Hydrological and hydrochemical behavior of a riparian zone in a high-order flatland stream.

Hydrological and hydrochemical processes occurring within riparian zones in temperate mid-latitudes flatland areas have significant implications for water management by controlling nutrient transfer between the watershed and the stream system. The riparian zone in a high-order flatland stream located within a 7063-km2 agricultural watershed in Argentina was investigated to study its hydrological connectivity to upland zones, interactions with the stream, and their implications for groundwater hydrochemistry. The analysis was based on 9-year-long time series of groundwater/stream water levels collected along a 220-m-long transect comprising six piezometers, a river stage sensor, and hydrochemical information from 37 groundwater/stream water sampling campaigns. Samples were analyzed for electrical conductivity (EC), Cl-, SO4+2, (Ca+2 + Mg+2), pH, and redox potential (ORP). Data were interpreted using descriptive statistics, statistical tests, groundwater flux calculations, and identification of hydrological patterns and associated hydrochemical responses. The system was hydrologically controlled by shallow groundwater. Three representative landscape hydrological patterns were identified: disconnected, incipient-weakly connected, and fully connected. Groundwater hydrochemistry was closely linked to hydrological connectivity, which played an important role in the mobilization and fluxes of solutes. Overall, groundwater EC, Cl-, SO4+2, and (Ca+2 + Mg+2) concentrations decreased from upland to lowland. For full connectivity, Cl- concentrations reduced 33%, while SO4+2 reduced 42%, demonstrating the system's buffering capacity. This investigation constitutes the first attempt to formulate the riparian zone functioning in this agricultural region and has contributed to the understanding on the complex interactions between hydrologic regimes of large flatland-high-order streams and shallow groundwater systems in fine-texture sediments.

Investigating nitrate dynamics in a fine-textured soil affected by feedlot effluents.

Feedlots concentrate large volumes of manure and effluents that contain high concentrations of nitrate, among other constituents. If not managed properly, pen surfaces run-off and lagoons overflows may spread those effluents to surrounding land, infiltrating into the soil. Soil nitrate mobilization and distribution are of great concern due to its potential migration towards groundwater resources. This work aimed at evaluating the migration of nitrate originated on feedlots effluents in a fine-textured soil under field conditions. Soil water constituents were measured during a three-year period at three distinct locations adjacent to feedlot retention lagoons representing different degrees of exposure to water flow and manure accumulation. A simple statistical analysis was undertaken to identify patterns of observed nitrate and chloride concentrations and electrical conductivity and their differences with depth. HYDRUS-1D was used to simulate water flow and solute transport of Cl-, NO4+N, NO3-N and electrical conductivity to complement field data interpretation. Results indicated that patterns of NO3-N concentrations were not only notoriously different from electrical conductivity and Cl- but also ranges and distribution with depth differed among locations. A combination of dilution, transport, reactions such as nitrification/denitrification and vegetation water and solute uptake took place at each plots denoting the complexity of soil-solution behavior under extreme polluting conditions. Simulations using the concept of single porosity-mobile/immobile water (SP-MIM) managed structural controls and correctly simulated -all species concentrations under field data constrains. The opposite was true for the other two locations experiencing near-saturation conditions, absence of vegetation and frequent manure accumulation and runoff from feedlot lagoons. Although the results are site specific, findings are relevant to advance the understanding of NO3-N dynamics resulting from FL operations under heavy soils.

Water and chloride transport in a fine-textured soil in a feedlot pen.

Cattle feeding in feedlot pens produces large amounts of manure and animal urine. Manure solutions resulting from surface runoff are composed of numerous chemical constituents whose leaching causes salinization of the soil profile. There is a relatively large number of studies on preferential flow characterization and modeling in clayed soils. However, research on water flow and solute transport derived from cattle feeding operations in fine-textured soils under naturally occurring precipitation events is less frequent. A field monitoring and modeling investigation was conducted at two plots on a fine-textured soil near a feedlot pen in Argentina to assess the potential of solute leaching into the soil profile. Soil pressure head and chloride concentration of the soil solution were used in combination with HYDRUS-1D numerical model to simulate water flow and chloride transport resorting to the concept of mobile/immobile-MIM water for solute transport. Pressure head sensors located at different depths registered a rapid response to precipitation suggesting the occurrence of preferential flow-paths for infiltrating water. Cracks and small fissures were documented at the field site where the % silt and % clay combined is around 94%. Chloride content increased with depth for various soil pressure head conditions, although a dilution process was observed as precipitation increased. The MIM approach improved numerical results at one of the tested sites where the development of cracks and macropores is likely, obtaining a more dynamic response in comparison with the advection-dispersion equation.