title: Evaluating changes in matrix-based, recovery-adjusted concentrations in paired data for pesticides in groundwater.
abstract: Pesticide concentration data for waters from selected carbonate-rock aquifers in agricultural areas of Pennsylvania were collected in 1993-2009 for occurrence and distribution assessments. A set of 30 wells was visited once in 1993-1995 and again in 2008-2009 to assess concentration changes. The data include censored matched pairs (nondetections of a compound in one or both samples of a pair). A potentially improved approach for assessing concentration changes is presented where (i) concentrations are adjusted with models of matrix-spike recovery and (ii) area-wide temporal change is tested by use of the paired Prentice-Wilcoxon (PPW) statistical test. The PPW results for atrazine, simazine, metolachlor, prometon, and an atrazine degradate, deethylatrazine (DEA), are compared using recovery-adjusted and unadjusted concentrations. Results for adjusted compared with unadjusted concentrations in 2008-2009 compared with 1993-1995 were similar for atrazine and simazine (significant decrease; 95% confidence level) and metolachlor (no change) but differed for DEA (adjusted, decrease; unadjusted, increase) and prometon (adjusted, decrease; unadjusted, no change). The PPW results were different on recovery-adjusted compared with unadjusted concentrations. Not accounting for variability in recovery can mask a true change, misidentify a change when no true change exists, or assign a direction opposite of the true change in concentration that resulted from matrix influences on extraction and laboratory method performance. However, matrix-based models of recovery derived from a laboratory performance dataset from multiple studies for national assessment, as used herein, rather than time- and study-specific recoveries may introduce uncertainty in recovery adjustments for individual samples that should be considered in assessing change.
authors: U.S. Geological Survey, Pennsylvania Water Science Center, New Cumberland, PA, USA. tmzimmer@usgs.gov
authors: Tammy M
authors: TM
authors: Zimmerman
authors: Kevin J
authors: KJ
authors: Breen
mesh_terms: D004784:Environmental Monitoring; D060587:Groundwater / Q000737:chemistry*; D010414:Pennsylvania; D010575:Pesticides / Q000737:chemistry*; D013997:Time Factors; D014874:Water Pollutants, Chemical / Q000737:chemistry*
chemical_list: D010575:Pesticides; D014874:Water Pollutants, Chemical