The removal kinetics of dissolved organic matter and the optical clarity of groundwater | Cinétiques d’élimination de la matière organique dissoute et clarté optique des eaux souterraines La cinética de eliminación de la materia orgánica disuelta y la transparencia óptica del agua subterránea 去除地下水中溶解有机物及光学透明性的动力学 A cinética da remoção da matéria orgânica dissolvida e a transparência ótica da água subterrânea
2016
Chapelle, Frank | Shen, Yuan | Strom, Eric W. | Benner, R. (Ronald)
Concentrations of dissolved organic matter (DOM) and ultraviolet/visible light absorbance decrease systematically as groundwater moves through the unsaturated zones overlying aquifers and along flowpaths within aquifers. These changes occur over distances of tens of meters (m) implying rapid removal kinetics of the chromophoric DOM that imparts color to groundwater. A one-compartment input-output model was used to derive a differential equation describing the removal of DOM from the dissolved phase due to the combined effects of biodegradation and sorption. The general solution to the equation was parameterized using a 2-year record of dissolved organic carbon (DOC) concentration changes in groundwater at a long-term observation well. Estimated rates of DOC loss were rapid and ranged from 0.093 to 0.21 micromoles per liter per day (μM d⁻¹), and rate constants for DOC removal ranged from 0.0021 to 0.011 per day (d⁻¹). Applying these removal rate constants to an advective-dispersion model illustrates substantial depletion of DOC over flow-path distances of 200 m or less and in timeframes of 2 years or less. These results explain the low to moderate DOC concentrations (20–75 μM; 0.26–1 mg L⁻¹) and ultraviolet absorption coefficient values (a ₂₅₄ < 5 m⁻¹) observed in groundwater produced from 59 wells tapping eight different aquifer systems of the United States. The nearly uniform optical clarity of groundwater, therefore, results from similarly rapid DOM-removal kinetics exhibited by geologically and hydrologically dissimilar aquifers.
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