Origin and migration of dissolved organic carbon fractions in a clay-rich aquitard: 14C and δ13C evidence

Abstract

The origin and long-term transport of dissolved organic carbon (DOC) and its high molecular weight (HMW) and low molecular weight (LMW) fractions were investigated in a clay-rich till aquitard system. DOC concentrations decreased systematically with depth from 168 mg L-1 in the surficial fractured zone (1.2 m) to minimum values of 12-16 mg L-1 from 15 to 43 m depth in the nonfractured aquitard. The HMW and LMW DOC fractions and their corresponding δ13C values also decreased systematically with depth. Numerical transport simulations of the downward migration of DOC from the soil zone into the aquitard closely approximated field-measured values and show transport of DOC was controlled by diffusive mixing. The depth trends for HMW and LMW DOC fractions and δ13C support hypothesized transport via diffusion, as well as earlier findings that microbial respiration in this nonfractured aquitard was negligible over the time period studied. The 14C content of the HMW DOC fraction decreased systematically with depth from 87 percent modern carbon (pmC) at 1.2 m to 13-15 pmC from 15 to 43 m depth. A simple model accounting for diffusive mixing and radioactive decay described the 14C profile in the aquitard. Radiocarbon measurements, transport modeling, and δ13C data suggest DOC and its fractions were derived from two distinct end-members: soil organic carbon formed since the Holocene (∼10 ka B.P.) and DOC in connate pore water (∼15 ka B.P.). Copyright 2005 by the American Geophysical Union.