Length of domain Lx=Ly=200 m . Nodal spacing, x=y= 2 m .

1. INFLUENCE OF UNCERTAINTY IN LEAK LOCATION ON DETECTION OF CONTAMINANT PLUMES RELEASED AT LANDFILL SITES Contaminants are introduced in groundwater by planned human activities rather than by the natural phenomena. Landfills, represent a widespread and significant threat to groundwater quality. Therefore monitoring well networks at these sites are of vital importance in detecting plumes. However, it is often difficult to ensure a specific network which will detect all of the contaminants released from the landfill because of the numerous significant uncertainties that are involved. GOALS • To examine the influence of uncertainties due to subsurface heterogeneity and contaminant leak location on detection probability. • To investigate the influence of dispersivity of medium on detection probability. • To examine influence of number of wells in monitoring system on detection probability. • To analyze the effect of the initial contaminant source size on detection probability. HYPOTHETICAL MODEL • Length of domain Lx=Ly=200 m. • Nodal spacing, x=y= 2 m. • Steady state groundwater flow model is used. • Random walk transport model is used. • Y=ln(K) modelled as a Gaussian stationary distribution. • Variance of ln(K), Y2=0, 1 and 2 and x= y =5 m. • A Monte Carlo based approach is used to generate random leak locations. • Random leak locations follow a uniform distribution. • Failure is modelled as a point and a small arealsource (2 m x 2 m). • L=0 m, T=0 m; L=0.5 m, T=0.05 m; L=1.5 m, T=0.15 m. • Contaminants are assumed to be conservative. A plan view of model domain with selected single row monitoring system. RESULTS OF ANALYSIS Influence of dispersivity of medium on individual monitoring wells for point contaminant source case. Influence of hydraulic conductivity variance on monitoring systems composed of 3, 5 and 10 wells for point contaminant source case. Influence of dispersivity of medium on monitoring systems composed of 3, 5 and 10 wells for areal contaminant source case (2Y =0). Influence of hydraulic conductivity variance on monitoring systems composed of 3, 5 and 10 wells for areal contaminant source case. CONCLUSIONS • The detection probability of contaminant plumes released from a landfill highly depends on size of the plume, subsurface heterogeneity, and numberof wells in a monitoring system. • The probability of detection increases as the dispersivity of medium increases. • The probability of detection increases as the initial size of the contaminant source increases. • The probability of detection shows tendency to decrease as the subsurface heterogeneity increases. • The detection probability of the monitoring systems increases as the number of wells in the system increases. • The efficiency of 3- well system of current practice, particularly in medium with relatively low dispersivity, is quite low.