NATIONAL WATER QUALITY LABORATORY
TECHNICAL MEMORANDUM 97-10
June 20, 1997
To: Chief, Office of Water Quality
Assistant Chief, Office of Water Quality
Assistant Chief Hydrologist for Technical Support
Regional Hydrologists
Chief, NAWQA
Chief, Office of Ground Water
Assistant Chief, Office of Ground Water
Chief, National Water Information System
Chief, Office of Hydrologic Research
Chiefs, Branches of Regional Research
District Chiefs
Regional Water Quality Specialists
Assistant Regional Hydrologists for NAWQA
District Water Quality Specialists
Chiefs, NAWQA Study Units
Chief, Quality Water Service Unit, Ocala
Chief, Yucca Mountain Project
QA Manager, Yucca Mountain Project
Chief, Branch of Technical Development & Quality Systems
Employees, National Water Quality Laboratory
From: Peter F. Rogerson, Chief
National Water Quality Laboratory
Branch of Analytical Services
Subject: Change in Ammonia Minimum Reporting Limit
Author: Charles J. Patton, Methods Research & Development Program
(303) 467-8084, cjpatton@usgs.gov
Revision: None
Supplemental: None
SCOPE
This technical memorandum pertains to changes in the minimum reporting
limit (MRL) for dissolved ammonia nitrogen in chilled and chilled-acidified
samples (see table below), effective October 1, 1997. Specifically it
announces a change in the MRL for these tests from 0.015 milligrams
of nitrogen per liter (mg-N/L) to 0.02 mg-N/L.
Lab Parameter Procedure Name Bottle MRL
Code Code Type mg-N/L
1976 608 F Colorimetry, ASF, salicylate- FCC 0.015
hypochlorite, N, Ammonia, FIL
1991 608 G Colorimetry, ASF, salicylate- FCA 0.015
hypochlorite, N, Ammonia,
FIL, acidified
BACKGROUND
The rationale for this change is outlined below, along with a summary of
events that led to an inadvertent change in the MRL for these tests that
occurred on October 1, 1994.
On October 1, 1994, the U.S. Geological Survey discontinued the practice
of adding mercuric chloride to samples collected for nutrient analysis as
announced and discussed in National Water Quality Laboratory (NWQL)
Technical Memorandum 94.16. Up to that time, the MRL stated for these tests
in NWQL analytical services catalogs and in Techniques for Water Resources
Investigations (TWRI) methods was 0.01 mg-N/L. However, when new laboratory
codes were created to document this change in field processing protocol, the
MRL for these tests was changed to 0.015 mg-N/L by mistake. Note that this
is the concentration of the lowest standard used to calibrate these tests
at the NWQL, at least as far back as 1989. Because of this inadvertent
MRL change, the lowest concentration of ammonia reported between
October 1, 1994, and September 30, 1997, is 0.015 mg-N/L.
This inadvertent increase in the MRL for these tests was actually
fortuitous, because blind blank data and method detection limit
estimates collected over the past 3 years indicate that an MRL of
0.015 mg-N/L is a more accurate reflection of data quality. An MRL
of 0.015 mg-N/L, however, invites the assumption that ammonia concentrations
greater than 0.015 mg-N/L are reliable to three decimal places, which
clearly exceeds the analytical capabilities of these methods. For this
reason, the established reporting convention for these tests--two decimal
places for concentrations up to 0.99 mg-N/L and two significant figures
for concentrations greater than or equal to 1.0 mg-N/L--has been retained.
On October 1, 1997, the MRL for these tests will change to 0.02 mg-N/L.
Data for these tests released prior to that date will not be modified,
but we suggest that concentration values of < 0.01 mg-N/L, 0.01 mg-N/L,
and < 0.015 mg-N/L be interpreted as < 0.02 mg-N/L.
Impact on Data Base: Interpretive only--see narrative
Key words: ammonia, MRL
Supersedes: None
Distribution: See above plus the Netnews usgs.labnews and .water.quality,
WRD Secretaries; Field and Project Offices;
Hydrologic Technicians; and http://wwwnwql.cr.usgs.gov/