For the six States and the District of Colombia surrounding the Chesapeake Bay area the TMDL’s will now be mandatory (Chapter 5.2) (Perez, Cox and Cook, 2009). In the TMDL process modelling and monitoring can play an important part in allocating pollutant loads to various sources, such as helping to determine the relative contributions of arable crops, intensive livestock operations and urban sources to loads of nutrients and pathogens observed in large water catchments (Helmers et al, 2007). The resulting classification for each water body is reported to the European Commission (Johnes, 2007). The ecological status ranges which can range from “high” to “bad”, is determined by a combination of biological quality elements (aquatic flora, benthic invertebrate fauna and fish fauna) and physico-chemical quality elements (such as oxygenation conditions, nutrient conditions, salinity, as well as specific pollutants).