., 2010), maintains a large collection of archived samples from large-scale USGS and state monitoring programs which can be analyzed for isotopic composition, and can provide assistance with coordinating with other USGS research groups for sample collection at other on-going study locations. These capabilities will allow the postdoctoral fellow to select a suite of isotope analyses most appropriate for the specific project goals, and allow flexibility for incorporating additional analyses as the project progresses.


Alexander, R.B., Smith, R.A., Schwarz, G.E., Boyer, E.W., Nolan, J.V. and Brakebill, J.W. (2008). Differences in phosphorus and nitrogen delivery to the Gulf of Mexico from the Mississippi River Basin. Environmental Science & Technology 42, 822-830.

Cravotta III, C.A. (2002) 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, 99 pgs, available at:

Kendall, C., Young, M.B, Silva, S.R. (2010) Applications of stable isotopes for regional to national-scale water quality monitoring programs.  In: West, J.B., G.J. Bowen, T.E. Dawson, K.P. Tu (Eds,), Isoscapes: Understanding Movement, Pattern, and Process on Earth through Isotope Mapping, Springer Publ., p. 89-111.

Shih, J.-S., Alexander, R.B., Smith, R.A., Boyer, E.W., Schwarz, G.E., and Chung, S. (2010), An initial SPARROW model of land use and in-stream controls on total organic carbon in streams of the conterminous United States. U.S. Geological Survey Open-File Report 2010–1276, 22 pgs, available at:

Proposed duty station:  Menlo Park, CA

Area of Ph.D.:  Isotope geochemistry, geochemistry, hydrology, biogeochemistry, geology, or related fields (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered).

Qualifications:  Applicants must meet one of the following USGS qualifications - Research Hydrologist, Research Geologist, Research Chemist, Research Ecologist.

(This type of work is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant’s background, education, and research proposal. The final classification of theposition will be made by the Human Resources specialist).

Research Advisors:  Carol Kendall, (650) 329-4576,; Megan Young, (650) 329-4544,; Richard Alexander, (703) 648-6869,

Human Resources Office Contact: Jennifer Daberkow, (303) 236-9566,

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U.S. Department of the Interior, U.S. Geological Survey
URL: Kendall.htm
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Last modified: 17:58:07 Tue 23 Jul 2013
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