# Re: Phreeqc

```
> I am writing to check how are the CEC determinations supposed to be done
for PHREEQC modelling. Could you please send me the recipe? I believe they
should be determined in whole sediment and not just in the fine fraction.
I have all my CEC values determined for fine fraction. When I tried to
model cation exchange using these values they seemed to be too high. Then I
tried to recalculate these CEC values considering the amount of fine
material in the sediment and I got CEC values about one order of magnitude
less and that simulated better the results. However, I am not sure it is
correct to do this approximation.

The number you want for PHREEQC is the number of equivalents of exchange
sites per liter (kilogram) of water. I'm not sure what you are modeling,
but if it is an aquifer, and you have CEC in meq/100 g of fine sediment,
assume 2.7 kg/L is the density of the sediment particles, f is the number
of kilograms of fine sediment per kilogram of sediment, and p is porosity,
then the calculation is as follows:

CEC/100  /1000 (meq/eq)   * 1000 (g/kg) = eq/kg fine

and

(1-p)/p    * 2.7 (kg/L)   * f (kg fine/kg sed) = kg fine/L water,

so

CEC/100  /1000 (meq/eq)   * 1000 g/kg   * (1-p)/p    * 2.7 (kg/L)   * f
(kg fine/kg sed) = eq/L

You may have a different situation, but you should be able to adapt the
calculation so that you get equivalents of exchanger per liter of water.

David

David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

"Teresa Melo"
<tmelo@xxxxxxxxx         To:      <dlpark@xxxxxxxx>
>                        cc:
Subject: Phreeqc
03/24/02 10:23
AM

Dear Sir,

I am writing to check how are the CEC determinations supposed to be done
for PHREEQC modelling. Could you please send me the recipe? I believe they
should be determined in whole sediment and not just in the fine fraction.
I have all my CEC values determined for fine fraction. When I tried to
model cation exchange using these values they seemed to be too high. Then I
tried to recalculate these CEC values considering the amount of fine
material in the sediment and I got CEC values about one order of magnitude
less and that simulated better the results. However, I am not sure it is
correct to do this approximation.
Could you please clarify for me this question?

Teresa Melo

M. Teresa Condesso de Melo
Departamento de Geociências
Campus Universitário de Santiago
3810-193  Aveiro
PORTUGAL

Phone: +351-234-37.02.00 ext. 23213
Fax: +351-234-37.06.05
http://www.ii.ua.pt/

```

• References:

Complete Water Resources Division Software