Pollution risk assessment based on QUAL2E-UNCAS simulations of a tropical river in Northern India Richa Babbar Received: 18 August 2013 /Accepted: 18 June 2014 /Published online: 3 July 2014 # Springer International Publishing Switzerland 2014 Abstract This paper exemplifies the application of U.S. Environmental Protection Agency's water quality model, QUAL2E-UNCAS in assessing the pollution risk of a tropical river. The rivers selected for study were Hindon (main river) and Kali (its tributary) flowing through Uttar Pradesh district of Northern India. The model application to the two rivers revealed poor water quality in terms of dissolved oxygen (DO), biochemical oxygen demand (BOD), and ammonia concentrations. Monte Carlo simulations were performed on two differ- ent data sets that were confirming to marked seasonal variations. The Monte Carlo simulation (MCS) derived 95 % confidence level for these parameters strengthened the fact that all point sources were exploiting the assim- ilative capacity of the two rivers. In order to ascertain probabilistically the risk at which two rivers were falling short of desired water quality, probability curves based on effluent standards and available water quality were prepared. On mapping the two curves, it was found that at 95 % probability, Hindon River was flowing with 53 to 100 % less of desired DO, up to 100 % more of minimum BOD, and probability with which ammonia concentration would not be more than the desired con- centration was found to fall downstream. The Kali headwaters showed better quality during low river tem- perature but worsened downstream with up to 100 % violation in all the above observed parameters. It is expected that similar studies wherein the dependable levels with which a polluted river can be understood to fall short of desired water quality can prove to be useful in ascertaining the efficacy of effluent standards and/or follow-up of pollution control measures. Keywords Water quality modeling . Monte Carlo simulation . Effluent standards . Confidence level . Probability curves Introduction In this study, U.S. Environmental Protection Agency's water quality model, QUAL2E-UNCAS has been used to evaluate and perform risk analysis for highly pollution-impaired Hindon River system flowing through Northern India. QUAL2E-UNCAS is an en- hancement to QUAL2E model, which allows user to perform uncertainty analysis along with water quality simulation. The QUAL2E, as a stand-alone simulation model, is a one-dimensional, steady state, deterministic model that can simulate up to 15 water quality constit- uents in any combination desired by the user. In its prototype representation, the model allows for multiple waste discharges, abstractions, tributaries, and non- point inflows applicable to dendritic streams that are well mixed. This model has an established record of being one of the most comprehensive water quality model used time and again for regulatory as well as policy decision-making purpose (USEPA 1995; Barnwell et al. 2004). The core of the model has not changed since 1987 (Brown and Barnwell Jr 1987), but Environ Monit Assess (2014) 186:6771–6787 DOI 10.1007/s10661-014-3888-1 R. Babbar (*) Department of Civil Engineering, Thapar University, Patiala, Panjab, India e-mail: richa.babbar@gmail.com