Environmental Monitoring and Assessment (2006) 116: 81–90 DOI: 10.1007/s10661-006-7228-y c  Springer 2006 COMPARISON OF NITRATE LEVELS IN RAW WATER AND FINISHED WATER FROM HISTORICAL MONITORING DATA ON IOWA MUNICIPAL DRINKING WATERSUPPLIES PETER J. WEYER 1,* , BRIAN J. SMITH 2 , ZHEN-FANG FENG 1 , JIJI R. KANTAMNENI 1 and DAVID G. RILEY 1 1 Center for Health Effects of Environmental Contamination, The University of Iowa, 100 Oakdale Campus, N203 OH, Iowa City, IA 52242-5000, USA; 2 Department of Biostatistics, College of Public Health, The University of Iowa ( * author for correspondence, e-mail: peter-weyer@uiowa.edu) (Received 20 August 2004; accepted 12 May 2005) Abstract. Nitrate contamination of water sources is a concern where large amounts of nitrogen fertilizers are regularly applied to soils. Ingested nitrate from dietary sources and drinking water can be converted to nitrite and ultimately to N-nitroso compounds, many of which are known carcinogens. Epidemiologic studies of drinking water nitrate and cancer report mixed findings; a criticism is the use of nitrate concentrations from retrospective drinking water data to assign exposure levels. Residential point-of-use nitrate data are scarce; gaps in historical data for municipal supply finished water hamper exposure classification efforts. We used generalized linear regression models to estimate and compare historical raw water and finished water nitrate levels (1960s–1990s) in single source Iowa municipal supplies to determine whether raw water monitoring data could supplement finished water data to improve exposure assessment. Comparison of raw water and finished water samples (same sampling date) showed a significant difference in nitrate levels in municipalities using rivers; municipalities using other surface water or alluvial groundwater had no difference in nitrate levels. A regional aggregation of alluvial groundwater municipalities was constructed based on results from a previous study showing regional differences in nitrate contamination of private wells; results from this analysis were mixed, dependent upon region and decade. These analyses demonstrate using historical raw water nitrate monitoring data to supplement finished water data for exposure assessment is appropriate for individual Iowa municipal supplies using alluvial groundwater, lakes or reservoirs. Using alluvial raw water data on a regional basis is dependent on region and decade. Keywords: cancer risk, exposure assessment, historical nitrate data, Iowa municipal water supplies 1. Introduction Nitrate (NO 3 ) contamination of water sources is a concern in areas where large amounts of nitrogen fertilizers are regularly applied to soils (Burkart and Stoner, 2002). The human health impacts of long-term exposure to nitrate in drinking water derived from impacted water sources are unclear. Epidemiologic investigations of nitrate in drinking water and cancer risk have mixed results (Weyer, 2001). For example, recent studies in Iowa (US Midwest) have reported conflicting findings on the risk for bladder cancer (Weyer et al., 2001; Ward et al., 2003). A criticism of drinking water nitrate studies is the use of nitrate concentrations from retrospective