flow paths

Germanium-silicon as a flow path tracer: Application to the Rio Icacos watershed

Kurtz, A. C., F. Lugolobi, and G. Salvucci (2011), Germanium‐silicon as a flow path tracer: Application to the Rio
Icacos watershed, Water Resour. Res., 47, W06516, doi:10.1029/2010WR009853.

We use dissolved silicon together with its “geochemical twin” germanium for the first time as a hydrologic tracer to study water delivery to the stream during storm events in the Rio Icacos watershed, Puerto Rico. Ge and Si were measured on base flow, stormflow, springwater, and soil water samples. Compositions of all of these waters appear to reflect varying contributions from three components, which we attribute to solutes released from bedrock weathering (groundwater), from short-term soil-water interaction (quick soil water), and longer-term soil-water interaction (matrix soil water). Base flow stream waters have high Si and moderate Ge (Ge/Si ratio ∼0.29 μmol/mol), consistent with a predominantly bedrock weathering source as indicated by their similarity with water sampled from springs emerging from the saprolite-bedrock boundary on a hillslope landslide scar. During storm events there is a shift toward more dilute compositions (but higher Ge/Si ratios) similar to those measured on water samples from temporary depression storage and overland flow (quick soil water). Geochemical mass balance shows that 80%–90% of the stream chemistry can be explained by mixing groundwater with this quick soil water composition, which we infer to reflect new water traveling as shallow throughflow. Stream water δ18O values decrease to more negative values typical of precipitation supporting rapid delivery of rainwater to the stream channel during stormflow. The third component, with a Ge-rich composition characteristic of soil matrix water sampled by tension lysimeters, is required to explain higher stream water Ge/Si ratios measured during hydrograph recession. We infer from this an additional, slower, and less dominant pathway for delivery of soil water to the stream channel.

The Role of Rapid Flow Paths for Nitrogen Transformation in a Forest Soil: A Field Study with Micro Suction Cups

Hagedorn F, Mohn J, Schleppi P, Flu¨ hler H (1999) The
role of rapid flow paths for nitrogen transformation in
a forest soil: A field study with micro suction cups.
Soil Sci Soc Am J 63:1915–1923

Preferential flow is a common phenomenon in soils. This study was conducted to investigate the significance of rapid flow paths for N transformation in a forested Humaquept in central Switzerland. Fifty micro suction cups, each with a surface area of 12 mm2, were installed in a regular grid in the uppermost 5 cm. First, the location of each "microcup" relative to main flow paths was estimated based on the response to applications of a dye, SO2-4, and Cl-. Then, a N-addition experiment was carried out to study the N transformation at locations along flow paths and within the soil matrix. Only 23 of 50 microcups responded to the application of the dye within the first 24 h, which indicates that a large portion of the soil volume is not in contact with the infiltrating rainwater. Those microcups which responded to the added dye were regarded to be located along flow paths. At depths below 2 cm, under temporarily reducing conditions, sampling locations in or near flow paths had higher NO-3 concentrations (20-25 microM) than those of the soil matrix (below 12 microM). Within 24 h after a simulated rainfall, the IMGf1.gif" BORDER="0"> ratio decreased more in the flow paths (between -2.4 and -4.9 mol mol-1) than in the soil matrix (-0.7 to -0.8 mol mol-1), which indicates an enhanced denitrification at these locations. In the subsequent dry period, nitrification started 2 d earlier and was more pronounced along flow paths. The results of this study suggest that flow paths are microhabitats with an increased N transformation compared with the soil matrix.

Hydrologic flowpaths influence inorganic and organic nutrient leaching in a forest soil

Asano, Y., Compton, J. E. & Church, R. M. Hydrologic flowpaths influence
inorganic and organic nutrient leaching in a forest soil. Biogeochem. 81,
191-204 (2006).

Hydrologic pathways through soil affect element leaching by determining the relative importance of biogeochemical processes such as sorption and decomposition. We used stable hydrogen isotopes of water (dD) to examine the influence of flowpaths on soil solution chemistry in a mature spruce–hemlock forest in coastal Oregon, USA. Soil solutions (50 cm depth, n = 13) were collected monthly for 1 year and analyzed for dD, major ions and dissolved organic carbon (DOC) and nitrogen (DON). We propose that the variability of dD can be used as an index of flowpath length and contact time. Throughfall variability in dD was much greater than soil solution variability, illustrating that soil solution integrates the variation in inputs. Lysimeters with greater variation in dD presumably have a greater proportion of flow through rapid flowpaths such as macropores. The variation in soil solution dD for individual lysimeters explained up to 53% of the variation in soil solution chemistry, and suggests that flowpaths influence leaching of some constituents. Soil solutions from lysimeters with greater dD variation had higher DOC and DON (r2 = 0.51 and 0.37, respectively), perhaps because transport via macropores reduces interaction of DOM with the soil matrix. In contrast, nitrate concentrations were highest in lysimeters with a small variation in dD, where long contact time and low DOC concentrations may yield higher net nitrification. Our results demonstrate the utility of stable isotopes to link flowpaths and soil solution chemistry, and illustrate how the spatial complexity of soils can influence ecosystem- level nutrient losses.
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