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AR226-2252 f^w^i (O\il\we /(/4jte(Ei7iC&w~) 7-?-? - ^o<x< ^ DRAFT Modeling of the Leaching Potential of MoM Mark R Russell Modeling and Environmental Risk Assessment DuPont Crop Protection January 31,2002 I. Background The leaching potential ofmobile and persistent chemicals was evaluated using the USEPA Pesticide Root Zone Model (PRZM 3.12b) using soil and weather data representing fee 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 unsateated soil systems within and immediately below the plant root zone. This daily time 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 of generalized soil parameters, including field capacity, wilting point and saturation water content. Chemical transport is represented using a convection-dispersion equation. The goal of this modeling effort is to estimate potential chemical concentrations in. shaBow groundwater following continuous application, of a mobile and persistent chemical to the land surface. This work will permit estimation ofpotential groundwater concentrations as a function of chemical application rate (kg/ha/yr) and me rateof chemical degradation in soiL 2; Model inputs ^dAssompfioBir - -.-----.- .......----.--. - -.--..-.- ----.-.-.. 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 for the various required input data: 2.1 Chemical data Appin rate: For modeling purposes, it is necessary to specify the chemical application rate to soil (mass/area/time)- 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 of 48 quarterly applications of6.025 kg/ha, resulting in a semicontixsuons application of chemical to the soil. This application frequency was selected because PRZM can. only handle 50 applications and it is necessary to apply the chemical over an extended period of time to accurately simulate potential accumulation of chemical residues in the soil profile over time. The 48 quarterly ' applicationsresult in twelve years of semi-continuous exposure. For estimating groundwater concentrations resulting from other application rates, it is important to note that fee simulated concentrations are directly proportionalto me applicationrate. Sorption: Koc ^ 25 ml/g (an assumed value, representativeof a highly mobile chemical) Halflifeinsoil: assumed values-- 1 yr, 3 yr, 5 yr and 10 yr. Uniform degradation was assumed mroughotit the sofl profile. Typically, chemicals which are miCTobially degraded demonstrate slower rates of degradation with derpetlha.tiFveolry'(shliosws,imthuelaatdiodnit,iosninacle me degradation rates were already effect of me degradation rate decreasing with depth was neglected. 2.2 Soil profile data Information from geologic boring logs indicates the following basic statigraphy in . the area of interest: Depm interval (ft) 0--14 14 -- 27 27 - 34 34--60 Textnral description brown clay brown clayey sand brown sand brown sandy gravel The typical depth to shallow groTindwater at me sitcehoarfainctteerriezsetdisas15hafeveint,g a high indicating that the soil profile should generally be day content The USEPA soils database (DBAPE, based on Soils 5 soil profile data) was searched for high clay content soils in Washington County OH and me data for a Licking silt loam underlain by a high clay layer was selected for nse in simulations. Based on the data in DBAPE and the PRZM manual, me properties of the selected soil profile are as follows: CAB000135 EID546984 DRAFT Table 1: Soil profile data for Licking silt loam (Hydrologic Group C) Soil depth (cm) Soil texture (sand/sitt/clav%) Bulk density (g/cm3) Organic matter <%) 10 bar (PC) moisture (cm3/cm3) 15bar(WP) moisture (cm3/cm3) 0-30 silty loam 1.40 2.5 0.33 (20/55/25) 0.17 30-170 day 1.50 1.0 0.42 028 (20/30/50) 170- day 1.50 0.7 0.42 340 (20/30/50) 0.28 340+ clay (assumed) (20/30/50) 1.50 (assumed) OS. (assumed) 0.42 (assumed) 0.28 (assumed) In general, mean values were used for fhe reported soil texture, bulk density, organic matter and water contents. Soil data was not available beyond a depth, of 340cm so the fhe values of fhe 170-340 cm layer were used for <he 340+ layer, adjusting fhe organic matter from 0.7% to 0-2%. A licking silt loam is listed as a hydrologic group C soil which implies that this profile has a>relatively low organic matter, is Tisually high in clay and has a " minimum infiltration rate of0.1-0.4ctD/hr under saturated conditions. This soil has a moderate rate of surface nmoff (typically 5-15%) and is likely to temporarily pond water on relatively flat areas. 2.3 Crop / agronomic data The presence of perennial grasses and/or brush was assumed, resulting in selection of the following nmoff curve numbers: Table 2: Rimoff curve numbers selected for grasses / brush Crop condition fallow (winter) cropping (spring/summer) residue (autumn) Runoff curve number 86 80 86 Curve numbers represent fhe tendency ofprecipitation to horizontally rurioff from a soil surface rather than infiltrate the soil profile. These numbers range from 0 to 100 with higher numbers representing an increasing tendency to nmoff. Curve numbers of 80 to 86 are appropriate for a Hydrologic Group C soil that is continuously vegetated with senescence of the grasses and brush over fhe winter and no spring cultivation. CAB000136 EID546985 DRAFT 2.4 Climatic data The closest available daily meteorological file is from McConnelsvffle, OH (~ 30 Washington County, OH). This ffle consists of mdailielys -NweWathoefrtphaersaimteeotefrisntfeorreaspt iendod of 36 years, extending irom January 1,1957 to December 31,1992. Daily data in me file include precipitation, pan precipitation for tins evaporation and mean ah: temperature. The 36-year-average tshitee aisre1a0(5P2armkemrswbuhrigc,hWagVre:es97re6ansuonn,ab1l9y4w1-e1l9l 7w0i;thCohtharelrelsotonng,-tWerVm:av1e0r3a5gemsmfo,r 1941-1970). 2.5 Calibration of hydrology Based on the soil properties,the assumed curve numbers and the climatic data, two simulation cases were considered fer the Washington County site: a . "normal" recharge case and a'low" recharge case. Annual average value simulated over a period of 36 years Input Precipitation: 1052 r^m (~ 41 in) Output ^XfRunoffi ^^LRecharge:. y Evapotranspiration: Change in water storage: "normal" recharge case 113mm(ll%ofpredp) 272mm(26%ofpredp) 643mm(61%ofprecip) 24 mm ( 2% ofpredp) 'low" recharge case 113 mm (11%) 190 mm (18%) 726 mm (69%) 23 m-m ( 2%) 3. Simulation, results The continous application of a mobile, persistent chemical was simulated, in PKZM as a sedes of repetitive quarterly appliacpaptiloicnasteioxnteenvdenintsg. oTvheirsa appeprriooadcohfwtwaseltvaekeyneabresc, ause rPeRsuZlMtincgainn oa.ntloytahlaondf4le8admiscarxeitme um of 50 application events. The simulation of a semicontinuous application of chemical over an extended period of time permits the estimation of long-term average concentrations in shaDow groundwater. The groundwater concentration at a depth of 5 m (equivalent to ~16 ft) was simulated by The flux of chemical moving past this depth by the infiltration volume. at a depth of 5 m dividing fee mass simulated concentration represents fee concentration in soil pore water loaded onto the top of the surficial and is equivalent to the concentration that is being, is loaded into the aquifer at me study rite. When a relatively uniform concentration period of time, this concentration will correspond reasonably aquifer well to ova: an extended concentrations measured in monitoring wells with relatively short screens (i.e. ~2 m) that intersect the top of the local water table. , CAB000137 EID546986 DRAFT Two concentration -values have been calculated for groundwater; the highest daily the highest 1 concentration shmflated at a depth of 5 mco(fneceenpteraaktiocno)n.cBenottrhatvioanlu)easnadre directly tperrompoarvteiorangael tvoaflhuee a(sasfuivmee-ydeaanr-nauvaelraapgpelication, rate of 0.1 kg/ha. Groundwater concentrations resulting fiom alternative application rates can easily be estimatedthrough linear adjustment ofthe reported concentrations. Table 3: Simulated groundwaler concentrations for a mobile, persistent chemical In Washington County, OH applied at 0.1 hg/ha/yrfor 12 consecutive years ("normal" recharge case) Degradation half-life (years) 1 3 .5 10 Peak concentration at 5 m depth (ug/U 0.11 5.2 11.6 22.0 Highest 5-yr-average cone at 5m depth (ug/U 0.067 4.4 10.4 20.4 The simulated concentration in shallow grouanpdpwlaicteartio(5nmof) 0is.1akfgu/nhcatioofnaomf tohbeilheaclfh-elimfeicoafl twhiethchaermeliactailvienlythseloswoirlapteroofifled.egCraodnatitniounouins soil (i.e. a soil degradationhalf-life of 5-10 years) can result in groundwater concentrations of approximately 10-20'ug/L. Less persistent chemicals are simulated to have significantly lower groundwater concentrations. The peak concentration and fee five-year-average concentration, are reasonably similar indicating that fhe simulated application for a period of 12 years provided a reasonable estimate of a longer-term, steady-state concentration. Over a pearpiopdrooxfimfiavteeylyea5rs* , 3th60engnrnouonrd1w8a0t0ermremc.hoafrgreecfhraormgea, nwahpicphlicwaotiuolndsaiptepwrooxuimldately the result in occupy the top 3.6 m ofthe surficial aquifer (assuming. 50% porosity). As a result, 5-yr-average concentration is likely to correspond to the concentration measured in a monitoring well which provides samples of surficial groundwater. For reference, fhe simulated results reported in Table 3 are also plotted in Figure 1. This similarity between me dafly peak concentration and fhe 5-yr- faivgeurraegcelceoanrlcyesnhtroawtisonfh. e To a first approximation, the groundwater concentration is proportional to the chemical half-life. CAB000138 EID546987 DRAFT Figure 1: 3 5 7 Chemical half-life in soil (years) Simulateci-concentration at a mobile, persistent chemical in shallow groundwater in Washington, OH . . Lower rates of recharge can result in lower concentrations &r more rapidly degrading chemicals since Qas situation provides increased, residence time in fhe unsaturated zone and additional time for degradation. Howevever, for chemicals which degrade relatively slowly (i.e. those with half-lives greater than 5 years), 1he effect of increased residence time is outweighed by the reduced recharge volume. As a result, estimated groundwater concentrations for slowly degrading chemicals are slightly higher under conditions of low recharge, as shown in Table 4. Table 4: Simulated groundwater concentrations for a mobile, persistent chemical In Washington County, OH using two recharge conditions, "normal" and "low" recharge (same application rate sequence as In Table 3) Groundwater Recharge Typical Low Annual ' recharge (mm) 272 (26%) 190(18%) Chemical half-life In soil (years) 1 10 1 10 Peak concentration in groundwater (ugfl.) 0.11 22.0 0.023 25.7 5-yr-ave cone in groundwater (ugA.) 0.067 20.4 0.012 23.6 CAB000139 EID546988 DRAFT 4. Conclusions Continuous application equivalent to 0.1 kg/yr of a mobile, persistent chemical can result in concentrations of 1 to 20 ug/L in shallow ground-water in a setting with a clay sou profile and 300-400 mm ofgroimdwater recharge per year. The simulated concentration is groundwater is a distinct function offhe half-life of the chemical in the soil profile, with longer half-lives resulting in higher concentrations. Lower rates ofrecharge wfll reduce the predicted, concentrations for rapidly degrading chemicals hut 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 of 0.1 kg/ha. 5. References Carsel, RJ?., J.C. WbaS, P-R. Hummel, JM. Cheplick, and A.S. Domgian, Jr., 1996. "PRZM3, 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