Stormwater pond efficiency determinations with the effluent probability method : the use of mass versus concentration parameters

Use of the concentration parameter has in the past generally been favoured in stormwater structure efficiency determinations, and specifically with use of the Effluent Probability Method. However, efficiency is by definition related to the amount of substance removed within a stormwater structure, and concentration, being a mathematical construct, cannot be "removed". The purpose of this study was to investigate whether there is substantive proof for the use of substance concentration as a proxy for mass in the Effluent Probability Method to ascertain the amount of substance removed by a structure, i.e. its efficiency. Theoretical considerations and results of data analyses did not support this, and it is therefore recommended that mass, and not concentration, be used in efficiency determinations with the Effluent Probability Method.