Policy and technical guidance for conversion of sediment concentration from parts per million (ppm) to milligrams per liter (mg/L)




In Reply Refer To:                                  August 9, 1993
Mail Stop 415



OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 93.21

SUBJECT:  Policy and technical guidance for conversion of 
          sediment concentration from parts per million (ppm) 
          to milligrams per liter (mg/L)

Beginning with the 1972 Water Year, conversion factors given in 
Techniques of Water-Resources Investigations (TWRI), Book 5, 
Chapter C1, page 4, "Laboratory theory and methods for sediment 
analysis," were superseded by conversion factors given in a new 
table in Quality of Water Branch Technical Memorandum No. 72.10.  

Since Memorandum 72.10 was issued, there have been a few instances 
where Water Resources Division (WRD) sediment laboratory computer 
programs have produced different concentrations, expressed as mg/L, 
than those computed following guidelines in Memorandum 72.10.  The 
differences apparently result from the use of different rounding and 
interpolation methods.  The reported differences have been small, 
but their occurrence indicates that guidelines are needed to ensure 
that concentration data are processed uniformly throughout WRD.

The purpose of this memorandum is to provide guidelines for the use 
of standard conversion factors to convert sediment concentrations, 
expressed in parts per million (ppm), to milligrams per liter (mg/L).

The following equation should be used as the primary method for 
converting concentrations, expressed in ppm, to values expressed 
in mg/L.  The equation provides an exact solution which is 
suitable for laboratory computer programs.  Table 1, which is a 
revised version of the two tables in Memorandum 72.10, is also 
acceptable for use under most conditions (see assumptions under 
which Table 1 was derived).  Interpolation is required for values 
that are outside of the concentration ranges given in the table.

The equation for converting ppm to mg/L is:

    Milligrams per liter         Ds x Dw x 1,000,000 
C = -------------------- = --------------------------------- 
    Parts per million      Ds x 1,000,000 - [(Ds - Dw) x ppm] 

where

      C   = conversion factor (The concentration, in ppm is 
            multiplied by C to obtain concentration, in mg/L)

      Ds  = density of sediment, in grams per cubic centimeter

      Dw  = density of water, in grams per cubic centimeter

      ppm = sediment concentration, in parts per million

For sediment with a specific gravity of 2.65, The equation reduces 
to:

    Milligrams per liter               2,650,000
C = --------------------   =   ------------------------ 
    Parts per million          2,650,000 - (1.65 x ppm)


Table 1.--Conversion factors for sediment concentration: 
Parts per million to milligrams per liter.  (Revised from
Quality of Water Branch Technical Memorandum No. 72.10)

[The factors are based on the assumption that the density of water is 1.00, 
the range of temperature is 0-20o C, the specific gravity of sediment is 2.65, 
and the dissolved solids concentration is less than 10,000 ppm.]
__________________________________________________________________
     Concentration     C        Concentration      C
      range (ppm)                range (ppm)
__________________________________________________________________
       0-    7,990   1.00     327,000  336,000    1.26
   8,000    23,700   1.01     337,000  346,000    1.27
  23,800    39,200   1.02     347,000  356,000    1.28
  39,300    54,300   1.03     357,000  366,000    1.29
  54,400    69,200   1.04     367,000  375,000    1.30
  69,300    83,700   1.05     376,000  385,000    1.31
  83,800    98,000   1.06     386,000  394,000    1.32
  98,100   112,000   1.07     395,000  403,000    1.33
 113,000   126,000   1.08     404,000  412,000    1.34
 127,000   139,000   1.09     413,000  421,000    1.35
 140,000   153,000   1.10     422,000  429,000    1.36
 154,000   166,000   1.11     430,000  438,000    1.37
 167,000   178,000   1.12     439,000  446,000    1.38
 179,000   191,000   1.13     447,000  455,000    1.39
 192,000   203,000   1.14     456,000  463,000    1.40
 204,000  *216,000   1.15     464,000  471,000    1.41
*217,000   227,000   1.16     472,000 *479,000    1.42
 228,000   239,000   1.17     480,000  487,000    1.43
 240,000   251,000   1.18     488,000  495,000    1.44
 252,000   262,000   1.19     496,000  502,000    1.45
 263,000   273,000   1.20     503,000  510,000    1.46
 274,000   284,000   1.21     511,000  517,000    1.47

Table 1.--Conversion factors for sediment concentration:  Parts 
per million to milligrams per liter.  (Revised from Quality of 
Water Branch Technical Memorandum No. 72.10) (Continued)

[The factors are based on the assumption that the density of water is 1.00, 
the range of temperature is 0-20o C, the specific gravity of sediment is 2.65, 
and the dissolved solids concentration is less than 10,000 ppm.]
__________________________________________________________________
     Concentration     C        Concentration      C
      range (ppm)                range (ppm)
__________________________________________________________________

 285,000   295,000   1.22     518,000  525,000    1.48
 296,000   306,000   1.23     526,000  532,000    1.49
 307,000   316,000   1.24     533,000  539,000    1.50
 317,000   326,000   1.25       ---      ---       ---
_________________________________________________________________

*Revised from Quality of Water Branch Technical Memorandum 72.10




                               Charles W. Boning
                               Chief, Office of Surface Water

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