Articles on the subject of interpreting data with nondetects.
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Helsel, D. R. (2006). Fabricating Data: How substituting values for nondetects can ruin results, and what can be done about it. Chemosphere 65 (11), 2434-2439.

Helsel, Dennis R., (2005) Insider Censoring: distortion of data with nondetects. Human and Ecological Risk Assessment 11, pp. 1127-1137.

Lee, Lopaka, and Helsel, Dennis (2005) Statistical analysis of water-quality data containing multiple detection limits: S-language software for regression on order statistics, Computers & Geosciences 31, 1241-1248

Helsel, D.R., (2005). More Than Obvious: Better methods for interpreting nondetect data. Environmental Science and Technol. 39 (20), p. 419A–423A.

Lee, L, and D. R. Helsel (2005). Baseline models of trace elements in drinking water of the United States. Applied Geochemistry 20, 1560-1570.

Helsel, Dennis R. (1990). Less Than Obvious: Statistical Treatment of Data Below the Detection Limit, Environmental Science and Technology 24(12), p. 1766-1774.

Helsel, Dennis R., and Cohn, Timothy A. (1988). Estimation of Descriptive Statistics for Multiply Censored Water Quality Data, Water Resources Research 24(12), p. 1997-2004, December 1988.

Gilliom, Robert J., and Helsel, Dennis R. (1986). Estimation of distributional parameters for censored trace-level water-quality data. I. Estimation Techniques, Water Resources Research, v. 22, 135-146.

Helsel, Dennis R., and Gilliom, Robert J. (1986). Estimation of distributional parameters for censored trace-level water-quality data. II: Verification and Applications, Water Resources Research, v. 22, p. 147-155.