THE REMOVAL KINETICS OF DISSOLVED ORGANIC MATTER AND THE OPTICAL CLARITY OF GROUNDWATER

Concentrations of dissolved organic matter (DOM) and ultraviolet absorbance decrease systematically as groundwater moves through the unsaturated zones overlying aquifers and in the saturated zones of aquifers. These changes occur over distances of tens of meters 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 two-year record of dissolved organic carbon (DOC) concentration changes in groundwater at a long-term observation well. Estimated rates of DOM loss from the dissolved phase 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 0.0026 to 0.011 per day (d^-1). Applying these removal rate constants to an advective-dispersion model illustrates substantial depletion of chromophoric DOM over flow-path distances of two hundred meters or less and in timeframes of two years or less. These results explain the low to moderate DOC concentrations (20 -75 µM; 0.26 – 1 mg/L) and ultraviolet absorption coefficient values (a₂₅₄ < 5 m^-1) observed in older groundwater produced from 59 wells tapping eight different Principal Aquifers 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.