Infiltration mechanism controls nitrification and denitrification processes under dairy waste lagoon

Maintaining intensive agriculture overlying aquifers using the threshold nitrate root-uptake phenomenon

Reducing nitrate leaching from agricultural land to aquifers has been a high priority concern for more than a half century. This study presents theory and observations of a threshold concentration of nitrate in the root zone (Cmax), which the leachate concentration increases at higher rates with increasing root-zone nitrate concentration. The value of Cmax is derived both by direct results from container experiments with varying nitrogen (N) fertigation and as a calibration parameter in N-transport models beneath commercial agricultural plots. For five different crops, Cmax ranged between 20 and 45 mg L^-1 of NO₃ -N derived from experiments and models. However, for lettuce (Lactuca sativa L.), which was irrigated with a large leaching fraction, Cmax could not be defined. In crops irrigated and fertilized in the warm/dry season (corn [Zea mays L.] and citrus), the experiments show a dramatic change in leachate-concentration slope, and simulations reveal a wide range of sensitivity of leachate NO₃ -N concentration to Cmax. In annual crops irrigated and fertilized in the cool/wet season (e.g., potato [Solanum tuberosum L.] in a Mediterranean climate), the experiments show a distinct Cmax that is less dramatic than that of the summer-irrigated crops in the container experiment and a smaller impact of Cmax in N-transport models. The simulations show that, for summer-irrigated crops, maintaining fertigation at C < Cmax has a significant effect reducing deep leachate concentrations, whereas for the winter annual crops the simulations revealed no threshold effect. It is suggested that for summer-irrigated crops, fertigation below Cmax robustly serves the co-sustainability of intensive agriculture and aquifer water quality; this is also suggested for winter crops, but the benefits are not robust. For short-season, small root-system crops (e.g., lettuce), efforts should be made to detach the crop from the soil.

Oligotrophic Nitrification and Denitrification Bacterial Communities in a Constructed Sewage Treatment Ecosystem and Nitrogen Removal of Delftia tsuruhatensis NF4

Oligotrophic nitrifiers and denitrifiers play important roles in the removal of nitrogen from wastewater. Here, we studied the dominant bacterial populations of the sewage treatment ecosystem (STE) water from different processes and those of culture on oligotrophic heterotrophic nitrification (OHN) medium and oligotrophic aerobic denitrification (OAD) medium, using co-analysis of Illumina HiSeq DNA sequencing and traditional culture methods. The results showed that the STE water had no dominant population of oligotrophic nitrifiers or oligotrophic denitrifiers. However, after culturing on OHN medium and OAD medium, the core genera Pseudomonas, Aeromonas, and Acinetobacter that have the nitrogen removal capacity in oligotrophic environments, dominated in the bacterial community. The principal component analysis (PCA) showed that the bacterial community in the constructed rapid infiltration (CRI) effluent water of STE had high similarity with those of cultures on OHN medium and OAD medium, which prompt the special purification role of nitrogen in the CRI system. The sodium alginate immobilized OAD bacteria strain Delftia tsuruhatensis NF4 was isolated from the CRI system, with total nitrogen (TN) removal efficiency of 43.3% in sterilized STE influent water, and 60.1% in OAD medium on day three. The immobilization significantly influenced the TN and nitrate removal efficiency in OAD medium (p < 0.05), but not in sterilized STE influent water (p > 0.05). This study would lay the foundation for resource discovery of oligotrophic heterotrophic nitrifiers and aerobic denitrifiers in STE and further functional application of them on the bioremediation of wastewater.

Oligotrophic nitrifiers and denitrifiers play important roles in the removal of nitrogen from wastewater. Here, we studied the dominant bacterial populations of the sewage treatment ecosystem (STE) water from different processes and those of culture on oligotrophic heterotrophic nitrification (OHN) medium and oligotrophic aerobic denitrification (OAD) medium, using co-analysis of Illumina HiSeq DNA sequencing and traditional culture methods. The results showed that the STE water had no dominant population of oligotrophic nitrifiers or oligotrophic denitrifiers. However, after culturing on OHN medium and OAD medium, the core genera Pseudomonas, Aeromonas, and Acinetobacter that have the nitrogen removal capacity in oligotrophic environments, dominated in the bacterial community. The principal component analysis (PCA) showed that the bacterial community in the constructed rapid infiltration (CRI) effluent water of STE had high similarity with those of cultures on OHN medium and OAD medium, which prompt the special purification role of nitrogen in the CRI system. The sodium alginate immobilized OAD bacteria strain Delftia tsuruhatensis NF4 was isolated from the CRI system, with total nitrogen (TN) removal efficiency of 43.3% in sterilized STE influent water, and 60.1% in OAD medium on day three. The immobilization significantly influenced the TN and nitrate removal efficiency in OAD medium (p < 0.05), but not in sterilized STE influent water (p > 0.05). This study would lay the foundation for resource discovery of oligotrophic heterotrophic nitrifiers and aerobic denitrifiers in STE and further functional application of them on the bioremediation of wastewater.

High Resolution Monitoring Above and Below the Groundwater Table Uncovers Small-Scale Hydrochemical Gradients

Hydrochemical solute concentrations in the shallow subsurface can be spatially highly variable within small scales, particularly at interfaces. However, most monitoring systems fail to capture these small scale variations. Within this study, we developed a high resolution multilevel well (HR-MLW) with which we monitored water across the interface of the unsaturated and saturated zone with a vertical resolution of 0.05-0.5 m. We installed three of these 4 m deep HR-MLWs in the riparian zone of a third-order river and analyzed for hydrochemical parameters and stable water isotopes. The results showed three distinct vertical zones (unsaturated zone, upper saturated zone, lower saturated zone) within the alluvial aquifer. A 2 m thick layer influenced by river water (upper saturated zone) was not captured by existing monitoring wells with higher screen length. Hydrochemical data (isotopes, total ions) were consistent in all HR-MLWs and showed similar variation over time emphasizing the reliability of the installed monitoring system. Further, the depths zones were also reflected in the NO₃-N concentrations; with high spatial variabilities between the three wells. The zonation was constant over time, with seasonal variability in the upper saturated zone due to the influence of river water. This study highlights the use of high resolution monitoring for identifying the spatial and temporal variability of hydrochemical parameters present in many aquifer systems. Possible applications range from riparian zones, agricultural field sites to contaminated site studies, wherever an improved understanding of biogeochemical turnover processes is necessary.

Using the natural biodegradation potential of shallow soils for in-situ remediation of deep vadose zone and groundwater

In this study, we examined the ability of top soil to degrade perchlorate from infiltrating polluted groundwater under unsaturated conditions. Column experiments designed to simulate typical remediation operation of daily wetting and draining cycles of contaminated water amended with an electron donor. Covering the infiltration area with bentonite ensured anaerobic conditions. The soil remained unsaturated, and redox potential dropped to less than -200mV. Perchlorate was reduced continuously from ∼1150mg/L at the inlet to ∼300mg/L at the outlet in daily cycles. Removal efficiency was between 60 and 84%. No signs of bioclogging were observed during three operation months although occasional iron reduction observed due to excess electron donor. Changes in perchlorate reducing bacteria numbers were inferred from an increased in pcrA gene abundances from ∼10⁵ to 10⁷ copied per gram at the end of the experiment indicating the growth of perchlorate-reducing bacteria. We proposed that the topsoil may serve as a bioreactor to treat high concentrations of perchlorate from the contaminated groundwater. The treated water that infiltrates from the topsoil through the vadose zone could be used to flush perchlorate from the deep vadose zone into the groundwater where it is retrieved again for treatment in the topsoil.

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.

Assessing the impact of dairy waste lagoons on groundwater quality using a spatial analysis of vadose zone and groundwater information in a coastal phreatic aquifer

Dairy waste lagoons are considered to be point sources of groundwater contamination by chloride (Cl(-)), different nitrogen-species and pathogens/microorganisms. The objective of this work is to introduce a methodology to assess the past and future impacts of such lagoons on regional groundwater quality. The method is based on a spatial statistical analysis of Cl(-) and total nitrogen (TN) concentration distributions in the saturated and the vadose (unsaturated) zones. The method provides quantitative data on the relation between the locations of dairy lagoons and the spatial variability in Cl(-) and TN concentrations in groundwater. The method was applied to the Beer-Tuvia region, Israel, where intensive dairy farming has been practiced for over 50 years above the local phreatic aquifer. Mass balance calculations accounted for the various groundwater recharge and abstraction sources and sinks in the entire region. The mass balances showed that despite the small surface area covered by the dairy lagoons in this region (0.8%), leachates from lagoons have contributed 6.0% and 12.6% of the total mass of Cl(-) and TN (mainly as NO3(-)-N) added to the aquifer. The chemical composition of the aquifer and vadose zone water suggested that irrigated agricultural activity in the region is the main contributor of Cl(-) and TN to the groundwater. A low spatial correlation between the Cl(-) and NO3(-)-N concentrations in the groundwater and the on-land location of the dairy farms strengthened this assumption, despite the dairy waste lagoon being a point source for groundwater contamination by Cl(-) and NO3(-)-N. Mass balance calculations, for the vadose zone of the entire region, indicated that drying of the lagoons would decrease the regional groundwater salinization process (11% of the total Cl(-) load is stored under lagoons). A more considerable reduction in the groundwater contamination by NO3(-)-N is expected (25% of the NO3(-)-N load is stored under lagoons). Results demonstrate that analyzing vadose zone and groundwater data by spatial statistical analysis methods can significantly contribute to the understanding of the relations between groundwater contaminating sources, and to assessing appropriate remediation steps.