Determination of dispersivity in the subsurface foreshores of River Mersey Outer Estuary

This paper determines contaminant dispersivity in the coastal sand of River Mersey Outer Estuary using results of field measurement from dye solute injection. Dispersivity is an alternative parameter for describing the characteristic behaviour of porous media. In this case, 3-D plumes in 2-D Photo-images were taken from multiple injection zones across the foreshore. The colour images were calibrated, standardized and the optical intensity converted into concentration maps. Apparent dispersivity of the subsurface foreshore was graphically determined from spatial covariance tensors. The variances were obtained by subtracting the products of the zeroth moments and square of the mean location of mass centers from the second order moments, and results were divided by the zeroth moment. The variance tensors (�xʹxʹ2, and �yʹyʹ2) were plotted against their respective travel distance intervals. The result predicts linear increase of the variance of spread distance, enhancing estimate of dispersivity using the best fit relations and for the x-x and y-y planes. The dispersivity coefficient varied with increase in travel distance. It quantifies the rate of spreading of the mean concentration of the tracer front which shows dependence on the distance traveled by the front. The effect on the mean concentration at the injection point is therefore described by the area of the effective dispersivity coefficient controls.