Modelling Zonal Compliance

The calibration of the zonal compliance model was an iterative process that involved matching predicted compliance LCR data to that observed by the Utility, and it is important to put this into perspective ... 

Presently, too little data has been provided to be able to calibrate the zonal compliance model to the extent possible for Cities A and B. However, a speculative exercise has been undertaken, based on the pipe-work characteristics of Cities A and B and assuming plumbosolvency factors ranging from Af= 0.03 to 0.05 and = 45 to 75. These plumbosolvency factors reflect (i) the lead concentrations observed in the sequential sampling surveys, and (ii) reference to extensive UK data from laboratory testing for low orthophosphate doses to moderately high alkalinity waters. 

The models, which are described in more detail elsewhere (Van der Leer et al, 2002), enable the most relevant features of a water supply zone to be incorporated in the prediction of zonal compliance with lead standards, as a function of boA plumbosolvency (corrosivity of the water to lead) and the zone s physical characteristics. A zonal model simulates the emissions of lead at individual simulated houses, through time, across an entire water supply zone or area of supply. It uses a single pipe model to determine the lead emission profile at each simulated house, the characteristics of each simulated house being the outcome of the random ascription of variables, which follows the Monte Carlo method for establishing a probabilistic firame-work. 

Table 10.2 Use of zonal model to predict RDT sampling compliance for a range of plumbosolvency reductions when 50% houses have a lead pipe...

From 1999 to 2002, 17.5% of random daytime samples had lead >10 pg/1 and 7.9% >25 pg/1. Laboratory plumbosolvency testing gave an average lead concentration after 30 minutes contact of around 50 pg/1 Zonal modelling indicated that an average ortho-phosphate dose in the range 0.6 to 0.9 mg/1 (P) should achieve >98% compliance with the fiiture lead standard of 10 pg/1. 

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