The efficacy of a newly developed method of controlled calcium carbonate introductions for anthropogenic acidification mitigation in a naturally acidic riverine environment
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Historical anthropogenic acidification in the West Dart River, southwest England, represents a significant threat to surface water quality and aquatic ecology through flushing induced pH troughs and elevated bioavailable Aluminium (Al) levels. The West Dart River is failing to meet good ecological status established under the Water Framework Directive (2000/60/EC) (WFD) for pH (Upper West Dart, Cycle 2 classifications 2013-2019 = Moderate, physico-chemical quality elements 2013-2019 pH = Moderate). The aim of this study was to develop and test a method of controlled calcium carbonate (CaCO3) introductions, in order to preserve the naturally acidic riverine state and the biological communities therein, yet to prevent the river pH from falling below the H+ ion concentration critical to the survival of salmonids – pH 5.5. For the first time, a method of encasing known, evaluated quantities of CaCO3 in removable bags with constant in-situ monitoring of water chemistry parameters and the ecology to WFD standards post characterisation of the study river. Changes in water chemistry; pH, turbidity, dissolved organic carbon (DOC), metals present in concentrations around a critical threshold determined by risk modelling, bioavailable Al and nutrients, as well as changes in the ecological communities; macrophytes, diatoms and aquatic vii macroinvertebrates, evaluated as ecological quality ratios (EQRs). The CaCO3 represented little risk to the system in terms of hazardous inputs and three tonnes were introduced in controlled dosing over the course of nine months (2017). Observed negative effects on the water quality were negligible with a decrease of 50.85% (bad) and 67.1% (poor) WFD classifications for pH downstream of the dosing site. Moderate and high/good classifications increased by 11.9% and 67.5% respectively. However, overall, the 5th percentile pH shifted from 4.8 to 5.2 whilst the 95th only shifted from 6.2 to 6.5. Immediately following dosing there was also an observable decrease in bioavailable Al concentrations, leading to potentially fewer negative consequences for fish species, such as Salmo salar which have been suffering poor at sea return rates within the West Dart. The post-dosing EQR for monitored sites revealed a decrease for combined phytobenthos and macroinvertebrates however, this was mirrored in the control site and is likely attributable to uncontrollable environmental factors. The major toxic effects of Al on fish occur at the gills and can be summarised as osmoregulatory disturbances due to net loss in ion uptake (Na+, Cl- and Ca2+). Al toxicity mechanisms are complex, and a protocol was devised to examine respiratory binding on Gammarus pulex as macroinvertebrates have also been shown to accumulate Al on respiratory surfaces and therefore were considered a substitute to fish gills in the face of ethical and timing considerations. However, the greatest surface concentrations of Al were found on the uropods of G. pulex. Though this may be linked to Al concentrations on gills due to large surface areas and regular water movement across the appendages. However, due to the speciation of Al in acidic watercourses, restoration of the alkalinity and pH levels within the watercourse reduces the concentration Al which is in a bioavailable form and thereby reduces the toxicity.
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