Slow sand filters are used in rural regions where source water may be subjected to antimicrobial contaminant loads from waste discharges and diffuse pollution. A numerical model (LETA) was derived to calculate aqueous antimicrobial concentrations through time and depth of a slow sand filter and estimate accumulating contaminant mass in the schmutzdecke. Input parameters include water quality variables easily quantified by water system personnel and published adsorption, partitioning, and degradation coefficients. Simulation results for the tetracycline, quinolone, and macrolide classes of antimicrobials suggested greater than 3-log removal from 1 microg/L influent concentrations within the top 40 cm of the sand column, with schmutzdecke antimicrobial concentrations comparable to other land-applied waste biosolids. A 60-day challenge experiment injecting 1 microg/L tylosin to a pilot slow sand filter showed an average 0.1mg/kg of the antimicrobial remaining in the schmutzdecke layer normally removed during filter maintenance, and this value was the same order of magnitude as the sorbed concentration predicted by the LETA model.