In their pure form, chlorinated solvents are dense non-aqueous phase liquids (DNAPLs) that are, by definition, heavier than water. So, plumes of chlorinated solvents dissolved in groundwater must sink, right? I’ve had many conversations over the years where this claim was made, and I always inwardly cringed a bit. Let’s take a closer look at it.
The density of water is a function of its temperature and the amount of dissolved matter it contains. The presence of dissolved substances in groundwater can increase its density under certain conditions. For our discussion, we’ll assume the temperature of shallow groundwater varies seasonally within a narrow range, which allows us to focus on density differences related only to the dissolved load in groundwater.
Laboratories report analytical results in either milligrams per liter (mg/L – the mass of solute per 1 liter of water) or parts per million (ppm – the mass of solute per the mass of 1 liter of water). These same relationships hold for micrograms per liter (µg/L) and parts per billion (ppb). We typically assume the two sets of units are equivalent, which is true for the range of concentrations we usually see during environmental investigations. However, the equivalency between these two unit sets is based on the assumption that 1 liter of water weighs 1 kilogram (K), or unit density. For you geeks, this assumption is strictly true only for pure water at 3.89ºC, but ignoring this level of scientific “purity” doesn’t affect our discussion.
The error introduced by assuming unit density isn’t greater than any other type of analytical errors until the concentration of dissolved matter in water exceeds about 7,000 mg/L1 (for comparison, the dissolved load of naturally occurring inorganic salts in groundwater in Indiana is usually 500 to 1,000 mg/L). For concentrations greater than 7,000 mg/L, density starts becoming a factor, and a correction for density should be applied when converting from mg/L to ppm, or vice versa. We can extend this thought to state that the amount of dissolved matter in a plume of contaminated groundwater must be at least, and probably greater than, 7,000 mg/L before the density contrast between the body of contaminated groundwater (a “plume”) and the surrounding groundwater is sufficient to create a density gradient that could cause the plume to sink or “dive” within the flow system.
Let’s consider the aqueous solubilities (the maximum amount of a substance that can be dissolved in water) of the common chlorinated compounds we deal with every day – tetrachloroethene (PCE), 206 mg/L; trichloroethene (TCE), 1,280 mg/L; and 1,1,1-trichloroethane (TCA), 1,290 mg/L.2 I think you see where this is going. Based on these solubility limits, not enough of any of these compounds can dissolve from a pure-phase product source into groundwater to create a plume that is denser than the surrounding groundwater. Nor does the fact that the solute is derived from a dense chlorinated compound make any difference. To claim otherwise is to believe, for example, that 10 mg of TCE dissolved in 1 liter of groundwater (10 mg/L) is denser than 10 mg of sulfate (or any other solute) dissolved in a liter of groundwater – 10 mg is 10 mg, regardless of the substance.
So, how did groundwater 60 feet directly beneath my dry cleaner site become contaminated with PCE, you may ask? Only a few explanations are plausible. Unless a nearby pumping well has drawn the PCE downward within the flow system, the most likely cause is that PCE in pure-phase product (DNAPL) form has moved downward beneath the site and created a vertical source column that has dissolved into the groundwater. Vertical dispersion may also occur as a plume moves away from a source, and a plume may “dive” when vertical recharge from overlying soils or strata pushes it deeper into the flow system, but these are typically shallow phenomena, neither is related to plume density, and both would affect plumes of any dissolved substance.
So, with a little basic physics and chemistry, we’ve debunked the misconception that plumes of dissolved chlorinated compounds in groundwater dive or sink as a result of their perceived greater density. Unless the laws of physics are repealed, we have to rely on them to explain what we observe – part of the process of developing conceptual site models that assist us in accurately characterizing and remediating contaminated sites.
1Hem, J.D., 1985, Study and Interpretation of the Chemical Characteristics of Natural Water, US Geological Survey Water-Supply Paper 2254, 3rd Ed. (https://pubs.usgs.gov/wsp/wsp2254/pdf/wsp2254a.pdf).
2US EPA, 2016, Regional Screening Level (RSL) Chemical-specific Parameters Supporting Table May 2016. (https://www.epa.gov/sites/production/files/2016-06/documents/params_sl_table_run_may2016.pdf).