Environmental Risk of Metal Mining Contaminated River Bank Sediment at Redox-Transitional Zones

Abstract

Diffuse metal pollution from mining impacted sediment is widely recognised as a potential source of contamination to river systems and may significantly hinder the achievement of European Union Water Framework Directive objectives. Redox-transitional zones that form along metal contaminated river banks as a result of flood and drought cycles could cause biogeochemical changes that alter the behaviour of polyvalent metals iron and manganese and anions such as sulphur. Trace metals are often partitioned with iron, manganese and sulphur minerals in mining-contaminated sediment, therefore the dissolution and precipitation of these minerals may influence the mobility of potentially toxic trace metals. Research indicates that freshly precipitated metal oxides and sulphides may be more "reactive" (more adsorbent and prone to dissolution when conditions change) than older crystalline forms. Fluctuations at the oxic-anoxic interface brought about through changes in the frequency and duration of flood and drought episodes may therefore influence the reactivity of secondary minerals that form in the sediment and the flux of dissolved trace metal release. UK climate change models predict longer dry periods for some regions, interspersed with higher magnitude flood events. If we are to fully comprehend the future environmental risk these climate change events pose to mining impacted river systems it is recommended that research efforts focus on identifying the primary controls on trace metal release at the oxic-anoxic interface for flood and drought cycles of different duration and frequency. This paper critically reviews the literature regarding biogeochemical processes that occur at different temporal scales during oxic, OPEN ACCESSMinerals 2014, 4 53 reducing and dry periods and focuses on how iron and sulphur based minerals may alter in form and reactivity and influence the mobility of trace metal contaminants. It is clear that changes in redox potential can alter the composition of secondary iron and sulphur minerals and influence the sorption of toxic trace metals and susceptibility to dissolution when further redox potential changes occur. However further work is needed to determine: (i) The extent to which different duration and frequency of wet and dry cycles influences the dissolution and precipitation of iron and sulphur minerals in mining contaminated river bank sediment; (ii) The temporal effects on mineral reactivity (sorption capacity and susceptibility to dissolution); (iii) The key biogeochemical processes that control the mobility of contaminant trace metals under these dynamic redox potential conditions.

Critical control of flooding and draining sequences on the environmental risk of Zn-contaminated riverbank sediments

“…Principal component 1 was found to represent redox potential conditions, and a gradient of change was observed for individual samples where those with longer wet periods scored more highly against PC1, along with EQS for total concentration of Zn for the protection of surface water is 8 μg l −1 (annual average value) at 0-50 mg l −1 CaCO dissolved Zn. It is possible that the dissimilatory reduction of Fe and/or Mn (hydr)oxides occurred in response to a fall in redox potential conditions due to prolonged flooding (Lynch et al 2014). However, although dissolved Fe was measured in pore water, redox potential conditions did not decline low enough for the reductive dissolution of Fe (hydr)oxide (<300 mV, pH 5) (Gotoh and Patrick 1974;Emerson et al 1979).…”

“…Summer river flows are expected to decline, and Q95 (flow that is exceeded 95% of the time) may reduce by 26% by 2050 and 35% by 2080 in western Wales using medium emission (P50) scenarios (DEFRA 2012b) that could increase the length of antecedent dry period along riverbanks. As discussed in a previous paper by the current authors (Lynch et al 2014), we hypothesised that flooding and drought brought about by future climate change will cause changes in river stage that will alternately expose and submerse riverbank sediment over varying temporal scales, alter redox potential conditions and pH and influence the mobilisation of trace metal contaminants. In mining-impacted catchments, Zn is often partitioned with Fe, Mn hydroxides (Desbarats and Dirom 2007;Du Laing et al 2009) and sulphur-bearing minerals (Carroll et al 1998) in the sediment.…”

“…During high flow events, contaminated sediment can be eroded from riverbanks and floodplains and re-introduce particulate forms of Zn back into the river channel (Dennis et al 2003;Young et al 2007). However, more bioavailable, dissolved Zn can be re-introduced where river stage variability and riverbank inundation results in changes in redox potential and drying of sediment (Buckby et al 2003;Du Laing et al 2009;Lynch et al 2014).…”

The fluctuation of calcium and magnesium concentrations in the floodwater in the Nemunas and Minija lowlands

“…Alteration in pH and redox reactions will change the bioavailability of nutrients in the soil (Lynch et al, 2014). pH, ammonium and phosphate levels increase, when the soils become anaerobic (Beumer et al, 2007;Loeb et al, 2007).…”

Urban Aquatic Ecosystems: the good, the bad and the ugly

“…Consequently, many urban rivers have elevated nutrient, heavy metal, pesticide and other organic contaminant concentrations (Paul and Meyer, 2001;Gurnell et al, 2007;Lynch et al, 2014) and, significantly modified biogeochemical cycles (Kaushal and Belt, 2012). Rapid urbanisation in developing and emerging economies has caused marked declines in water quality, for example in Shanghai, China (Zhao et al, 2006;Wang et al, 2009).…”