Environmental Engineering Consulting

In order to better understand the chemical processes which occur during weathering, the authors explains that steel slag, is composed of metal and calcium silicate minerals (Roadcap, Kelly, and Bethke, 2005, Page 809).  The presence of calcium silicate in the Lake Calumet region was confirmed during the drilling of a monitoring well.  Since calcium silicate appears in multiple chemicals forms, two common forms were considered, which are rankinite (Ca3Si2O7), and larnite (Ca2SiO4).  As well as akermanite (Ca2MgSi2O7), which is typically found in slag (Roadcap, Kelly, and Bethke, 2005, Page 810).

The authors effectively uses a cross-section to visualize the stratification of the subsurface pit, which shows a layer of unweathered slag sandwiched between weather slag regions; the lower of the weathered regions is mostly below the ground water table (Roadcap, Kelly, and Bethke, 2005, Figure 2, Page 810).  The aquifer discharge into to a nearby wetland formed a thin calcite (CaCO3) layer, which precipitated because of slag weathering.   The authors correctly note that the presence of carbonate (CO3) is generated by a source external to the slag since carbonate is not a product of the slag weathering equations (1), (2), and (3) (Roadcap, Kelly, and Bethke, 2005, Page 810).

The authors introduce a plausible source of the carbonate by stating that atmospheric CO2 diffusion, bicarbonate (HCO3-) from precipitation and underlying sand and soils, and carbonic acid (H2CO3) from precipitation, led to the calcite precipitation at the wetlands entrance, according to equations (4) and (5) (Roadcap, Kelly, and Bethke, 2005, Page 810).  Since the addition of CO2 to the ground water produces additional H+, pH decreases with additional CO2.  It was on the basis, that the authors introduced CO2 sparging as a potential remediation scheme.  The authors also make the important point that that pH neutral waters behave much differently to CO2 introduction than the extremely alkaline waters considered in the article, which is another reason special consideration is required when considering remediating them.

The authors continue their detailed chemical analysis of the waters samples taken from springs at sites 2 and 7.  Reaction path modeling was used to determine the source of alkalinity in the springs, as well as ground water in the test pit (site 4) and pond (site 3), which demonstrated the source of pH in each.  The modeling results demonstrated high levels of Ca2+ that when interacting with carbonate, produced by the introduction of CO2, formed calcite to precipitate, according to equation (5) (Roadcap, Kelly, and Bethke, 2005, Page 811).

REFERENCES

Roadcap, George S., Kelley, Walton R., and Benthke, Craig M. 2005. Geochemistry of Extremely Alkaline (pH > 12) Ground Water in Slag-Fill Aquifers. GROUNDWATER, 43(6), 806-816.