It has been known that water leached out from actual Acid Sulphate Soil (ASS) acidified water in canals in ASS areas. However, the leaching of toxic metals from ASS in the Mekong Delta-Vietnam is poorly known and the amounts of these toxic metals in soils have not been studied. This study aims at (1) assessing pH and amounts of 9 toxic metals (Al, As, Cd, Cu, Fe, Mn, Ni, Pb, Zn) in first, second, and third-order canals and in ground water in seven canal systems in severe (SASS), moderate (MASS) and potential ASS (PASS) in the beginning of the rainy season in ASS Long Xuyen Quadrangle in the Mekong Delta, in comparison to those in three non-ASS areas, and (2) quantifying amount of Cu, Fe, Mn, and Zn extracted by 0.05M NH4-EDTA in B and C horizons and its relations to concentration in water.
Results showed that pH of canal water was low in the order of SASS < MASS < PASS < non-ASS, and was very low (2.6-3.2) in the third-order (the smallest canal), low (3.5-5.2) in second-order (the intermediate canal) and high (5.8-6.4) in the first-order canals (the largest canal). Average concentration of Al (40.2 mg/l), Cu (30.8 µg/l), Fe (54.2 mg/l), Mn (2.9 mg/l), Ni (109.9 (µg/l), Zn (0.5 mg/l) in third-order canal water were highest in the SASS and MASS in comparison to PASS and non-ASS soils. The concentration of Al, Cd, Cu, Fe, Mn, Ni, and Zn in the first, second and the third-order canal in severe ASS was 17 - 296, 5 -37, 2-12, 11-176, 17-50, 12 - 233, and 4 - 54 times higher than those in non-ASS, respectively. Concentration of Cu, Fe, Mn, Ni and Zn in ground water, however, was varied. Concentration of As, and Pb was found not related to the occurrence of ASS because its concentration in ground water table in non-ASS was even higher than in ASS.
In the soils where the third-order canal located, average surface soil pH was low in SASS (3.9) and in MASS (4.1) and higher in PASS (4.2) and non-ASS (5.4). Average soil pH of the oxidized B horizons was also very low in SASS ( 2.9), in MASS (3.5) and low in PASS (4) and non-ASS (4.4). Average fresh soil pH of the reduced C horizon was low in SASS (3.4) and MASS. The parent material of C horizons is all pyritic materials because its pH after oxidizing by H2O2 dropped below 2.
Average Cu (NH4-EDTA) concentration of air-dried soil in B horizon and of fresh soil in C horizon in ASS and non-ASS group was comparable, varied from 6.0 to 9.4 mg/kg and from 3.2 to 5.2, respectively; except for high Cu(NH4-EDTA) in PASS (12.4 mg/kg). Concentrations of Cu in the third-order canal water were found correlated with air-dried soil pH of A and B horizons and of fresh soil pH of C horizon, but not with Cu (NH4-EDTA) concentrations in soil. High Cu (NH4-EDTA) concentration in PASS, however explains high Cu concentration in ground water in this soil group.
Average Fe(NH4-EDTA) concentration of air-dried soil in B was higher in SASS and MASS (791 and 887 mg/kg) than in PASS (393 mg/kg) and non-ASS (123 mg/kg). Similarly, Fe(NH4-EDTA) concentrations of dried soil in C horizon were high in SASS (2323 mg/kg), MASS (2689 mg/kg), lower in PASS (1615 mg/kg), and lowest in non-ASS (633 mg/kg), and were all higher than Fe(NH4-EDTA) in fresh soil C horizons. The oxidation of soil samples during air-drying decreases soil pH and increases NH4-EDTA extractable Fe, which may explain high Fe concentration in third order canal of ASS in comparison to non-ASS.
In contrast to Fe, average Mn(NH4-EDTA) concentration of air-dried soil in B horizons was lower in SASS and MASS (13.6 and 19.3 mg/kg) than in PASS (52.3 mg/kg) and non-ASS (45.6 mg/kg). Similarly, Mn (NH4-EDTA) concentrations of dried soil in C horizon horizons were low in SASS (38.2 mg/kg), MASS (40.9 mg/kg), higher in PASS (89.7 mg/kg), and highest in non-ASS (186 mg/kg), and were all higher than Fe(NH4-EDTA) in fresh soil C horizons. High Mn(NH4-EDTA) in PASS and non-ASS can explain high concentration of Mn in ground water in these soil groups than in SASS and MASS.
Average Zn(NH4-EDTA) concentration of air-dried soil in B horizons was high in SASS and MASS (10.9 and 7.3 mg/kg) than in PASS ( 5.8 mg/kg) and non-ASS (4.1 mg/kg), and similarly for fresh soil in C horizons in SASS (28.3 mg/kg), MASS (17 mg/kg), PASS (14.2 mg/kg), and non-ASS (16.6 mg/kg). High Zn concentration in third-order canal water in SASS than in other soils therefore can be explained by high NH4-EDTA extractable Zn in B and C horizons.
Results of the study clearly showed that ASS is a source of contamination of Al, Cd, Cu, Fe, Mn, Ni, Zn to water environment, and amounts of Cu, Fe, Mn, Zn extracted by 0.05M NH4-EDTA in B and C horizons were related to either the concentration of these metals in third-order canal water or in ground water. Further drainage, reclamation of severe ASS for crop cultivation should now be seriously considered.