1. ^aNew York State Department of Health, Troy, New York, USA
2. ^bNational Center for Environmental Assessment, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina, USA
3. ^cNew York City Department of Environmental Protection, 59-17 Junction Boulevard, Flushing, New York, USA
4. ^dDepartment of Environmental and Radiological Health Sciences, Colorado State University, Fort Collins, Colorado, USA

Correspondence: Wayne Richter, New York State Department of Health, Bureau of Water Supply Protection, 547 River Street, Troy, NY 12180, USA. Tel.: +1 518 402 7712; Fax: +1 518 402 7799; E-mail: wxr04@health.state.ny.us

Received 5 March 2007; Accepted 20 March 2008; Published online 28 May 2008.

Abstract

Epidemiological studies of the relationship between disinfection byproducts (DBPs) and adverse birth outcomes often use a single quarterly sample result to characterize an exposure period during a pregnancy. Concentrations of trihalomethanes (THMs), a frequently studied class of DBPs, can fluctuate considerably between sampling periods so that a single point measurement in time may not adequately characterize levels over an exposure period. In addition to obtaining compliance samples that are required quarterly at a limited number of sampling sites, the New York City water supply system takes monthly THM samples at a large number of sites. We used these monthly data to investigate two methods for interpolating between quarterly measurements: cubic splines and linear interpolation. We based interpolations on 1 month in each calendar quarter and checked the interpolated values against the actual data values from the other 2 months of the quarter. Both methods produced generally acceptable fits: 90% of the observed discrepancies between fitted and actual values were less than 14 g/l and over half were less than 5 g/l for total trihalomethanes (THM4). These numbers are about half the change in THM4 level from one quarter to the next and compare favorably with the range of 0–80 g/l or more used in many studies. Comparable results were obtained for chloroform and bromodichloromethane, the two THMs with levels high enough to analyze. Linear interpolation generally performed better than the spline fit, producing lower average discrepancies and fewer large discrepancies. Linear interpolation can be a useful and easily applied method for developing improved exposure estimates using quarterly sampling data.