Polydimethylsiloxane (PDMS) is a widely used silicone polymer that is intro
duced into wastewater treatment systems where it is removed with sludge. PD
MS subsequently enters the terrestial environment as a result of sludge ame
ndment to soil. Laboratory studies have shown that PDMS extensively breaks
down into monomeric units when in contact with dry soils. The byproducts of
hydrolysis eventually biodegrade or evaporate. The objective of this study
was to develop a computer model that can predict the degree of PDMS breakd
own based on level and duration of soil drying under different climatic con
ditions. The framework of the model was the SHAW (Simultaneous Heat and Wat
er) model that predicts daily water content distribution in soil over the c
ourse of a year. The soil water contents predicted from the SHAW model were
then linked to PDMS degradation rate data for Various soils to predict soi
l and climate impacts on PDMS losses. Field testing of the model at Columbu
s, OH showed that the model was able to predict the general trends in PDMS
degradation over 2 years. Predicted PDMS concentrations remaining in the 0-
10 cm depth 2 years after sludge addition were 19.8 mg/kg of soil compared
to the measured values of 23.0 mg/ kg of soil. The sensitivity analysis of
the model showed that >95% of PDMS degraded at the soil surface in Bayamon
sandy clay loam (San Juan, PR), Miamian loam (Columbus, OH), and Wedowee sa
ndy clay loam (Athens, Ga) soils within 365 days after application. However
in some years, >50% of applied PDMS was still remaining at 2.5-cm depth 36
5 days after its application. at any given day, there was less PDMS remaini
ng in soil at San Juan, PR, and at Athens, GA, than at Columbus, OH. This i
s because of (1) higher rates of PDMS degradation in Bayamon and Wedowee so
ils than in Miamian soil and (2) better soil drying conditions in Puerto Ri
co and Georgia than in Ohio.