PCA variables that must NOT be log-transformed before the chemistry PCA

Every other PCA variable is concentration-like — strictly positive and strongly right-skewed, spanning orders of magnitude — so it is log10-transformed before centring/scaling. Without that, the PCA is dominated by a handful of high-magnitude major ions (e.g. Cl, SO4, TDS) and the leading axes mostly track absolute ionic strength rather than the multiplicative covariance structure that drives metal/organic behaviour.

Usage

.PCA_NO_LOG_VARS

Format

An object of class character of length 4.

Details

The exclusions are the variables for which a log is meaningless or undefined:

• pH — already a logarithmic scale (−log10 of H+ activity).

• temperature — interval scale (°C); zero/negative values are valid.

• ORP — redox potential (mV); routinely negative.

• DO — dissolved oxygen (mg/L); legitimately ~0 in anoxic leachate plumes and only spans a narrow, near-linear range.