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Life and element cycling on Earth is directly related to electron transfer (or redox) reactions. An understanding of biogeochemical redox processes is crucial for predicting and protecting environmental health and can provide new opportunities for engineered remediation strategies. Energy can be released and stored by means of redox reactions via the oxidation of labile organic carbon or inorganic compounds (electron donors) by microorganisms coupled to the reduction of electron acceptors including humic substances, iron-bearing minerals, transition metals, metalloids, and actinides. Environmental redox processes play key roles in the formation and dissolution of mineral phases. Redox cycling of naturally occurring trace elements and their host minerals often controls the release or sequestration of inorganic contaminants. Redox processes control the chemical speciation, bioavailability, toxicity, and mobility of many major and trace elements including Fe, Mn, C, P, N, S, Cr, Cu, Co, As, Sb, Se, Hg, Tc, and U. Redox-active humic substances and mineral surfaces can catalyze the redox transformation and degradation of organic contaminants. In this review article, we highlight recent advances in our understanding of biogeochemical redox processes and their impact on contaminant fate and transport, including future research needs.

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Nutrient input through submarine groundwater discharge (SGD) rivals river inputs in certain regions and may play a significant role in nutrient cycling and primary productivity in the coastal ocean. In this paper, we review the key factors determining the fluxes of nitrogen (N) and phosphorus (P) associated with SGD and present a compilation of measured rates. We show that, in particular, the water residence time and the redox conditions in coastal aquifers and sediments determine fluxes and ratios of N and P in SGD. In many coastal groundwater systems, and especially in contaminated aquifers, N/P ratios exceed those in river water and are higher than the Redfield ratio. Thus, anthropogenically driven increases in SGD of nutrients have the potential to drive the N-limited coastal primary production to P-limitation. River input of N and P to the coastal ocean has doubled over the past 50 yr. Results of a dynamic biogeochemical model for the C, N and P cycles of the global proximal coastal ocean (which includes large bays, the open water part of estuaries, deltas, inland seas and salt marshes), suggest that this has led to a factor 2 increase in primary production and biomass and a decline in water column N/P ratios, i.e. the system has become more N-limiting. With the same model, we show that an increase of SGD-N fluxes to , 0.7-1.1 Tmol yr 21 (with a SGD N/P ratio of 100; equal to ,45 -70% of pre-human riverine N-inputs) is required to drive the coastal ocean to P-limitation within the next 50 yr. q

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