Model and Scenario Development Guidance for Estimating Pesticide Concentrations in Groundwater Using the Pesticide Root Zone Model
December 20, 2012
U.S. Environmental Protection Agency, Office of Pesticide Programs, Environmental Fate and Effects Division
Health Canada, Pesticide Management Regulatory Agency, Environmental Assessment Directorate
On this Page
- Groundwater Conceptual Model
- Implementation of the Conceptual Model into PRZM
- Selection of Scenario Locations
- Groundwater Scenario Development Guidance for PRZM
- Appendix 1. Guidance on Setting up Degradation in the Profile
This guidance describes the methods for developing scenarios for the PRZM-GW model. PRZM-GW is an implementation of the Pesticide Root Zone Model (Suarez, 2006) in which a pesticide moves downward into an aquifer according to the NAFTA conceptual model (see below). Scenarios are the subset of input parameters that describe the spatial characteristics of the PRZM-GW parameters. These include weather, soil, and crop characteristics.
Groundwater Conceptual Model
Figure 1 depicts the conceptualization of the groundwater scenario that evolved from meetings between the USEPA and Canada's PMRA and from the 2005 N-Methyl Carbamate SAP meetings (USEPA, 2005a, 2005b). The model represents vulnerable private drinking water wells in the vicinity of agricultural environments. In this conceptualization, the pesticide is applied to the soil surface or plant canopy, and precipitation or irrigation drive pesticide into the soil. Meteorological, crop, biological, chemical, and management processes affect the transport of the pesticide as it moves through the soil and into a saturated zone. Horizontal surface movement of a pesticide (via runoff or erosion) and subsequent removal is assumed negligible for this model.
The saturated zone of the conceptual model is a shallow unconfined aquifer with a water table depth that corresponds to the scenario location. The well-screen extends from the aquifer surface to 1 meter below the surface, but this length is also adjustable according to common practices. The default well-screen length of 1 meter was chosen to sample the higher concentrations expected to be closest to the water table; however, this can be adjusted based on site-specific data. Pesticide concentrations in the well are taken as the average concentration in the screened zone.
The conceptual model includes meteorological events that can significantly affect pesticide transport, including precipitation, evaporation, snow, temperature, and wind. These weather processes vary and are simulated with daily resolution. Daily resolution is required by risk assessment applications within OPP's and PMRA's Health Effects Division.
Soil temperature variations with depth and time have important implications on transport and are thus included in the model. Temperature simulations on and below the field are more important for groundwater simulations than for surface water simulations. In a groundwater simulation, the entire duration of the simulation occurs on or below the field; whereas in a surface water simulation the relevant pesticide is in the field's zone of relevance (top 2 cm) for much shorter periods and will be less likely to be in that surface zone during periods of extreme cold when temperature effects will be significant. It is likely that in many scenarios temperature in the subsurface could be quite different from surface temperatures or from the laboratory experiment temperature of the degradation experiments.
Crop descriptions are needed only in regard to how they impact hydrology (i.e., water balance) or pesticide transport. In most cases, parameterization of pesticide hold up and wash off of foliage will be difficult due to lack of data, thus prudence should be considered in attempting to simulate these processes. A description of canopy coverage and root depth is incorporated to capture the most relevant plant influences on pesticide transport.
Agricultural management practices that could dramatically affect pesticide transport such as irrigation are included in the conceptual model. Because irrigation provides significant amounts of water and is an important transport driver, irrigation is included in those areas where it is used. Simulation of other management practices such as soil manipulation, tile drainage, and pesticide application methods may be desirable but are not considered in the present conceptualization because there is a lack of consensus regarding their importance to transport. Such processes may be considered in subsequent versions if more evidence of their impact becomes available.
The conceptual model includes consideration for pesticide degradation in the soil profile. In general, degradation occurs faster in the top of the profile and decreases at lower depths. As depicted in Figure 1 (by the black line on the left of the figure), the pesticide fate conceptualization uses the pesticide's aerobic soil degradation rate in the top 10 cm, and a linearly declining rate with depth to 1 meter, below which only abiotic processes are assumed to occur. In some cases, there may be evidence that certain pesticides behave differently than this depiction. The conceptual model does not preclude alternative degradation schemes when there is compelling evidence that a pesticide may behave differently than the default conceptualization.
Implementation of the Conceptual Model into PRZM
PRZM is a one-dimensional, finite-difference vertical-transport model that accounts for pesticide fate. PRZM can simulate specific site, pesticide, and management properties, including field properties (organic matter, water holding capacity, bulk density, general hydrology), pesticide application parameters (application rate, frequency, incorporation, and methods), and pesticide environmental fate and transport properties (aerobic soil metabolism half-life, soil-water partitioning coefficients, foliar degradation and dissipation). PRZM is the standard EPA model for simulating transport of pesticides off of a field and into a surface water body.
In order to use PRZM to simulate transport to groundwater rather than to surface water, environmental scenarios need to be developed to represent the conceptual model. These scenarios will differ from surface water scenarios in particular for a few key parameters, including field capacity, irrigation, dispersion, near surface degradation, the non-consideration of erosion and runoff, temperature simulation, and relevant output. The basis for these changes is described below. Detailed guidance for selection of these and other parameters is included in the section—Groundwater Scenario Development Guidance.
Although PRZM was not originally developed to simulate saturated conditions, a review of the model's structure and coding showed that saturated conditions can be effectively simulated by redefining the field capacity parameter. Soil compartments in PRZM are maintained at "field capacity" unless losses occur by evapotranspiration. Below the zone of evapotranspiration (which occurs at the bottom of the defined root zone or at a minimum defined evaporation depth), PRZM maintains the water content at "field capacity." Thus, a saturated zone can be created in PRZM by setting the scenario's field capacity parameter (called THEFC in PRZM) equal to the porosity. By doing this, a constant water table can be simulated (Figure 2).
PRZM-GW output concentrations are calculated from the spatial average over the depth from the top of the saturated zone to the bottom of the well screen (1 meter default). Depending on precipitation and evapotranspiration characteristics of the particular scenario, this spatial averaging represents the temporal average of six months to greater than one year (i.e., 1 meter of aquifer can hold six months of water from the FL scenarios to greater than a year for eastern Washington scenarios; scenarios described later). Note that during this time period, degradation within the aquifer still occurs, and thus this conceptualization is more realistic with regard to concentrations that may be pulled from a well than the temporal averaging described by the FOCUS group, in which post-breakthrough degradation does not occur.
In addition, analysis of prospective groundwater (PGW) studies indicates that actual water retention in the soil profile is better characterized by a value between field capacity and saturation (Young and Carleton, 2012). Therefore, the soil profile water holding capacity (field capacity in PRZM) should be set to a value that is the midpoint between field capacity and saturation. In this way, the travel time to the aquifer will be better simulated.
In Figure 2, note the unique adjustments to PRZM groundwater scenarios: "field capacity" is set to porosity to simulate a saturated zone; well-screen of 1 meter is the default length.
Irrigation may be an important contributor to vertical pesticide transport and in many scenarios may be the dominant water source. When a scenario specifies irrigation, it is handled according to OPP guidance on PRZM irrigation (USEPA/OPP, 2007), which modifies the guidance provided in the PRZM manual. PRZM irrigation guidance that appears in the PRZM manual may be inappropriate for deeply rooted plants such as orchard trees. PRZM attempts to satisfy water demand to the bottom of the root zone, whereas in reality water demand would only be satisfied as a result of irrigation to several centimeters depth. Thus too much irrigation would occur unless preventative measures are taken. This should be considered in scenario development if irrigation is to be simulated. Guidance is provided for this case later in the parameter selection section of this document.
Dispersion (whether numerical or explicitly modeled) has an especially important impact on pesticide concentrations when there is a limited degradation zone that applies only over a short distance, such as in this groundwater conceptual model. In these cases, dispersion will quickly remove the pesticide from the degradation zone into areas of low or no degradation. Thus, higher dispersion leads to higher pesticide concentrations, the opposite of what would be expected for a conservative non-degrading tracer. This characteristic is analogous to the efficiency difference between plug flow and completely mixed reactors, which is well known in chemical and environmental engineering fields. Thus, it is important to standardize the discretization within PRZM.
In PRZM, the vertical discretization is the dominant means of applying dispersion, which is caused by PRZM's use of backwards spatial differencing for the pesticide transport finite differencing. Although backward differencing is effective for elimination of oscillation at boundaries, it produces high but quantifiable numerical dispersion. Additional numerical dispersion also results from the fully implicit discretization of the temporal term (1 to 2 cm), but users have no control over this characteristic (Young and Carleton, 2012). For a backward finite difference, dispersivity is equal to one half of the spatial discretization.
EFED evaluation of PGW studies shows dispersivity values much greater than the 2.5 cm used by the European Union, as described in the FOCUS guidance. In an analysis of bromide tracers in PGW studies [see Figure 3], dispersivity increased with depth and ranged from about 10 cm at 1 m, increasing to around 40 cm at 5 m. In order to capture the dispersivity in the PRZM scenarios, the scenarios use discretization of 1 cm for the top 10 cm. For the next 10 cm, discretizations are 5 cm. From 20 cm to 1 m, discretizations are 20 cm. The standard scenarios use discretizations of 50 cm for all depths below 1 m, but the PRZM-GW user interface allows other discretizations if the user has justification for alternate schemes.
Near Surface Degradation
Aerobic soil metabolism is modeled as declining with depth and becomes zero at 1 m below the surface. Because typically submitted soil degradation studies do not allow differentiation of water-phase from solid-phase degradation, only total system degradation is typically known. Thus the rates of both solid phase and water phase degradation should be set identically and should decline at the same rate with depth.
For the groundwater conceptualization, this can cause some difficulties when hydrolysis is significant, especially when the degradation rate in water drops below the hydrolysis rate, creating apparent inconsistencies. In order to overcome this problem, the following recommendations apply:
both solid and aqueous degradation rates (represented initially by aerobic soil metabolism rate) are to decline in a linear fashion with depth;
if the aqueous degradation rate drops below the hydrolysis rate, then the hydrolysis rate will be used for the aqueous degradation rate, while the solid degradation rate remains unaffected by the hydrolysis rate and continues to decline with depth.
An example of the use of these recommendations is given in Figure 4. Figure 4 shows that the aqueous-phase degradation rate does not decline below the hydrolysis rate, while the sorbed-phase rate continues to decline.
In this case, the hydrolysis rate is 0.15 times the aerobic soil metabolism rate. Hypothetical partitioning conditions with the compound distributed in water and solids is the ratio of 2.8:1
The temperature routine in PRZM allows for heat transfer between the bottom boundary of the simulation (set at a constant temperature) and the soil surface. The temperature routine requires that solar radiation be included in the meteorological file. Solar radiation has not previously been included in the surface water meteorological files; nevertheless, all current weather files have included solar radiation. Thus, standard weather files will work correctly. There are a few other parameters that are needed to run the temperature routine, which are new to the standard scenario development (e.g., albedo, emissivity), and which are not readily available for specific scenarios. Without further specific information regarding this parameterization, typical values should be chosen as specified in the guidance below.
Selection of Scenario Locations
Appropriate scenario locations should be created in regions that are vulnerable to groundwater contaminations. This includes areas where the depth to groundwater is shallow (<100 feet), where there is high net leaching (or high rainfall with respect to evaporation), and where specific characteristics of the location may make a pesticide more prone to persistence (e.g., high or low pH depending on the pesticide).
Groundwater Scenario Development Guidance for PRZM
Guidance is presented in Table1 on how to develop PRZM scenarios for groundwater estimates. The output of these model runs are contained in the PRZM "zts" file, which will give the daily spatially averaged (vertically over 1 m) aqueous concentration. These values represent high-end drinking water concentrations from a rural well. The scenario file can be constructed in the following manner:
|PRZM Record||Parameter||Input Value||Note|
|3||PFAC - Pan factor||See note||PRZM 3 Manual, Figure 5.9 (Suarez, 2006). Select the value for the specific region based on the location of the crop scenario.|
|SFAC - Snow melt factor (cm/°C day)||See note||For row crops, use 0.36|
For orchard crops, use 0.16
Where snow persists less than a day, use 0.
See PRZM 3 Manual, Table 5.1 (Suarez, 2006).
|IPEIND - Pan factor flag||0||Pan data is read from weather file.|
|ANETD – Minimum Depth to which evaporation is extracted (cm)||See note||PRZM Manual Figure 5.2 (Suarez, 2006). Use the mid-point of the range of values based on the location of the scenario. If a region crosses one or more boundaries, select the average of the midpoints.|
|INICRP - Initial crop flag||1||Indicates no crop before first emergence date.|
|ISCOND - Surface condition of initial crop||1||For groundwater scenarios, this parameter is a placeholder and will not affect the results.|
|DSN - Weather data (5 values)||Leave blank||Specifies reading weather data from sources other than the standard weather files.|
|6||ERFLAG - Erosion flag||0||The erosion routine is not used in the groundwater scenarios.|
|8||NDC - Different crops in a simulation||1||Simulates 1 crop|
|9||ICNCN - Crop number of the different crop||1||Simulates 1 crop|
|CINTCP - Max interception storage of crop (cm)||See note||PRZM 3 Manual (Suarez, 2006) Table 5.4 provides intercept storage for a limited number of crops. For orchard crops, the value ranges from 0.25 to 0.30.|
|AMXDR - Max rooting depth of crop (cm)||See note||This parameter controls depth within the soil profile from which water can be removed to satisfy potential evapotranspiration.|
When irrigation is used, this parameter also controls the amount of water that is supplied during an irrigation event. Enough water will be applied to fill the profile to a depth of AMXDR up to field capacity. Root depth is set to satisfy irrigation requirements in preference to actual root depth. Depth of irrigation can be obtained from agriculture extension agents or extension bulletins.
|COVMAX - Max aerial canopy coverage (%)||See note||Extension agents, USDA crop profiles, or other authoritative sources can provide this information.|
|ICNAH - Surface condition of crop after harvest||1||This parameter is a placeholder as it is not relevant for groundwater scenarios and only affects runoff.|
|CN - Curve number for runoff (3 values)||10||CN is set to the low value of 10 to essentially eliminate runoff.|
|WFMAX - Max dry weight of crop at full canopy (kg/m2)||0||WFMAX is a placeholder since it is not used in groundwater scenarios.|
|HTMAX - Max height of canopy at maturation (cm)||See note||HTMAX, a placeholder for volatilization, is currently not implemented.|
Values for selected crops in PRZM Manual (Carsel et al., 1998) Table 5-16 and Table A-1.
|10||NCPDS - Number of cropping periods||See note||This parameter will be the same as the number of years in the met files.|
|11||EMD/EMM/IYREM - Day, month and year of crop emergence||See note||Sources of information include crop profiles, extension agents, and Table 5.9 in the PRZM manual (Suarez, 2006)|
|MAD/MAM/IYRMAT - Day, month and year of crop maturation||See note||See sources of information above.|
|HAD/HAM/IYRHAR - Day, month and year of crop harvest||See note||See sources of information above.|
|INCROP - Crop number associated with NDC||1||Only one crop is modeled.|
|12||PTITLE - Label for pesticide title||See note||Pesticide specific text notes.|
|13||NAPS - Total number of pesticide applications||See note||Pesticide specific. Typically this value is the number of years times the number of applications per year.|
|NCHEM - number of pesticides in the simulation||1||Simulation for one pesticide only.|
|FRMFLG - Flag for testing ideal moisture conditions for pesticide applications||0||Not used.|
|DKFLG2 - Flag to allow bi-phasic half-life||0||Not used.|
|15||PSTNAM - Pesticide name for output file||See note||Pesticide specific text notes.|
|16||APD/APM/IAPYR - Day, month and year of pesticide application||See note||Pesticide specific.|
|WINDAY - Number of days soil moisture||0||Not used.|
|CAM - Chemical application method||See note||Pesticide specific.|
|DEPI - Depth of pesticide application||See note||Pesticide specific.|
|TAPP - Application rate (kg/ha)||See note||Pesticide specific.|
|APPEFF - Application efficiency to target (fraction)||1.0||The conceptual model for groundwater represents a large area such that any losses out of the system (e.g., spray drift) are negligible.|
|DRFT - Spray drift fraction to pond or reservoir||0.0||Irrelevant to the groundwater scenario.|
|17||FILTRA - Filtration parameter||0.0||This method is not used.|
|IPSCND - Condition for deposition of foliar pesticide after harvest||See note||Assume 1 unless crop is year round evergreen or if specific management practices are being modeled.|
|UPTKF - Plant uptake factor||0||Not used.|
|18||PLVKRT - Pesticide Volatilization decay from plant foliage (days-1)||0||Not used.|
|PLDKRT - Pesticide decay rate on plant foliage (days-1)||0||Not used.|
|FEXTRC - Foliar extraction coefficient for pesticide washoff per cm of rainfall||0.5||Default value is 0.5. In some rare cases, data may be available to better assess this value.|
|19||STITLE - Label for soil property title||See note||User defined text notes.|
|20||CORED - Flag for total depth of soil core (cm)||Scenario specific||The depth to GW is scenario specific. The default is a 10-meter soil profile, with the last meter simulated as a saturated zone.|
USDA Soil Data Mart or Soil Landscapes of Canada are the primary data sources for soil properties for the GW scenarios.
In many cases, soil characterizations will not be available for depth below 1 meter. In these cases, the deepest known profile can be extended using reasonable judgment regarding the physical properties soils (e.g., reductions in organic carbon content with depth).
|BDFLAG - Flag for bulk density||0||Not used.|
|THFLAG - Field capacity||0||Not used.|
|KDFLAG - Soil/pesticide adsorption coefficient||0||Not used.|
|HSWTZ - Drainage flag||0||Not used.|
|MOC - Methods of characteristic||0||Not used.|
|IRFLAG - Irrigation flag||Crop and site specific||Set the flag to 0 for no irrigation, 1 for year-round irrigation, or 2 for irrigation during the cropping period only.|
|ITFLAG - Soil temperature simulation flag||1||Use temperature simulations|
|IDFLAG - Thermal conductivity and heat capacity flag||1||Calculate thermal properties|
|BIOFLAG - Biodegradation flag||0||Not used.|
|26||DAIR - Diffusion coefficient in air||0||Not used.|
|HENRYK – Henry's Law Constant||0||Not used.|
|ENPY - Enthalpy of vaporization||0||Not used.|
Record 27 is used only if IRFLAG (Record 20) = 1 or 2
|IRTYP – Irrigation type||Set to 3 or 4||See USEPA/OPP (2007).|
|FLEACH – leaching factor||See note||See USEPA/OPP (2007).|
|PCDEPL – fraction of water capacity trigger for irrigation||See note||See USEPA/OPP (2007).|
|RATEAP – maximum rate at which irrigation is applied (cm/hr)||See note||See WQTT Advisory Note: Irrigation Guidance for Developing PRZM Scenarios|
|31||ALBEDO||0.20||0.20 is a typical value for crops in the PRZM manual, Table 5.21. Use the same value for all 12 instances of ALBEDO.|
|EMMIS emissivity||0.97||This value is taken from Table 5.22 of the PRZM manual|
|ZWIND wind reference height||10.||Met data is taken at 10 meters.|
|32||BBT groundwater temperature (°C)||See note||Enter temperature of ground at 10 m below surface. Various sources, including USGS, can be used. Enter typical value for all 12 instances.|
If ground temperature data are not available, annual average air temperature from the associated weather file can be used.
|32A||QFAC (Q10 value)||2||Standard assumption in EPA/OPP|
|TBASE (reference temperature of degradation values)||See note||Pesticide specific. Enter the temperature at which the degradation values were derived|
|33||NHORIZ - Total number of horizons||8||USEPA: For the upper horizons, use USDA Soil Data Mart.|
PMRA: Use Soil Landscapes of Canada
Resolution need not be less than 1 cm in the top portion of the profile and not less than 20 cm in the remaining profile.
The top profile is resolved into 1 cm increments in order to allow for accurate applications of pesticides into the soil surface.
Below 20 cm, discretization is increased to 20 cm in order to simulate realistic dispersion.
See Appendix 1 for guidance.
|34||HORIZN - Horizon number||1 to 8||See Appendix 1.|
|THKNS - Horizon thickness (cm)||Fixed||See Appendix 1.|
|BD - Bulk density (g/cm3)||See note||Use the USDA Soil Data Mart or Soil Landscapes of Canada. Use the midpoint of the reported range.|
|THETO - Initial soil water content (cm3/cm3)||See note||Use the field capacity (see below)|
|AD - Soil drainage parameter||0||Not used.|
|DISP - Pesticide hydrodynamic solute dispersion coefficient||0||Dispersion will be simulated by numerical dispersion as implemented through spatial discretization.|
|ADL - Lateral soil drainage parameter||0||Not used.|
|36||DWRATE/DSRATE/DGRATE - Dissolved, adsorbed, and vapor phase pesticide decay rates. (Day-1)||Pesticide specific||See Appendix 1 for guidance on converting laboratory degradation rates to rates that decline with depth. Also see USEPA OPP (2002) for values used for pesticide properties.|
|37||DPN - Thickness of compartments in horizon (cm)||Fixed||See Appendix 1.|
|THEFC - Field capacity in the horizon (cm3/cm3)||See note||For the soil profile, use the 33 kPa water content in the soil data. Input the midpoint between the field capacity and porosity.|
For the saturated zone, set field capacity to porosity.
|THEWP - Wilting point (cm3/cm3)||See note||Use the 15-bar or 1500 kPa water content in the soil data. Use the midpoint of the reported range. This parameter has no effect at depths below the evapotranspiration zone.|
|OC - Organic carbon (%)||See note||Use the organic carbon value, percent converted to a fraction, reported in the soil data. Use a depth-weighted average of all the soil horizons in the PRZM layer.|
|KD - Partition coefficient (ml/g)||See note||This value is chemical specific. See USEPA OPP (2002). If Koc is used, this parameter has no effect. Refer to input parameter guidance.|
|38||SPT (initial temperature of profile)||See note||Use BBT value from Record 32.|
|SAND||See note||Use the USDA Soil Data Mart or Soil Landscapes of Canada. Use the midpoint of the reported range.|
|CLAY||See note||Use the USDA Soil Data Mart or Soil Landscapes of Canada. Use the midpoint of the reported range.|
|40||ILP - Flag for initial pesticide level||0||Assume no prior pesticide applications.|
|CFLAG - Conversion flag for initial pesticide levels||0||Assume no prior pesticide applications.|
|42||Archaic Parameters||Place Holders: Copy from previous run|
|45||Archaic Parameters||Place Holders: Copy from previous run|
|46||PLNAME||DCON||Specifies dissolved (Pore water) concentration|
|INDX||Set to 1||Only 1 chemical is simulated.|
|MODE||TAVE||Specifies vertical spatial averaging|
|IARG||Variable||Set to the number of compartments that represent the depth to the top of the water table.|
|IARG2||Variable||Set to the number of compartments that represent the depth to the bottom of the well screen.|
|Const||1.0e3||Provides conversion to ppb.|
Suarez, L.A., 2006. PRZM-3, A Model for Predicting Pesticide and Nitrogen Fate in the Crop Root and Unsaturated Soil Zones: Users Manual for Release 3.12.2. EPA/600/R-05/111 September 2006, revision a.
USEPA, 2005a. N-Methyl Carbamate Pesticide Cumulative Risk Assessment: Pilot Cumulative Analysis, February 15 - 18, 2005, Docket Number: OPP-2004-0405
USEPA, 2005b. Preliminary N-Methyl Carbamate Cumulative Risk Assessment, August 23 - 26, 2005, Docket Number: OPP-2005-0172
USEPA OPP, 2002. Guidance for Selecting Input Parameters in Modeling the Environmental Fate and Transport of Pesticides, available on the OPP Water Models web page.
USEPA/OPP, 2007. WQTT Advisory Note: Irrigation Guidance for Developing PRZM Scenarios, May 9, 2007.
Young, D.F. and Carleton, J.N. (2012) Estimating Dispersivity, Mass Recovery and Water Hold Up in Field-Scale Leaching Studies by Use of a Capacity Model, Abstract H11B-1173 presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec.
Guidance on Setting up Degradation in the Profile
The PRZM implementation of the groundwater conceptual model uses eight soil layers as shown in Table A1 and also depicted in Figure A1. The first layer is 10 cm and is discretized at 1 cm increments to allow some precision in defining root depths and evaporations zones and pesticide incorporation into the soil. The next layer is 10 cm thick and is discretized at 5 cm increments. The next four layers are all 20 cm and contain one compartment. These first six layers make up the top 1 meter of soil and represent the aerobic degradation zone. Degradation in the top meter declines proportionally with depth after the first 10 cm. Degradation reduction factors are given in the table below. The degradation rate in each layer is to be multiplied by the respective factor from the table. The next layer represents the depth between the aerobic zone and the water table. This layer is also discretized at 50 cm intervals. The eighth layer represents the water table and also has 50 cm discretization. Aerobic degradation does not occur in the seventh and eighth layers; however, hydrolysis is not precluded in these or any other horizons.
|Layer||Multiplication Factor for Aerobic Degradation Rate||Depth (cm)||Δx (cm)||Nodes|
|1||1.0||0 to 10||1||1 to 10|
|2||0.94||10 to 20||5||11 to 12|
|3||0.78||20 to 40||20||13|
|4||0.55||40 to 60||20||14|
|5||0.33||60 to 80||20||15|
|6||0.11||80 to 100||20||16|
|7||0||100 to Water Table Depth||50||17 to x|
|8||0||Water Table Depth to 1000||50||x+1 to x+2|