The removal kinetics of dissolved organic matter and the optical clarity of groundwater

Francis H Chapelle,Yuan Shen,Eric W Strom,Ronald Benner

doi:10.1007/s10040-016-1406-y

Francis H Chapelle, Yuan Shen + Show 2 more

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https://doi.org/10.1007/s10040-016-1406-y

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Abstract

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.

Highlights

Among the most remarkable and valued properties of groundwater, and the spring waters derived from groundwater systems, is their striking optical clarity (Davies-Colley and Smith 1995)
dissolved organic carbon (DOC) concentrations in SC Piedmont groundwater show a strong inverse correlation to the thickness of the overlying unsaturated zone (Fig. 2a) indicating substantial dissolved organic matter (DOM) removal during transport. This reflects the fact that CDOM in soil and surface water must traverse an unsaturated zone comprised of clayey saprolite in order to reach the water table (Shen et al 2015)
Previous studies (Meier et al 1999; Guo and Chrover 2003) have shown that sorption of DOM to mineral surfaces can result in the removal of the more aromatic and hydrophobic molecules that contribute to CDOM

Summary

Introduction

Among the most remarkable and valued properties of groundwater, and the spring waters derived from groundwater systems, is their striking optical clarity (Davies-Colley and Smith 1995). This clarity, and the perceived healthfulness of groundwater relative to many surface waters, has had a profound effect on the history of human water use. Many of the mythologies that have traditionally surrounded groundwater (Bord and Bord 1985) can be traced directly to its clarity relative to most surface waters. The earliest scientific studies of groundwater geochemistry can be traced directly to curiosity about the chemical and optical purity of spring waters (Back et al 1995). The fundamental hydrologic and geochemical processes that lead to the nearly uniform optical clarity of groundwater are not widely understood

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