Nitrate Collection for d15N and d18O Analysis in Dilute Waters Cecily Chang, Michael Riggs, Don Campbell, Steve Silva and Carol Kendall

Abstract

A method for collecting and recovering nitrate for d¹⁵N and d¹⁸O analysis from low ionic strength waters with modest dissolved organic carbon (DOC) concentrations (1-10 ppm) and low NO₃ concentrations (0.7-10.0 µM) is described.¹ This method uses ion exchange columns, accommodates the large volumes of  water  (7-70 L)  required due to the low NO₃ concentration, and minimizes DOC accumulation.²  Problems developed when an earlier method designed for waters with higher NO₃ concentrations was applied to alpine waters DOC accumulation on the anion column competed for NO₃ sites and clogged the column.

These problems were solved by:

•  pre-filtering the sample  (0.45 microns) using a filter with a larger surface area,
•  using a coarser resin, enabling more rapid sample flow,
•  increasing the volume of resin and using AG2X anion resin,
•  putting a cation column in front of the anion column to protonate the DOC, making DOC less likely to adhere to the anion column.

This method was designed for alpine watersheds where field sites are far from lab facilities, thereby necessitating on-site collection with minimal equipment.

Problem

To date, none of the current collection methods for dual isotope analysis of NO₃ are suitable for waters where nitrate concentrations are extremely low  (0.7-10.0 µM), nor are the potential problems of DOC accumulation adequately addressed.
 

Melting the snow and loading the sample onto a column.

When nitrate concentrations are low, larger sample volumes are required (5-70 L) The large sample volumes result in extended sample loading time and greater DOC accumulation on the exchange column. Accumulation of DOC can clog the column, occupy sites on the anion exchange resin that would otherwise be available for nitrate and may contribute unwanted oxygen to the 18O analysis.

Solution

The use of a filter cartridge, application of the cation column, and using a courser sized anion resin made it possible to successfully concentrate nitrate from large volumes of water. Nitrate recovery after processing has also been more consistent using AG2X anion resin.  Key to the success of this method (cat-AG2X) is the ability to prevent DOC accumulation on the anion column. This was accomplished by putting a cation column in front of the anion column. It is critical to prevent accumulation of DOC because DOC can clog the anion column, occupying sites that would otherwise be available for nitrate, and because oxygen from the DOC may be transferred to the ¹⁸O sample.

Field Method
 

Digging a snow pit in Tahoe Basin, California

Preliminary steps before collecting a NO₃ field sample for d¹⁵N and d¹⁸O:

• Determine the concentration of the NO₃ and chemical characteristics of the water including DOC.
• Calculate volume of water required to obtain 100-300 µ moles of NO₃. Calculate the total MEQ of anions in the sample volume and determine whether the capacity of the anion column (6 MEQ) will be exceeded. 
• If the total MEQ of the water sample is >6 MEQ, then more than one column must be used to collect the sample.

Getting to the site in Lochvale, Colorado.

Conclusion 

Samples can be collected in the field without transporting large volumes of water. It is now possible to obtain nitrate isotope analysis from access-limited sites such as alpine watersheds, giving researchers the opportunity to identify nitrate sources and transformations in snow, rain, snowmelt and streams.
 
 

Author, Cecily, standing on 7 feet of snow.

References 

¹C.C.Y. Chang, J. Langston, M. Riggs, D.H.  Campbell, S.R. Silva, and C. Kendall. 1999. A Method for Nitrate Collection for d¹⁵N and d¹⁸O Analysis from Waters with Low Nitrate Concentrations. Canadian Journal of Fisheries and Aquatic Sciences, 56: 1856-1864.
²S.R.  Silva, C. Kendall, D.H. Wilkison, A.C. Ziegler, C.C.Y. Chang, and R. Avanzino. 2000. A New Method for Collection of Nitrate from Fresh Water and the Analysis of Nitrogen and Oxygen Isotope Ratios, J. of Hydrology, 228: 22-36.

The URL of this page is: http://wwwrcamnl.wr.usgs.gov/isoig/guidelines/nitrate/poster.htm/
Page created by Carla Chamberlain, Daniel Steinitz and Cecily Chang. 
Please contact Carol Kendall (ckendall@usgs.gov) for questions and comments regarding this page. 
This page was last changed on July 19, 2001. 
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