How Important Are Those Fracture Zones? Scale Dependent Characteristics Revealed Through Field Studies and an Integrated Hydrological Model of a Mountain Headwater Catchment

Modeling groundwater flow in bedrock can be particularly challenging due to heterogeneities associated with fracture zones. However, fracture zones can be difficult to map, particularly in forested areas where tree cover obscures land surface features. This study presents the evidence of fracture zones in a small, snowmelt-dominated mountain headwater catchment and explores the significance of these fracture zones on groundwater flow in the catchment. A newly acquired bare earth image acquired using LiDAR identifies a previously undetected linear erosion zone that passes near a deep bedrock well at low elevation in the catchment. Borehole geophysical logs indicate more intense fracturing in this well compared to two wells at higher elevation. The well also exhibited a linear flow response during a pumping test, which is interpreted to reflect the influence of a nearby vertical fracture zone. The major ion chemistry and stable isotope composition reveal a slightly different chemical composition and a more depleted isotopic signature for this well compared to other groundwaters and surface waters sampled throughout the catchment. With this evidence of fracturing at the well scale, an integrated land surface – subsurface hydrologic model is used to explore four different model structures at the catchment scale. The model is refined in steps, beginning with a single homogeneous bedrock layer, and progressively adding 1) a network of large-scale fracture zones within the bedrock, 2) a weathered bedrock zone, and 3) an updated LiDAR-derived digital elevation model, to gain insight into how increasing subsurface geological complexity and land surface topography influence model fit to observed data and the various water balance components. Ultimately, all of the models are considered plausible, with similar overall fit to observed data (snow, streamflow, pressure heads in piezometers, and groundwater levels) and water balance results. However, the models with fracture zones and a weathered zone had better fits for the low elevation well. These models contributed slightly more baseflow (~14% of streamflow) compared to models without a weathered zone (~1%). Thus, in the watershed scale model, including a weathered bedrock zone appears to more strongly influence the hydrology than only including fracture zones.

GEOPHYSICAL SURVEY AS A TOOL TO REVEAL SUBSURFACE STRATIFICATION AT A SMALL AGRICULTURAL HEADWATER CATCHMENT: A CASE STUDY

Catchment drainage area is a basic spatial unit in landscape hydrology within which the authorities estimate a water balance and manage water resources. The catchment drainage area is commonly delineated based on the surface topography, which is determined using a digital elevation model. Therefore, only a flow over the surface is implicitly considered. However, a substantial portion of the rainfall water infiltrates and percolates through the soil profile to the groundwater, where geological structures control the drainage area instead of the topography of the soil surface. The discrepancy between the surface topography-based and bedrock-based drainage area can cause large discrepancies in water balance calculation. It this paper we present an investigation of the subsurface media stratification in a headwater catchment in the central part of the Czech Republic using a geophysical survey method - electrical resistivity tomography (ERT). Results indicate that the complexity of the subsurface geological layers cannot be estimated solely from the land surface topography. Although shallow layers copy the shape of the surface, the deeper layers do not. This finding has a strong implication on the water transport regime since it suggests that the deep drainage may follow different pathways and flow in other directions then the water in shallow soil profile or shallow subsurface structures.

Sulfur-Based Denitrification in Streambank Subsoils in a Headwater Catchment Underlain by Marine Sedimentary Rocks in Akita, Japan

Sulfur-based denitrification may be a key biogeochemical nitrate (NO3−) removal process in sulfide-rich regions, but it is still poorly understood in natural terrestrial ecosystems. We examined sulfur-driven NO3− reduction using streambank soils in a headwater catchment underlain by marine sedimentary rock in Akita, Japan. In a catchment exhibiting higher sulfide content in streambed sediment, we sampled two adjacent streambank soils of streambank I (two layers) and of streambank II (eight layers). Anaerobic long-term incubation experiments (40 days, using soils of streambank I) and short-term incubation experiments (5 days, using soils of streambank II) were conducted to evaluate variations of N solutes (NO3−, NO2−, and NH4+), N gases (NO, N2O), and the bacterial flora. In both experiments, two treatment solutions containing NO3− (N treatment), and NO3− and S2O32− (N + S treatment) were prepared. In the N + S treatment of the long-term experiment, NO3− concentrations gradually decreased by 98%, with increases in the SO42−, NO2−, NO, and N2O concentrations and with not increase in the NH4+, indicating denitrification had occurred with a high probability. Temporal accumulation of NO2− was observed in the N + S treatment. The stoichiometric ratio of SO42− production and NO3− depletion rates indicated that denitrification using reduced sulfur occurred even without additional S, indicating inherent S also served as an electron donor for denitrification. In the short-term incubation experiment, S addition was significantly decreased NO3− concentrations and increased NO2−, NO, and N2O concentrations, especially in some subsoils with higher sulfide contents. Many denitrifying sulfur-oxidizing bacteria (Thiobacillus denitrificans and Sulfuricella denitrificans) were detected in both streambank I and II, which dominated up to 5% of the entire microbial population, suggesting that these bacteria are widespread in sulfide-rich soil layers in the catchment. We concluded that the catchment with abundant sulfides in the subsoil possessed the potential for sulfur-driven NO3− reduction, which could widely influence N cycling in and NO3− export from the headwater catchment.