TRANSPORT OF BROMIDE, SIMAZINE, AND MS-2 COLIPHAGE IN A LYSIMETER CONTAINING UNDISTURBED, UNSATURATED SOIL

The effect of rate-limited adsorption on transport of environmental co ntaminants is difficult to characterize at the field scale. This study investigated transport, during unsaturated water flow, of pulse input s of bromide, simazine (2-chloro-4,6-bis(ethylamino)-s-triazine), and MS-2 coliphage in a field lysimeter (0.8 m x 0.8 m square) containing undisturbed Tujunga loamy sand (mixed, thermic, Typic Xeropsamment). S ixty-four fiberglass wick soil solution samplers collected drainage fr actions from the exit surface (30 cm depth) following daily 2-cm water inputs applied at 0.5 cm h(-1). After 19.7 cm of cumulative drainage, the soil above 10 of the 64 locations was sampled to determine final depth distributions of simazine and virus. Most of the bromide was lea ched from the transport volume, while the sorbing pesticide and virus remained in the soil. Variance analysis indicated that local dispersio n processes contributed more to the observed bromide spreading than di d differences in local water velocities. A linear, first-order, kineti c adsorption submodel was incorporated into a generalized linear trans port model relating the bromide flux concentrations to the simazine an d virus final resident concentrations. Least squares fitting showed th at area-averaged bromide transport could be described reasonably well by the two-parameter convection-dispersion model (CDM), although the m obile-immobile water model provided a slightly better representation o f effluent tailing. The CDM parameters fitted to the bromide data were then held constant while the two parameters of the adsorption submode l were varied to fit the pesticide soil concentrations at the end of t he experiment at 10 days. A good fit was obtained for simazine, and th e fitted value 0.54 d(-1) of the rate coefficient was in the range cha racterizing nonequilibrium adsorption. A batch adsorption/desorption e xperiment produced Freundlich isotherms describing nonlinear adsorptio n (exponent m 0.85) and hysteresis in desorption. There was poor agree ment between the retardation factor (R) estimated from a linearized ba tch distribution coefficient K-d and the R fitted to lysimeter data. V irus concentrations fitted to the model yielded coefficients implying strong adsorption (R = 254) and rapid inactivation (inactivation rate coefficient of 1.64 d(-1)), whereas the laboratory sorption study impl ied that the virus should be very mobile in soil. The difference in fi eld and laboratory sorption may be due to air-water interfacial forces in the unsaturated field experiments.