Underground » Environment - Subsidence and Mine Water
Temperate Highland Peat Swamps on Sandstone (THPSS) consist of both ephemeral (natural ephemeral) and permanent swamps, which are a distinctive feature of highlands in the Sydney Basin Bioregion. These swamps support distinctive vegetation which rely on high soil water content and organic matter content. THPSS are defined by plant communities, which effectively respond to the hydrological conditions of swamps. Thus, the significant natural variability in plant species present within and between individual swamps can be used to infer the baseline hydrology of swamps. Some THPSS of the Sydney Basin lie over underground coal mines and there are concerns regarding the impact of mining on the swamps' hydrology, and thus vegetation cover. This project aimed to assess the resilience and sustainability of the Temperate Highland Peat Swamps on Sandstone (THPSS) of the Sydney Basin Bioregion in response to fluctuations in soil water availability resulting from climate variability and/or underground mining activities.
Ten study sites were selected based on their accessibility, underground mining history and historical groundwater level fluctuations. Soil water potential sensors were installed at five different depths at five of these sites, to quantify soil moisture fluctuations. The remaining sites were monitored using Sentek sensors which were already installed as part of an ongoing swamp monitoring program. Soil samples were collected from five different depths at each study site to characterise the soil physical properties required for developing a soil hydrological model. Although soil hydraulic properties varied among the swamps, the soil characterisation suggested the soil water retention curve parameters can be generalised for modelling purposes, yet wherever possible, site-specific parameters should be used. This project suggested bulk density and organic matter content as well as sand content, mainly control the soil physical properties of swamps.
The soil moisture monitoring program at the Newnes Plateau was affected by fire in December 2019. The obtained data indicated a beneficial effect of fire on the soil water storage of swamps. Considerably higher soil water potentials (wetter soil conditions) were observed after fire, compared to those before fire. This appears to be to a large extent due to an increase in the total porosity of soil that coincides with drought followed by three years of high precipitation amounts. The measured soil profile water potentials of the study sites ranged between saturation and -100 kPa after fire, and generally between saturation and -1400 kPa before fire.
Although the observed soil water potentials at all sites did not approach permanent wilting point, a swamps' vegetation specific critical threshold, beyond which plants are considered to be severely affected by a deficiency in water, may be reached at much higher water potentials than for typical terrestrial vegetation. The plant (leaf) water potential of presented species in the study sites were measured individually and compared with the observed soil water potential of their relevant sites. The comparison of leaf water potentials of plants (indicator of water stress) and soil water potential, suggested that current plants can create a much lower potential to move water to their leaves without showing water deficiency status. The comparison of measured leaf water potential and observed soil water potential proposed that current plants can tolerate much lower soil water potential to uptake water.
It was established that the Sentek sensors installed in the study sites of Upper Nepean were not measuring absolute soil water content. Therefore, where possible, recorded soil water contents were corrected using an in-situ calibration in order to estimate absolute soil water content. The corrected soil water contents indicated that soil moisture fluctuations are more rainfall responsive after mining, with a brief wetting up corresponding to the magnitude of rainfall events. A lower soil water storage of swamps was also observed after mining, and it can be assumed that for the Newnes Plateau sites, a change in hydrology will likely change the plant community in longer term.
To project changes in the soil water availability of swamps and their vegetation in response to climate change/variability and mining impact, 1D and 2D soil hydrological models were developed and calibrated using the observed soil water potentials of the study sites, by solving an inverse simulation problem. At the post fire sites, the post fire soil physical properties were used for the modelling. The models were then validated using statistical analyses, where observed soil water potentials were compared with the simulated soil water potentials. It is concluded that the 2D model is a better representation of the study sites, particularly as lateral flow and rock fractures, both critical features for the performance of water flow in the THPSS, can be directly considered in a 2D model. The developed 2D models are applicable to a range of swamps under changed inputs and parameter values. In this context, parameter optimisation to in-situ conditions is highly recommended. Sensitivity analyses were conducted to determine the fracture properties that have the most influence on the soil moisture fluctuations. Increasing the number of fractures was reflected in a decrease of soil water potentials as drier soils were simulated. Although the response of soil water potentials to changes in hydraulic conductivity and width of a fracture was site specific, generally a drier 'subsoil' was simulated in response to increasing hydraulic conductivity of a fracture and width of a fracture.
The 2D models were used to evaluate the potential resilience and sustainability of the THPSS of Sydney Basin Bioregion in response to fluctuations in soil water availability caused by climate variability, climate change and mining activity. A 60-year climate change scenario and a 30-year historical climatic scenario were applied to the 2D model of each study site. This study indicated that a generally higher soil water storage was observed under the climate change scenario. This was potentially influenced by projected climate change rainfall patterns for the study Upper Nepean swamps. Fire altered soil properties and projected climate change rainfall patterns were likely responsible for a greater soil water storage for the studied Newnes Plateau swamps. The simulated soil profile water potentials of the study sites were continually high (mostly between saturation and -200 kPa) under both climatic scenarios. Based on the simulated soil water potentials of the study sites and measured leaf water potentials of existing plant species in the sites (an indicator of plant water stress status), the current vegetation of the study swamps would not be affected under both climate change and climate variability scenarios, and is unlikely to experience drought stress conditions, assuming the vegetation communities are in equilibrium with the current soil moisture conditions after mining for Upper Nepean study swamps. In the bushfire affected swamps in the Newnes Plateau this is also the case, but changes in the hydrology of swamps after mining is still implied, even with three years above average rainfall. Assuming the soil moisture is the reflection of the equilibrium between plant and soil moisture after mining and potential fracture in sandstone, any vegetation alteration in swamps will be unlikely to occur under climate change or climate variability scenarios. It is concluded that the “current evidence” regarding soil moisture impacts of mining does not support the conclusion that there will necessarily be vegetation impacts in mined under swamps of Upper Nepean. As such changes will occur over long periods of times, further long term monitoring of soil moisture and vegetation at impacted and non-impacted sites is required to observe any possible delay in vegetation response to soil moisture changes and establishing equilibrium. For bush fire affected swamps, vegetation changes and plant regrowth seem to be solely depended on post fire hydrology of swamps. A more precise projection of vegetation changes requires a better understanding of threshold water potential of each species, as well as the soil moisture conditions of a non-impacted swamp.