Flow‐covariate prediction of stream pesticide concentrations

Potential peak functions (e.g., maximum rolling averages over a given duration) of annual pesticide concentrations in the aquatic environment are important exposure parameters (or target quantities) for ecological risk assessments. These target quantities require accurate concentration estimates on nonsampled days in a monitoring program. We examined stream flow as a covariate via universal kriging to improve predictions of maximum m ‐day ( m  = 1, 7, 14, 30, 60) rolling averages and the 95th percentiles of atrazine concentration in streams where data were collected every 7 or 14 d. The universal kriging predictions were evaluated against the target quantities calculated directly from the daily (or near daily) measured atrazine concentration at 32 sites (89 site‐yr) as part of the Atrazine Ecological Monitoring Program in the US corn belt region (2008–2013) and 4 sites (62 site‐yr) in Ohio by the National Center for Water Quality Research (1993–2008). Because stream flow data are strongly skewed to the right, 3 transformations of the flow covariate were considered: log transformation, short‐term flow anomaly, and normalized Box‐Cox transformation. The normalized Box‐Cox transformation resulted in predictions of the target quantities that were comparable to those obtained from log‐linear interpolation (i.e., linear interpolation on the log scale) for 7‐d sampling. However, the predictions appeared to be negatively affected by variability in regression coefficient estimates across different sample realizations of the concentration time series. Therefore, revised models incorporating seasonal covariates and partially or fully constrained regression parameters were investigated, and they were found to provide much improved predictions in comparison with those from log‐linear interpolation for all rolling average measures. Environ Toxicol Chem 2018;37:260–273. © 2017 SETAC