Chemical impact of the acidic red soil on seawater

Abstract

Red soil contamination into marine ecosystems such as coral reefs and estuaries is a prevalent environmental problem in the Okinawa Islands. This study was conducted to examine the chemical impact of Okinawan red soil on seawater. The different soil weights were mixed and shaken with 100 mL of seawater for four hours to obtain the extracts. The pH, concentrations of Na+, K+, Ca+, Mg2+, Al3+ and dissolved silica were then determined. The soil is acidic with a pH value of 5.03, dominated by SiO2 and Al2O3. The pH of the Zampa cape seawater was 8.23 and decreased with soil weight to the lowest value of 4.06. The Al concentration in the seawater was 0.0256 megL-1 and increased with soil to solution ratio to the maximum value of 5.95 megL-1. The base metal concentrations decreased from the seawater by 23.5%, 8.04%, and 3.59% in extracts for K+, Ca2+and Mg2+, respectively, while Na+ was relatively stable to change. The dissolved SiO2 increased from 0.430 mgL-1 in the seawater to the maximum value of 20.0 mgL-1 in ground soil. The Summation of decreasing K+, Ca2+and Mg2+ concentrations and increasing Al3+ and W in the extracts showed a high correlation coefficient (P<0.001), which largely suggests an exchange process. The significant correlations between logarithmic concentrations of Al3+ and silica vs. pH (P<0.001) also suggest the importance of W to release soluble Al as well as hydrogen-promoted weathering of silicates. Physical features such as sedimentation and turbidity do affect coral reefs, but possibly chemical effects such as the elevated Al concentration observed in coastal seawaters may also interfere with the coral's well being at significant red soil-affected areas in the Okinawa Islands.