Accumulation and mobility of ions from sea water in shrimp pond soil profiles and their impacts on environment and soil resource in Amphoe Ranote, Songkhla

Results revealed that Ca was leached away at the depth interval of 100-130 cm from ground level and accumulated beyond depth of 140 cm by sea water, while Mg was leached away from the whole study profiles. Mn and Cu were neither leached away nor accumulated in the profiles. However, the accumulation of Zn at the depth of 100-120 cm was observed and its amount declined with depth. None of microelements was found accumulated in the profiles, except that minor amounts of Ba and Ni were detected at the ponds' bottom. The accumulated elements found in this study has no sufficient amounts to have adverse impacts on environmental integrity and soil resource. Three zones could be designated on the basis of chemical reactions and ion mobility characteristics in the profiles. Firstly, leaching zone (100-120 cm), where the high concentration of Na+ in sea water substituted for exchangeable Ca++ at the surface of clay minerals and organic matter as in the reaction. 2Na+(aq.) + Ca++(exch.) --Ca++(aq.) + 2Na+(exch.). This resulted in the gain of Na but depletion of Ca in the profiles. Secondly, transition zone (120-140 cm) has neither Ca accumulation nor Ca depletion. The leachate contained less Na+ and more Ca++ than that of the leaching zone, resulting in decreasing influence of the aforementioned reaction. Finally, accumulation zone, (140-150 cm) possessed the leachate with high Ca content and low amount of Na resulting in triggering the following reaction. Ca++(aq.) +2Na+-- +2Na+(aq.) + Ca++(exch.). Based on this chemical signature, two major stages of Ca and Na mobility in the soil profiles were reconstructed as follows: 1. The initial stage took place at the beginning of shrimp culture. In this stage Na+ substituted for exchangeable Ca++ in all the three zones since Na with monovalence travelled faster than divalent ions (Ca++) causing the gain of Na in the whole profiles. 2. The mature stage occurred when the pond bad been used for some time. Thus, Ca++ in solution arrived at the accumulation zone and substituted for exchangeable Na+ resulting in Ca accumulation in this zone.