Liying Wang (Florida State University, firstname.lastname@example.org)
Ming Ye (Florida State University, email@example.com)
Paul Z. Lee (Florida Department of Environmental Protection, firstname.lastname@example.org)
Richard W. Hicks (Florida Department of Environmental Protection, Richard.email@example.com)
What does it do?
Nitrate, as a commonly identified groundwater and surface-water pollutant, is associated with a number of adverse health and environmental impacts. One major source of nitrate in the environment is due to wastewater treatment using Onsite Sewage Treatment and Disposal Systems (OSTDS) (a.k.a., septic systems), whose effluent is directly discharged into soils and subsequently groundwater. Therefore, it is important to simulate nitrogen transformation and transport in the vadose zone and groundwater due to septic systems. We have developed an ArcGIS-based Nitrate Load Estimation Toolkit (ArcNLET) to simulate nitrate fate and transport in groundwater and to estimate nitrate load from septic systems to surface water bodies such as lakes and rivers (read the ArcNLET blog). In this blog, we introduce VZMOD, Vadose Zone MODel, a recently developed software for simulating nitrogen transformation and transport in the vadose zone. VZMOD can be used as a pre-processor of ArcNLET to provide source plane concentrations of individual septic systems from the vadose zone to groundwater.
VZMOD is based on ArcGIS 10.0 and developed using the Python programming language. The software simulates
- one-dimensional (1-D) unsaturated, steady-state flow in the vertical direction,
- nitrification process for transformation of ammonium to nitrate and denitrification process for transformation of nitrate to nitrogen gas, and
- steady-state transport of ammonium and nitrate.
The end-product of this model is vertical profiles of ammonium and nitrate concentrations for either one, or multiple septic systems.
What ArcGIS input files does it need?
The software can be used for the following four modeling scenarios:
- a single septic system,
- multiple septic systems with heterogeneous hydraulic conductivity and porosity (other parameters being homogeneous),
- multiple septic systems with heterogeneous depth to water table (other parameters being homogeneous), and
- multiple septic systems with heterogeneous hydraulic conductivity, porosity and depth to water table, i.e., combination of scenario (2) and (3).
When running VZMOD for the latter three modeling scenarios, functions of ArcGIS 10.0 are used to handle spatial variability of septic system locations, hydraulic conductivity, porosity, and depth between the drainfield of septic systems to the water table.
When running VZMOD for scenario (1) with a single septic system, you do not need to prepare any ArcGIS layers. In the graphic user interface (GUI) of VZMOD shown below, you select the type of soil below the drainfield of the septic system, and VZMOD activates literature values of a number of input parameters specific to the soil type (such as soil hydraulic parameters and parameters related to nitrification and denitrification processes). You may adjust these values when site-specific measurements are available.
When running VZMOD for multiple septic systems with heterogeneous hydraulic conductivity and porosity (scenario (2)), in addition to selecting the soil type, you need to check the boxes of “Multiple sources” and “Heterogeneous Ks and ?s” in the GUI, which will activate three boxes for inputting three ArcGIS layers:
- the ArcGIS point layer of septic system locations,
- raster layers of hydraulic conductivity and
- raster layers of porosity.
When running VZMOD for multiple septic systems with heterogeneous depth to water table (scenario (3)), in addition to selecting the soil type, you need to check the boxes of “Multiple sources” and “Calculate depth to water table” in the GUI, which will activate three boxes for inputting three ArcGIS layers:
- the ArcGIS point layer of septic system locations
- a raster layer of digital elevation model (DEM), and
- a raster layer of smoothed DEM that represents the shape of water table and can be obtained by running Groundwater module of ArcNLET.
Since scenario (4) is a combination of scenarios (2) and (3), you need to check all three boxes for inputting all five ArcGIS layers for scenarios (2) and (3). This is the most complicated modeling scenario that VZMOD can handle.
Example Scenario: Using VZMOD to simulate spatial distribution of ammonium and nitrate concentration
VZMOD was used to estimate spatial distribution of ammonium and nitrate concentration in the Julington Creek neighborhood located in Jacksonville, FL, USA, where nitrate due to septic systems is believed to be one of the reasons for nutrient enrichment. We present below the modeling procedure and results for this neighborhood under modeling scenario 4. (The data used for this example is available on the VZMOD webpage.)
Step 1: Determine soil types and prepare ArcGIS files for septic system locations
The first step in VZMOD is to determine soil types of the modeling domain. As shown in Figure 1 above, VZMOD considers twelve soil types based on soil texture information that can be extracted from the Soil Survey Geographic (SSURGO) database available at http://soildatamart.nrcs.usda.gov/. The clay, silt, and sand contents of SSURGO horizons are first aggregated to the component level, and subsequently components are aggregated to the map unit level. Thickness of horizons and percentage of components are used as weights during the aggregation. The clay, silt and sand percentages at the map unit level are used to determine soil types. As shown in the figure below for Julington Creek, the soil type for most of the septic systems is sand; only three septic systems are located in soil of type sandy loam. You thus need to prepare two ArcGIS point layer files of septic system locations for the two soil types. The GUI above shows the ArcGIS layer file for the sand soil.
Step 2: Prepare four more ArcGIS input files
Next, prepare four ArcGIS raster files for heterogeneous hydraulic conductivity, soil porosity, DEM, and smoothed DEM for the entire modeling domain. (The former three files are also input files of ArcNLET, and the file of smoothed DEM is an output file of the groundwater flow module of ArcNLET.) The figure above shows the delineation of the soil units for the Julington Creek area; within each unit, the hydraulic conductivity and soil porosity (labeled in red and purple, respectively) are homogeneous. The raster files (hydrau_con.img for hydraulic conductivity and porosity.img for porosity) are generated from the SSURGO database. The LiDAR DEM file (DEM.IMG) is used in the Julington Creek modeling, and it is the background of the figure above. The LiDAR DEM has a horizontal resolution of 5 × 5 ft2. If a LiDAR DEM is not available, alternative DEM raster files can be used. One alternative is the NED, which is a DEM raster file with horizontal resolution of 1/3 arc seconds. The smoothed DEM (smoothedDEM.img) is generated by running the Groundwater flow module of ArcNLET (more details can be found at the Esri Hydro blog).
Step 3: Run VZMOD and post-process modeling results
After selecting the soil type and inputting all the ArcGIS files, specify the output folder to save the modeling results and then click the Run button shown in the VZMOD GUI to run VZMOD for each soil type. Information of the model run is written to the rightmost window of the VZMOD GUI.
The vertical profiles of ammonium and nitrate concentrations in the vadose zone can be plotted by clicking the “Check Results” button. You can view the profiles of different septic systems by changing the FID of the septic systems and then clicking the “OK” button. The figure below shows the profiles for two septic systems located in sand (FID 100) and sandy loam (FID 391). The profiles show that during the downward transport, ammonium concentration decreases while nitrate concentration increases because ammonium becomes nitrate due to nitrification. The process of nitrification is faster in sandy loam (FID 391) than in sand (FID 100) because of the different degrees of saturation for the two soil types.
The simulated ammonium and nitrate concentrations are saved in the default output file “results.txt” located in the output folder specified by the users at the bottom of VZMOD GUI. As shown in the figure below for a small segment of the output file, the data are organized by septic systems. You can use this file for more post-processing of the modeling results.
VZMOD also directly generates ArcGIS files for visualization and post-processing. The figure below plots the simulated nitrate concentrations at the water table for the entire modeling domain. This also updates the ArcGIS file of septic systems used as a VZMOD input file by creating a new field in the attribute table to save the simulated nitrate concentrations. The updated ArcGIS file can be used directly for ArcNLET modeling, because the simulated nitrate concentrations are the source plane concentrations used as inputs of ArcNLET.
The site provides the user’s manual, sample data and even technical support of VZMOD!
Special thanks to Ming Ye for providing this post. Questions for Ming: firstname.lastname@example.org