Document 0Jj2V5K6rp753r9DOnpV4enRx

AR226-2252 Mark H. Russell Modeling and Environmental Risk Assessment DuPont Crop Protection January 31,2002 1. Background The leaching potential o fmobile and persistent chemicals was evaluated using the USEPA Pesticide Root Zone Model (PRZM 3.12b) using soil and weather data representing the conditions along the Ohio River in Washington County, OH. PRZM is a one-dimensional, dynamic, compartmental model that can he used to simulate chemical movement-in unsataated soil systems within and immediately below the plant root zone. This daily tim e step model has two major components: hydrology and chemical transport The hydrologic companent for calculating runoffand erosion is based on the USDA Soil Conservation Service curve number technique and the Universal Soil Loss Equation. Infiltration is simulated by the use o f generalized soil parameters, f in d in g field capacity, w ilting point and saturation water content. Chemical transport is represented using a convection-dispersion equation. The goal ofthis modeling effort is to estimate potential chemical concentrations in shallow groundwater following continuous application o f a mobile and persistent chemical to the land surface. This work w ill permit estimation o fpotential groundwater concentrations as a function o f chemical application rate (kg/ha/yr) and the rateof chemical degradation in soiL 2. Model m puts an d Assum ptions........................ ............... ........... ............... The PRZM model requires four types of inputs in order to perform a leaching simulation: Chemical data Soil profile data , ' Crop / agronomic data Climatic data The following values were used forthe various required input data: . 2.1 Chemical data Applnrate: For modeling purposes, it is necessary to specify the chemical ^ application rate to soil (mass/area/tune). The base case application assumed in this modeling work is 0.1 kg/ha/yr. Since the model CAB000134 EID546983 DRAFT cannot handle a continuous application, this loading is simulated as a series o f48 quarterly applications o f0.025 kg/ha, resulting m a semi continuous application of chemical to the soil. Tins application frequency was selected because PR2M can only handle 50 applications and it is necessary to apply the chemical over an ten d ed period oftime to accurately simulate potential accumulation o f chemical residues in the soil profile over time. The 48 quarterly ' applications resultin twelve years o fsemi-continuous exposure. For pgtimatmg groundwater concentrations resulting from other application rates, it is important to note that the simulated concentrations are directly proportional to the applicationrate. Sorption: K oc= 2 5 ml/g (an assumed value, representative o f a highly mobile chemical) Halflife in soil: assumed values - 1 yr, 3 yr, 5 yr and 10 yr. ^ _ Uniform degradation was assumed throughout the soil profile. Typically, chemicals which are microbiaUy degraded demonstrate slow er rates o f degradation w ith depth. For this sim ulation, smce tire degradation rates w ere already relatively slow , the additonm ^ 2.2 Soilprofile data . Information from geologic boring logs indicates the follow in g basic statigraphy in file area o f interest: n ^ th interval ffft 0 -1 4 14 --27 27 --34 34 --60 Textural description brown clay brown clayey sand brown sand brown sandy gravel The typical depth to shallow groundwater at the site o f interest is 15 feet, imiiVating that the soil profile should generally be characterized as having a high day content The USEPA soils database (DBAPE, based on Soils 5 soil l ^ e f f , searched for high clay content soils in Washington County OH and the data for a Licking silt loam underlain by a high clay layer was selected for uaem simulations. Based on the data in DBAPE and the PRZM manual, the properties ofthe selected soil profile are as follows: CABO00135 EID546984 DRAFT Table 1: Soil profile data for Licking silt loam (Hydrologic Group C> Soil depth (cm) 0-30 Soil texture silty loam (20/55/25) Bulk density (qfcm3) 1.40 Organic matter (%) 2.5 1/3 bar (FC) 15 bar (WP) moisture moisture (cm3/cm3) (cm3/cm3) 0.33 0.17 30-170 170 340 day (20/30/50) day (20/30/50) 1.50 1.50 1.0 0.7 0.42 0.42 023 023 340+ clay (assumed) (20/30 / 50) 1.50 02 (assumed) (assumed) 0.42 (assumed) 028 (assumed) In general, m ean values were used for the reported soil texture.bu llt d en si^ , 340 cm so the Ihc values o f the 170-340 cm k y er wore used lor the 340+ layer, adjusting the organic matter from 0.7% to 0.2%. A U tin g sat loamis lisfcd a, ahydrologic grow C TMffle h i aMlatively low orgBiicmalte, is usuallyb # '*!/"* has a m oderate rate o f surface runoff (typically 5-15/4) an temuorarily pond water on relatively flat areas. Y ,,n Crop / agronomic data The presence o f perennial grasses and/or brush w as assum ed, resulting in selection o f the follow ing runoff curve numbers: Table 2: Runoff curve numbers selected for grasses / brush Crop condition fallow (winter) cropplnq (spring/ summer) residue (autumn) Runofif curve number 86 ~~~ 80 86 a soil surface rather than mMtrate m e son prom t. " "" 100 with higher numbers representing an increasing tendency numbers o f 80 to 86 are appropriate for a H ydrologic continuously vegetated w ith senescence o f the grasses andbrus and no sm ine cultivation. "n - ter CABOOOl36 EID546985 DRAFT 2.4 Climatic data ,, H ie closest available daily meteorological file is : miles NW o fthe site o f interest in Washington Comity, daily weather parameters for a period o f 36 years, extending from January 1,1957 to December 31 1992, Daily data in the file include precipitation, pan evaporation andmean air temperature. The 36-year-average preapitation for to s sitete 1052 mm which agrees reasonably well w ith o te 1ns^ ; a^ | ^ the area (Parkersburg, WV: 976 mm, 1941-1970; Charleston, WV. 1035 mm, 1941-1970). 2.5 Calibration o f hydrology Based on the soil properties, the assumed curve numbers and theclim atic data, . two simulation cases were considered for the Washington County site, a "normal" recharge case and a "low" recharge case. Annual average value simulated over a period o f 36 years Input Precipitation; 1052 mm (~ 41 in) Output Rimofi: .^ X R ech a rg e:. -y Evapotranspiration: Change in water storage: "normal" recharge case 113 mm (11% of precip) 272 mm (26% o fpredp) 643 mm (61% o fpredp) 24m m ( 2% o fpredp) "low" recharge case 113 mm (11%) 190 mm (18%) 726 mm (69%) 23m m ( 2%) 3. Simulation, results The commons application o f a mobile, persistent chemical was TM 33 a series o f repetitive quarterly applications extending over a period oftw elve years, ^ s i t i n g in a total o f48 discrete application events. This approach was taken becaus PRZMcan onlyhandle a maximum of50 application evento. The sim ulation^ a semi continuous application o f chemical over an extended period o ftime permits th estimation of long-term average concentrations in shallow groundwater. The groundwater concentration at a depth o f 5 m (equivalent to ~16 ft) was simulated by flux o f chemical moving past this depth by the simulated concentration represents the concentration m sod pore water at a dep* o f 5 SanTa iss eqmvv<Sucmt ow tuh*e cem en__ta_t_ion that_i_s_bem--gloaded onto theitsolpoaodfetodeinstoorftihceial w d fto c o n ^ tr S o L n JaS re d in mortitormg wells with relatively short screens (U m) that intersect the top o f the local water table. . CAB000137 EID546986 DRAFT Two concentration -values have been calculated for groundwater: the Kghest concentration simulated at a depth o f 5 m (the peak concentration) and foe highest long term average value (a five-year-average concentration). Both values aredirectly proportional to foe assumed annual application rate o f 0.1 kg/ha. , resulting fiom alternative application rates can easilybe estimated forougti linear adjustment offoe reported concentrations. Table 3- Simulated groundwater concentrations for a mobile, persistent chemicaj in Washington County, OH applied at 0.1 Jcg/ha/yrfor 12 consecutive yearn ("normal" recharge case) Degradation half-life ivoarst 1 3 5 10 Peak concentration at 5 m depth (uqfl-1 0.11 5.2 - 11.6 22.0 Highest 5-yr-average cone at 5 m depth ..... <ug/L) _ 0.067 4.4 10.4 20.4 . The simulated concentration in shallow groundwater (5 m) is a function offoe foJf-H e of S e a l infoe soilprofile. Continuous application o f 0.1 k ^h a o f anm bde chemical with a relatively slow rate o f degradation in sod (i.e. a soil degradationhalf-life of5-lU years) can result in groundwater concentrations o fapproximately 10-20 ug/L. Less persistent chemicals are simulated to have significantly lower groundwater concentrations. lY./K^ting that foe simulated application for a period of 12 years provided a reasonable estimate of a longer-term, steady-state concentration. Over a period o f five years, the groundwater recharge to rn an application site ^ d d result in approximately 5 * 360mm or 1800 mm o frecharge, which would approxmmtely occupy foe top 3.6 m offoe surficial aquifer (assuming. a 5-yr-average concentration is 1 _ _ monitoring w ell w hich provides sam ples o f surficial groundwater. For reference, foe sim ulated results reported in Table 3 are also plotted in J 11"8 figure clearly show s foe sim ilarity betw een foe daily peak concentration andfoe 5-yr- average concentration. To a first approximation, the groundwater concentration is proportional to foe chem ical half-life. EID546987 Figure 1: Simulated-concentration of a mobile, persistent chemical In shallow groundwater in Washington, OH . Lower rates recharge t^ r e sd tm lower chemicals since this situation provides increased residence t o e m , .e relativeiy and additional tim e for degradation. Howevever,for ^ ^ re sid e n c e slowly (i.e. those w ith half-lives greater than 5 5*)**' S ^t n a^g ro im d w ater recharge, as shown in Table 4. Table 4: SWimasuhlaintegdtognroCuonudnwtya,teOrHcounsicnegnttwraotiorencshfaorrgae tmnditlons normal ana . recharge (same application rate sequence as in Tame jj Groundwater Recharge Low Annual recharge 272 (26%) 180 (18%) Chemical Peak concentration half-life in soil in groundwater (vearsl fuofU 1 0.11 10 22.0 1, 0.023 10 2S.7 5-yr-ave cone in groundwater (ugA.) 20.4 23.6 CABO 00139 EXD546988 DRAFT 4. Conclusions Continuous application equivalent to 0.1 kg/yr o f a mobile, persistent chemical can result in reHifftntratirYnR 0f 1 to 20 ug/L in shallow groundwater in a setting with a clay soil profile and 300-400 mm o fgroundwater recharge pear year. The gmmlated concentration in groundwateris a distinct function o fthe half-life o fthe chemical in the soil profile^ with longer half-lives resulting in higher concentrations. Lower rates o frecharge will reduce the predicted concentrations for rapidly degrading chemicals but will slightly increase simulated concentrations for chemicals which degrade slowly. The simulated concentrations are directly proportional to the application rate. All concentrations shown in this report correspond to an annual application rate o f 0.1 kgha. 5. References Carsel, RJ?., J.C. Tnihnff, PR. Hummel, JM . Cheplick, and A S. Donigian, Jr., 1996. `TRZM3, A Model for Predicting Pesticide and Nitrogen Fate in the Crop Root and Unsaturated Soil Zones: Users Manual for Release 3.0", USEPA, Center for Exposure Assessment Modeling. CAB000140 EID546989