RATES OF REDOX REACTIONS DERIVED FROM DATED GROUNDWATERS WITH APPLICATIONS TO AGRICULTURAL WATERSHEDS

Rates of biogeochemical redox reactions in groundwater range over at least 8-10 orders of magnitude. Low reactions rates may be difficult or impossible to measure in controlled experiments, but they can be derived in some cases by comparing in situ gradients of reaction progress and groundwater age. Rates of oxygen reduction, denitrification, and sulfate reduction on the order of 10^-5 to 10^-4 µmol/L/d have been derived from groundwaters dated by ¹⁴C, whereas rates on the order of 10^-2 to 10⁰ µmol/L/d have been derived from groundwaters dated by modern anthropogenic atmospheric tracers such as CFCs, SF₆, and ³H-³He. Potential complications to this approach include (1) practical time scales of dating methods that do not span the complete time scale over which the reactions occurred, (2) difficulty in sampling and measuring reaction gradients where reactions occur abruptly at redox boundaries; and (3) non-steady-state gradients caused by changes in the rate and chemical composition of recharge. Agricultural irrigation and fertilizer use have altered substantially the recharge fluxes of water and biogeochemical reactants such as oxygen and nitrate in large areas of the world. Increased fluxes of aqueous electron acceptors have important but variable effects on the distributions and rates of water-rock reactions in aquifers underlying agricultural land, depending in part on the relative importance of different rate-limiting processes such as large-scale transport or local reaction kinetics. Examples include suppression of denitrification by rapid fluxes of oxygen in irrigated alluvial sediments, and enhanced oxidation of pyrite by agricultural nitrate contamination in glacial outwash sediments. Concentrations of redox-sensitive species such as oxygen, nitrate, and sulfate in discharge to wells and streams in agricultural watersheds have been altered as a result of changing recharge conditions and reaction rates.