| dc.description.abstract | We investigated groundwaters in the vicinity of a coal ash site near an electric generating
station in the western U.S.A. The purpose of the study was to ascertain why fine particles or
colloids appear in some subsurface water samples there. If such fine particles are merely introduced
during bailing or pumping operations which suspend otherwise immobile soil colloids, we should
exclude these particulate materials from the water samples before analysis intended to quantify
what is moving through the aquifer. However, if the colloids were truly suspended and moving with
the groundwater flow in situ, then we should include their contribution to our assessment of the
mobile loads.
Application of very careful sampling techniques (slow pumping rates, no atmospheric exposure)
did not cause the large quantities of colloids observed previously to disappear from well water in
which they occurred. Additionally, the same sampling procedures did not cause similar abun-
dances of colloids to appear in waters collected from neighboring wells installed and' developed in
the same manner and in the same geologic strata. Thus we believe sampling artifacts do not explain
the colloids' presence in the groundwater samples.
On the other hand, the groundwater chemistry and the nature of the suspended colloids (size,
composition) strongly suggest these fine particles were suspended and therefore moving with the
groundwater flow. At wells exhibiting large amounts of suspended colloids (- 10-100mg L-), the
water was enriched in CO2 and depleted in 02 relative to nearby locations. The colloids were
typically between 0.1 and 2 gm in size and were primarily silicates. These results suggest to us that,
where infiltrating water is percolating through a site that has been mixed with coal ash, the
secondary carbonate minerals in the soils are being dissolved; removal of this cementing carbonate
phase may consequently release soil silicate colloids to be carried in the flowing water.
Such processes may enhance contaminant transport in groundwater by augmenting the
pollutant load moving in the groundwater, and increasing the permeability of the porous medium
to pollutant infiltration with waste water and/or rainwater. | en |
