Impact of Water Halinity on the Presence of Hypersulfidic Materials in Estuarine Tidal Marsh Soils, Chesapeake Bay (USA)

Abstract

In brackish tidal marsh soils, sulfate reduction processes commonly lead to the formation of Fe sulfide minerals, and if the accumulated potential acidity exceeds the ability of other components for neutralisation, can lead to the occurrence of hypersulfidic soil materials, which if disturbed and oxidised, can become extremely acid (sulfate) soils. In estuarine/riverine marshes that are fed by fresh water flowing into an estuary, a pronounced halinity gradient exists along the course of the stream, with upstream portions being fresher and downstream sections being more strongly influenced by salts. Thus, it is expected that hypersulfidic materials will be less prevalent in upstream sections, and this is reflected in the concepts used in soil mapping of the marshes in the Chesapeake Bay estuary (hypersulfidic materials not being recognised when stream halinity is lower than about 2 ppt). This study was designed to examine tidal marsh soils that span a halinity gradient in estuarine/riverine marshes. Soils at eight sites were identified for study that had stream halinity ranging between 0.10 and 8.8 ppt. Soil morphology was described and samples collected from each horizon, which were examined by documenting pH change during moist aerobic incubation (MAI). Surprisingly, all soils, even those with halinity between 0.10 and 1.0 ppt, contained horizons that became extremely acid (pH < 4.0) within 14 weeks during MAI. Examination of salts that developed in the samples during MAI were demonstrated by X-ray diffraction to be mainly sulfate salts, confirming that the acidity was derived from oxidation of sulfide minerals. We expect that occasional pulses of sulfate enriched water, such as occurs during storm events, may provide sufficient stream water sulfate to lead to formation and accumulation of Fe sulfide minerals sufficient to form hypersulfidic materials. Continued rising sea levels under the current warming climate scenario might also exacerbate this worldwide. These observations suggest that a review of the mapping paradigm used in Chesapeake Bay may be in order. Potential modifications to existing soil maps of marshes around Chesapeake Bay should perhaps recognise soils with hypersulfidic materials extending further up the tidal estuary than previously recorded. This work may also have implications for mapping of similar estuarine tidal marsh soils in other parts of the country or the world.