ABSTRACT

The oxidation of pyrite in acid sulfate soils (ASS) produces high concentration of strong acid (H₂SO₄) in the soil. The acid reacts with soil minerals and dissolves aluminum and other acid-soluble metals. During rainfall, they are flushed and discharged into terrestrial and aquatic environments. The authors examined the chemical characteristics of the runoffs from ASS in Arashiyama and in Ooshitai, which are located in the northern part of Okinawa Island and evaluated their potential effects on surrounding environments. The results show that the runoffs were very acidic with pH range of 2.87 to 4.29, and minimum values were close to the pH values of the soils. The dominant species were SO₄^2-, Al³⁺, and Fe²⁺ and they were well correlated with pH. The acidity of the runoffs caused dissolution of metals in the following order: Mn > Zn > Cu = Cd. The quality of runoffs was found to be poor and was mainly governed by the interaction of rain and soil. A comparison between the stream waters collected during rain event and on fine day showed a marked changes in stream water chemistry during rainfall as follows: alkalinity was reduced by 3.7 times, sulfate concentration was increased by 3 times whereas pH was decreased by 0.18 units. These changes are attributed to the contamination by runoff from ASS. Moreover brown flocky sediments were observed to have covered entire streambed at the discharging point. The chemical composition of the sediments shows that as those of runoffs, they were dominated by Fe, Al, and SO₄. These results indicate that the sediments were mainly formed as the results of mixing runoffs and stream water.