Hydro-geomorphic controls on the development and distribution of acid sulfate soils in Central Java, Indonesia and robust remote sensing and GIS methods for acid sulfate soil mapping

Abstract

The exploitation of coastal areas for aquaculture in Indonesia has occurred without awareness of the impact of acid sulfate soils (ASS) on the environment in which aquaculture takes place. Coastal planning policy and regulation for aquaculture has not included ASS as a consideration for or constraint on development. As a result, aquaculture is undertaken in areas of unsuitable soil and often fails after a short initial period of production. Accordingly, there is a need to provide spatial information that is able to show suitable areas for aquaculture based on an understanding of the vertical and horizontal distribution of ASS. The general aim of this study was to produce a mapping approach based on an understanding of the hydro-geomorphic controls on the distribution and formation of ASS based on hydro-, with a focus on sedimentary processes driven by hydrology. The study focussed on mapping ASS within the pyrite-bearing coastal landscapes in Central Java. The underlying approach was to identify associations between ASS development and distribution within estuarine geomorphological units. Patterns of ASS distribution and the characteristics of the soils are assumed to be associated with hydro-geomorphic processes that influence soil development.

This study utilized a multi-level methodology involving multi-resolution remotely-sensed data and GIS analysis to generate multi-scale hydro-geomorphic information (geomorphic setting, water sources and hydro-dynamics). The method includes the identification of Geo-climatic Regions (GcR), a new estuary classification system, and use of hydro-geomorphic units (HGU) for Central Java to establish a hydro-geomorphology-based classification scheme to aid mapping ASS distribution. Field and laboratory assessment of soil properties were undertaken to identify the vertical and horizontal distribution of ASS of the HGUS in each type of estuary.

This study identified four estuary types that represent two GcRs on the north coast: river-dominated estuary (Rambut Estuary); tide-dominated estuary (Jajar Estuary), and two GcRs on the south coast: wave-dominated estuary (Serayu Estuary) and one new type of estuary class which is wave-dominated estuary with pre-existing barrier (Bengawan Estuary). Fifty two HGUs were generated from different types of landforms, land-uses, vegetation types, water table depth and distance from the brackish water resources, from those four selected estuaries. Soil analytical results show that these HGUs represent different ASS physical and chemical properties in coastal sediments especially in the estuary zones of tidal shores and where river deposition occurs. The HGUs in the estuaries on the north coast have less disparity and pyrite concentration, despite their low energy environment, compared to the south coast. It was found that intensive aquaculture ponds and dredging activities were the main factors contributing to the absence of ASS in these estuaries. In contrast with some previous studies, in the south coast, the combination of high river and wave energy in wave-dominated estuaries created scattered subaqueous soil landforms. These environments comprise of a high variety of HGUs with medium to very high pyrite concentration (4-9%). The HGUs with high pyrite concentrations are mostly developed in landforms with low energy environments overlaying former high river energy environments.

Statistical analysis showed that the specific combination of river, tide and wave energy levels influenced the development and distribution of ASS. The distance of the HGUs to the river and sea, was identified as one factor that influenced the vertical distribution of pyrite concentration. It was shown that the differing ASS properties in each HGU for each of the different estuary types, could be used to explain and map differences pyrite accumulation.

The study showed that a multi-level ASS mapping approach using geo-climatic regionalization as a basis for grouping hydro-geomorphic features catchment and their coastal marine environments indeed assist in understanding of coastal evolutionary processes in Central Java. The uses of multi-resolution remotely sensed data show the effectiveness of the approach in cost and labour, especially conducting doing field survey. The information generated for each HGU facilitated the development of a scientifically robust ASS mapping model that incorporates knowledge on the relationship between soil and landform formation in Central Java estuaries which is essential in minimizing the risk of environment degradation. The resulting maps and mapping methods will improve land capability assessment and site selection criteria for brackishwater aquaculture.