Michael D Melville

Chemical controls on acid discharges from acid sulfate soils under sugarcane cropping in an eastern Australian estuarine floodplain

Abstract Chemical controls on acid discharges from acid sulfate soils (ASS) were investigated in an eastern Australian estuarine floodplain cropped with sugarcane. The results show that the acid export was controlled mainly by the combined effect of soil hydrological and chemical processes. During high evapotranspiration spells, lowering of the watertable allowed air to penetrate into the upper part of sulfidic subsoils and the oxidation products of sulfides were then transferred upwards by capillary action. Since the hydraulic gradient during these periods was from drains towards soils, the upwardly moved acid materials tended to be temporarily stored in the non-sulfidic upper soil layer (jarositic layer) in both soluble and buffered forms. During the alternating low evapotranspiration spells, the acid materials accumulated in the jarositic layer were exported from the soils to the drainage system after heavy rain events. However, the amount of acid exported into the drainage system during rainfall events appears to be limited, providing a large proportion of acidity stored in the upper soil layer was in buffered forms through ion adsorption and formation of basic Fe and Al sulfates (predominantly jarosite). Release of this buffered acidity did occur but tended to be very slow under successive water extraction in the laboratory. Field observation showed that the drianwater pH of the study site hardly dropped below 3.5. Such a pH value is greater, compared to that observed from the drains in the estuarine floodplains with limited drainage (e.g. Sammut et al., 1996 . Marine Freshwater Research 47, 669–684.). Under sugarcane farming conditions in the study site, the intensive drainage has intensified soil acidification by increasing the frequency and magnitude of low watertable conditions which enhanced the oxidation of sulfides in the subsoils, but at the same time, the creation of highly oxidized conditions in the upper soil layer due to this artificial drainage also caused the accumulation of jarosite which appears to have an important effect on preventing more extreme acidification from occurring in the soil-drainage system.

Preliminary Investigation into the Suitability of Permeable Reactive Barriers for the Treatment of Acid Sulfate Soils Discharge

Publisher Summary The generation and release of acidic water from acid sulfate soils are an environmental problem of international importance. This chapter focuses on the suitability of permeable reactive barriers packed with neutralizing agents such as calcite in the treatment technology for assisting in the management of drainage from acid sulfate soils. However, various factors need to be resolved prior to installation of such a barrier in an acid sulfate soils region. Preliminary data required for design include the water chemistry and local hydrology of the region. It is also particularly important to develop an understanding of the variability in flow and acid discharge through storm events, as these potentially constitute the times of greatest impact with respect to acid transport. Although significant concern exists that armoring by iron and aluminum oxide precipitates may limit the reactivity and longevity of the permeable reactive barrier, maintenance of saturated conditions and installation of effective subdrainage and/or flushing procedures can be effective in overcoming problems and yield an effective, low maintenance solution to an increasingly troublesome problem.

Characteristics of the Acidity in Acid Sulfate Soil Drainage Waters, McLeods Creek, Northeastern NSW, Australia

Environmental Context. Acid sulfate soils are found in many low-lying coastal areas, but they can also be encountered in inland areas of Australia and other parts of the world. These soils typically contain iron sulfides, primarily pyrite (FeS2) and mackinawite (FeS), and the products that result from oxidation of these iron minerals. Acidic and metal-rich waters can be produced when the pyrite in soil is oxidized by natural means or accelerated when the soil is drained, which typically occurs when it is developed for agriculture or urban use. In general, acid sulfate soils become a problem when oxidation products are transported from the soil profile into nearby streams and estuaries, which can severely affect the ecology, biodiversity, economic development, and the aesthetics of adjacent waterways. The key contributors to acidity in drainage waters from the site examined are Al3+, AlSO4– and, under particular circumstances, Mn2+ and Fe2+, but the principal species contributing to acidity are strongly time variant and would be expected to vary from site to site. Abstract. Catchments that contain acid sulfate soils can discharge large quantities of acid and dissolved metals into waterways. At McLeods Creek in far northern NSW, Australia, the acidity from the hydrolysis of dissolved metal species, particularly aluminium and iron, contributes to greater than 70% of the total acidity. Therefore, a poor relationship exists between both calculated and titrated acidity and pH because of the dominant influence of these hydrolyzable metal species. Determination of the so-called ‘cold acidity’ by direct titration with NaOH yields results that are difficult to replicate because of the buffering effects of suspended solids, carbon dioxide ingassing, and/or MnII and FeII oxidation in the sample as the titration end-point is approached. Samples that are pre-treated with sulfuric acid and hydrogen peroxide produce results (of ‘hot acidity’) that can be easily replicated and are similar to calculated acidities based on elemental analysis and speciation calculations. The cold acidity values for titrations of 105 water samples from the chosen field site are often higher than hot acidity values as a result of the loss of carbonate acidity during pre-treatment of samples for hot acidity analysis.

Field-based measurements of sulfur gas emissions from an agricultural coastal acid sulfate soil, eastern Australia

The emissions of biogenic hydrogen sulfide (H2S) and sulfur dioxide (SO2) play important roles in the global atmospheric sulfur (S) cycle. Field-based investigations using ultraviolet fluorescence spectroscopy show that drained acid sulfate soils (ASS) are a potentially unaccounted source of biogenic H2S and SO2. Significant diurnal variations were evident in SO2 fluxes, with average daytime measurements 9.3–16.5-fold greater than night-time emissions. Similar diurnal patterns in H2S fluxes were observed but proved statistically insignificant. The results from simultaneously collected micrometeorological measurements suggest that emissions of SO2 and H2S are most likely occurring via different processes. The SO2 fluxes are closely linked to surface soil temperature and moisture content, whereas H2S is constantly emitted from the land surface at the two study sites. Drained ASS are most likely mapped as agricultural lands rather than drained backswamps. Therefore, these areas are likely to be assigned H2S and SO2 flux values of zero in greenhouse gas species inventories. These findings suggest a need to expand these measurements to other drained ASS areas to refine regional (and possibly global) atmospheric S budgets. Further research is necessary to elucidate the sources of measured S compounds, and specifically whether they are limited to individual agricultural drainage patterns in ASS.

From Conflict to Industry- Regulated Best Practice Guidelines: a Case Study of Estuarine Flood Plain Management of the Tweed River Eastern Australia

Different sectors of society claim rights to use valuable coastal ecosystem resources. Conflicts over their use are therefore inevitable. In eastern Australia, government-encouraged development and drainage of coastal flood plains, principally for agriculture, resulted in accelerated oxidation of acid sulphate soils and export of toxic acidic drainage to coastal streams. Major impacts on infrastructure, ecology, fisheries and aquaculture resulted. In 1987/1988, all gilled organisms were killed in 23 km of the Tweed River estuary by acid outflows from canelands. This generated major conflicts among fishers, environmentalists and sugarcane producers farming the flood plain. Here, we describe the evolution of a collaborative learning approach to coastal flood plain management involving cane farmers, local government and researchers, and the institutional response to this fish kill. Existing knowledge in Australia was inadequate and efforts centred on providing information and options for better management and regulation of sulphidic estuarine areas and on mitigating impacts on downstream ecosystems. Farmers, researchers and local government officers working collaboratively generated information on the properties and management of sulphidic flood plains under the highly variable rainfall conditions common in Australia. This provided options for management that were rapidly translated into practice and underpinned mandatory best management guidelines for the NSW sugar industry. Increases in productivity and decreases in acid water discharge have resulted. Essential features of the collaborative partnership are analysed and the institutional response, which led to the adoption of Australia’s national strategy for the management of acid sulphate soils, is described.

Treatment of drainage from acidic canelands using a constructed wetland

This paper describes trials of a constructed freshwater wetland to treat highly acidic drainage from acid Sulfate soil in a sugarcane farm. A constructed freshwater wetland was used to treat acidic discharge from drained acid sulfate soils on a sugar cane farm in the Tweed River flood plain, northern New South Wales (NSW). The bunded 1.44 ha wetland was laser levelled into 6 segmented bays with an overall hydraulic gradient of 0.13%. Water retention time varies between 19 and 82 days dependent on the prevailing evapotranspiration rate. The wetland, which receives about 12% of runoff from a hydraulically isolated 100 ha sugarcane area by pumping, is designed to treat the highly acidic groundwater-dominated recession phase of drainage with large concentrations of dissolved aluminium, iron and manganese. Common couch grass (Cynodon dactylon) and Eleocharis reeds were established in the wetland by natural recruitment. Transects of water quality wetland during filling and while in operation revealed that pH increased while electrical conductivity (EC), dissolved oxygen (DO), redox potential (RP), dissolved sulfate, total potential and total actual acidity (TPA and TAA) decreased dramatically through the wetland. Iron oxyhydroxide flocks are deposited in the inlet bay of the wetland and the redox-pH relation is consistent with ferrolysis. As water moves further into the wetland it is titrated by the organic matter present, resulting in the reduction of protonic acidity, sulfate, dissolved metal concentrations and EC. The advantages and disadvantages of using the wetland as a practical method to treat drainage within a farming system are discussed. The wetland successfully treated acid drainage.