European Journal of Scientific Research - EuroJournals

Toxicity of Arsenic in the Ground Water of Comarca-Lagunera (Mexico) 874
Generalized residual alkalinity
Residual alkalinity (Van Beek and Van Breemen, 1973), resulting from the concept of residual
carbonate (Eaton, 1950), it is obtained by difference between carbonated alkalinity (Alcc) and the total
concentrations of the divalent cations in solution:
AR = Alcc-2(Ca 2+) T-2(Mg 2+ )T (1)
It is expressed in equivalents per litter of solution. The evolution way of the solution
concentrations under evaporation conditions can be predicted according to the sign of the residual
alkalinity. This concept was generalized with successive precipitation of some minerals such as
calcite-gypsum sequence (Al-Droubi, 1976; Valles, 1987; Valles and al, 1989; Valles and al, 1991). It
corresponds to a ionic balance which let foreseen the main geochemical interactions.
Until little time ago, this method was used to foreseen the way of evolution of a concentrating
solution or to precise the geochemical group of solutions. Since not so far time ago, the conservative
character of residual alkalinity was theoretically predicted (Valles and al, 1991) and was showed on
field conditions (Ribolzi and al., 1993). Consequently this concept can be used in the applications to
quantitative purpose such as the decomposition of a flood flow hydrogram (Giusti and Neal, 1993;
Ribolzi and al, 1996). It was largely used by Gonzalez Barrios (1992) to explain the behaviour of
major chemical elements in the groundwater of the Comarca Lagunera. The results of this study are
synthesized in the following part.
Results and discussion
The arsenic content of 52 groundwater samples was taken from Gonzalez Barrios (1992) as well as the
major chemical elements. The groundwater data bases are reported in appendix 1.1 and 1.2.
Statistical study of arsenic concentrations and other elements
This part focused the danger of toxicity by mineral arsenic. The groundwater analysis was made in
Mexico at the laboratory of INIFAP CENID-RASPA in 1989, with traditional procedures:
thermoelectric atomic absorption method.
Table 1: Statistics on arsenic and other chemical elements concentrations
Variable Average Minimum Maximum Variance
Standar
deviation
Variation
Coefficient
Unit
Carbonated Alkalinity 2,86E-03 1,30E-03 6,40E-03 1,28E-06 1,13E-03 3,95E-01 Eq L -1
H 3,02E-05 5,01E-06 1,00E-04 6,07E-13 2,46E-05 8,15E-01 mg L -1
Na 1,84E+02 4,60E+01 8,14E+02 1,16E+00 1,63E+02 8,90E-01 mg L -1
Ca 1,11E+02 2,00E+01 5,49E+02 2,66E-01 1,03E+02 9,28E-01 mg L -1
Mg 2,80E+01 1,22E+00 2,15E+02 7,19E-02 4,18E+01 1,50E+00 mg L -1
Cl 8,15E+01 1,06E+01 2,66E+02 1,23E-01 6,59E+01 8,09E-01 mg L -1
SO4 5,24E+02 5,28E+01 2,88E+03 3,19E+00 5,53E+02 1,06E+00 mg L -1
Pb 2,49E-02 2,07E-04 2,30E-01 1,26E-08 5,12E-02 2,06E+00 mg L -1
Zn 2,16E-02 6,54E-05 6,00E-01 1,12E-07 8,56E-02 3,96E+00 mg L -1
Cu 5,13E-04 6,35E-05 1,70E-02 9,34E-11 2,43E-03 4,75E+00 mg L -1
As 9,96E-02 1,00E-02 5,50E-01 2,29E-07 1,31E-01 1,32E+00 mg L -1
EC 1,59E+00 3,67E-01 6,40E+00 1,44E+00 1,20E+00 7,55E-01 dS cm -1