Analytical Methods--Turbidimetric Sulfate Method


WGS-Mail Stop 412:                         December 21, 1989



OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM NO. 90.04

Subject:  Analytical Methods--Turbidimetric Sulfate Method

In early March 1989, the National Water-Quality Laboratory 
(NWQL) discovered a positive bias in the turbidimetric method 
for sulfate analysis.  The bias appears to result from not 
applying a blank correction to the measured values.  The 
magnitude of the bias is about 2 milligrams per liter (mg/L) 
median difference for concentrations less than 75 mg/L.  
The period of affected data is from October 1982 through 
July 1989.  Attachment 1 describes (1) the origin of the 
problem, (2) actions taken by the NWQL to address the 
problem, (3) implications for data stored in the water-
quality data base, and (4) the need for District-by-District 
evaluation of the bias.

Personnel who update the water-quality data base are 
instructed to make no adjustments to affected sulfate data 
until the Office of Water Quality (OWQ) and the Branch of 
Quality Assurance (BQA) have determined what adjustments can 
or should be made.  It is unlikely that a national adjustment 
to the affected sulfate values can be made because natural 
waters in the United States exhibit a complex range of 
dissolved ions (matrix) that probably will prevent 
identification of a nationally consistent correction for the 
bias.

To determine whether the magnitude of bias can be defined on 
a geographic basis, the NWQL will relate the values for 
affected samples to a wide array of chemical and water source 
factors that might cause the bias.  In addition, each 
District and project office is encouraged to make comparisons 
between blank corrected and uncorrected sulfate values.  The 
NWQL will assist this effort by providing paired analytical 
results until April 30, 1990.  Each office electing to make 
this comparison should follow the guidelines in attachment 2.  
Results of each comparison should be forwarded to Bill 
Shampine of the BQA by May 30, 1990.  Bill will coordinate 
review of the District comparisons with the NWQL, the BQA, 
and the Systems Analysis Branch.  Following review, 
individual offices and projects that have satisfactorily 
established a geographical pattern for the bias will be 
permitted to adjust values in subsequent investigative 
reports.

While the comparative evaluation of methods continues, a 
warning flag has been added to the WATSTORE data base for 
sulfate.  The flag reads:  "Sulfate values (00945) below 
75 mg/L have a median positive bias of 2 mg/L above the true 
value between 1982 and 1989.  See OWQ Memorandum 90.04."

If the comparative evaluations fail to enable a national 
correction, the WATSTORE data will remain uncorrected and the 
sulfate bias flag will become a permanent feature of WATSTORE 
tabling routines.




                              David A. Rickert
                              Chief, Office of Water Quality

Attachments

This memorandum does not supersede any previous memorandum.

Key words:  Laboratory analysis, sulfate

WRD Distribution:  A, B, S, FO, PO