Pesticide in a well-mixed water body is increased through addition of mass in inflow, resuspension and diffusion from the sediment layer. The amount of pesticide in a well-mixed water body is reduced through removal in outflow, degradation, volatilization, settling and diffusion into the underlying sediment.
Pesticide is removed from the water body in outflow. The amount of dissolved and particulate pesticide removed from the water body in outflow is:
pstsol,oβ=QβVFdββpstlkwtrββ 8:4.1.14
pstsorb,oβ=QβVFpββpstlkwtrββ 8:4.1.15
where pstsol,oβ is the amount of dissolved pesticide removed via outflow (mg pst), pstsorb,oβ is the amount of particulate pesticide removed via outflow (mg pst), Q is the rate of outflow from the water body (m3 H2βO/day), Fdβ is the fraction of total pesticide in the dissolved phase, Fpβ is the fraction of total pesticide in the particulate phase, pstlkwtrβ is the amount of pesticide in the water (mg pst), and V is the volume of water in the water body (m3 H2βO).
Table 8:4-1: SWAT+ input variables that pesticide partitioning.
Pesticides will partition into particulate and dissolved forms. The fraction of pesticide in each phase is a function of the pesticideβs partition coefficient and the water bodyβs suspended solid concentration:
8:4.1.1
8:4.1.2
where is the fraction of total pesticide in the dissolved phase, is the fraction of total pesticide in the particulate phase, is the pesticide partition coefficient (m/g), and is the concentration of suspended solids in the water (g/m).
The pesticide partition coefficient can be estimated from the octanol-water partition coefficient (Chapra, 1997):
Pesticide in the dissolved phase is available for volatilization. The amount of pesticide removed from the water via volatilization is:
8:4.1.8
where is the amount of pesticide removed via volatilization (mg pst), is the volatilization mass-transfer coefficient (m/day), is the surface area of the water body (m), is the fraction of total pesticide in the dissolved phase, is the amount of pesticide in the water (mg pst), and V is the volume of water in the water body(m HO).
The volatilization mass-transfer coefficient can be calculated based on Whitmanβs two-film or two-resistance theory (Whitman, 1923; Lewis and Whitman, 1924 as described in Chapra, 1997). While the main body of the gas and liquid phases are assumed to be well-mixed and homogenous, the two-film theory assumes that a substance moving between the two phases encounters maximum resistance in two laminar boundary layers where transfer is a function of molecular diffusion. In this type of system the transfer coefficient or velocity is:
Pesticide in the particulate phase may be removed from the water layer by settling. Settling transfers pesticide from the water to the sediment layer. The amount of pesticide that is removed from the water via settling is:
8:4.1.13
where is the amount of pesticide removed from the water due to settling (mg pst), is the settling velocity (m/day), is the surface area of the water body (m), is the fraction of total pesticide in the particulate phase, is the amount of pesticide in the water (mg pst), and . is the volume of water in the water body (m HO).
Pesticides in both the particulate and dissolved forms are subject to degradation. The amount of pesticide that is removed from the water via degradation is:
8:4.1.6
where is the amount of pesticide removed from the water via degradation (mg pst), is the rate constant for degradation or removal of pesticide in the water (1/day), and is the amount of pesticide in the water at the beginning of the day (mg pst). The rate constant is related to the aqueous half-life:
8:4.1.7
where is the rate constant for degradation or removal of pesticide in the water (1/day), and
Kdβ=3.085β10β8βKowβ 8:4.1.3
where Kdβ is the pesticide partition coefficient (m3/g) and Kowβ is the pesticideβs octanol-water partition coefficient (mg moctanolβ3β(mg mwaterβ3β)β1). Values for the octanol-water partition coefficient have been published for many chemicals. If a published value cannot be found, it can be estimated from solubility (Chapra, 1997):
log(Kowβ)=5.00β0.670βlog(pstsolβ²β) 8:4.1.4
where pstsolβ²β is the pesticide solubility (ΞΌmoles/L). The solubility in these units is calculated:
pstsolβ²β=MWpstsolβββ103 8:4.1.5
where pstsolβ²β is the pesticide solubility (ΞΌmoles/L), pstsolβ is the pesticide solubility (mg/L) and MW is the molecular weight (g/mole).
vvβ=KlββHeβ+RβTKββ(Klβ/Kgβ)Heββ 8:4.1.9
where vvβ is the volatilization mass-transfer coefficient (m/day), Klβ is the mass-transfer velocity in the liquid laminar layer (m/day), Kgβ is the mass-transfer velocity in the gaseous laminar layer (m/day), Heβ is Henryβs constant (atm m3 moleβ1), R is the universal gas constant (8.206 β 10β5 atm m3 (K mole)β1), and TKβ is the temperature (K).
For lakes, the transfer coefficients are estimated using a stagnant film approach:
Klβ=zlβDlββ Kgβ=zgβDgββ 8:4.1.10
where Klβ is the mass-transfer velocity in the liquid laminar layer (m/day), Kgβ is the mass-transfer velocity in the gaseous laminar layer (m/day), Dlβ is the liquid molecular diffusion coefficient (m2/day), Dgβ is the gas molecular diffusion coefficient (m2/day), zlβ is the thickness of the liquid film (m), and zgβ is the thickness of the gas film (m).
Alternatively, the transfer coefficients can be estimated with the equations:
Klβ=Kl,O2βββ(MW32β)0.25 8:4.1.11
Kgβ=168βΞΌwββ(MW18β)0.25 8:4.1.12
where Klβ is the mass-transfer velocity in the liquid laminar layer (m/day), Kgβ is the mass-transfer velocity in the gaseous laminar layer (m/day), Kl,O2ββ is the oxygen transfer coefficient (m/day), MW is the molecular weight of the compound, and ΞΌwβ is the wind speed (m/s). Chapra (1997) lists several different equations that can be used to calculate Kl,O2ββ.
: Pesticide settling velocity (m/day)
.lwq
LKPST_KOC
Kdβ: Pesticide partition coefficient (m3/g)
.lwq
LKPST_REA
kp,aqβ: Rate constant for degradation or removal of pesticide in the water (1/day)
.lwq
LKPST_VOL
vvβ: Volatilization mass-transfer coefficient (m/day)
.lwq
LKPST_STL