The Chloride/Sulfate Mass Ratio or CSMR is a measure of corrosion potential developed for the case of galvanic corrosion of Pb-soldered joints in copper and brass plumbing components. It has been extended to other situations as a general indicator of corrosion potential. See Edwards and Triantafyllidou 2007 for a general discussion)

The parameter is the simple ratio of chloride to sulfate in mass units:

CSMR = (Cl^- mg/L)/(SO₄^2- mg/L)

The reasoning behind this formulation is that Pb solubility is greatly increased by the presence of dissolved chloride, but elevated sulfate causes the precipitation of an insoluble lead sulfate, anglesite. This mineral, unlike most Pb compounds, is impervious to acid attack, so has the potential to be overlooked in analytical procedures such as ICP that require dissolving the scale.

The US Geological Survey has made an extensive study of groundwater compositions in which the Langelier Index and the CSMR were calculated (Belitz et al. 2016). They recommended the following categories of corrosion potential for the CSMR:

Belitz, K., Jurgens, B.C. and Johnson, T.D., 2016. Potential corrosivity of untreated groundwater in the United States (No. 2016-5092). US Geological Survey.

https://pubs.er.usgs.gov/publication/sir20165092

Distribution of CSMR values among a selection of U.S. distribution systems. (Full size)

The figure above shows a cross-plot of the two parameters used by the USGS in evaluating the CSMR index. Almost all US distribution systems fall in the "moderate" corrosion potential category.

Edwards, M. and Triantafyllidou, S., 2007. Chloride‐to‐sulfate mass ratio and lead leaching to water. Journal‐American Water Works Association, 99(7), pp.96-109.

The CSMR is not related to the amount of Pb measured in LCR sampling for a set of 20 US drinking water utilities, but does show a weak postive correlation to Cu. (click on the image for larger version)