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This study examines the distribution of soil organic carbon and carbon-isotopes with depth and among particle size fractions in 2 forest soil profiles of contrasting texture from Cape York Peninsula, Queensland, Australia. The profile on sand has a comparatively low inventory of carbon (557 mg/cm2 from 0–100 cm) and exhibits comparatively small variations in δ13C value. In contrast, the clay-rich profile has a much larger inventory of soil organic carbon (1725 mg/cm2 from 0–100 cm) and large variations in δ13C value occur both with depth in the profile and between different particle size fractions. The considerable differences in carbon inventories and δ13C values between the sites appear to be largely due to soil textural differences. In�the absence of fine minerals the trend in δ13C value with decreasing particle size is to similar or lower δ13C values, due to an increase in the relative abundance of low δ13C compounds in the residue left by microbial decomposition. In the presence of fine minerals, the trend is to higher δ13C values due to the stabilisation of the products of microbial decomposition by the fine minerals. Thus, the bulk δ13C value of soil organic carbon appears to be determined as much by the abundance of fine minerals in a soil profile as by isotope fractionation effects accompanying degradation. It is further postulated that an initial rapid rise in δ13C value in the upper soil layers is due to an increase in the relative importance of higher 13C, root-derived carbon immediately below the soil surface.

Extraneous flows in wastewater collection systems are conventionally evaluated solely on the consideration of discharge hydrographs, which often involves a great degree of subjectivity and oversimplification. To obtain reliable information on the material fluxes within the urban environment, the use of intrinsic tracers can be the optimal choice. We demonstrate the successful use of naturally occurring stable isotopes of water (18O/16O and D/H) to accurately quantify extraneous discharge of groundwater in a combined sewer network. Fresh water supply from a distant hydrological regime provided usable isotopic separations between drinking water (proxy for real foul sewage) and local groundwater (proxy for sewer infiltration) of 1.8 per thousand in delta18O and 11.7 per thousand in 62H. Diurnal variation of wastewater isotopic composition reflected both the varying rates of foul sewage production and irregular dispersion effects in the pipe network. The latter suggest the existence of larger cumulative backwater zone volumes, which have not been attended to yet. Infiltrating groundwater contributed 39% (95% confidence interval = +/- 2.5%) of the total daily dry weather wastewater discharge. This paper discusses all relevant aspects for practical application of the method. It presents a comprehensive framework for uncertainty analysis and details on the detection and discrimination of possibly interfering effects.